Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/11/21
Area Forecast Discussion
National Weather Service Columbia SC
643 PM EST Fri Dec 10 2021
Low clouds, showers, and cool temps will linger around through
early Saturday morning. An approaching cold front will warm
temps quickly Saturday afternoon, followed by a line of showers
and thunderstorms Saturday evening. Some of the storms could
produce gusty winds. High pressure builds in Sunday with near
average temperatures. A warming trend is expected next week with
little-no chance for rain.
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Main forecast concern is possibility of dense fog forming north
of a warm front that is progged to move into the FA tonight.
Models may be too quick to move the warm front through, though
latest ACARS soundings confirm only a very shallow in-situ
wedge and guidance indicates a southerly low level jet
increasing to 30-35 knots later tonight. Will monitor
observations and trends and a Dense Fog Advisory for portions
of the FA will likely be required for tonight. May start with
one area and expand later if/when confidence warrants.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
By early Saturday, deep southwest flow will overspread the
region as the sharp shortwave over the Midwest slides east. As
the pressure gradient increases ahead of the associated cold
front, we should finally mix into the warm sector by late
Saturday morning as strong south-southwesterly surface flow
ramps up. Mixing should be strong as 10-15mph surface combines
with 850mb flow near 50mph. There are some questions over how
quickly the wedge-like surface layer will mix out with guidance
somewhat split. The NAME and several of the hi-res members keep
the lowest 500-1000m saturated well into the afternoon, despite
depicting strong southwesterly surface winds. GFS and EC
quickly mix out the lowest levels, surging temps into the mid-
upper 70`s. With drier air just above any saturated surface
layer and strong mixing, the NAM solution seems unrealistic,
especially with a lack of pre-frontal showers and moisture.
While confidence is relatively low, current thinking more
closely follows the GFS solution but delaying how quick the
surface layer mixes out by a couple hours. So high temps are
expected to reach the low 70`s ahead of the cold front.
The front and associated line of precip will enter our western fa
between 20z and 22z, exiting to the east by roughly 06z Sunday.
The severity of the leading convective line will generally be
limited thanks to unfavorable mid-level lapse rates after days
of warm advection aloft. Given the uncertainty over afternoon
temps, the hi- res solution for maintaining a saturated and
cool surface layer would force all convection to be elevated and
reduce the severe risk to near zero. The warmer and mixed out
solution would yield a higher, albeit still low, severe threat
as surface/mixed layer CAPE could sneak up to several hundred
J/kg. Regardless of surface layer solution, the mid-levels will
not be favorable so the CAPE ceiling is low. Kinematics are not
a question mark however, with good agreement in a robust shear
environment ahead of the front. Consistent agreement across the
HREF with 20+ knots of 0-1km shear and 50+ knots of 0-6km shear,
all southwesterly and mostly parallel to the front/forcing.
Coupled with some drier air at mid-levels, this could yield a
fairly efficient mechanism to mix down some strong winds if any
instability and strong updrafts do develop. Overall, the severe
threat appears limited but some gusty winds are possible given
the impressive wind profiles.
Strong high pressure builds in behind the front Sunday with
good agreement across guidance. As is typical, the strong cold
advection will be somewhat offset by downsloping and solar
heating throughout the day, so while temps will still warm to
around normal. Clear skies and weakening surface flow will
produce a fairly ideal radiational cooling night heading into
Monday, with low temps dropping to around freezing.
.LONG TERM /MONDAY THROUGH FRIDAY/...
High confidence in strong upper level ridging developing over
the central CONUS with above average temperatures and little-no
chance for precip. GEFS and all other longer range guidance
continues to show a remarkably strong ridge developing next week
with the upper level axis centering to our west, the 850mb
ridge centered overhead, and surface ridging off the coast.
Since the lower level ridge ridge axis will be nearly centered
over the fa, we will not see the strong southerly flow and
advection component to cause extremely above average
temperatures (like areas in the MS Valley and Central US will
see). However, like the NAEFS summarizes will, our temps will
steadily increase each day through the week, starting near
average Monday and 10-15 degrees above average by Friday.
Moisture will remain near average throughout the period with
GEFS PWATs all remaining below 0.8" through Friday.
.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
LIFR dominate the region this evening and expect these
conditions to remain through the overnight. A warm front over
central Ga will be moving slowly north overnight. Expect frontal
fog to develop ahead of the warm front. In fact, observations
are indicating low visibilities developing in east central Ga.
So dense fog appears likely overnight despite a shallow cold
wedge and strong winds developing above the inversion by 06z.
There may be some visibility improvement at AGS/DNL overnight,
but uncertain. Non-convective Low level wind shear will develop
overnight with 35 to 40 knots southwest winds above a shallow
surface based inversion where winds light and variable. This
shear will likely continue after 12z and low stratus through at
least 15z. Think enough mixing by mid morning to see MVFR
ceilings by 18z. A cold front will be approaching from the west
late in the day and winds will shift to southwest and increase
to 10 to 15 knots with higher gusts. A few pre-frontal showers
may develop late in the afternoon but most guidance suggest
showers and possible thunderstorms moving through the area
around or after 00z Sunday.
EXTENDED AVIATION OUTLOOK...Restrictions possible through
Saturday evening as cold front moves through the area with
showers and a few thunderstorms. Drier air rest of the period
and restrictions not expected.
Area Forecast Discussion
National Weather Service Louisville KY
750 PM EST Fri Dec 10 2021
Issued at 750 PM EST Fri Dec 10 2021
Tornado Watch #554 has been issued for the western two-thirds of our
CWA until 3 AM EST. Scattered pre-frontal convection has already
developed across the lower Ohio/Mississippi River confluence and
will quickly race northeastward into our western CWA within the next
hour. The VAD wind profiler data from VWX/HOP radars have shown
increasing low level wind shear resulting in very strong helicity in
the lowest 1 km (resulting in >400 m2/s2). Model soundings along our
far western CWA do show a bit of an inversion and farther east
toward the I-65 corridor we are seeing that inversion show up on
AMDAR soundings. RAP soundings over the next few hours show dew
points slowly increasing west of I-65, which may be enough to allow
those storms to become surface based when they move into the CWA.
Do want to convey that we expect another Watch will be needed
farther east in our CWA for the pre-dawn and around sunrise time
frame, but that will likely come at a later time tonight.
.Short Term...(This evening through Saturday)
Issued at 344 PM EST Fri Dec 10 2021
...SIGNIFICANT EPISODE OF SEVERE WEATHER EXPECTED TONIGHT AND EARLY
Deepening mid-level trough axis over the Plains will translate
eastward this evening and overnight. The trough axis is forecast to
remain positive to neutrally tilted as it moves toward our region.
Initial mid-level vorticity lobe moving NE across KS/OK this
afternoon will lead to initial convective development across
MO/AR/WKY/WIN/SCIL as early as late this afternoon/evening. Deep
tropospheric wind fields are expected to overspread the Ohio Valley
along with a strengthening low-level flow that will transport
deep/rich Gulf moisture northward into the region. The net result
of this combination of increasing instability and shear will lead to
a severe weather episode over the Ohio Valley tonight and early
Saturday morning. Damaging winds in excess 60 MPH and isolated to
scattered reports of tornadoes are expected. In addition, copious
amounts of moisture will lead to heavy rainfall which may result in
some minor flooding issues across the area.
Meteorological Technical Discussion
Surface warm front is rather diffuse this afternoon, but boundary
looks to be crossing through the region at this time, based on band
of elevated convection oriented from W/C IN southeast through
eastern KY. This activity will continue to move to the northeast
this afternoon. Once this activity clears out, we will be in the
open warm sector with moisture advection continuing through the
evening hours. Dewpoints are currently in the mid 50s across the
region but are approaching 60 in our southwest areas. A stronger
dewpoint surge is currently in progress out along the MS river and
dewpoints have already risen into the upper 60s over far southwest
In the near term, a mid-level short wave evident in the GOES
moisture channel was moving NE across OK/KS. This feature will
translate northeastward into the Midwest this evening. This feature
will likely lead to initial convection firing over AR/MO/WKY/WTN and
SCIL later this afternoon/evening. Further east over our area,
continued moistening and theta-e advection will continue across our
region. The front edge of a strong low-level jet axis will work
northeastward into very lower part of the Ohio Valley. As this
occurs, additional cooling aloft is expected to overspread portions
of SWIN/WKY which should allow higher instability to develop as mid-
level lapse rates steepen. These two items will also help erode the
elevated mixed layer of warm air aloft and result in a possible
uptick in surface based convection across our western CWA this
evening. Given the parameter space, initial cells that develop will
like be discrete/scattered and given the shear profiles a mix of
multicell and supercells are expected. These storms will likely
race off to the northeast at 55-60 MPH. Threats with this activity
will be damaging winds, perhaps some marginally severe hail, and
Many members of the various short range ensemble models generally
support this idea, i.e., more members that not. However, the
eastward extent of pre-frontal convective development this evening
remains uncertain due to unknowns involving how much of the EML will
erode and whether surface based convective inhibition will remain.
In general, the best chances of convective development this evening
will be in areas along and west of the I-65 corridor.
Later in the overnight, it appears that additional convection will
develop given the instability and deep forcing resulting in more of
a linear QLCS. Given the strong shear and substantial front-
parallel components of the deep layer flow, downward momentum
transfer should yield instances of wind damage within the convective
line. Additionally QLCS mesovortices within the squall line where
bows and surges develop may produce several instances of embedded
tornadoes. This line will work through region during the late night
hours and continue into our eastern sections after sunrise Saturday.
The expected progressive nature of this line should result in
generally short residence time of heavy rainfall over any one
particular area. Additional QPF information may be found below in
the Hydrology section of the discussion.
We will continue to monitor observational data closely this
afternoon and evening especially upstream. As cells develop and the
convective evolution becomes more certain, we will be able to update
our timing and speak a bit more confidently with regards to the
possible magnitude of this severe weather event.
As we have mentioned over the past 24 hours, we want to stress the
danger in the overnight severe weather potential. Historically,
tornadic and widespread severe weather events in the month of
December are rare, although we`ve had a couple of events in the last
10 years (12/2013 and 12/2015). We would like to emphasize the
1. Have multiple ways to get warnings overnight. Leave your cell
phone on and have WEA alerts turned on. Check the status of your
weather radio and make sure it is functioning. Charge/replace
batteries in your alert devices.
2. Review your severe weather plans for your household. Review
where your lowest level interior rooms in your home are and have
space ready in there to shelter. Put as many walls as you can
between you and the outside of your home. If you have a basement,
make sure a shelter space in an interior room or under substantial
framing is available and ready in the event of a warning. Stay away
from exterior windows and doors.
3. If you live in a mobile home, consider finding a more substantial
shelter this evening with friends and/or family. Even in non-
tornadic winds, gusts of 60+ MPH can easily flip mobile homes
whether they are tied or bolted down.
4. These storms will be moving quickly with speeds of 55 to 60 MPH.
Warning lead time on these storms could be a challenge especially
involving mesoscale vortices within the anticipated squall line.
West of I-65: Medium-High
East of I-65: Low-Medium
High confidence across our region
Medium confidence across our region
.Long Term...(Saturday night through Friday)
Issued at 320 PM EST Fri Dec 10 2021
After an stormy start of the weekend, the latter part of the
forecast will certainly be more relaxed. Ejecting upper wave will
give way to anomalous ridging aloft and surface high pressure for
the easternmost two-thirds of the CONUS. Meanwhile, upstream flow
will feature a series of digging troughs which will slowly advance
towards the central US by midweek, eroding the high-pressure dome.
Sunday-Monday... Morning temperatures both days will be below
freezing thanks to a combination of CAA and radiational cooling.
Afternoon highs will gently move from the upper 40s on Sunday to the
lower 50s at the start of the work week. A dry air mass in place
secures clear skies and plenty of sunshine as the surface high
pressure moves to the Mid Atlantic states and an West Coast trough
moves to Central Plains.
Tuesday-Wednesday...The aforementioned large scale configuration
imposes a southerly wind and accompanying increased theta-e
advection. As a result, temperature readings will progressively
climb and the forecast reflects maximum temps around the upper 60s
south of the Parkways by midweek. Expect winds to start picking up
on Wed with gusts in the 10-15 mph range while cloud coverage spread
Thursday-Friday...As the Central Plains trough progresses to the
Midwest/Great Lakes region, it pushes an attendant cold front to the
Ohio Valley. However, strong ridge aloft anchored over the SE CONUS
will inhibit further southward momentum of the baroclinic zone,
therefore, it could stall south of our area or right above us. There
is still some timing uncertainty with this evolution as noted in the
WPC mid-level EOF cluster patterns. That being said, there is
increasing agreement for warmer temps on Thursday which could result
in record high low for SDF. The latest NAEFS ESA guidance has 850-mb
temps around the 90th climo percentile. Winds gusts will be blowing
from the southwest around 15-20 mph on Thursday as the front sweeps
by central KY and diminishing by Friday. Finally, precipitation is
boosted for the end of next week and probably next weekend with the
frontal passage and subsequent stalling nearby. This scenario is
also reflected by the slight risk of heavy precipitation delineated
by CPC during the Dec 17-20 timeframe as return flow from the GoM
leads to high rain chances.
.Aviation...(00Z TAF Issuance)
Issued at 649 PM EST Fri Dec 10 2021
- Low stratus early this evening with scattered showers
- Southerly winds increase become gusty tonight
- Gusts tonight and overnight of 30-35kt
- Strong to severe thunderstorms moving through overnight
- Strong/gusty southwest to west winds will be seen on Saturday
behind a cold front
A strong storm system will approach and pass the region during this
TAF cycle, strong & impactful storms, gusty gradient winds, low
ceilings at times. In addition, the most intense thunderstorms will
briefly reduce visibilities below 2 SM.
For the evening and overnight period, we expect scattered convection
to develop over the next 1 to 2 hours across western KY and
southwest Indiana. After scattered storms (some supercells), these
storms are expected to organize into a well developed squall line
that will race across the region. While not explicitly defined in
the TAFs, localized gusts over 50kts can be expected in this line in
some areas. Additionally wind fields aloft are very strong and LLWS
is expected at all the terminals this evening and into the overnight
Convection should clear the terminals around or just after sunrise
Saturday however, scattered rain showers and gusty southwest to
westerly winds are expected throughout the day.
Medium-high on all elements.
Issued at 320 PM EDT Fri Dec 10 2021
Deep, moist SW flow characterized by PW~1.5" is being advected along
the warm sector ahead of an amplifying upper-level trough. Attendant
cold front will swing through our area Friday night/Saturday morning
bringing a strong-to-severe line of thunderstorms. Heavy rain from
this event might provoke localized flooding and river flow rises,
especially streams that experienced flow peaks during the last event.
Current forecast package is considering QPF in the 1-1.5" range
associated with the main frontal passage. Meanwhile 3-hr FFG sits
between 2-3" area-wide and the 0-40 cm soil moisture is still
average per the latest NASA SPORT analysis. Therefore, rainfall
should be largely managable giving minimal runoff. Additionally,
HREF 1-hr probabilities for QPF greater than 1" is less than 20%,
which aligns well with the progressive nature of the system. Only
problem for stagnant water is in poor drainage areas and low-lying
locations. WPC still manintains a Marginal Risk (1 out of 4) of
excessive rainfall due to the strong linear forcing, theta-e
advection, and any localized areas of training convection given the
alignment and strenght of the LLJ and the cloud-layer mean wind.
Another factor to consider is the antecedent hydrological conditions
of rivers that experienced flooding/rises this past Monday and
respond quicly to short periods of heavy precipitation, such as:
Rochester Ferry, Paris, Peaks Mill,and Boston. Other rivers worth
monitoring during the next 24 hours per the lastest AHPS/HEFS
forecast are the Green and White Rivers. It is important to mention
that the OHRFC does not expect any widespread risk of significant
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
953 PM CST Fri Dec 10 2021
Issued at 952 PM CST Fri Dec 10 2021
Warm front has surged north and lies roughly from Chicago west-
southwest to southern DeKalb/northern LaSalle County. Modifying
recent ACARS soundings from MDW for observed T/Tds in the warm
sector over our CWA suggests there is a weak near surface
inversion present with very weak instability. Despite the minimal
instability and weak inversion near the surface, the strongly
forced convection is taking full advantage of the extreme low and
strong deep layer shear. Have seen transient circulations
developing within the small short line segments. Have also seen
some near severe/isolated severe gusts with these short line
segments, which isn`t surprising given 50kt winds about 1500ft agl
on the KLOT VWP.
Over the next 1-2 hours, storms should continue moving eastward
into northwest IN and into a very similar environment that they
are in now. Given this, not anticipating much if any weakening.
Primary threat should continue to be locally damaging wind gusts.
While there is a weak near surface inversion, the extreme shear
and strong dynamics suggests that the subtle inversion could be
overcome with more intense convection. Will need to closely watch
for any transient circulations or developing mesovortices for
tornado potential over the next hour or two.
Issued at 655 PM CST Fri Dec 10 2021
No significant updates early this evening as we continue to
monitor upstream convective trends as well the evolution of low-
level thermodynamics ahead of and along a warm front over central
Illinois. At this point, the rapidly develop convection west and
northwest of St. Louis is of most interest as the associated
airmass quickly advects NE into central Illinois over the next few
hours. The potential for surface-based convection into the CWA
remains unclear even at this hour.
Near-term CAMs still show variability in overall convective
evolution, but the fairly consistent trends in the HRRR along with
reasonable depiction of how convection has developed thus far
supports leaning more toward its guidance for this evening. Our
greatest severe threat will likely be tied to any upscale
growth/congealing of the recent convection in northeast Missouri.
Any mature convective line will have the capability of punching
stronger winds through any existing shallow inversion, and may
even have enough micro/mesoscale dynamic influence on the
environment to weaken existing near-surface stability immediately
ahead of the line. Additionally, recent convective growth with
some surface-based potential in southeast Iowa indicates low-level
stability can quickly be overcome within this dynamic
Even more, gravity wave packets within the significant shear
across the frontal inversion have been evident on radar this
evening. these features can be of concern for convective
initiation or potentially helping to bring down stronger winds
with any slightly elevated convection. With all that said,
concerns for severe convective winds into the CWA, primarily south
of I-88, remains warranted through at least midnight, with the
greatest focus in the 9pm-midnight period. The potential for local
thermodynamic modification by any mature convection bears close
monitoring for low- level rotation under any enhanced updrafts,
especially south of the Illinois/Kankakee River Valleys.
Issued at 320 PM CST Fri Dec 10 2021
Multi-faceted hazards into Saturday with the strong storm system
impacting the region, including:
* Dense fog threat into early evening
* Thunderstorms and associated severe threats later this evening
into the early overnight
* Strong winds from the southwest to west pre-dawn Saturday
through the early afternoon
* A period of wind blown mainly light snow Saturday morning for
portions of northern Illinois
Regarding the going Wind Advisory, main change was to expand the
Advisory to include Winnebago, Boone, Lee, and Ogle Counties, in
effect from 3 AM to 12 PM.
After the visibility improvement in the late morning, starting to
see visibility creeping back downward for locales south of I-80
and also deteriorating visibility on webcams in downtown Chicago.
As the warm front sharpens into the early evening, we may well
need to issue a new Dense Fog Advisory that could also include
counties farther north. May need to start with some initial
counties in an advisory in the near term (within next hour or two)
if trends continue and then assess from there.
[Thunderstorms and Severe Threat]
Lead warm advection wing showers spreading east-northeast as of
this writing have had isolated embedded thunder, so have included
slight chance/isolated wording over the next few hours with this
Shifting to the severe weather threat tonight, SPC outlook
20z update maintained the level 3 (Enhanced) risk into areas
south of US-24, with no changes on northward extent of level 2
(Slight) and level 1 (Marginal) risk areas that were expanded
northward earlier this morning. As can best be gleaned from
guidance trends and plausible scenarios, there is solid agreement
in robust deep moist convection initiating in the early to mid
evening (00z-02z) near the MS River and then quickly tracking
east-northeastward. Then toward through a few hours after midnight
immediately ahead of the sharp system cold front, there is
another window of thunderstorms.
See below for more details, but in summary, the evening round
could be mainly elevated but have a wind threat. Window for
surface based severe threat would be tied to the 2nd potential
round, with this also having a damaging wind threat and possibly a
brief tornado threat. All in all, the presence of sufficient
MUCAPE tonight for a high shear low CAPE setup amidst very strong
dynamics and kinematics supports the level 1-3 risks from SPC. And
speaking of the shear and low level SRH, it is extreme as you`d
expect for a wintertime setup.
We`re still not in the realm of mesoanalysis for the local area
yet being well north of the warm front, but there continue to be
some key elements to watch as the warm front advances northward.
An exceptionally juicy (upper 60s-lower 70s Td) air mass resides
in the open warm sector in AR, far southern MO, and TN. The 60F
isodrosotherm has pushed to or just north of the latitude of St.
Louis as of this writing. Guidance varies on how far north the
60F+ dew points get this evening, and HREF CAMs currently have a
decent handle, so it`s too soon to say which guidance is on the
right track, though we did lean toward the slightly more
Also have questions on how much if any role the existing fog/very
low stratus bank has on evolution of the air mass and available
instability this evening. Would think that initially prior to
stronger pressure falls, the fog would somewhat inhibit northward
advance but then quickly strengthening southerly flow aloft should
advance it northward. If anything, the existence of this very low
cloud cover and obvious low level inversion may shorten the time
window in which severe weather is possible this evening/tonight.
A consistent item seen in poring through model forecast soundings
south of the warm front this evening is a pretty distinct low
level inversion around 900 or 925 mb. This seems to be tied to the
extreme warmth at 925 mb (+17C at LZK/Little Rock and +16C at
SGF/Springfield MO) set to advect over our area. Most recent
SPC/RAP Mesoanalysis shows +15C isotherm at 925 mb reaching KSTL.
Even with the unseasonably warm and moist air mass in the warm
sector, this magnitude of warmth at 925 mb is not being overcome
by surface warming through the bulk of the evening. This leaves
low-level inversion in place and providing rather large SBCIN
except for maybe a very short 1-2 hour window where that capping
can be overcome. As mentioned earlier, the likely evening round,
depicted as a QLCS type or mixed with semi-discrete supercells on
some recent HRRR runs, quite possibly could remain mostly
elevated. Temperature trends will need to be closely monitored, as
observations being just a few degrees warmer would lessen the
The above being said, think the modeled low level inversion does
present uncertainty as to the magnitude and mode of the severe
threat within the CWA. At this time, think the primary threat will
be damaging winds in QLCS storm mode during the evening, as the
tremendous momentum aloft could be mixed down through the
inversion (perhaps not in a widespread manner). Suspect that any
tornado threat would be mainly of the QLCS variety, though it`s
tough to say how that antecedent low level inversion would play
into the tornado threat. In low topped supercell storm mode,
decent likelihood of discrete/ semi-discrete cells remaining
elevated may limit severe threat aside from rear-flank downdraft
punch and perhaps marginally severe hail.
Then overnight (~05z-08z), immediately ahead of and along the
front, strong low level advection could help the environment
recover for a short 1-2 hour window of a surface based severe
threat, per consensus on forecast soundings. SPC outlook is a
reasonable approximation of the relative threat during this time
as well, with points south more favored. Confidence is a bit
lower by this time, as we`ll need to see how much the air mass
gets worked over, and if the threat truly becomes surface based.
We may also start to see some drying and rising LCL heights at
this time, suggesting that perhaps damaging wind threat will be
primary again. The tornado threat, while certainly not one to
completely discount given the extreme low level shear profile, is
Needless to say, and to reiterate, observational trends will need
to be closely monitored this evening, as to how well the models
are handling the temperature profile and dew point surge. In
addition, ILX will be launching a special RAOB sounding at 02z or
03z, which hopefully will help get a better handle on the
environment that will be in place in the mid-late evening. For the
00z sounding, they will still be north of the warm front. Finally,
the cold front sweeping across the area toward and beyond 09z will
end the convective threat and shift our attention to the strong
southwest to west winds.
[Wind and Wind-blown Light Snow Threat Saturday]
Intense pressure rises on the order of 12+ mb/6 hrs coupled with
strong cold air advection and a 50 to 60 kt low level jet aloft
will generate strong winds late tonight through early Saturday
afternoon. This will be as the surface low deepens into the low-
mid 980s mb over central Lake Michigan by 12z Saturday. Forecast
soundings indicate that winds of 45 to 50 mph are expected to be
mixed down to the surface across the CWA, with a potential for
gusts to exceed 50 mph right on the leading edge of the rapid
onset of CAA with pressure rise/fall couplet, as well as in the
deeper mixing during Saturday morning.
The strongest gusts and best chance for ~50-55 mph gusts (and
non-zero threat for 60 mph gusts) are most likely across the
southeastern half or so of the CWA, but recent forecast guidance
does support gusts around 40 to 50 mph occurring in Winnebago,
Boone, Ogle, and Lee counties late tonight through Saturday
morning. Because of this signal, the wind advisory starting at 3
AM CST on Saturday has been expanded to include Winnebago, Boone,
Ogle, and Lee counties until noon on Saturday. The rest of the
area will remain under a wind advisory until 3 PM CST Saturday
afternoon. The winds will slowly begin to ease late Saturday
afternoon and throughout the evening with gusts around 15 mph
expected Saturday night.
The strong short-wave trough axis will be pivoting overhead
Saturday morning, so during this time, still looks like a flip to
mainly light snow is plausible near and north of I-80 and especially
for northern tier where highest PoPs are. Model soundings vary on
how deep saturation gets, with some models pretty dry in DGZ and
some sufficiently saturated up to -12 to -13C. This suggests that
the snowflakes will be poor quality/small. Temperatures will crash
to the lower-mid 30s behind the cold front, with profile quickly
becoming snow supportive, so the main question is the aforementioned
saturation depth. Given such mild temperatures late tonight ahead
of the cold front, think that road accumulations are pretty
unlikely and main impact would be reduced visibility. Can`t rule
out some slick spots on elevated surfaces, with accumulation
primarily a coating to a few tenths on grassy surfaces. The
precipitation will end by mid day, with clouds quickly eroding
west to east yielding sunny breaks before sunset.
Issued at 123 PM CST Fri Dec 10 2021
Saturday night through Thursday...
Forecast thinking has not changed during the period. Please
reference the previous discussion below for details.
The main forecast point of note in the long term is the
pronounced warmth next week, with all days looking to be above
normal and a period midweek with 20 to 25 degrees above normal,
including possibly a record high on Wednesday and warmest December
day in several years (at least since 2017).
As has been the case for several weeks, the upper air pattern this
coming week remains notably progressive across the country, even
with strong synoptic lows and highs. For the latter half of this
weekend, a 1030 mb high south of the area will be scooting
quickly to the east. This will keep the CWA under the pressure
gradient on its north promoting breezy southwest winds on Sunday.
Both Sunday and Monday should be sunny as well, and support highs
near 50. Monday may even overperform given 850-925 mb temperature
climatology for this time of year and the dry air mass. Have
leaned toward warmer guidance as has been the forecast trend for
this period. Neither of the days stand out as a fire weather
concern based on the stronger gusts on Sunday and lower relative
humidity values being on Monday.
On Tuesday into Wednesday, an upper level ridge will build
northward through the Great Lakes, with highly anomalous heights
values at 500 mb. Deep low pressure in the lee of the Rockies
will strengthen over the northern High Plains area. A warm front
will approach and then lift north of the area, replacing the dry-
ish air mass that starts the work week with a very moist one for
this time of year on Wednesday. Tuesday should see increasing mid
and high clouds at least as warm front lifts north, so temps may
be similar if not a degree or two warmer than Monday in spots.
Increasing pressure falls Tuesday night will result in increasing
southerly winds and rising temps, providing a springboard for
Wednesday`s near record or record warmth. It will be windy with
southerly gusts up to 30-35 mph, with strong surface advection of
the warmth and moisture. At least low-mid 50s Td appears likely
and any higher would increase upside potential for high temps.
Conceptually, this pattern still continues to favor low stratus
due to strong south-southwesterly flow. With isentropic ascent
from strong WAA, could even see some sprinkles or drizzle break
The 850 mb temps of at least 10-15C are still shown for Wednesday
and this again is near top of the charts for December, so with all
the above factors in play, comfortable with the 61-66F range from
the NBM. Daily record high on Wednesday (12/15) for Rockford is
58F and is 64F for Chicago, both set in 1971. Strong south-
southwesterly winds look to result in a positively spring-like
Wednesday night likely not dipping below 50F ahead of the cold
front. Uncertain timing of the cold frontal passage will dictate
how warm temps get until the frontal passage, with mid-upper 50s
in current forecast. Should be a band of strongly force showers,
possibly even with a few storms with the front, so chance PoPs in
gridded forecast appear reasonable, though would be shorter-lived
than in the current blend-provided, broad-brushed grids.
For the 00Z TAFs...
Concerns for the near term include low cigs and vsbys in the rain
shield ahead of an approaching center of low pressure. Current
values are not quite as low across the area as expected earlier,
so elected to bump the initial vsbys up to MVFR in the 3 to 5 mile
range. Thunderstorm chances are not great early this evening, but
do ramp up somewhat as midnight approaches, more for the Chicago
terminals than RFD, and with MDW and GYY probably favored a bit
more than ORD and DPA.
Winds veer from easterly early this evening to southerly by later
this evening, around to southwesterly by midnight, then to the
west during the morning. The flip from south to southwest toward
midnight will be accompanied by a significant increase in speeds
and gusts as a cold front pushes through. These magnitudes are
expected to persist through late morning and very gradually
subside during the afternoon. Before the surface winds increase
this evening, and before they really ramp up behind the front in
the early overnight hours, winds riding over the stable surface
layer will be quite strong, so we have maintained a mention of
The last item to mention is the chance of some light snow toward
mid-morning as an upper level wave of energy behind the low swings
across the area. At this time this is not expected to produce much
more than flurries, but there could be a brief period of vsby
Issued at 123 PM CST Thu Dec 9 2021
...Higher end westerly gales Saturday morning...
A strong and deepening low pressure system will track northeast
over the southern half of the lake very late tonight through
Saturday morning. Ahead of this system, southerly winds will be
blowing near or into Small Craft Advisory this evening into early
overnight before the cold front arrives. Also in the hours ahead
of the cold front, convection is likely, and some of this will
potentially be strong and mix down winds in excess of 35 kt.
After the cold front passes around 3-4 A.M., solid westerly gales
of 40-45 kt will occur with continued high confidence. A few
storm force gusts may even materialize during Saturday morning.
Winds will slowly ease during the afternoon into evening on
No major changes have been made to the going forecast, and thus
the Gale warning remains in effect from 3am through 3pm Saturday.
ILZ106-ILZ107-ILZ108...3 AM Saturday to 3 PM Saturday.
Wind Advisory...ILZ003-ILZ004-ILZ008-ILZ010...3 AM Saturday to
IN...Wind Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...3 AM
Saturday to 3 PM Saturday.
LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...3 AM
Saturday to 6 PM Saturday.
Small Craft Advisory...nearshore waters until 3 AM Saturday.
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Area Forecast Discussion
National Weather Service Phoenix AZ
517 PM MST Fri Dec 10 2021
.UPDATE...Updated Aviation Discussion.
A quiet weather pattern will temporarily return to the region with
a cold and dry air mass settling in. Freezing temperatures will be
possible in some areas on Saturday and Sunday morning, especially
in southwest Arizona and southeast California. Quiet weather will
return and colder air will settle in today and this weekend.
Another weather system will move through the region sometime
between late Monday and Wednesday and bring a good chance of
showers, along with cooler temperatures later in the week.
The shortwave trough that brought widespread rain showers to
south-central Arizona has shifted east into the southern Plains,
ushering in a drier and cooler air mass over the Southwest. ACARS
soundings show pronounced drying above 850-mb in the past 24 hrs,
while modest moistening closer to the surface was noted. This is
also being reflected by an isolated CU field east of Phoenix
within a steeper low-level lapse rate environment and enhanced
upslope flow. Another band of high clouds is streaming in from the
east and is ahead of a weaker and dry shortwave impulse off the
southern California coast.
The primary impact for the next couple days will be the potential
for isolated freezing temperatures across the lower deserts of
southwest Arizona and southeast California. A Freeze Warning
remains in effect for Saturday morning across parts of these
areas, including Blythe and Parker. NBM probabilities continue to
advertise a marginal and localized event, with readings between 30
and 32 degrees expected in the coldest low elevation spots. A
general repeat of these temperatures is expected once again Sunday
morning, although with slightly less coverage than Saturday, and
an extension of the Freeze Warning may be necessary at some point.
By early next week, attention turns to an impressive speed max
over the Pacific Northwest which will result in an amplification
of a trough along the West Coast. Given the orientation of this
evolving trough, most significant moisture advection and IVT will
be confined to the coastal ranges of California, but rain
probabilities continue to show a general increase area-wide
Tuesday and early Wednesday. Given the strength and compact nature
of this system and associated speed max, any deficiency in
moisture will likely be easily offset by the degree of forcing for
ascent. Experimental NBM 4.1 forecasts are highlighting some
impressive QPF amounts across parts of western Imperial County,
where localized spots over 3 inches are possible west of El
Centro. Localized amounts of 1 inch were also noted in terrain-
favored areas of south- central Arizona. While there`s still
considerable uncertainty in rainfall totals at this point, trends
suggest this could be a wetter overall event than what was just
experienced - especially for southeast California. Near to even
below normal temperatures are likely in the wake of the early week
storm system as mean troughing persists over the western CONUS.
.AVIATION...Updated at 0017Z
South-Central Arizona including KPHX, KIWA, KSDL, KDVT:
Current light westerly winds are expected to switch to typical
easterly drainage winds during the mid-late evening hours tonight.
There will be a slight uptick in northeasterly winds above the
surface inversion Saturday morning, up to 15-20 kts around 1 kft.
While this is not expected to have any significant impact to
operations, it may lead to a light southwesterly wind developing
at KPHX at times during the early morning hours before more
solid/stronger easterly winds develop during the mid/late morning.
Weak sfc pressure gradients will keep the usual late-afternoon
westerly winds quite light, with light/variable winds at
times.SCT/BKN cloud decks this evening to transition to mainly CLR
skies late tonight thru Saturday.
Southeast California/Southwest Arizona including KIPL and KBLH:
Northerly winds are expected through the early evening at both
terminals, then becoming northwesterly-westerly tonight and
Saturday. Few/SCT high cirrus cloud decks this evening to
transition to mainly CLR skies late tonight thru Saturday.
Sunday through Thursday:
Temperatures will be slightly above normal Sunday and show little
to a slight positive trend into Tuesday. No precipitation or
significant winds are expected during that period. Humidity values
will gradually trend upward. By Tuesday and into Wednesday, the
next potential system will move into the region with widespread
chances for wetting rain, cooler temperatures, and breezy
conditions possible. Drier conditions expected by Thursday with
near normal temperatures and light winds.
AZ...Freeze Warning from 11 PM this evening to 9 AM MST Saturday for
Freeze Warning from 5 AM to 9 AM MST Saturday for AZZ530-533-534-
CA...Freeze Warning from 10 PM this evening to 8 AM PST Saturday for
Freeze Warning from 4 AM to 8 AM PST Saturday for CAZ569.