Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/05/19
Area Forecast Discussion
National Weather Service Louisville KY
841 PM EDT Wed Sep 4 2019
Issued at 841 PM EDT Wed Sep 4 2019
The cold front has passed south of the area, and dew points are
slowly dropping off on a steady N wind across much of the area. For
instance, already seeing the upper 50s at SDF/LOU. In contrast,
southern KY is still holding onto lower 70s dew points. These values
should drop off into the 60s pretty quickly this evening, however
there is some concern that some fog may form across southern KY
toward dawn given the lingering low level moisture. A mitigating
factor could be the light but persistent N surface winds so don`t
fog to become dense, but will keep fog mention in the forecast.
Forecast looks on track this evening, and mainly just updated the
grids for latest obs and trends.
.Short Term...(This evening through Thursday)
Updated at 310 PM EDT Wed Sep 4 2019
AMDAR and model soundings as well as GOES Sounder data show quite a
bit of elevated instability across the region, as well as enough
moisture to work with for storms to develop...however the low-levels
of the atmosphere are fairly well capped by a strong inversion. The
question mark for any late afternoon development will be whether we
can get enough convergence along a cold front moving southeast
across the region...combined with differential heating boundaries as
well as the surface heating to get to a point to break that cap.
Most hi-res near term models are arguing against that development,
with 18Z soundings from ILN/OHX both showing convective temperatures
well into the 100s, will not carry any pops tonight. With loss of
heating tonight, expect any type of cumulus field to
diminish...though we may see some thin low stratus develop overnight
with moisture trapped under that inversion. Will lean on the
optimistic side for now...having the cold front clear through enough
of the region to have those clouds form more south of our KY/TN
With the assumption the clouds will be gone in the morning, Thursday
should be a very pleasant day. Highs will rise into the upper 70s to
lower 80s across the area, but a steady north wind should
help dewpoints fall into the 50s for many places by late afternoon.
.Long Term...(Thursday night through Wednesday)
Updated at 320 PM EDT Wed Sep 4 2019
Another dry cold front looks to come through the area Friday...not
really bringing a change in airmass, as we should see temperatures
return to normal for the weekend. Rain chances will be limited again
by capping aloft, and what looks to be even less available moisture
than the front we have today. Despite temperatures near normal for
the weekend, dewpoints again getting into the upper 50s to low 60s
should make each day relatively comfortable.
As we get into the next work week, the GEFS mean fields show a ridge
developing over the Deep South. Should this occur, it looks to keep
the storm track to our north with temperatures warming up each day.
The GFS and Euro both allude to this pattern as well, but the CMC
keeps more troughiness aloft over our region. Will hedge a little
toward climo 20 pops by Wednesday, given that model difference and
some weak signals in the GEFS QPF fields by then. We also have a
slight chance early Monday, with a warm front lifting north over our
northern CWA, as we transition into that pattern.
.Aviation...(00Z TAF Issuance)
Updated at 733 PM EDT Wed Sep 4 2019
A steady N wind is expected overnight behind a departing cold front.
These winds and the drier air behind the front should keep fog
formation in check, although can`t rule out some MVFR fog at BWG
toward dawn. Otherwise, steady N or NNE winds continue on Thursday
with a few flat cumulus around 3 K feet.
Area Forecast Discussion
National Weather Service Phoenix AZ
510 PM MST Wed Sep 4 2019
Updated 00Z aviation discussion.
Seasonably hot weather will persist through the week with excessive
readings this afternoon, and only a few degrees of cooling through
the end of the week. A small weather disturbance may spark a few
storms through much of the area late this afternoon into Thursday
morning, however drier air will steadily spread through the region
into the weekend. A stronger weather system and frontal boundary
will approach the region next week with fairly good rainfall chances
through south-central and eastern Arizona, as well as much cooler
Latest streamline analysis depicts a mean layer southeasterly flow
around the Monsoon High, which has migrated eastward into the
southern Plains. Meanwhile, a pocket of drier air is evident on
water vapor imagery across central Arizona. Further to the south and
west, thunderstorms across the northern Baja Peninsula are
indicative of a weak inverted trough, which extends into southwestern
Between the aforementioned trough near Puerto Penasco and the
anticyclone further east, the pressure gradient is relatively
strong, resulting in an anomalous u-component to the steering flow,
particularly across southern Arizona. This will favor propagation
of convection as far westward as southeastern California this
afternoon and evening.
Across the Phoenix Metro area, latest ACARS flights indicate a
mid-level isothermal layer, which will reduce CAPE this afternoon.
Consequently, a lack of convection further upstream across Gila
County will reduce the potential for an outflow boundary triggering
additional storms in the lower deserts. However, the intense
diabatic heating with temperature approaching 110 degrees should at
least should be sufficient for isolated to scattered air-mass
storms to initiate. DCAPEs approaching 1500 J/kg suggest that any
storms that develop will be capable of producing strong winds
and patchy blowing dust.
Latest consensus from both the HREF and local CAMs generally
highlight areas further west from Phoenix westward for thunderstorm
activity. This is likely due to the broader ascent from the inverted
trough and widespread instability (MUCAPE approaching 1000 J/kg).
Higher PWATs will also translate into an increased threat for flash
flooding as far west as JTNP.
Shower activity will likely linger into Thursday morning across
southeastern California before dissipating in the afternoon and PoPs
were raised in these areas. Consensus from the CAMs again points to
isolated storms across southwestern Arizona and southeastern
California Thursday afternoon.
Confidence is moderate to high that more resounding dry air through
the entire atmospheric column along with general subsidence will
spread into the region Friday and Saturday.
Quite a pattern shift is still being advertised among all
operational and ensemble members starting next week, although there
is rather significant differences in timing and depth of northern
stream waves entering the SW Conus and Great Basin. Recent
iterations of the GFS remain quicker and less amplified with a
series of shortwaves progressing into the western US while the ECMWF
carries more depth to troughing becoming established through the
western Conus (while ensemble members cover a wide range of
outcomes). A few consistent facets carry over all model output 1)
stronger subtropical ridging will temporarily be eroded with H5
heights falling to near 586dm and consequently temperatures
depressed back near or slightly below normal, and 2) unusually early
season height/pressure falls associated with this trough will force
stronger low level southerly flow through the eastern half of
Arizona drawing moisture north from Mexico. Some of this moisture
and potentially subtle waves may be associated with the long decayed
remnants of TS Fernand crossing westward from the Gulf of Mexico.
Regardless, models suggest ample cooling aloft with this trough, and
along with dynamical forcing, forecast soundings actually look quite
enticing for the eastern portions of the CWA during this transition
season type event. Have bumped up POPs generally for the entire Sun-
Tues time frame as model output indicates the potential for multiple
rounds of convection aided by deep upslope flow and dynamical
influences. Given all the uncertainty describe above, could not
really pinpoint a specific time frame more likely for rain/storms;
though conceptually with the forcing, instability, and stronger
shear parameters, prospects are excellent for at least one
convectively active day around and east of the Phoenix metro during
the first part of next week.
.AVIATION...Updated at 0009Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Very isolated convection will affect the Phoenix terminals for
the next few hours, with an isolated cell developing E-NE of FFZ
and additional cells possible with outflows later this evening.
Confidence on deep convection affecting any of the terminals is
low because of weak midlevel stability and drier conditions
relative to the last few days and further west. Thus, we have
included VCSH for now through 03Z at KIWA and KSDL with a 02-04Z
period for KDVT and KPHX, the latter of which would be due to
outflows from the S-SW and/or east if either materializes. Winds
will most likely favor south to southwesterly headings at most of
the Phoenix terminals this evening except KIWA where more south-
southeasterly headings will be favored. Light and variable or
easterly winds will return overnight through tomorrow morning,
with winds most likely veering to the south-southeast by late
morning or early afternoon. If winds do veer to the southwest
tomorrow, it will not be until very late in the afternoon.
Southeast California/Southwest Arizona including KIPL and KBLH:
Isolated to scattered thunderstorms with broken 10 kft ceilings
will continue to affect SW Arizona and SE California for at least
the next few hours, with VCTS included through 02-03Z at KIPL and
BLH. Although storms are generally remaining south of both sites,
outflows from the southwest at KIPL and southwest to southeast at
KBLH will be possible, with gusts to 25-30 knots and blowing dust
possible. Some hi-res models suggest convection may continue into
the evening and overnight hours with a potential inverted trough
moving in from Sonora, but the ongoing convection may stabilize
the atmosphere enough to prevent this from occurring or temper
coverage significantly. For now we have just VCSH through 05Z at
KBLH and 08Z at KIPL, but this is a low confidence forecast that
will likely change significantly with the next TAF cycle. In the
absence of convective outflows, winds should return to south-
southeasterly headings below 10 knots overnight through tomorrow
afternoon with lower chances of convection tomorrow.
Aviation Discussion not updated for amended TAFs.
Saturday through Wednesday:
Seasonably hot temperatures will persist through Saturday, however
much cooler weather will arrive next week as readings fall near or
slightly below average with an incoming storm system. There will
be a sharp gradient between relatively dry air to the west and
relatively moist air from Phoenix eastward. The storm system
next week will could result in fairly widespread wetting rains over
higher terrain area in the eastern districts while southerly winds
periodically become gusty around a frontal boundary.
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530-
CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ567-
Area Forecast Discussion
National Weather Service Blacksburg VA
759 PM EDT Wed Sep 4 2019
A cold front in the Ohio Valley will move southeast this
evening, crossing the Mid- Atlantic Region tonight into
Thursday. Hurricane Dorian will move slowly north, along the
southeast Atlantic coast into Thursday night before exiting the
Outer Banks on Friday.
.NEAR TERM /THROUGH THURSDAY/...
As of 230 PM EDT Wednesday...
Subsidence/dry air continuing to hold across the region this
afternoon with a weak downsloping component helping to confine most
cumulus to the far northwest ahead of the upstream cold front.
Latest analysis does show a decent ribbon of instability just in
advance of the boundary to our northwest, but appears any convective
development will be slow per limited convergence for now. Latest
short term solutions basically agree on only isolated coverage at
best until perhaps mixing decreases and the 85h front edges into
northeast sections this evening. This could result in some
convective development east of the mountains before the low level
boundary slides through after midnight. Developing northeast flow in
the wake of the front may also undercut warm air aloft by daybreak
resulting in some low clouds espcly Blue Ridge east as moisture
pools a bit below the inversion. Lows a bit milder tonight but still
appears enough dry air per current dewpoints, and subsidence to
allow most to fall back into the 60s with some 50s in the deeper
Front folds south of the area Thursday while Dorian slides
north/northeast to off the South Carolina coast late in the day.
Guidance depicts bands of deeper moisture working to the northwest
of Dorian on increasing easterly flow aloft. However at the same
time will continue to see some dry advection kick in behind the
front on northeast flow, with forecast soundings indicating deeper
moisture having a hard time overcoming residual pockets of dry air
through the column. This may tend to delay any rainfall outside of
very light passing showers until late in the day and mainly across
the piedmont. Thus only going with low pops increasing over the
southeast in the afternoon with some spotty chances elsewhere as
some light rain possibly sneaking through within the onshore flow.
May even see a few more showers over the southwest as well where
could get heating on the periphery of the cool pool.
Otrw more clouds east to start with moisture overspreading the
region if can completely overcome the dry air. Will become breezy as
well espcly south/east as the jet to the north of Dorian increases
and enough mixing likely helps bring down some 20-30 mph gusts far
southeast by late in the day. Highs tricky pending the degree of
clouds and just how fast cooling at 85h arrives from the north. For
now trended on the warmer side of Mos (70s to low 80s) given
potential for at least a few sunny breaks.
.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY NIGHT/...
As of 245 PM EDT Wednesday...
The biggest challenge during this portion of the forecast will
continue to be any impacts the region will have from the northwest
extent of Hurricane Dorian. The latest official forecast cone track
has the center of Dorian skirting the coast of South Carolina on
Thursday and passing over the Outer Banks of North Carolina on
Friday. During the portion of this progression where Dorian is near
the SC/NC border, its position and size would place the far
northwest section of its rain shield across the far southeastern and
then eastern sections of the forecast area during the Thursday
afternoon and Thursday night part of the forecast. Precipitation
totals are still forecast to be less than a quarter of an inch
across this part of the forecast area with wind gusts approaching 30
mph. For the latest information regarding Hurricane Dorian, you can
A secondary area of isolated to scattered light rain showers is
possible across far southwest Virginia, south into the Northern
Mountains of North Carolina on Thursday afternoon. Here northeast
flow on will advect moisture in the area of the old weak and washed
out frontal boundary. This combination may be just enough for some
light precipitation in this area.
As Dorian reaches the Outer Banks on Friday, its distance from us
will have increased enough to slacken the pressure gradient, and
winds will decrease through the day. Precipitation, will be isolated
at best across the far eastern and northeastern sections of the
region during the morning.
Friday night through Saturday night drier air will work its way into
the area from the northwest.
Temperatures during this portion of the forecast will average a few
degrees above normal.
Confidence in this portion of the forecast is moderate.
.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 130 PM EDT Wednesday...
Synoptic flow across the area will transition to zonal flow by the
end of the weekend. Heading into early next week, an upper level
ridge is expected to build over the Ohio River Valley, transitioning
our pattern aloft to a northwest flow. This northwest flow is
expected to bring a cold front into the region by Monday afternoon
where it will stall, and/or wash out over the region. This feature
is expected to bring isolated showers to primarily northern parts of
the area on Monday. On Tuesday the center of surface high pressure
is expected to shift east of the region, bringing low level flow
back tot he south across the area. This flow will help advect
slightly higher moisture levels into the region. The presence of the
washed out front, will help act as a focus for isolated to scattered
coverage of shower and storms. The greatest focus will be the
Northern Mountains of North Carolina where both differential heating
and upslope components will help maximize coverage across this region
of the forecast area. For Wednesday, the 850mb flow becomes
westerly. This will help to minimize convection in the east thanks
to downsloping, with an upslope component in the west keeping
isolated to scattered coverage of showers and a few storms in the
Temperatures during this portion of the forecast will experience
lows a few degrees above normal and high temperatures around normal
for this time of the year.
Confidence during this portion of the forecast is moderate.
.AVIATION /00Z THURSDAY THROUGH MONDAY/...
As of 745 PM EDT Wednesday...
Much of the TAF valid period will remain VFR, despite the
formidable and ominous Hurricane Dorian looming off the GA/SC
coast this evening and drifting slowly north toward the SC
coast. A weak cold front located to our north will essentially
dissipate in the face of large scale subsidence around the outer
periphery of Dorian. Cumulus clouds and isolated showers popped
up along the front mainly just north of the I-64 corridor and on
up toward DCA. All but the northeastern most activity has
dissipated at this point. A weak area of high pressure aloft is
sandwiched in between the weak front to our north and Hurricane
Dorian to our southeast. Meanwhile, the large cirrus shield
associated with Dorian has spread as far north as a TNB-BCB-LYH
line and should make further progress into Thursday toward the
western border of the CWA at which point it should stop drifting
any further west. Variable/southeast winds generally less than
10kts this evening will begin to respond to the approaching
Dorian and back to the northeast and east overnight and increase
in speed with low end gusts commencing shortly after 12Z Thu
across much of the area.
The easterly flow will increase the marine layer moisture and
likely result in a shallow layer of low clouds developing across
much of the region, especially along/east of the Blue Ridge
after daybreak or early in the morning. At this time, it appears
that these cloud ceilings could be in the mid to high end range
of the MVFR category. This should persist throughout much of the
day, especially east of the Blue Ridge, but may scatter out some
west of the Blue Ridge. Meanwhile, a thick deck of
cirrus/cirrostratus should reside above in the 200-250 range
through much of the TAF valid period.
Fog development in the morning is more questionable than normal
as a result of increased wind flow and clouds. However, do feel
that BCB and especially LWB should remain far enough removed
from the effects of Dorian for one more night to decouple and
see some MVFR to potentially LIFR fog conditions for a few hours
/Confidence in Forecast Parameters/
Ceilings - Moderate,
Visibility - Moderate to High,
Winds - Moderate to High.
.Extended Aviation Discussion...
Periods of sub-VFR cigs could continue espcly Blue Ridge east
into Thursday night within the onshore northeast flow around
Dorian. Bands of showers may also work into the KDAN vicinity
Thursday night with possible brief vsby restrictions at times
pending the future track of the hurricane. While the latest
model guidance keeps Dorian well off to our east, there still
will likely be enough wind to affect aircraft operations espcly
east of the mountains Thursday night into Friday. Aside from
Dorian, the overall trend after Friday morning will be for VFR
conditions as weak high pressure builds in behind the exiting