Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212354

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
654 PM CDT Tue Mar 21 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 300 PM CDT Tue Mar 21 2017

Overrunning precipitation along and south of I-70 will diminish this
evening and light sprinkles to the north with weaker upper level
shortwave to diminish as well. Otherwise, surface ridge to build in
with colder temperatures in the 30s tonight.

On Wednesday, mid and high clouds to prevail and with east wind,
temperatures will struggle to warm up much. Only expect highs in the
mid 40s to low 50s. Could see some sprinkles/light rain mainly
during the afternoon hours as another weak shortwave approaches
region on northwest flow. For now kept forecast dry.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 300 PM CDT Tue Mar 21 2017

The synoptic pattern at the start of the long term period will
feature generally northwest flow over the Midwest, due to a trough
over the northeast CONUS.  This flow regime will transition to more
southwesterly through the period as several longwave troughs build
into the southwestern CONUS.

By Wednesday night, a stalled front will be located across portions
of eastern OK and AR.  As an upper-level ridge axis slides across
the region allowing the flow aloft to back, the boundary will surge
northward through the day on Thursday.  As a low-level jet of 25-35
knots focuses into this boundary, isentropic ascent may be enough to
squeeze out some showers (especially across central MO) Thursday
morning.  The second half of Thursday into the first half of Friday
will likely be dry as the region sits in the warm sector of a
developing storm system out across the Central Plains.

There is still some timing differences noted with the system Friday
night into Saturday, with the GFS slower than the ECMWF.  The GEFS
mean is a bit faster than the operational GFS and closer to the
ECMWF and thus are the preferred solutions. This consensus is for a
line of showers with a few thunderstorms late Friday afternoon
through Friday night. Given the system will be fairly vertically
stacked as it approaches, instability does not look very impressive
given weak mid-level lapse rates and only modest low-level moisture
values (dewpoints in the mid/upper 50s), which should help limit a
more organized severe weather threat in our area.

On Saturday the mid-level dry slot will work into the region ahead
of the surface low pressure system and associated cold front.  As
the cooler temperatures aloft work in, guidance is suggestive that
some surface-based instability will build on Saturday (GFS is most
aggressive with this), leading to additional shower and thunderstorm

Conditions should slowly dry out through the day on Sunday as
surface ridging builds into the region.  However, guidance is in
general agreement that the active pattern will continue into Monday
as yet another trough/PV anomaly approaches the region bringing yet
another round of showers and a few thunderstorms.



.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 620 PM CDT Tue Mar 21 2017

VFR through the period with northeasterly winds gradually becoming
easterly as a surface high pressure center moves across the Great
Lakes. Increasing moisture may lead to increased coverage of
mid/high clouds after 22/18z.





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