Area Forecast Discussion
Issued by NWS St. Louis, MO

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851
FXUS63 KLSX 202039
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
339 PM CDT Mon Mar 20 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 339 PM CDT Mon Mar 20 2017

A cdfnt will continue to sink south tonight, pushing the threat of
precip south of the CWA. With the area remaining largely cloud free
overnight, expect temps to drop into the 40s overnight. While mixing
will persist overnight, have still trended twd the cooler guidance,
especially across the nrn portions of the CWA where temps maxed out
in the lower 70s today.

Precip is expected to redevelop across portions of southern MO ahead
of a developing sfc wave late tonight into Tues morning. These
storms are expected to propagate ewd and impact the srn half to
third of the CWA, mainly late Tues morning and thru the afternoon
hours.

Temps on Tues will be tricky and highly dependent upon cloud cover.
Mdl solns for today were too cloudy and believe that continues into
Tues. This seems to be especially true for the GFS. Some guidance
suggests areas north of the convection mixing to nearly 850mb and
have tailored temps twd that soln.

Tilly

.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 339 PM CDT Mon Mar 20 2017

The synoptic pattern at the start of the long term period will
feature generally northwest flow over the Midwest, due to a trough
digging into the northeast CONUS.  This flow regime will transition
to more southwesterly through the period ahead of a strong upper-
level low sliding across the south-central CONUS.

By Tuesday night, a cold front will be located well south of the
region across portions of northern AR.  However, a weak LLJ
ascending this boundary could push some showers and a few storms as
far north as southeast MO and southwest IL early Tuesday night,
before all activity shifts to the southeast by Wednesday morning.
Overnight lows will be chilly, dipping into the mid/upper 30s.  Will
have to watch cloud/temp trends over the next day or so as portions
of northeast MO into western IL may dip below freezing for a time
Wednesday morning which could lead to renewed agricultural concerns.

Heights will build aloft through the day on Wednesday, allowing the
previous cold front to begin to lift back northward as a warm front.
The GFS/GEFS solution seem a bit too wet for Wednesday given the dry
airmass in place, so preferred the drier ECMWF/GEM solutions for
this forecast.  By Wednesday night into early Thursday, however, a
low-level jet will focus into this lifting warm front providing the
ascent for renewed showers.  The front will have pushed north of the
region by late Thursday afternoon, allowing for another period of
dry conditions.

The first half of Friday will likely be dry, but attention will turn
to the west as a strong upper-level low approaches the region. There
is still some subtle timing differences, but the overall consensus
is for a line of showers with a few thunderstorms 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 may 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
development.

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 early
next week with a few additional chances of showers/storms.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1225 PM CDT Mon Mar 20 2017

A cdfnt will sink south of the area this evening keeping TSRA
south of the terminals. Much of today and tonight is expected to
be mainly clear. However, clouds will gradually build into the
area later tonight into Tues. Cigs are expected to remain VFR thru
the period.

Tilly

&&

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

$$

WFO LSX



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