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fxus63 klsx 092120 

Area forecast discussion
National Weather Service Saint Louis MO
320 PM CST Mon Dec 9 2019

Short term... (through late Tuesday night)
issued at 320 PM CST Mon Dec 9 2019

The cold front as of 2100 UTC is located near a kpof>>ksar>>kmto
line and continues to move southeastward. In its wake, expect a
sharp temperature drop (~20 degrees within 3-4 hours) and gusty
northwest winds. Sustained winds should be about 15-25 mph with
gusts as high as 35 mph. These blustery winds combined with
temperatures falling into the 20s and 30s will make it feel more
like the teens and twenties by late this evening. In other words, it
will feel much more like winter. Behind the front, upstream radar
imagery depicts some scattered echoes aided by weak midlevel
frontogenesis. However, surface observations show that nearly all of
this echo is not reaching the ground due to a dry layer between 600
and 850 hpa. If this layer can saturate sufficiently, we may see a
few very light rain showers or sprinkles. Also, patchy drizzle is
being observed within the first few hours behind the frontal passage
and this should continue heading toward the early evening. By late
this evening, strong low-level dry advection should cutoff any
threat for precipitation reaching the ground areawide.

A seasonably cold night is on tap and elected to cool overnight lows
across the bi-state area several degrees from the previous forecast.
This change was predominantly due to upstream observations in the
teens and twenties already as close as central Iowa. Lows tonight
should range from the mid teens to mid twenties from northwest to
southeast, which now are more in line with the 1200 UTC mav/met MOS

Seasonably cold and dry conditions are forecast to continue through
the remainder of the short-term forecast period. A secondary system
is expected to develop along the cold front moving through the mid
south which is expected to bring a round of rain changing to snow.
However, this system should stay well to the south of the area with
the only impact being a continued stream of thicker cirrus clouds,
mainly across portions of southeast Missouri and southwest Illinois
through Tuesday.

Any lingering cirrus should depart southern sections of the County Warning Area by
Tuesday evening, leaving a mostly clear sky for a majority of the
night. Another cold night should be on tap due to light/variable
winds associated with a surface high, low dewpoints, and the
aforementioned mostly clear sky. Leaned toward the cooler met most
guidance for lows Tuesday night, especially across favored valleys
in east-central and southeast Missouri.


Long term... (wednesday through next monday)
issued at 320 PM CST Mon Dec 9 2019

(wednesday - Saturday night)

Surface high pressure maintains control across the County Warning Area through
Wednesday night with seasonably cold temperatures continuing along
with dry weather. Temperatures are still expected to moderate for
the end of this work week as 850-hpa temperatures warm to near +5c.
Highs Thursday and Friday are forecast to be in the 40s to near 50
degrees, with lows Wednesday and Thursday night in the 20s to low

Models agree that a midlevel shortwave trough and a weak cold front
should move across the mid-Mississippi Valley Friday into early
Saturday. This system may produce some light precipitation, mainly
in the form of a cold rain as temperatures both at the surface and
aloft appear too warm to support snow.

(Sunday - next monday)

This is a potential time period of interest as deterministic
guidance suggests the next system to eject out of the Rocky
Mountains will have enough cold air to produce snowfall north of
the track of the surface low. Not surprisingly, ensemble members of
the gefs and the eps are quite dispersive with respect to the track,
timing, and strength of this system. However, there is a
decent signal for at least some accumulating snowfall associated
with this system. This is evidenced by a majority of the gefs/eps
members showing snowfall within or near the County Warning Area sometime between
Sunday and next Monday. At this early juncture, the northern County Warning Area
would be favored based on the ensemble means of the gefs/eps. In
addition, the location of the shortwave trough entering the Continental U.S.
(Southern Oregon/northern california) along with weak
midlevel ridging developing downstream suggest a track of the
midlevel impulse toward the northern or northwestern County Warning Area.

Given all of the uncertainty mentioned above, leaned with a blended
approach, which resulted in chance pops for a rain/snow mix, with
more snow wording the farther north you go and for Sunday
night/Monday morning. Temperatures should cool behind the first
system moving through on Saturday, so expect a return to slightly
below normal values.



Aviation... (for the 18z tafs through 18z Tuesday afternoon)
issued at 1131 am CST Mon Dec 9 2019

The cold front has just passed through kcou and KUIN and is
forecast to move through the Metro terminals by mid afternoon.
Patchy drizzle or a few showers are possible over the next few
hours, but the rest of the afternoon through the remainder of the
taf period will be dry. Ceilings will improve a few hours after
the front, but low MVFR conditions should dominate this afternoon,
with a brief period of IFR ceilings are expected. The other
significant change with frontal passage will be the winds. Look
for a sharp veering from the southwest to the northwest along with
gusts increasing to 25-30 mph. These gusty northwest winds will
continue tonight and slowly subside toward Tuesday morning.
Ceilings should improve several hours after frontal passage, with
only mid or high-level clouds expected tonight and Tuesday

Specifics for kstl:

The cold front is expected to pass through the terminal around
2100 UTC. Ahead of it, southwest winds along with low MVFR
ceilings are expected along with the possibility of patchy
drizzle. A brief period of IFR ceilings are also possible early
this afternoon. Behind the front, gusty northwest winds on the
order of 25-30 mph are expected. By early evening, ceilings are
forecast to rise to VFR levels and remain that way through the day
on Tuesday.



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