Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 290829
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
329 AM CDT Fri Mar 29 2024

.KEY MESSAGES...

- A cold front drops south into the region tonight. A few showers
  and thunderstorms are possible along it. It will then stall
  across the region on Saturday before lifting back north on
  Sunday.

- Additional rounds of showers and thunderstorms are possible
  Saturday night and again on Sunday. Storms on Sunday evening
  could produce large hail.

- Additional thunderstorms are possible on Monday as a broader
  storm system moves through the region. Some of these
  thunderstorms could become severe with all severe weather
  hazards possible.

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 320 AM CDT Fri Mar 29 2024

Mid level ridging moves overhead today as a shortwave trough moves
out into the Northern Plains. As this wave moves east, low pressure
emerges out of the lee of the Rockies and tracks northeast toward
Iowa this evening. We`ll see an increasing pressure gradient today
leading to stronger southerly winds and gusts to 30 mph or so.
It`ll be warm with temperatures rising into the 70s area wide.
Dewpoints will be on the increase as well as the wave passing to
the north pulls in a bit better moisture. By this evening we`ll
actually see some weak to moderate instability developing across
northern Missouri as low pressure moves by to the north and a cold
front drops in behind it. The best chances for showers and
thunderstorms will be further north across Iowa and central to
northern Illinois along the warm front this evening, but some
convective development further south into northeast Missouri and
western Illinois is possible into the overnight hours as the front
drops south. Due to a lack of stronger instability, severe
weather is unlikely.

The cold front will stall across the center of the forecast area on
Saturday, perhaps a bit further south than prior forecasts.
Temperatures will be cooler north of the front, mainly in the 60s,
while south of the front we`ll warm at least into the 70s again. The
uncertainty on the frontal position has led to greater uncertainty
in the temperature forecast on Saturday than normal. NBM
interquartile range is 5 to 8 degrees across the center of the
forecast area due to that uncertainty on how far south the front
gets.

Kimble

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 320 AM CDT Fri Mar 29 2024

The front begins to lift back north Saturday night into Sunday as
low level flow turns southerly to the south of it ahead of the next
subtle shortwave trough rounding the top of the ridge. The moisture
push back north will lead to another chance for showers and
thunderstorms along and north of the boundary Saturday night into
Sunday. Instability is somewhat weak initially, but grows stronger
Sunday evening when there will be a better chance for large hail.
The new SPC Day3 outlook highlights the northern part of our
forecast area in a Marginal risk for severe thunderstorms due to
this threat for elevated thunderstorms producing large hail.

Meanwhile to the west, a large upper trough will be settling into
the Southwest US while another, stronger shortwave trough moves out
into the Plains on Monday. This sends another wave of low pressure
eastward along the front with another surge of Gulf of Mexico
moisture ahead of it. There is some uncertainty on where the frontal
boundary will be at this point as Sunday`s wave initially moves the
front north but then shoves it back south a behind it. In the
warm sector on Monday we`ll see a much more unstable air mass with
temperatures warming potentially into the 80s and dewpoints
rising into the 60s. With surface winds out of the south and mid
level winds rather strong out of the southwest we`ll also see
plenty of vertical shear as well. This will set up an environment
where supercell thunderstorms will be possible with all severe
weather hazards. This threat remains conditional, though, as the
effective warm front will likely be stretched east to west
somewhere in our forecast area. Along and north of this boundary
the low level air mass will be considerably cooler and any
instability elevated in nature. Thunderstorms in this environment
would be primarily a hail threat.

With the increasing low level moisture, there`s also some
uncertainty on cloud cover even in the warm sector which could
significantly dampen overall instability and severe weather
potential. The operational GFS for example has morning showers and
thunderstorms which prevent the development of more robust
instability in the afternoon. This uncertainty is shown in part by a
large range in potential high temperatures on Monday. The NBM
interquartile range is from 8 to 16 degrees across the area on
Monday due primarily to that uncertainty on cloud cover. On the high
end we could see surface based CAPE values well over 1000, but with
more cloud cover this could be limited to just a couple hundred.

Given the overall synoptic setup, the more likely area for surface
based convective initiation Monday afternoon would be along the
dryline in central Oklahoma north into central Kansas where it
intersects with the surface low and advancing cold front. This is
where instability and low level convergence is maximized. Storms
which initiate in this region will then track east northeast
potentially into our area Monday evening. Given the environment with
at least the potential for strong instability and strong shear,
supercell thunderstorms with all severe weather hazards will be
possible. Locally heavy rain will also be possible considering the
humid air mass, especially if a slow moving stratiform rain region
develops from more widespread convection.

As the wave moves east Monday night into Tuesday, the front will
move south and push the cooler air mass through the entirety of the
forecast area. Some rain may linger behind the departing low on
Tuesday and temperatures stay mostly in the 50s. There`s some
uncertainty on how long the cooler conditions last and how quickly
ridging builds back in. This is noted in the NBM interquartile range
becoming extreme (15 to 20 degrees) for the last half of the week. A
trend back toward warmer weather is expected, but it`s the speed of
that warm up that`s uncertain.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 320 AM CDT Fri Mar 29 2024

South southwest winds increase during the morning with gusts to 25KT
or so through the day. Gusts may continue into the night tonight. A
cold front dropping into the area from the north may bring a shower
or thunderstorm as it moves through. This chance is greatest at
Quincy but not high enough to mention in the TAF at this stage.

Kimble

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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