Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250921

Area Forecast Discussion
National Weather Service Saint Louis MO
321 AM CST Tue Feb 25 2020

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 314 AM CST Tue Feb 25 2020

Ongoing light precipitation (mainly in the form of light
rain/drizzle very early this morning) is associated with a
deformation zone and the slowly departing surface low currently
near the lower Ohio River. As temperatures continue to cool over
the next few hours, rain is expected mix with and changeover to
snow in portions of northeast Missouri and west-central Illinois.
Due to the warm/wet ground and temperatures remaining largely
above freezing, any accumulations (if any) appear very light and
confined to grassy/elevated surfaces. By afternoon, nearly all of
the precipitation will be back over to rain as temperatures climb
a couple of degrees into the mid/upper 30s. Therefore, cancelled
the ongoing advisory in northeast Missouri, with these same
counties as well as the adjacent ones further east in Illinois
beginning late this afternoon as temperatures cool and the rain
changes back over to snow.

A system of more interest, immediately on its predecessor`s heels,
will begin to impact the area very late this afternoon lasting
through about midday on Wednesday. Currently, there is a closed
midlevel low near the Nebraska/Kansas border which is forecast to
move east/southeast over the next 24 hours, slowly deepening with
time. Upper-level forcing for ascent associated with this feature
is expected to combine with weak midlevel frontogenesis and
upper-level divergence associated with the left-exit region of a
120+ knot jet streak moving through the mid south. Precipitation
late this afternoon will initially be predominantly across
northeast Missouri and west-central Illinois before slowly moving
southeastward through the evening and early overnight hours. As
temperatures drop, the rain/snow line will also progress
southeastward. All snow is likely around 6PM along/NW of a
KCOU>>KPPQ line, and along/NW of a KVIH>>KSET>>K3LF line around
9PM. Light snow is then forecast to continue for the remainder of
the overnight hours through Wednesday morning after the changeover,
focused on east- central Missouri and western Illinois. By the
time the snow departs around midday on Wednesday, accumulations of
1-3" are likely for northwestern sections of the forecast area.
The highest amounts are most likely to be near Hannibal, MO to Mount
Sterling, IL due to a faster changeover from rain to snow late
this afternoon. The main changes compared to yesterday`s forecast
were to shift the axis of heavier snowfall amounts quite a bit
further to the southeast, as well as also expanding lighter
amounts (1-2") through the St. Louis metropolitan area in addition
to parts of south-central Illinois. This was a result of a fairly
substantial shift of the track of the midlevel shortwave trough
to the south in latest model guidance. Due to this shift southward
in the snowfall amounts, elected to expand the winter weather
advisory as well where accumulations of 2-3" are expected along
with temperatures falling into the mid/upper 20s toward dawn
Wednesday. While the majority of the accumulating snow will likely
be on grassy/elevated surfaces, due to the falling temperatures
well below freezing and snow falling at night, at least some minor
accumulations appear probable on road surfaces, especially
untreated pavement.


.LONG TERM...  (Wednesday Night through Monday)
Issued at 314 AM CST Tue Feb 25 2020

An active northwest flow pattern is likely Thursday and Friday with
two potential light QPF/snow events to watch. The first one (and
more robust looking at the moment) is late Thursday
afternoon/evening. For a clipper system, it is fairly impressive
with a well-defined surface low tracking somewhere through the mid-
Missouri Valley just west of the Mississippi River into
southwestern Illinois. The combination of low-level convergence
near/just northeast of the track of this low, low-level warm/moist
advection, midlevel vorticity advection, and weak low/midlevel
frontogenesis should yield a northwest to southeast traveling band
of light snow. Boosted PoPs substantially into the high chance
category along the Mississippi River for this timeframe, and
further increases very well may be warranted as we get closer.

A second shortwave trough is forecast to move through the area
Friday night, but this feature is a bit weaker than its predecessor
and model guidance is more variable with its track/timing.
Therefore, only introduced slight chance PoPs for light snow with
this package.

A quiet period of weather is likely this weekend with midlevel
ridging coming across the central CONUS. Moderating temperatures
back above normal are forecast by Sunday with highs in the upper 40s
to mid 50s.

Another period of active weather looks to begin early next week due
to strong southwest flow aloft downstream of a slow-moving closed
low in the desert southwest. Way too early to get into specifics 6-
7+ days out, but multiple rounds of rainfall are becoming
increasingly likely. Given the already elevated mainstem river
levels and abnormally moist soils, a very wet period if it pans
out could cause hydrological problems.



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1100 PM CST Mon Feb 24 2020

IFR stratus continues to hold the region in its grips as the
surface low responsible for tonight`s weather has slid to the
Bootheel of Missouri. Persistent moisture will keep low cigs
through the TAF period even when precipitation is not occurring.
Visibilities have been inconsistent over the evening and are
expected to continue to jump between categories over the remainder
of the TAF period as light precipitation moves in and out of the

Precipitation continues to be spotty over the bi-state region,
often too shallow to show up on radar. As temperatures cool
tonight in northeast Missouri and west central Illinois snow will
mix in with any rain that falls. Over the remainder of the evening
and into Tuesday morning a second weather system is expected to
move into the area and the interaction with the current surface
low will result in more widespread precipitation Tuesday morning
across the area. Temperatures will gradually drop below freezing
Tuesday night, causing any lingering precipitation to fall as



MO...Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Wednesday for Knox MO-Lewis MO-Marion MO-Shelby MO.

     Winter Weather Advisory from 6 PM this evening to noon CST
     Wednesday for Lincoln MO-Montgomery MO-Pike MO-Ralls MO.

     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Wednesday for Audrain MO-Boone MO-Callaway MO-Moniteau MO-
     Monroe MO.

IL...Winter Weather Advisory from 6 PM this evening to noon CST
     Wednesday for Calhoun IL-Greene IL-Jersey IL-Macoupin IL-
     Pike IL.

     Winter Weather Advisory from 3 PM this afternoon to noon CST
     Wednesday for Adams IL-Brown IL.



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