.SHORT TERM... (Through Wednesday Night)
Issued at 342 AM CST Mon Jan 23 2023
Key Messages:
1) Confidence is high (>90%) that a winter storm will
produce accumulating snow and impact the region Tuesday night into
Wednesday evening.
2) Impacts to travel are expected area-wide, especially during the
Wednesday morning commute for areas along and south of I-44 in
Missouri and I-70 in Illinois.
Current water vapor imagery shows an upper-level trough over the
Midwest pushing eastward, with northwest flow on the backside of the
trough moving over the CWA. At the surface, an area of high pressure
centered over the Central Plains has the CWA in weak northwesterly
flow at the surface. Through the day, the surface high will quickly
move eastward, causing winds to gradually shift to out of the
southwest through the day. Depending on how soon this occurs, we
may see temperatures rise above what is currently forecast due to
downsloping off the Ozarks.
Upper-level ridging will briefly move over the Middle Mississippi
Valley tomorrow ahead of a trough digging across the Desert
Southwest. This, and sustained southerly to southwesterly flow will
help temperatures rise by a couple of degrees over what is expected
today.
Our primary focus remains on a winter storm that is set to move
into the area Tuesday night with impacts expected through
Wednesday. As the aforementioned trough ejects out into the
Southern Plains, the right entrance and left exit regions of two
jet streaks will combine to deepen a surface low as the low enters
the Mid South. As it does so, isentropic lift will lead to
precipitation overspreading the CWA from the south/southwest to
the north/northeast. This precipitation is expected to fall
primarily as snow, as the consensus among model soundings is that
the DGZ will be fully saturated, leading to ice crystal
development and growth. Additionally, temperatures through the
middle and lower atmosphere are expected to be below freezing, so
melting aloft is not expected. Lift focused in the DGZ and weak
winds through the lower atmosphere support SLRs generally at to
just above climatology. While temperatures for locations mainly
across southeastern Missouri and southwestern Illinois will
initially be just above freezing at the surface, high snow rates
(approaching 1" an hour at times) in this portion of the area will
overcome these temperatures to produce accumulating snowfall. In
fact, snowfall will likely cause temperatures to drop by a few
degrees thanks to dynamic and evaporative cooling. The snowfall
rates are expected to be highest leading up to sunrise and the
Wednesday morning commute.
Given the expected trajectory of the surface and 850mb low,
climatology favors locations along and south of I-44 in Missouri and
I-70 in Illinois seeing the greatest snowfall, with accumulations
here toping out roughly 6-8". For these locations, a Winter Storm
Watch is in effect and was expanded slightly to account for a bump
in totals a little further northward. North of the Watch area,
totals are expected to be generally 3-5". There is still enough
spread among ensembles that leads me to believe that there could
still be a slight shift in where the heaviest snowfall occurs. A
slight adjustment of the system to the north could center the axis
of greatest accumulation along the I-44 MO/I-70 IL corridor.
While the heavier snow is expected to taper off through late
Wednesday morning and early afternoon, the chance for light snow
capable of roughly another 0.5" of accumulation will continue into
the evening as the deformation zone lingers across the area.
Elmore
