Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250448
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1148 PM CDT Fri Mar 24 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 405 PM CDT Fri Mar 24 2017

Main focus will be precip chances and potential for severe threat
thru the period.

SHRA with sct TS continues across portions of central MO this
afternoon. These will slowly propagate ewd this evening and
overnight. Ahead of theses SHRA, the area has destabilized somewhat
tho limited as dewpoints mix out into the lower 50s. This area also
remains capped with warming in the 800 mb region. There appears to
still be a limited window, in both time and space, where a severe
threat will linger late this evening. Appears the main threat will
be limited to the srn portions of the CWA with wind as the main
threat where convection can organize, or remain organized as it
propagates into the area.

Going forecast may have too much TS mentioned overnight and may
become more of a shower event thru the night.

As the stacked low continues to approach the area, expect the dry
slot to clear out precip over much of the area Sat morning. This
lull shud be brief as convection develops and fills in the area.
Still appears at least a hail threat will be exist with any storms
that develop on Sat. Can not rule out an isolated tornado threat,
but will depend on where the occluded fnt sets up. Currently, area
of most concern is nrn portions of the CWA.

The stacked low continues to fill and is expected to weaken somewhat
as it approaches the area. However, will need to monitor expected
wind speeds for Sat as an advisory may be needed. Area of most
concern are ern half to third of the CWA.

Tilly

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 405 PM CDT Fri Mar 24 2017

First in the series of cutoff upper lows will be over central MO as
of 00z Sunday. So for Saturday evening, will see co-located surface
low lifting northward through eastern MO and into southeastern
IA/northwestern IL. It will slowly drag the cold front through with
showers and isolated thunderstorms. Still some concern for isolated
severe due to cold core low. The latest SPC Day 2 Outlook keeps the
region in a Marginal risk, but this could be upgraded in later
forecasts as instability trends become more clear.

Showers to taper off towards daybreak on Sunday, but could still see
some isolated showers Sunday morning due to being in cyclonic flow
on back side of system. Otherwise, most locations will be dry on
Sunday before next cutoff upper low lifts out of the desert
southwest towards region Sunday night into Monday. So will see
another round of showers and thunderstorms through Monday evening.

Active pattern to persist with weak ridging building in for Tuesday-
Tuesday night before next system moves in on Wednesday. Will see
best chances of showers and thunderstorms late Wednesday night
through Thursday Night.

As for temperatures through the period, will see lows in the 40s and
highs mainly in the 60s.

Byrd

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1145 PM CDT Fri Mar 24 2017

Main forecast issue is trying to separate mainly wet periods from
periods that won`t be as wet. One wet period ongoing and will take
most of the night to get thru UIN and STL metro sites while it
should be ending in COU soon. Another main round of SHRA and TSRA
expected to then re-fire Saturday afternoon with the heating of
the day and then diminish in the evening. VFR conditions should
prevail at the TAF sites for much of the valid period in a warm
sector setup but as the cold front slowly moves thru beginning
late Saturday afternoon and Saturday night, this should result in
lower CIGs building in from the west and deteriorating prevailing
conditions. Could end up with IFR, especially late Saturday night
with better probs further north at UIN. Surface winds from the
SE-S will prevail for much of the period until cold FROPA with
either a shift from the W-SW for COU and STL metro very late, or
becoming more variable at UIN closer to the track of the surface
LO.

SPECIFICS FOR KSTL: Two main periods of rain...one ongoing until
about 09/10z and another main round for Sat afternoon. Otherwise,
VFR should prevail until late Saturday night with lowering CIGs.

TES

&&

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

$$

WFO LSX



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