Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200513

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1213 AM CDT Sat May 20 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 338 PM CDT Fri May 19 2017

New tornado watch has just been issued for much of central and parts
of east central Missouri along the quasi-stationary front.  Latest
GOES-16 imagery is showing cloud streets ahead of a QLCS over
southwest Missouri.  This is south of the stationary front that
extends front south of Kansas City and Columbia to north of St.
Louis.  The atmosphere has destabilized to the south of the warm
front with MLCAPES now between 500-2000 J/kg.  Deep layer shear is
also sufficient for severe storms including supercells and organized
multi-cells including the QLCS.  Runs of the CAMS have been showing
one QLCS moving across the CWA this evening and another moving
across the CWA late tonight.  There will the potential for all
hazards with these storms.  There could also be some locally heavy
rainfall across the area.

Showers and thunderstorms will begin to move out of the area on
Saturday from west to east as a cold front begins to move across the


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 338 PM CDT Fri May 19 2017

(Saturday Night - Wednesday Night)

Cold front will exit the CWA early on Saturday night from west to
east. Along and ahead of the front, there will exist at least a
chance of showers and thunderstorms. Severity is highly unknown as
atmosphere may be overworked and stabilized ahead of the actual cold
front due to antecedent convection. Severe threat regardless
would likely be confined to the evening hours.

Story behind the front continues to be the much cooler airmass which
will quickly filter into the mid-Mississippi Valley beginning after
the frontal passage. Below to much below normal temperatures will
likely be the rule through at least Wednesday night as longwave
trough amplifies overhead, and pivots southeastward toward the area
by Tuesday night/Wednesday. A prolonged chance of showers appears
possible beginning Monday night through Wednesday, with a closed low
forming at midlevels. Cannot rule out a few thunderstorms on Monday
and Tuesday, particularly during peak heating.

Coolest temperatures will likely be on Wednesday as 850-hPa
temperatures drop below +5C. Look for highs in the low to mid 60s
for most locations. Coldest night will likely be Wednesday Night
with lows in the mid to upper 40s.

(Thursday - Next Friday)

A quick moderation in temperatures is likely to end the work week
along with drier conditions as midlevel shortwave ridge moves toward
the bi-state region. Highs by Friday look to be back to just below



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1213 AM CDT Sat May 20 2017

Main TAF concern will be convective trends overnight into
Saturday. We continue to track a convective line across
southwestern MO. This line will attempt to lift east/northeast
overnight, but will likely weaken as it lifts north of a front
which has sunk south of all terminals. Showers and storms will be
likely at all terminals overnight, but reductions to IFR will
likely be fairly brief. Outside of convection, an IFR or low-end
MVFR deck has built into the terminals. This deck is ragged and
locally augmented by rainfall, but expect it to slowly lift north
out of all terminals through the early to mid morning hours as
the warm front lifts northward. After a brief lull in the
convective activity later this morning, expect additional
thunderstorms to develop along the synoptic cold front this

Main TAF concern will be convective trends. Line of storms across
southwestern MO will lift northward overnight, although likely
weakening as it lifts north of a front that lies across south-
central MO. These showers/storms will bring reductions in vsbys,
perhaps to IFR at times. Outside of convection, an IFR/low-end
MVFR deck has built in. This deck should hold firm until the
convection moves in around 09Z, before lifting through Saturday
morning. Another round of showers/storms will then develop along
the synoptic cold front Saturday afternoon.





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