Area Forecast Discussion

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FXUS63 KLSX 261723
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1123 AM CST Wed Feb 26 2020

.SHORT TERM... (Through Thursday Night)
Issued at 312 AM CST Wed Feb 26 2020

(Today)

An area of light snow oriented southwest to northeast will continue
this morning. Snow will be initially focused across east-central
Missouri and western Illinois before pulling out to the east toward
midday as the midlevel shortwave trough passes by to the south. Some
breaks in the cloud cover are likely to occur for western sections
of the area this afternoon, but should quickly fill back in with
stratocumulus clouds. CAMs suggest some snow shower activity may be
possible this afternoon due to increasing low-level instability.
Left this out of the forecast for now, but did introduce some snow
flurries for now. Temperatures with plenty of cloud cover and
moderately strong cold air advection should struggle to rise much
from this morning`s lows. Readings across the the bi-state region
should top out in the mid 30s, which is about 10 degrees below
normal for most locations.


(Tonight)

A surface ridge of high pressure is forecast to quickly slide
southeastward out of the mid-Missouri Valley into our region late
tonight. As a result, a mostly clear sky with light winds should
help promote a cold night due to radiational cooling. Lows in the
low to mid 20s are forecast across the region.


(Thursday - Thursday night)

Thursday should start off mostly clear and chilly, with increasing
cloudiness into the afternoon ahead of a clipper-like system. Models
continue to suggest that light precipitation may move into the
region from the west/northwest late Thursday afternoon into the
early evening hours. But the details (track/amounts) remain
uncertain. The dominant precipitation type should be mostly snow as
wetbulb temperatures remain at or below freezing. While
precipitation could start off as rain due to surface temperatures
well above freezing, wetbulbing/evaporative cooling should cool the
column enough to produce snow. Not expecting any significant event
by any stretch of the imagination as the system is a bit disjointed.
More specifically, the vorticity maxima and ascent associated with
it are well southwest of the stronger low-level convergence and
moisture advection (albeit weak). Therefore, any precipitation that
falls likely will be on the light side. That being said, cannot rule
out some light (less than a half inch) snow accumulations Thursday
evening after the sun goes down. For now, kept the PoPs in the
schc/chc category due to the uncertainties and limitations mentioned
above with this particular system.


Gosselin

.LONG TERM... (Friday through Tuesday)
Issued at 312 AM CST Wed Feb 26 2020

(Friday - Friday Night)

Another shortwave trough is forecast to move through the area in
northwest flow aloft, about 24 hours after the one moving through
late Thursday. There is even less certainty with the possibility of
light rain/snow so do not have any mentionable PoPs. However, cannot
rule out some light rain/snow depending on the timing of any
precipitation. Timing into the evening would be more favorable for
snow and Friday afternoon likely would lead to more rain due to
warmer boundary layer temperatures. Speaking of temperatures,
Friday`s highs should be similar to Thursday, generally in the upper
30s to mid 40s. The coldest locations should be across western
Illinois and warmest conditions in central/southeastern Missouri.


(Saturday - Sunday)

A quiet couple of days is likely this upcoming weekend along with
moderating temperatures. Highs by Sunday should reach into the 50s
areawide, or slightly above normal for late February.


(Sunday Night - Tuesday)

An active pattern is likely to start off next week with deep
southwest flow aloft and a slow-moving cold front. Several waves of
showers, and possibly some thunderstorms, appear increasingly likely
toward the end of the extended forecast. The concern will be on the
possibility of heavy rainfall and the resulting impacts (mainstem
river flooding and flash flooding). However, it is still way too
early for any specifics other than to say it certainly looks like a
wet pattern across the bi-state region. Where the heaviest rainfall
axis sets up will be a critical question. Odds favor southeast of
the CWA as the aforementioned cold front should be able to slide
southeast of the area with a lack of a very strong mid/upper level
ridge across the southeastern CONUS. In addition, the main shortwave
ejecting out of the desert southwest has trended a bit further south
with time over the past 24-48 hours. Given how much this past storm
trended southeast with time, there is the possibility that rainfall
amounts stay light enough not to cause too many issues in our neck
of the woods. Bottom line, this will be a period of interest for the
possibility of heavy rainfall but odds right now do favor the mid
south into the Ohio Valley.

Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1109 AM CST Wed Feb 26 2020

Snow is moving out of the area, so mainly dry conditions are
expected other than some possible flurries this afternoon.
Ceilings between 2000-3000FT AGL are expected at the terminals,
mainly between 18-22Z before VFR conditions are expected the rest
of the period. Northwest winds are expected through this evening
with some gusts to 20KTS before they diminish by this evening.
The winds will back to the west-southwest tonight.

SPECIFICS FOR KSTL:

Mainly dry conditions are expected other than some possible
flurries this afternoon. Ceilings between 2000-3000FT AGL are
expected at the terminals through mid afternoon before VFR
conditions are expected the rest of the period. Northwest winds
are expected through this evening with some gusts to 20KTS before
they diminish by this evening. The winds will back to the west
tonight.


Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until noon CST today for Crawford MO-
     Franklin MO-Jefferson MO-Saint Charles MO-Saint Louis City
     MO-Saint Louis MO-Washington MO.

IL...Winter Weather Advisory until noon CST today for Bond IL-Calhoun
     IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-
     Monroe IL-Montgomery IL-Saint Clair IL.

&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion