Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/29/21
Area Forecast Discussion
National Weather Service Denver/Boulder CO
813 PM MDT Sat Aug 28 2021
Issued at 813 PM MDT Sat Aug 28 2021
Another surge/weak cold front will drop south across northeast
Colorado later this evening. This may trigger a few additional
showers and thunderstorms late this evening and into the overnight
hours. Confidence in this is not high, so will keep PoPs on the
low side. Low clouds will form late tonight and linger into Sunday
morning. Highs will be about 5-10F cooler Sunday with lower to
mid 80s across northeast Colorado. Going forecast looks good with
only minor changes expected.
.SHORT TERM...(This evening through Sunday)
Issued at 218 PM MDT Sat Aug 28 2021
The surface front has pushed into northeastern Colorado with winds
turning northeast with gusting from 15 to 20 mph. The northeast
surge will be reinforced by boundary layer cooling and lagging
increasing moisture into the evening and overnight hours. Currently
there is Cu building over the higher elevations with lingering
moisture and daytime heating. There could be just enough instability
to allow for a weak isolated shower across portions of Park County
and the southern foothills late this afternoon. Various convective
allowing models are now showing some isolated showers pushing onto
the plains during the early evening, however, the window for
convection potential will be small. ACARs soundings do show a
weakening inversion over Denver that could help to sustain a shower
or two over the metro area.
Overnight the moisture surge will push southwest and across the
plains bringing increased stratus for Sunday morning. This will
improve through the day but leave high temperatures up to 10 degrees
cooler than today. This will help to stabilize much of the plains
with the cooler temperatures making it difficult to reach convective
highs, however there continues to be a slight chance over portions
of Park county and east by the late afternoon with brief rain and
gusty winds possible.
.LONG TERM...(Sunday night through Saturday)
Issued at 240 PM MDT Sat Aug 28 2021
Upper ridging with 30-45 knot zonal flow aloft will be the rule
Sunday night through Monday night. By Tuesday and Tuesday night,
the upper ridge builds into Colorado with west west-northwesterly
flow aloft. There is little in the way of any synoptic scale
energy for the forecast area through Tuesday night. Models
continue to show little moisture through Tuesday night; just bits
in the mid and upper levels here and there. Temperatures should be
well above normals both Monday and Tuesday, with maybe near record
highs on Tuesday.
For later in the week, Wednesday through Saturday, the upper ridge
pushes east with one more hot day on Wednesday. However, moisture
increases in the southwesterly flow aloft. A decent upper trough is
in place over the northwestern U.S. on Thursday and Friday. There
is still plenty of moisture progged for the CWA into next weekend.
A cooler airmass moves in Thursday and sticks around through
Saturday. So will leave some decent pops in through the period.
.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 813 PM MDT Sat Aug 28 2021
Northeasterly winds are expected to continue through tonight and
shift southeast after 12Z. A second surge from the north should
bring winds gusts to 25 knots for a few hours after 03Z. This will
bring a slight chance for a shower or thunderstorm through 08Z.
Low clouds are expected to form after 06Z and persist through most
of Sunday morning. Ceilings heights look to be 2000-4000 feet.
Issued at 240 PM MDT Sat Aug 28 2021
Fire danger continues to be elevated to critical across the
northern mountains near the Wyoming border and into fire weather
zones 211 and 217. Relative humidity values will reach into the
lower teens with gusting around 25 mph through the late afternoon
hours. Smoke has hindered mixing slightly with winds on the lower
end of the range, however with the fuels and dry conditions will
maintain the Red Flag Warning that is currently in place.
There could be some elevated fire danger with low humidities,
mainly over Jackson County Monday afternoon.
Issued at 240 PM MDT Sat Aug 28 2021
Dry conditions are expected across the burn areas through Sunday.
There is not a threat of flash flooding Sunday night through Tuesday
night. There looks to be better moisture in place Wednesday through
Saturday, so the threat of flash flooding will increase, especially
over the burn areas in the mountains.
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
659 PM CDT Sat Aug 28 2021
251 PM CDT
Through Sunday night...
Visible satellite imagery and AMDAR soundings out of MDW indicate
that the cap is quickly weakening across the area as temperatures
continue to warm into the low 90s amidst mid to upper 70s dew
points. For this reason, similar to the past few afternoons, we
are likely to see some isolated shower and thunderstorm activity
develop across parts of the area late this afternoon. Coverage of
these showers and storms will remain low, and be most favored
along any subtle low level convergence zones. In the near term,
it appears the best chances for their development will be near the
lakeshore, where a lakebreeze is hugging the lakeshore, and
across north central IL where an enhanced band of agitated
cumulus is noted in satellite imagery. Not everyone will see
them, but where they occur a localized heavy down pour and
isolated wind gust will be possible. This activity will quickly
wane with sunset.
Otherwise, the story tonight will be a very warm and muggy night.
Overnight lows in the city may not drop below 80 degrees, and
even the outlying areas will remain in the mid 70s. This will set
the stage for another hot and humid day on Sunday ahead of an
approaching cold front. It appears our area will remain free of
storms tonight, but we will have to watch for the possibility of
more storms to our northwest pushing an outflow boundary towards
northern IL by early Sunday morning. This could in turn spark a
few widely scattered early morning storms over parts of northern
IL early in the day, similar to what we saw this morning. However,
or best shot and scattered thunderstorms looks to be in the
afternoon and early evening.
During the afternoon, there continues to be some uncertainty in
the timing of storm development, but at this time it does appear
that as at least some widely scattered storms may develop along a
remnant outflow/effective cold frontal boundary as it begins to
shift eastward across northern IL during peak heating. These
storms may thus develop right overhead across parts of northern IL
during the afternoon, before they gradually propagate
southeastward along the boundary. Deep layer shear will remain
relatively weak, but with the hot and humid and unstable airmass
in place, expect the potential for some isolated instances of
strong wind gusts with the storms, along with torrential
The threat of storms should gradually sag south of I-80 during
the evening hours on Sunday. Heavy rain and possible localized
flooding will be possible, especially in areas where individual
storm motions result in training cells.
223 PM CDT
Monday through Saturday...
Conditions will be transitioning with temperatures in the lowers 80s
for afternoon highs and dew points in the 60s through the week.
However, the week will start in a transitional period with chances
for showers and thunderstorms for Monday and Tuesday.
On Monday, the cold front is forecast to be in the southern portions
of the CWA. Due to the forecast location of the front and the
possibility of some lingering outflow boundaries from Sunday`s
forecasted scattered convection we could see some convection
around Monday morning with better chances for showers and
thunderstorms in the afternoon mainly along and south of I-80.
Some of the convection that may develop in the area could have the
typical isolated gusty downdrafts and bring with it some locally
heavy rainfall given the draped front and precipitable waters
near 2 inches.
Additionally, a low amplitude impulse in the flow aloft is forecast
to move through the area late Monday night into the day on Tuesday
which will keep a chance for showers and possibly even a storm or
two. High pressure will begin to move in from the north heading into
Wednesday which will bring us pleasant conditions for the
remainder of the work week. However, dangerous swimming conditions
could be present at the lake on Wednesday due to northeasterly
winds which may build waves to an average of 4+ feet.
For the 00Z TAFs...
Aviation Forecast Concerns:
* Isolated TS dissipating early this evening.
* Breezy west-southwest winds Sunday, with gusts around 25 kts.
* Chance of TSRA Sunday afternoon, mainly east of KRFD.
* Cold frontal passage Sunday evening turns winds NW.
Scattered thunderstorms developed over KRFD and KDPA/KORD areas
this afternoon, focused along weak convergent lines in southwest
low level wind field. Rapid decrease in coverage/intensity
occurring at this time, as diurnal heating wanes. Recent satellite
and radar trends suggest little/no additional convective threat
beyond 00Z. A few lingering cells along the IL/WI border and out
over the lake north of ORD should move out of C90 airspace early
this evening. Otherwise, VFR conditions expected overnight with
modest southwest surface winds. Southwest low level jet 25-30 kts
develops 1000-2000 ft AGL overnight, and may allow for a few
sporadic gusts (perhaps 15-18 kts) but shear is expected to
remain below TAF LLWS criteria.
Fairly strong mid-level disturbance will track across the upper
Midwest and northern Great Lakes late tonight and Sunday.
Deepening surface low pressure will pass well north of the
forecast area across Ontario, but will trail a cold front across
the terminals late Sunday afternoon/evening. Some potential for
thunderstorms will exist ahead of the front Sunday afternoon,
though details of evolution and extent of coverage remains
somewhat low confidence at this time. Some high-res CAM guidance
suggests that outflow from current storms across MN/SD/NE could
reach the area after sunrise Sunday, with a low but non-zero
potential that isolated convection could develop along it as
early as 14-16Z. Higher likelihood that convection would develop
ahead of the front itself closer to 19-20Z per CAMS, primarily
affecting the Chicago metro terminals through 23-00Z before moving
east and southeast. Current CAM runs don`t show very high
coverage however, and appear to favor areas off to the
east/southeast of Chicago for greater coverage by Sunday evening.
Low level flow veering more west-southwest ahead of the front may
support weaker convergence and thus a lower convective threat.
With somewhat lower confidence have maintained prob30 mention for
Chicago metro terminals during the afternoon, with that timing
still very reasonable.
Southwest winds look to become gusty as mixing commences after
sunrise, with gusts in the 25 kt range expected. Gusts may east by
mid-late afternoon as low level flow veers more west-southwest and
eventually west ahead of the front. Surface front then moves
through the area during the early- mid evening hours, shifting
winds from west to northwest.
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Area Forecast Discussion
National Weather Service Phoenix AZ
503 PM MST Sat Aug 28 2021
.UPDATE...Updated 00Z aviation discussion.
The best storm chances will be over higher terrain areas of
Arizona this weekend, but a few isolated showers or storms and
gusty winds are likely to impact the south-central Arizona
deserts. Hot temperatures with highs just above 110 degrees are
expected this weekend across southeast California and southwest
Arizona, while highs are likely to fall just short of 110 degrees
in the Phoenix area. A much more unsettled weather pattern is
expected for a good portion of next week as tropical moisture
surges into the region by midweek. This will bring very good
chances for rainfall through the middle of next week, likely
focused across south central Arizona, while also bringing much
cooler temperatures as well.
Latest RAP streamline analysis depicts an upper-level anticyclone
across central Arizona. This will generally result in a weak
northeasterly steering flow this afternoon and evening. Meanwhile,
in the lower levels, a well-defined gulf surge from a northern
Sonora MCS pushed through the lower Colorado River Valley this
morning. Gusts up to 30 mph were observed near Yuma, while further
west in the Imperial Valley dewpoints reached the upper 70s,
briefly yielding oppressive heat indices as high as 120 degrees.
Across the Phoenix metro area, the morning moisture has also
taken a bite out of afternoon temperatures. Current temperatures
are generally running 5 to 7 degrees cooler than they were at this
time yesterday. This is also evident in afternoon ACARS
soundings, which depict the cooler boundary layer and the area of
CIN just above that. However, further aloft temperatures are
relatively cool, right around -7 deg C at 500 mb. Consequently,
latest mesoanalysis indicates relatively steep mid-level lapse
rates and a widespread area of 1000+ J/kg MLCAPE across the lower
Consensus from the CAMs points to scattered shower and
thunderstorm activity across the Mogollon Rim and higher terrain
approaching the foothills north and east of Phoenix. Given the
elevated DCAPE, an outflow boundary will likely push westward from
this activity into the lower deserts. However, there is
considerable uncertainty whether this boundary will be strong
enough to break through the aforementioned CIN and generate new
convection across the Valley. Latest HREF indicates roughly a 50%
chance of an outflow boundary capable of producing winds up to 35
mph along with patchy blowing dust. Across southwestern Arizona,
the chance of storms will be somewhat higher as the outflow
boundary encounters a more favorable environment where CAPEs
currently exceed 2000 J/kg. The strongest storms will also be
capable of producing localized gusts up to 60 mph, particularly
across portions of Pinal, southern Maricopa and La Paz counties.
Temperatures today will cool slightly off of the previous couple
days, but the Excessive Heat Warning with highs of 110-114 degrees
will remain across southeast California and southwest Arizona
through Sunday. The increase in boundary layer moisture across
southern Arizona should keep temperatures mostly in a 107-109
degree range in the Phoenix area, but it will still feel quite
hot. Sunday`s temperatures look to be a near mirror image of
today, but boundary layer moisture is expected to lower a bit due
to the lack of a Gulf surge tonight. Afternoon scattered storms
are again expected over the Arizona high terrain on Sunday, but
due to increasing subsidence over the region the activity is
likely to be more subdued. Slightly lower moisture levels and
reduced MUCAPEs over the lower deserts on Sunday is likely to
limit or even inhibit any storm development over the south-central
For next week, there remains high confidence in a tropical
moisture surge first starting on Monday, but intensifying Tuesday
into Wednesday. A rather complex pattern is likely to take shape
for the first half of next week. An upper level trough just to
our southwest will become a major factor for our weather while
combining with the increasing tropical moisture well out ahead of
Hurricane Nora which is expected to gradually weaken as it tracks
into the Gulf of California.
Monday is likely to be a fairly limited storm day across most of
southern Arizona as subsidence aloft and a capping inversion is
likely to keep most storms confined to higher terrain areas.
Increasing moisture into southeast California on Monday may
present a favorable environment for some isolated to scattered
storms, possibly strong if the GFS forecast sounding for KIPL
comes to fruition. As a strengthening upper level jet associated
with the trough to our west moves northward across southern
California into the Great Basin on Tuesday, much of Arizona will
eventually become perfectly positioned within a favored vertical
ascent region. The combination of the deep tropical moisture
overtaking Arizona Tuesday into Wednesday and the favorable ascent
fields should give rise to fairly widespread shower and
thunderstorm activity sometime late Tuesday, likely lasting
through a good portion of Wednesday. Model ensembles continue to
support this main area of likely moderate to at times heavy
rainfall falling somewhere across south-central Arizona, but we
are still a few days out and it`s still possible for some shifting
in the expected heaviest rainfall. There is also a distinct
possibility of some isolated to scattered strong thunderstorms at
some point next Tuesday and Wednesday where higher instability and
shear falls, likely farther west over southeast California and/or
southwest Arizona. Forecast QPF amounts of greater than an inch
are seen over a large swath of south-central into central Arizona
for late Tuesday through Wednesday, but it is still a bit early to
know the specific details of timing and location of the greatest
heavy rainfall threat.
By late Wednesday, the upper level support should be waining as
the jet max lifts well to our north, but some model guidance shows
at least some jet dynamics supporting our region into late next
week. What happens with the remnant circulation of Nora is
impossible to know at this point, but it seems unlikely an intact
circulation will make it as far north as Arizona. Even without a
direct impact of Nora, the increased moisture is likely to stick
around over much of our region through the end of next week
resulting in at least a chance for showers and storms.
Forecast temperatures for next week take a big dive into the
middle part of the week as highs are likely to dip into the 90s
across a good portion of the lower deserts as early as Tuesday,
but definitely by Wednesday. For now it seems likely the areas
that see fairly continuous rainfall, or at least thick cloud cover
on Wednesday will struggle to reach 90 degrees. The below normal
temperatures are then likely persist through the rest of next
week, especially if the deeper moisture sticks around the region.
.AVIATION...Updated at 0003Z
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Primary aviation concern through the TAF period will be the
potential for convective impacts, mainly outflows from decaying
thunderstorms. While the chances for VCTS at any terminals now looks
to be quite low (10-20%), strong winds with dust from surroundings
storms are more probable. Current thinking is that a relatively weak
boundary may move thru first from the east or east-northeast. Wind
speeds will likely be on the low side (gusts into the 20-25kt range
at most). Then another outflow from more of a southeasterly
direction will attempt to move into the region, bringing slightly
strong winds and (perhaps) a bit of blowing dust int the Phoenix
area during the 03-05Z timeframe. This 2nd outflow is expected to
keep elevated southeasterly winds going thru sometime after
midnight, with more typical lighter southeasterly winds thereafter.
Cloud bases should remain elevated around 12 kft and above,
outside of storms. There are indications high-based ACCAS showers
may develop Sunday morning, but outside some brief gusty winds and
shifts, they should be minimal impact. Another round (similar to
today) of showers/TS is expected to develop along the Rim/White
Mountains Sunday afternoon/evening, bring the likelihood of outflows
back into the Phoenix area during the evening, with VCTS chances
once again remaining quite low (10-20%).
Winds to remain mainly out of a southeasterly direction at KIPL and
a southerly direction at KBLH through the TAF period. Thunderstorms
are still not anticipated at either terminal, but outflow winds from
storms southwest of KIPL and east of KBLH cannot be completely ruled
out. Elevated convection may also move into the region Sunday
morning from the east, but confidence is too low to add any mention
to the TAFs. Cloud bases should remain aoa 12 kft.
Monday through Friday:
Increasing moisture and rain chances will be seen early next week
with temperatures eventually cooling to below normal by Tuesday.
Shower and thunderstorm chances are likely to peak Tuesday into
Wednesday with widespread wetting rains likely as tropical
moisture surges into the region. Rain chances are likely to
persist to some degree late in the period, but is likely to be
more of the scattered variety. Minimum RH values will increase
from around 25% on Monday to 40-50% by Wednesday before gradually
lowering late in the period. Winds should be relatively light with
an afternoon upslope component and overnight drainage expected for
a good portion of next week, but southerly winds are also likely
to be seen a bit more than typical.
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
AZ...Excessive Heat Warning until 8 PM MST Sunday for AZZ530>533-535-
CA...Excessive Heat Warning until 8 PM PDT Sunday for CAZ560>570.
Area Forecast Discussion
National Weather Service Great Falls MT
747 PM MDT Sat Aug 28 2021
Skies clear overnight. Patchy frost may be possible for some
higher elevation valleys. Fair weather continues Sunday into
Monday, with gradually warmer temperatures. The next Pacific
weather system will likely bring a return to cooler than normal
temperatures and unsettled weather for the mid- week period into
With ridging dominating the region, a more stable atmosphere
prevails, and a quiet night is forecast. The current forecast is
working nicely; therefore, no update will be made, at this time.
Shallow cumulus persist this afternoon due to weak instability
associated with cooler air aloft. Isolated light showers remain
over the higher terrain of central MT, but overall are dissipating
as drier air air moves into the region behind the departing
shortwave trough. Expect mostly clear skies tonight. Some of the
higher valley locations could experience patch frost early Sunday
morning. Fair weather prevails through Sunday, with gradually
warmer temperatures associated with a dry westerly flow aloft. PN
Monday through Tuesday...The next system to impact the area will
start to move into the Pacific Northwest on Monday, shifting the
flow aloft from westerly to southwesterly. The inverted surface
trough will remain over the area, which will help continue the
warmer than normal temperatures. The flow aloft will bring a few
more clouds to the area, and possibly a period of smoke once
again. A weak disturbance in the flow may also bring a few very
light rain showers to the plains east of Interstate 15 Monday
night. The slow eastward progression of the low pressure trough
across the Pacific Northwest will help push the inverted surface
trough east across the forecast area on Tuesday as a Pacific cold
front, bringing a chance of showers to much of the area, as
temperatures cool back below normal for much of the area. There is
a slight chance for a few thunderstorms, but it is less certain
that they will develop. Winds behind the front over the plains
will become gusty out of the northwest, but along and ahead of the
front, gusty southwest winds will be more likely in southwest
Wednesday through next Saturday...Overall, forecast models
indicate that the low pressure trough will dig south over the
Pacific coast states. This will continue the pattern of cooler
than normal temperatures with a chance of showers and
thunderstorms each day. This seems to be handled well by the
National Blend of Models (NBM), so will not deviate from it at
559 PM MDT Sat Aug 28 2021 (29/00Z TAF period)
VFR conditions prevail throughout this 00Z TAF period. Mildly gusty
winds impacting light aircraft are occurring along The Northern
Rocky Mountain Front for the first couple hours. Then, light winds
Refer to weather.gov/zlc for more detailed regional aviation weather
and hazard information.
As temperatures warm across north central and southwest Montana
Sunday and Monday, afternoon relative humidity will generally
fall into the 15 to 25 percent range, which will persist across
southwest Montana into Tuesday ahead of a Pacific cold front.
Overall, though, fire weather concerns will be fairly low, as
winds will mostly remain light. However, afternoon winds in far
southwest Montana (south of Interstate 90) will become somewhat
breezy (southwest 10 to 15 mph with gusts to 25 mph) on Monday
and Tuesday, which will need to be monitored, but does not pose a
threat at this time.
.PRELIMINARY POINT TEMPS/POPS...
GTF 44 81 46 86 / 0 0 0 0
CTB 43 77 43 80 / 0 0 0 0
HLN 47 83 51 87 / 0 0 0 0
BZN 41 81 44 86 / 0 0 0 0
WYS 29 72 33 77 / 0 0 0 0
DLN 39 79 43 84 / 0 0 0 0
HVR 48 79 49 85 / 0 0 0 0
LWT 45 78 48 84 / 0 0 0 0