Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/15/21

Area Forecast Discussion

National Weather Service Denver/Boulder CO

 730 PM MDT Tue Sep 14 2021

.UPDATE...
Issued at 719 PM MDT Tue Sep 14 2021

The upper level longwave trough has pushed into the four corners
region with drying upstream seen on water vapor. Pressure rises
have pushed across the county warning area with increasing stable
conditions across the region. This has ended convection over the
area with scattering out from northwest to southeast. This will
continue overnight leaving mostly clear skies and lows dipping
into the mid-50s for the lower elevations and 30s for the high
country.

Tuesday will be the start of a warming and dry trend with the
introduction of an upper ridge over the region. Current forecast
is on track with highs reaching back into the lower 90s for
portions of the plains and Denver metro with elevated fire
conditions, especially over north and middle parks.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 250 PM MDT Tue Sep 14 2021

Strong to severe storms are ongoing over the Palmer Divide this
afternoon with severe hail and damaging wind gusts the primary
threats. SPC mesoanalysis shows up to 1,500 j/kg of surface-based
CAPE in Elbert and Lincoln counties with effective bulk wind shear
above 40 knots. These factors, along with QG ascent ahead of
a shortwave trough and upslope flow, will allow a couple
supercells to develop and push eastward as the afternoon
progresses. A Severe Thunderstorm Watch was issued for Elbert and
Lincoln counties until 9pm this evening although the watch may be
able to be cancelled early as the atmosphere may be worked over by
the first round of storms. Some of these storms have already
split and straight hodographs would indicate splitting storms will
be possible through this event. A left mover could make its way
into southern Washington County and produce severe hail but the
risk was not high enough to include that area in the watch. For
the rest of the plains, including Denver and the northeast corner,
it is too stable for much, if any, thunderstorm development.
ACARS soundings show a couple inversions around 750 and 650mb
which will be too much to overcome to produce severe storms. There
have been one or two storms that have made it onto the plains
after forming in the foothills and that`s about the most coverage
that will occur north of I-70.

Showers and storms come to an end this evening as instability
weakens. The rest of the night will be dry with slight above
normal temperatures.

Our forecast area will see subsident flow under the right exit
region of an upper level jet streak on Wednesday. With much drier
air in place, there will be dry conditions. High temperatures
will increase tomorrow due to warmer air aloft moving into
Colorado and downslope flow forming over the plains. Denver will
have a chance at seeing 90 degrees for a high although much of the
metro area will top out in the upper 80s.

The only concern on Wednesday will be elevated to near critical
fire weather conditions across the northern mountains and
foothills as well as North Park. This threat is discussed in the
fire weather section below.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 250 PM MDT Tue Sep 14 2021

Moisture will shift east Thursday as dry southwest flow aloft
brings drier and warmer conditions to the area. High temperatures
will rise a couple of degrees, particularly on the plains,
reaching the high 80`s and low 90`s. Winds aloft will remain relatively
healthy into the weekend, producing breezy conditions for much of
the mountains and foothills each afternoon.

On Friday, a weak trough brushing the area to our north will
temporarily lower temperatures into the low to mid 80`s for
daytime highs, with upper level winds primarily out of the west.
Some enhancement in moisture is possible Friday but does not look
sufficient for any precipitation.

High pressure will nudge northward Saturday, resulting in a quick
return to warmer and drier conditions with highs inching into the
low 90`s in the plains. Warm temperatures will continue into
Sunday, albeit with a slight increase in upper level moisture
which could be enough to squeeze out an isolated shower or
thunderstorm in the mountains.

As we head into early next week, all eyes will be on the deepening
longwave trough across the Intermountain West, which now features
fairly prominently in most guidance. There continues to be
significant uncertainty, particularly related to timing of the
trough, but a notable trend toward cooler temperatures, windier
conditions and possible precipitation looks likely sometime
between Monday and Wednesday. Approximately 75-80% of GFS and EC
ENS solutions indicate some precipitation early next week, with
the GFS ENS system clearly the wetter of the two. With that, there
will be some potential for mountain snowfall and possibly for
foothills too if the colder and wetter solutions verify. Either
way, it appears we`re in store for a little taste of fall.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 719 PM MDT Tue Sep 14 2021

Expect VFR conditions through the TAF period. The showers from
this afternoon and early evening have cleared the terminals and
skies should continue to clear out overnight. Dry conditions will
set in and skies should remain mostly clear through the rest of
the TAF period. It looks like there is a weak cyclonic circulation
centered roughly over KDEN, and winds should turn to normal
drainage through the evening as it breaks down. Drainage winds
should be replaced by easterly upslope winds late in the day or
early in the afternoon Wednesday, then around back to drainage
again by Wednesday night.

&&

.FIRE WEATHER...
Issued at 250 PM MDT Tue Sep 14 2021

A dry airmass under a ridge aloft will result in relative
humidities dropping to the low teens across the higher terrain and
northern plains. A jet stream will be to the north of Colorado
which will increase winds along the Colorado/Wyoming border. Gusts
could reach up to 25 mph across Jackson and Larimer counties which
will result in elevated to near-critical fire weather danger. A
Fire Weather Watch was not issued since winds will be just below
the threshold in most of the area.

Elevated fire danger will be in place, Thursday through Saturday
over Middle and North Park during the afternoon hours.

&&

.HYDROLOGY...
Issued at 250 PM MDT Tue Sep 14 2021

No threat for flash flooding.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Bowen
SHORT TERM...Danielson
LONG TERM...Rodriguez
AVIATION...EJD
FIRE WEATHER...Danielson/Rodriguez
HYDROLOGY...Danielson



Area Forecast Discussion

National Weather Service Louisville KY

 1001 PM EDT Tue Sep 14 2021

.Forecast Update...
Issued at 1001 PM EDT Tue Sep 14 2021

A narrow broken band of showers and thunderstorms has developed this
evening along and just ahead of a cold front stretching roughly from
Detroit to St Louis to Wichita in conjunction with a weakly digging
5H trough. The convection is within a corridor of 1.7-1.9"
precipitable water values that will sink southward with the front
tonight, eventually meeting up with a similarly humid atmosphere
already in place over Kentucky by morning.

An upper jet from the Midwest to the eastern Great Lakes will
intensify overnight and the Ohio Valley will increasingly be under
its right entrance region...especially on Wednesday.

ACARS sounding out of SDF a couple of hours ago still showed a
healthy mid-level cap overhead, but sounding progs show it weakening
overnight with cold air advection in the 5H-7H layer associated
with the advancing upper trough to our northwest.

RAP data accordingly show CIN decreasing overnight. Fortunately any
low level jet tonight looks to be weak, effective bulk shear will be
very weak, and lapse rates are weak at the surface and aloft with
CAPE on soundings showing up as tall, thin, and slightly elevated.

So, given the forcing, moisture, and available instability ahead of
the advancing front, will hold on to scattered showers and storms in
the forecast during the late night hours. Not everyone will see
rain, as evidenced by the large gap that has opened up in the line
over the Wabash Valley. The latest HRRR and 3km NAM capture this
well and actually bring little if any rain into the LMK CWA...but
would prefer to keep PoPs in the forecast given the aforementioned
moisture, forcing, and instability. Given the latest, will reduce
PoPs but not entirely eliminate.

&&

.Short Term...(This evening through Wednesday)
Issued at 245 PM EDT Tue Sep 14 2021

Isolated to widely scattered showers have developed across south-
central and western Kentucky this afternoon. Capping near 700mb has
limited overall growth and strength of these convective showers,
resulting in most echo tops not going beyond 20kft. Up to this
point, very little to no lightning activity has been detected in
this precipitation, and environmental conditions will not change
much over the next several hours, so expect any thunderstorm
activity to be largely isolated. One thing to keep an eye on this
afternoon is high DCAPE (>1,000J/KG) values across the region where
gusty winds could be possible in the stronger showers or isolated
storms, assuming they can overcome the warmer air/cap around 700mb.

We should see a break in the action near sunset as diurnal heating
wanes and convective showers taper off. A broken line of storms
forming ahead of a cold front this evening will then race toward our
region, potentially arriving after midnight. Much of this activity
will weaken by the time it approaches us, and any severe threat will
be very low with the line. Though unlikely, it is entirely possible
the line could dissipate before it reaches our southern IN counties
as a couple of CAMs show (3km NAM, WRF ARW). Model soundings do show
elevated instability persisting through much of the overnight and
early morning hours tomorrow, so will keep mention of storms in the
forecast as outflow boundaries from weakening convection could
generate new storms.

The cold front should reach our southern Indiana counties close to
sunrise tomorrow and gradually push through the region during the
day. As the front moves through, we could see additional development
of showers and storms along it. Depending on how much clearing we
see in the wake of the first round of showers/storms, there may be a
low-end severe threat for gusty winds and small hail with storms
that fire off along the cold front. SPC Day 2 Marginal covers that
threat area well.

.Long Term...(Wednesday night through Tuesday)
Issued at 255 PM EDT Tue Sep 14 2021

At the start of the extended period, expect to find the upper level
trough exiting east out of the area while the associated surface
front initially draped along the Ohio River Wednesday night
gradually sinks southward through the Commonwealth, likely washing
out in our vicinity. Showers and storms will be ongoing Wednesday
night ahead of and along the cold front with precip chances tapering
off from north to south overnight as the frontal boundary moves
through.

The end of the week will then feature ridging off of the East Coast
over the Atlantic while an upper trough lifts northeast across
central Canada, placing our region in relatively weak flow aloft.
Although the remnants of Nicholas are expected to remain well to our
south, will see lingering moisture across southern portions of the
area on Thursday associated with the washed out frontal boundary
then increasing moisture across the area Friday as we return to
southerly flow, keeping PWATs in the 1.2 - 1.6 inch range through
the remainder of the period. In a warm and marginally unstable
environment, cannot rule out isolated to scattered showers and
storms during the afternoon and evening hours Thursday into the
weekend. PoPs on Thursday will be confined to southern KY where the
better moisture is with precip chances then spreading across the
entire area the other days. By Sunday, as broad upper ridging moves
in over the east-central CONUS, expect mostly dry weather.

Temperatures will warm slightly through the period, with highs
initially in the lower to mid 80s Thursday then into the mid to
upper 80s by Sunday. Low temperatures will generally be in the mid
to upper 60s.

&&

.Aviation...(00Z TAF Issuance)
Issued at 731 PM EDT Tue Sep 14 2021

This evening a cold front is stretched out along a APN-CMI-SGF line
with a narrow broken band of showers and thunderstorms along and
just ahead of it. These features will sink southward tonight,
bringing showers and scattered storms to southern Indiana and
central Kentucky in the early hours of Wednesday.

Showers and storms will decrease in intensity and slightly in
coverage during the daylight morning hours Wednesday but then
renewed convection may develop in the afternoon, primarily affecting
LEX and BWG.

Forecast confidence is low on ceilings. Timing of showers and storms
will need to be adjusted as the system approaches.


&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&

$$

Update...13
Short Term...DM
Long Term...KDW
Aviation...13