.SHORT TERM... (Through Wednesday Night)
Issued at 337 AM CST Tue Jan 24 2023
Key Messages:
1) Wet snow will overspread the region this evening
into the overnight hours from north to south. Snowfall rates are
expected to overpower surface temperatures around freezing to
produce accumulation.
2) A majority of the snowfall will fall overnight into the early
morning hours, including the Wednesday morning commute. Impacts to
travel are expected.
3) Accumulation will be greatest along and south of the I-44
corridor in Missouri and I-70 corridor in Illinois where a total of
4-8 inches expected with locally higher amounts possible.
Accumulation of 2-5 inches is expected north of this corridor. The
area of greater accumulation has been shifted slightly northward.
Water vapor imagery currently shows weak ridging building into the
Middle Mississippi Valley and a trough digging across the Desert
Southwest. Southwesterly flow through the mid to low levels of the
atmosphere is increasing moisture over the region, leading to
increasing cloud cover that will only become denser through the day.
Despite the cloud cover, weak southerly surface flow will advect
warmer air into the CWA, leading to temperatures peaking this
afternoon as warm as or a degree or two warmer relative to
yesterday.
As we get into this evening, the upper-level trough will eject out
of the Southwest and enter the central CONUS. As this occurs, the
left exit and right entrance regions of two upper-level jet streaks
will couple to produce ample upper-level divergence over the Mid
South. This will allow a surface low moving out of the Southern
Plains to deepen as it enters the Mid South and tracks toward the
Ohio Valley. Ahead of the low, isentropic ascent will lead to precip
overspreading the CWA this evening roughly from north to south
starting around 5-6PM. This precipitation shield will reach the I-70
corridor at approximately 9PM, and cover northeastern Missouri and
west-central Illinois by around midnight.
Precipitation will start off briefly as rain before temperatures in
the lower atmosphere cool to the wetbulb. Then, a saturated DGZ and
below freezing thermal profile through the lower atmosphere will
lead to snowfall. Despite surface temperatures hovering right around
freezing through the morning, snowfall rates are expected to
mitigate melting and lead to accumulation. Given the track of the
surface and 850mb low, climatology favors the heaviest snowfall
occurring along and south of the I-44 corridor in Missouri and I-70
corridor in Illinois. Here, roughly 4-8 inches of accumulation is
expected, with totals in southeastern Missouri approaching 9-10
inches in localized areas by the time the heavier snow tapers off
late Wednesday morning. Northward of that corridor through
northeastern Missouri and west-central Illinois, generally 2-5
inches is expected.
It is worth noting that spread among ensemble and deterministic
guidance in regards to the low track still remains relatively high
at this lead time. Courser resolution guidance seems to be trending
slightly southward with the track of the low over the last several
initializations, while higher resolution guidance has been trending
northward since the storm system came into range. Given the idea
that higher-resolution guidance will better resolve the key features
of the system, the updated forecast leans more toward the northern
solution and has pulled greater totals slightly northward, including
more of the I-44 MO/I-70 IL corridor in the higher amounts. If the
trend among hi-res guidance continues and becomes reality, we could
see the aforementioned corridor become the central focus for the
heavier amounts, placing the St. Louis metro area squarely beneath
the heavier amounts too. The majority of the forecasted snowfall
will occur prior to and during the early portion of the Wednesday
morning commute, with travel impacts expected.
While heavier snowfall will tapper off through the late morning,
light snow or a rain/snow mix will continue through the day into the
overnight hours. An additional one to two tenths of an inch of
accumulation is possible. This light snow will have trouble
accumulating on surfaces not covered in snow thanks to temperatures
rising a couple of degrees above freezing during the day.
Elmore
