Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172331
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
631 PM CDT Sun Mar 17 2024

.KEY MESSAGES...

- A hard freeze (lows <= 28F) is likely tonight with most
  locations bottoming out in the low to mid 20s. Another freeze
  is likely Monday night, but most locations should be warmer than
  tonight with the possible exception of valleys in southeast
  Missouri.

- Temperatures warm back up quickly Tuesday, but there is low
  confidence in temperature trends thereafter.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 220 PM CDT Sun Mar 17 2024


The main story in the short-term portion of the forecast is on the
well-below (10-15 degrees) normal temperatures and the expectation
for subfreezing temperatures both tonight and Monday night.

Anomalous midlevel troughing (500-hPa heights below the 2nd
percentile of climatology) is working through the mid-Mississippi
Valley/Great Lakes late this afternoon. Moderately strong low-level
cold air advection is ongoing across the region, with 850-hPa
temperatures forecast to drop below -10C overnight (12-14C below
normal for the date). Looking upstream across the Upper Midwest with
this incoming air mass, lows were in the upper teens to mid 20s
earlier this morning. We are expecting something pretty similar
tonight, though northwest winds staying up around 10 mph east of the
surface high should help limit radiational cooling. Therefore, lows
in the low to mid 20s are favored across the area. Probabilities of
a hard freeze (<= 28F) on the latest LREF are 80 to near 100 percent
for most of the area, with sub 25 degree lows most likely (60-80
percent) across parts of northeast Missouri and west-central
Illinois. This hard freeze may impact susceptible early-blooming
vegetation.

The chilly start to the work week will feel even colder with the
northwest winds. Wind chill values early Monday morning are expected
to be in the teens. Plenty of sunshine is expected on Monday
however, which will help temperatures warm up into the low to mid
40s across the area. Northwest winds will also lighten through the
day as the seasonably strong (~1028 hPa) surface anticyclone moves
into southeastern Kansas and the pressure gradient relaxes.

This surface high is expected to move into the lower Mississippi
Valley Monday night, with return flow increasing in its wake. Winds
will turn to the southwest and gradually increase in speed, first in
northeast Missouri and then spreading east and south with time. Due
to these increasing southwest winds, the temperature drop will stop
and then readings will rise late Monday tonight. Most if not all of
the area is likely to drop below the freezing mark however before
warming back up. Coldest locations (lows in mid 20s) are expected to
be in lower terrain (i.e., valleys) in southeast Missouri where
winds will pick up around/just after 0900 UTC Tuesday.


Gosselin

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 220 PM CDT Sun Mar 17 2024

There remains high confidence in a rapid warmup heading into Tuesday
as strong low-level warm air advection continues ahead of an
approaching cold front. The 850-hPa thermal ridge by early afternoon
is expected to be over the I-70 corridor, with surface winds veering
more to the west/southwest ahead of the boundary. This is a near
ideal setup for warming combined with what should be plentiful
sunshine as the aforementioned surface winds downslope off of the
Ozark Plateau. This mainly impacts parts of central, east central,
and southeast Missouri. In these locations, highs in the upper 60s
to near 70 degrees is forecast. These values still may be too low
given the setup, but for now, did lean more toward the 90th
percentile of available model guidance. Further warming may be
required over the next 24-36 hours as it is often very difficult to
go warm enough with this type of pattern.

Forecast uncertainty begins to increase behind the cold frontal
passage, mainly in terms of temperatures. All signs continue to
point toward a fairly strong low-level temperature gradient, with
warm air to the southwest and much cooler air to the northeast. This
gradient likely will oscillate north/south through the end of the
work week and into the weekend as the track/strength of each
midlevel shortwave trough moving across the Great Lakes/northeast
helps to modulate its position. The spread between the 25th/75th
percentile of the NBM gradually increases through the period,
exceeding 10 degrees by Thursday and is about 20 degrees by the end
of the weekend. This high degree of spread helps illustrate the low
confidence forecast with respect to temperatures and is generally
the difference between near to slightly below normal and well-above
normal temperatures.

Precipitation wise, it still looks like a mostly dry week. There
remains a low chance (20-40%) Wednesday night - Thursday night, but
any rain that does occur likely would be light. Probabilities over a
24-hour period from the LREF for at least a tenth of an inch of
rainfall are only 20-30% at maximum anywhere in the CWA. A much
better chance for widespread rainfall may be in the offing to end
the weekend/early the following week as a longwave trough deepens
over the western CONUS.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 631 PM CDT Sun Mar 17 2024

VFR conditions are forecast through the period. Northwesterly
gusts will relax overnight before returning mid-morning Monday.

Maples

&&

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

$$

WFO LSX


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