Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221738
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1238 PM CDT Fri Mar 22 2024

.KEY MESSAGES...

- A band a light rain will move across the area this afternoon and
  evening (30-70% chance). Better chances (90-100%) for widespread
  rain and a few thunderstorms will be on Sunday night into
  Monday night as a cold front moves through the area.

- Temperatures will vary up to 20 degrees from normal through next
  week as a series of fronts move through the area.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 351 AM CDT Fri Mar 22 2024

Regional analysis shows an upper level low traversing the Gulf
States with broken/overcast cloud cover extending from southern
Missouri/Illinois southward toward the Gulf of Mexico. Another bank
of clouds is moving west to east over the Upper Midwest and extends
southwestward along a mid-level trough, which then meets up with a
weak surface low over the central Plains. A relative void exists in
the cloud cover over portions of the CWA, ranging from mostly clear
north of I-70 to mostly cloudy to the south.

This void will begin to fill in late this morning into the afternoon
as the weakening surface low tracks almost due eastward along the
southward advancing mid-level trough. Above normal temperatures run
ahead of the front, where highs in the mid-60s to near 70 degrees,
which contrasts from cooler air over northern Missouri and west-
central Illinois. As the front shifts southeastward through the day,
a narrow area mid-level ascent resides atop the surface boundary and
will result in a few showers. Unfortunately, trends aren`t looking
favorable at all for beneficial rainfall. Surface dewpoint
depressions run 10-20 degrees across the forecast area with dry air
extending as high 850mb, along with it being a narrow window (couple
of hours) for showers along the front. HREF ensembles show low
probabilities (30% or less) for measurable rainfall dotting the
front with a general decrease in potential with southward progress.

Temperatures cool about 20 degrees behind the front Saturday. Upper
40s to low-50s cover a large majority of the area. Dry conditions
likely hold through the day with easterly surface flow wrapping
around the southern end of the surface high over the Great Lakes.
While there is some potential (20%) for precipitation Saturday
night, much of it is confined to northern sections of Missouri. If
precipitation develops at all, a few snowflakes (inconsequential)
cannot be ruled out.


Maples

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 351 AM CDT Fri Mar 22 2024

An upper level trough is expected to dig southward over the
Intermountain West as a surface low rounds the base of the trough
and ejects northeast through the central Plains late Sunday into
Monday. Easterly surface flow begins to strengthen as it backs out of
the southeast and south, with return flow drawing moisture northward
out of the Gulf of Mexico. While this still looks like the best
potential (90-100%) for widespread rainfall, the rate at which the
surface low deepens could present some issues with overall rainfall
amounts.

Dp/Dt shows earlier in the week a positively tilted system fed
moisture northeastward with better potential for widespread, soaking
rainfall. The latest GFS/ECMWF have little difference between the
position of the surface low and northeast jaunt through the western
sections of Iowa. This places a narrow corridor of higher moisture
(dewpoints of 50-55 degrees) along the Mississippi River Monday
afternoon into Monday evening. The lead upper wave largely remains
north with any upper ascent holding off until a second upper wave
ejects northeast by late Monday evening into Tuesday. While showers
and a few embedded thunderstorms could develop along the front
Monday afternoon, convective potential doesn`t look too promising
either, as SBCAPE remains at or below 500 J/kg Monday afternoon.
Another piece to watch is whether the rapid deepening results in dry
slotting over northeast Missouri, where just a few days ago, it
looked like a soaking rain could fall. These trends would place less
rainfall over the northwest portions of the CWA with most of the
beneficial rain falling over southeast Missouri and southwest
Illinois. LREF ensembles produce a wide distribution across much of
the CWA, ranging from approximately 0.25-0.50 for the 25th
percentile and 1.00-1.25 for the 75th percentile. So, while some
rainfall is likely, the magnitude is still in question.

Much of the remainder of the week remains dry with cooler midweek
temperatures (10-15 degrees below normal) moderating back to
near/slightly above normal by the end of the week. The next chance
of rain may not come until late Friday into early next week when
another upper level low pushes west to east within a generally zonal
pattern.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1145 AM CDT Fri Mar 22 2024

A cold front will continue to sink south through the forecast area
today, with gusty north winds, MVFR ceilings, and scattered showers
following it. MVFR ceilings will persist until late tonight - mid-
morning depending on the time of onset, scattering to VFR
thereafter. Embedded light showers will be largely unimpactful, but
the most steady showers may result in brief reduced visibilities at
a given terminal. Showers are expected to diminish with southeast
extent, so this will be more applicable to areas generally north of
I-70. Elevated winds will follow a similar temporal trend, ending
overnight for our northern extent and tomorrow morning in the south.
Until then, gusts of 20 - 25 kts are expected behind the front with
bouts of 25 - 30 kts possible.

Jaja

&&

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

$$

WFO LSX


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