Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240954

Area Forecast Discussion
National Weather Service Saint Louis MO
354 AM CST Sat Feb 24 2024


- Highs today will be near average before quickly climbing to near record
  warmth Sunday through Tuesday.

- Very warm, dry, and breezy conditions will result in the
  potential for elevated fire danger on Sunday, possibly Monday
  and Tuesday as well.

- There is a potential for strong to severe thunderstorms late
  Tuesday into Wednesday. There is still uncertainty regarding
  exact timing and location.


.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 340 AM CST Sat Feb 24 2024

This morning`s cold front has exited the area and cool air is
filtering in behind it. Morning lows are forecast to drop into the
upper 20s to mid-30s by 12z. Despite these lows still being above
average for late February, brisk northwest winds will make it feel
even cooler as wind chills fall in the 20s areawide. Although we`ll
have plenty of sun to work with, cold air advection will influence
our high temperature forecast for today. This will keep them near
average, with south central Illinois leaning below average (low 40s)
and central Missouri leaning above (low 50s). The reason for this
temperature gradient is the passage of the ridge axis within the
middle and low levels of the atmosphere by Saturday afternoon. Winds
will back to westerly in the mid-levels and will back to
southwesterly in the low levels and surface in response to the
passage. This will allow for strong, deep warm air advection to
begin. The western CWA will experience this wind switch early in the
afternoon, allowing them to warm more than their Illinoisan
counterparts and resulting in the temperature gradient.

By Sunday, the entire CWA will feel the effects of the increasing
southwest flow which will be enhanced by an approaching dying cold
front. The largest effect this front will have is boosting
southwesterly flow during the day via a tightening pressure
gradient. Morning lows will start off "cool", landing in the low 30s
to low 40s in the same gradient as discussed previously. By 21z,
we`re forecasting a near 30 degree rise in temperatures with near
record daily highs across the region (see Climate section). In
contrast, dewpoints will likely be quite low due to daytime mixing.
The mix of elevated boundary layer winds, low relative humidity, and
dry fuels raises the potential for elevated fire danger on Sunday,
especially for areas west of the Mississippi River where dewpoints
will be lower and finer fuels will be drier.



.LONG TERM...  (Sunday Night through Friday)
Issued at 340 AM CST Sat Feb 24 2024

By Monday morning, an upper level trough will beach the coast of the
Pacific Northwest and begin to dig its way through the CONUS. In
response, a surface low pressure system will develop in the Northern
Rockies and be shoved southeastward by an Arctic airmass. The first
effect we`ll experience from these dynamics will be an increase in
southwesterly flow as the surface pressure gradient tightens over
the Mid-Mississippi Valley. This will strengthen warm air advection
even more, pushing us further into record-breaking territory with
highs reaching widespread 70s by Monday and even the low 80s in some
spots by Tuesday. As discussed in previous forecasts, southwest flow
regimes like this tend to underestimate how warm we`ll actually get.
We`ve accommodated for this in the forecast by warming our
temperature forecast, but it`s worth noting that even after that,
our forecast high of 80 degrees for KSTL is still below the NBM 25th
percentile. This already record breaking forecast will likely need
to be increased again.

Warm air isn`t the only thing that will advect into the area early
next week. Southwesterly flow will pull Gulf moisture into the Great
Lakes by Tuesday afternoon with its path crossing southeast Missouri
and Illinois. This moisture will be anomalously rich, with 50s
and 60s dewpoints threatening the Tuesday daily dewpoint record at
KSTL. There is evidence among ensemble and deterministic guidance
that there will be a gradient between these elevated dewpoints
and drier air Tuesday afternoon and evening with the highest
dewpoints being largely confined to southeast Missouri and
Illinois. LREF probabilities of 60 degree dewpoints are 30-50% in
these areas and quickly drop to <10% elsewhere. This gradient will
likely be much tighter, especially as the surface low and its
attendant cold front near the region and compact the moisture.

A very sharp and potent cold front will approach the region to the
northwest Tuesday evening into early Wednesday morning as the
surface low continues its trek through the CONUS. This cold front
will bring the potential for thunderstorms to the Mid-Mississippi
Valley as it arrives. We previously discussed the anomalous heat
and moisture ahead of the front. These ingredients will result in
impressive instability within the moist corridor. The caveat to
this is the presence of an elevated mixed layer within this
region. Although this feature will steepen lapse rates and
increase instability, it may also keep any surface based
instability capped until just before the front arrives. Frontal
timing is trending toward Tuesday evening/night, which may dampen
the amount of instability present for thunderstorms to utilize. On
the other hand, if the cap is weaker than expected and the
moisture gradient provides enough forcing for ascent, we could see
thunderstorms develop as early as Tuesday afternoon. With
deterministic guidance suggesting up to 1000 J/kg of MUCAPE, 50-60
kts of effective bulk shear, and PWATs approaching 1 inch in
southeast Missouri and southern Illinois, there is the potential
for strong to severe thunderstorms in these areas with all hazards

There is uncertainty regarding exactly where thunderstorms will
initiate within the aforementioned locations late Tuesday, and this
has to do with where the cold front will coincide with the moisture
tongue. Deterministic guidance sets this gradient up in our east and
naturally pushes it toward our southeastern border as the front
approaches. If the moisture is pushed outside of our border too
soon, the threat of strong to severe thunderstorms may be pushed out
with it. With a tight gradient expected, low resolution models that
will wash it out, and discrepancies in overall timing, it`s nearly
impossible at this juncture to predict where the moisture will place
and when and where the cold front will interact with it. In short,
there is the potential for strong to severe thunderstorms where
these two features meet. This may occur within our southeastern CWA
or outside of our border to the southeast.

Frigid Arctic air will follow the cold front. Temperatures will
plummet 20+ degrees very quickly. A non-negligible number of
ensemble members do keep a small amount of precipitation behind
the front (though it`s weighted by the GEFS). If this is the case,
it`s possible that colder locations in the north see a brief
flake of snow before the cold front exits completely. Highs on
Wednesday will struggle to reach 40 degrees in areas north of
I-70. The cold won`t last for long. Well above average
temperatures will return late next week, with 50s and 60s forecast
to kick off March.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1037 PM CST Fri Feb 23 2024

VFR conditions are expected to prevail through the 06Z TAF period.
At the start of the period, broken low-vfr stratus will impact
many local terminals, with a low probability of a brief period of
MVFR ceilings. These clouds will largely clear local terminals by
15Z and remain clear the remainder of the period. Meanwhile,
breezy northerly winds will gradually weaken during the first 12
hours of the TAF period, then slowly shift to the southwest
through the afternoon. A gradual increase in wind speeds is then
expected through the evening.



Issued at 257 PM CST Fri Feb 23 2024

Record climate information for St. Louis, MO (KSTL); Columbia, MO
(KCOU); and Quincy, IL (KUIN).
Daily Record High Temperatures:
     |__SUN 2/25___|__MON 2/26___|__TUE 2/27___|
KSTL   76 F (1996)   78 F (1996)   79 F (1981)
KCOU   74 F (1981)   77 F (1996)   75 F (1976)
KUIN   71 F (2000)   73 F (1996)   73 F (1976)
Daily Record Maximum Low Temperatures:
            |__TUE 2/27___|
KSTL          51 F (1896)
KCOU          48 F (1981)
KUIN          46 F (1948)
February Record High Temperatures:

KSTL       85 F (2/29/1972)
KCOU       82 F (2/29/1972)
KUIN       77 F (2/20/2017)




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