Area Forecast Discussion

000
FXUS63 KLSX 190452
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1052 PM CST Mon Jan 18 2021

.UPDATE...
Issued at 839 PM CST Mon Jan 18 2021

Have dropped the western half of the advisory. We have had reports
up to 0.8" of snowfall across Missouri earlier before the band of
snow moved into Illinois. This band has now quickly moved into
the eastern CWA now that the vort max vort has moved into central
Missouri. Radar is showing that the most intense precipitation is
falling as rain, with lighter snow falling back to the northwest
over the advisory area. Expect this snow to move out of the area
in the next few hours as the vort continues to move east and may
cancel the advisory early if this trend continues.

Otherwise introduce areas of fog later tonight into tomorrow
morning across parts of southeast and central Missouri where last
few SREF/HRRR model runs have been consistent in visibilities
falling below 1 mile in these areas late tonight. Will hold off on
issuing an advisory at this point, but will need to watch the
trends overnight.

Otherwise, the rest of the forecast looks on target and made few
changes.

Britt

&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 233 PM CST Mon Jan 18 2021

Main short term issue is snowfall potential the remainder of the
afternoon through the evening associated with a clipper low-
amplitude short wave currently located across the central Plains.
The anticipated band of precipitation has been slow to develop thus
far but a combination of regional radars and surface obs are showing
there has been an uptick in the last hour or so. The combination
of large scale ascent associated with the short wave and largely
mid level frontogenetic forcing should allow the band to continue
becoming better organized the remainder of the afternoon and into
the early evening. Evaporative cooling is supporting snow within
the band across western MO, and this trend should continue as the
band spreads east into the evening. That said the initial
temperatures ahead of the band are in the middle to upper 30s. I
think these initially warm temperatures, combined with somewhat
warm ground temperatures, and the fast system motion is going to
limit accumulations. Probably through the remainder of the
afternoon it will be hard to get accumulations on much other than
elevated or grassy surfaces, and then after sunset the potential
of getting at least some accumulation on roads will increase. That
area would be from eastern MO into southwest IL. The axis of the
band looks like it should be near a Moberly-Vandalia-Litchfield
line and a narrow band of up to an inch of snow is possible,
mainly on the eastern half of this axis. Accumulating snow should
be east of the area by midnight. I have removed some of the far
western counties out of the advisory, not that I don`t think it
will snow there, but I am not expecting any meaningful impacts
before the snow ends by early evening.

Clouds and seasonably cold temperatures will dominate on Tuesday
with the area remaining in northwest flow aloft and the next short
wave looking to pass to the north of the area.

Glass

.LONG TERM...  (Wednesday through Next Monday)
Issued at 233 PM CST Mon Jan 18 2021

Heights aloft are on the rise Wednesday and this combined with a
return of south-southwest low level flow and WAA should bring a
return of warmer temperatures/above normal that will continue
into Thursday. A cold front will then push through the region on
Thursday night ushering a return of seasonably cold winter
temperatures for Friday and Saturday as a expansive surface high
pressure system moves into and dominates the area.

The deterministic and ensembles indicated a pattern change will occur
heading into this weekend with an active split flow. The southern
stream appears to be the more dominate of the streams with a
closed low developing along the west coast, an upper low near Lake
Winnipeg, and increasingly west-southwest flow aloft over the
Nation`s midsection. The next threat of precipitation could occur
as early as late Saturday night into Sunday with return flow and
WAA which could include wintry precipitation at the onset before
the low level warm. Still pretty far out and a good deal of uncertainty
in the details at this time range.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1043 PM CST Mon Jan 18 2021

Expect IFR/LIFR ceilings and visibilities through mid morning at
KCOU where these conditions have already developed. IFR ceilings
have already developed at KSUS, and expect similar ceilings to
develop in the next hour or two at KSTL and KCPS. These ceilings
will likely clear out by 12Z. Fog with MVFR visibilities are also
possible at KUIN between 09-14Z. There will also be another round
of MVFR ceilings during the afternoon into the early evening
before conditions turn VFR.

SPECIFICS FOR KSTL:

Expect IFR ceilings to develop early in the period as low clouds
move into the terminal. These clouds will move out by 12Z before
another batch of low clouds move in during the late afternoon and
early evening with MVFR ceilings. Dry weather is expected through
the period.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion