Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211640

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1140 AM CDT Wed Mar 21 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 325 AM CDT Wed Mar 21 2018

The synoptic pattern early this morning features an amplifying
trough across the eastern CONUS in response to a strong PV
anomaly digging into the southeast coast. This will place the
region within northwest flow aloft through the period with
generally building heights aloft.

We continue to track a few sprinkles/flurries out there this
morning across portions of central MO. However, think those will
be short-lived as the responsible PV anomaly is pushing off to
the east/southeast and shortwave ridging is building in. There
continues to be a fairly solid band of clouds across portions of
central MO extending northwest into western IA. Guidance is in
general agreement that the combination of large scale subsidence
and diurnal mixing should be enough to dissipate this cloud band
through the day today. Have some suspicion guidance may be
advertising this clearing too quickly, thus have leaned on the
slower end of things. Nonetheless, it appears most areas should go
at least partly cloudy by late this afternoon which should help
temperatures climb into the upper 40s/low 50s.

After a partly cloudy to mostly clear sky the first part of
tonight, some mid/high clouds will begin to stream into the
region from the northwest. Therefore, expect the warmest lows
over northeast/central MO (low 30s) with upper 20s/low 30s
expected across eastern MO and Illinois where clearing should
prevail for a longer period.


.LONG TERM...  (Thursday through Tuesday)
Issued at 325 AM CDT Wed Mar 21 2018

Model guidance for late this week and this weekend seems to be in
slightly better agreement compared to the last several nights, and
there is even a hint of continuity with important surface
features. This produces slightly higher confidence in temp/pcpn
trends for the weekend compared to 24hrs ago.

Upper ridging will move across the central CONUS late this week
while a large closed low settles over the PacNW and several
disturbances rotate through its associated trough near the west
coast. One such PV anomaly will sail headfirst into the upper
ridge axis on Fri/Sat. Even though it will likely be weakening
and/or shearing out as it moves across the LSX CWA, this system
will still produce showers and isolated thunderstorms across the
area from Fri aftn through late Sat evening.

Precipitation chances on Thu/Thu night are tied to H85 moisture
convergence along an elevated warm frontal boundary associated
with the surface reflection of the aforementioned PV anomaly. As
the previous shift noted, limited moisture may limit the extent of
any pcpn which develops.

Temperature trends for Thu through Sun will be complex and highly
dependent on the location and position of surface frontal
boundaries. In general, the southern half of the CWA will be
slightly warmer each day between Thu and Sat, while the northern
one-quarter of the CWA may very well see cooler temperatures on
Sat due to the combination of clouds, pcpn, and a baroclinic zone
which will have shifted slightly southward as a cold front while
the surface low rides along it. Overnight lows on Sat night will
be cooler everywhere compared to the previous night based on the
forecast position of the baroclinic zone, which will be sinking
farther southward on Sat night and is forecast to have reached the
MO/AR border by 25/06z.

Models seem rather quick to open up the closed PacNW system into
an open wave, and there are some differences in timing and
location between the ECMWF, GFS, and GEM. If this system does
become an open wave as depicted by the models, then the resulting
flow would support another chance of precipitation on Mon/Tue.



.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1114 AM CDT Wed Mar 21 2018

Surface ridge over eastern portions of KS and OK will shift
eastward through the taf sites tonight. Gusty northwest surface
winds will become light this evening. Just scattered diurnal
cumulus clouds this afternoon, especially in the COU area. Mid
level cloudiness will advect southeastward into our area Thursday
morning. Maybe sprinkles in UIN and COU Thursday morning.

SPECIFICS FOR KSTL: Surface ridge over eastern portions of KS and
OK will shift eastward through the STL area tonight. Gusty
northwest surface winds will become light this evening. Just
few-scattered diurnal cumulus clouds this afternoon. Mid level
cloudiness will advect southeastward into the STL area Thursday.
Light showers possible in STL Thursday afternoon.





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