Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261959
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
259 PM CDT Tue Mar 26 2024

.KEY MESSAGES...

- A hard freeze (temperatures less than 29 degrees) is expected
  tonight for central and northeast Missouri, as well as parts of
  east central Missouri, and west central Illinois.

- Below normal temperatures are expected through Wednesday night,
  but temperatures will warm back up above normal for the latter
  half of the week and into the weekend.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 258 PM CDT Tue Mar 26 2024

...Key Messages...

Winds across the area will turn more to the west and northwest
tonight as low pressure currently over the Upper Midwest moves into
Canada.  This will pull a cold high pressure system into the Plains
and Midwest tonight and Wednesday.  This will result in below normal
temperatures for tonight through Thursday morning.  Temperatures
across much of Missouri and parts of Illinois will drop into the mid
and upper 20s by early Wednesday morning.  Agriculture interests are
still reporting "mixed susceptibility" to freezing temperatures, but
this freeze looks impactful enough with temperatures at or below 28
degrees in many areas to warrant the first Freeze Warning of the
season.  Afternoon highs on Wednesday will struggle into the 50s in
most locations after the chilly start to the day, early morning
clouds, and continuing northwest flow.  Winds become light and
variable Wednesday night as the ridge axis drifts across Missouri
and Illinois, and clear skies will allow good radiational cooling.
With that said, guidance is showing temperatures a few degrees
warmer Wednesday night than tonight.  Even the lower quartile
temperature guidance stays above 28 degrees in most locations, so
another hard freeze Wednesday night looks unlikely.  However there
will be frost potential there will be some frost potential due to
the light winds and relative humidity reaching 90-95 percent in some
locations.

Carney

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 258 PM CDT Tue Mar 26 2024

The rest of the forecast period for Thursday through next Tuesday
looks to feel much more like Spring and even early Summer.  The high
amplitude upper level pattern currently over the U.S. becomes more
zonal by this weekend, but not before a ridge builds over the
Mississippi Valley late Thursday into Friday.  South to
southwesterly flow at the low levels will bring warmer temperatures
back from the southern Plains into the Mid-Mississippi Valley, and
temperatures will rise back above normal.  I have pretty high
confidence in the temperature forecast for Thursday and Friday as
the spread in the ensemble guidance is only 2-3 degrees with highs
in the 60s and 70s respectively.

Confidence in the temperatures begins to fall Saturday with the
pattern shift to more zonal flow aloft, and a weak short wave moves
across the Mississippi Valley, potentially touching off some
thunderstorms, and bringing a surface cold front down into Missouri
and Illinois.  Both the deterministic GFS and ECMWF show the front
stalling across Missouri and Illinois, near the I-70/I-64 corridors.
 Both models also show some modest moisture convergence on the nose
of the low level jet late Friday night/early Saturday ahead of the
front which produces some precipitation.  The combination of clouds
and potentially some showers and thunderstorms along with the
presence of a stalled front throws a wrench in the temperature
forecast.  While guidance is showing above normal temperatures in the
70s both days, the spread between the 25th and 75th percentile
increases to between 7 and 10 degrees.

The upper pattern begins amplifying again late Sunday into Monday
with a progressive long wave trough digging into the Desert
Southwest and a sharpening ridge over the eastern U.S. This puts the
Mid-Mississippi Valley into deep southwest flow and cranks
temperatures up to late May/early June levels according to the
deterministic NBM guidance.  Of course, with this high
amplification, there are a number of solutions in the ensembles, and
while the majority favor a warmer solution, there are enough colder
solutions to produce an 11 degree spread between the 75th and 25th
percentiles.  Given the potential for increasingly warm and humid
air to move into the region ahead of the next cold front associated
with the aforementioned wave, there is a chance for strong to severe
storms early next week.  With that being said, confidence in the
forecast for early next week is low at this point due to the wide
variability in the ensembles.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1234 PM CDT Tue Mar 26 2024

MVFR ceilings and gusty west-southwest winds will prevail this
afternoon. The wind will turn more to the west and then northwest
this evening, and gusts will subside, although wind speeds
will likely stay in the 6-10kt range. Getting conflicting signals
about ceilings rising this evening...some guidance indicates the
MVFR will continue to prevail tonight, other guidance shows
ceilings lifting to 3500-4500ft. I feel with drier air filtering
into the region and the storm system itself moving farther to
the north tonight, we are more likely to see some improvement this
evening. However, there`s still a chance that ceilings do not lift
until Wednesday morning before scattering out.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Freeze Warning from 3 AM to 9 AM CDT Wednesday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Moniteau
     MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Shelby
     MO-Warren MO-Washington MO.

IL...Freeze Warning from 3 AM to 9 AM CDT Wednesday for Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX


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