Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 282313

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
513 PM CST Wed Feb 28 2024


- Expect temperatures to fall quickly below freezing tonight to
  bottom out in the upper teens to low and mid 20s as high
  pressure builds across the area.

- The wind will turn to the south Thursday and persist in a
  southerly direction through the weekend which will warm
  temperatures back to well above normal levels.

- The next cold front is expected to move through the area on
  Monday. While temperatures will cool off behind the front,
  relatively mild weather will continue into midweek.


.SHORT TERM...  (Through Late Thursday Night)
Issued at 245 PM CST Wed Feb 28 2024

High pressure centered over the east central Plains will move east
across Missouri and Arkansas tonight into the Tennessee Valley by
Tomorrow morning.  A clear sky and light wind tonight should produce
excellent radiational cooling conditions at least through 06-09Z for
most of the area.  By the 09-12Z time frame, the center of the high
will be east of the area and southerly flow will likely end the
overnight cooling for most of Missouri and Illinois.  That being
said, I expect temperatures to drop pretty quickly given the
conditions, and with dew point temperatures in the single digits the
ensemble guidance lows look too warm to me.  I`m not confident
enough to go with the coldest single-model temperatures, but I think
a blend of NBM 25th percentile and NAM MOS guidance is a good
compromise. This yields lows mainly in the upper teens to low 20s
across the area, which is 5 to 7 degrees below the NBM.  With The
wind turning to the south on Thursday and plenty of sunshine at an
increasing late Winter/early Spring angle (at least through the
early afternoon), we`ll see a strong rebound in temperatures, and
the NBM agrees well with traditional GFS/NAM MOS guidance to push
highs into  the upper 40s to low 50s in most locations.

The next upstream short wave will move from the central Plains
across Missouri Thursday night into Friday.  All deterministic
guidance is showing weak to moderate low level moisture convergence
ahead of the trough, focused across southern Missouri and Illinois
Thursday night.  HREF 6hr probabilities for precip > 0.01" are in
excess of 70 percent across southern Missouri and Illinois between
06-12Z Friday, and WPC seems to be on board with precip as well
giving us up to 0.1 inch across parts of Reynolds, Iron, St.
Francois, and Ste. Genevieve counties.  I`ve therefore bumped up
PoPs over NBM`s 30-40 percent to around 60% in these areas. Want to
leave a little wiggle room in the forecast for subsequent shifts, so
I didn`t go categorical just yet.  Forecast soundings are cold
enough (at least initially) for snow in this time frame, so have
intro`d a mix of rain/snow.  The snow looks like will won`t last
long, nor will it be heavy or fluffy.  Add above freezing surface
temps to this and it looks like any accumulations will be light and
limited to grassy areas.



.LONG TERM...  (Friday through Next Wednesday)
Issued at 245 PM CST Wed Feb 28 2024

The remainder of the forecast continues to look like a temperature
roller coaster ride.  Southerly flow will persist across the Mid-
Mississippi Valley through Sunday and temperatures will warm back
into the 50s on Friday, the mid 60s to low 70s, and up to the 70s to
near 80 by Sunday.  The differences between the bottom and top
temperature quartiles in the ensembles is pretty low, only 3-4
degrees, so confidence in these temperatures is pretty good.
However, another vigorous short wave moves along the U.S./Canada
border Sunday night into Monday which pushes a cold front through the
Mississippi Valley.  Current indications are that instability ahead
of this front will be limited, and while there is some chance for
thunder, severe storms do not look likely if the current guidance
pans out. Temperatures do look colder behind the front, but the wave
will be moving east-northeast into Canada Monday night into Tuesday,
so the push of colder air behind Monday`s front won`t be as strong
as yesterday`s.  Tuesday and Wednesday therefore look cooler than
Sunday and Monday, but still relatively mild with highs mainly in the
50s to low 60s.



.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 510 PM CST Wed Feb 28 2024

VFR conditions will persist through the TAF period as high
pressure slides across the forecast area tonight and pushes to our
east by tomorrow morning. Winds will become light and variable as
the sun sets this evening before shifting to the south tomorrow
morning. Winds will pick up tomorrow afternoon as upper level
clouds begin to stream into the region.





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