(Thursday Night - Saturday)
The Canadian high pressure is forecast to slide eastward into the
Great Lakes Thursday night with easterly flow expected for our area
around the southwestern periphery of the anticyclone. Low-level
moisture advection intensifies late night into Friday morning
coincident with a vorticity maxima cresting the midlevel ridge axis.
Precipitation is expected to move into central Missouri first, and
then spread east/northeast with time. The aforementioned cold, dry
retreating airmass will be key as soundings strongly suggest
everyone should start off as snow due to wetbulbing/sublimational
cooling. The strong low-level warm advection will lead to the
development of a strengthening warm nose from southwest to
northeast Friday morning through the afternoon. Therefore,
expectation is for the dominant precipitation type to transition
over to sleet and then freezing rain. As surface temperatures warm
above freezing in the afternoon/evening, a plain cold rain is
expected. However, it does appear that there will be several hours
of frozen/freezing precipitation between late Thursday night and
Friday evening across parts of the area. While confidence is
fairly high in the thermal profiles for this event, there is more
uncertainty in the onset timing of the precipitation late
Thursday night/Friday morning and the axis of heavier
precipitation amounts. Right now, the favored areas where the
stronger forcing will reside is across portions of northeast
Missouri and west-central Illinois. This is where at least a
couple of inches of snow (including some sleet) is favored, with
lesser amounts the further southeast you are located. In terms of
freezing rain (ice) amounts, they should be on the light side,
generally ranging from a light glaze to as much as a tenth of an
inch across the area. Lighter amounts are favored because it looks
like there should only be a few hours where the thermal profiles
favor a full melt of snowflakes aloft AND with surface
temperatures remain below freezing. In addition, freezing rain
occurring with temperatures near or just below freezing and with a
deepening warm nose aloft (hence warmer water droplet
temperatures reaching the surface) will not be very efficient
accreting.
Rain is favored areawide from late Friday evening through the
overnight hours with the surface low traveling north of the CWA. The
trailing cold front should move through the bi-state area late
Friday night through early Saturday morning. This looks like a
classic Schmocker rule scenario, where the precipitation shuts off
before the air cold enough to support snow arrives.
Drier, much colder weather is favored for the remainder of the day
Saturday. Decreasing cloudiness along with falling temperatures is
expected.
