Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211646
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1146 AM CDT Fri Apr 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 355 AM CDT Fri Apr 21 2017

Shortwave over Rocky Mountains will close off and move east into the
Great Plains today and tonight.  Low level cyclogenesis is already
occurring over the eastern Rockies and southerly flow is bringing
moisture out of the Gulf into the eastern Plains and the Mississippi
Valley.  The a 20-30kt low level jet pointed at the baroclinic zone
over southern Missouri is providing lift for widespread showers and
a few thunderstorms across much of southwest Missouri into parts of
southeast Missouri.  The baroclinic zone will lift slowly northward
today and the low level jet will increase this evening and overnight
as the shortwave moves over Missouri and the low level system moves
along the Missouri/Arkansas border.  Highest chances for precip will
be along and south of I-70 through the period.  Should be enough
instability through the day today for isolated to widely scattered
thunderstorms across southeast Missouri as well, but instability
falls of tonight so have only general rain showers in the forecast
after 00Z.  Current total QPF through 12Z Saturday is 1.5 to 2
inches across the eastern Ozarks.  There could be some localized
water issues with this much QPF...but flash flood guidance values
are 2 inches or more, so don`t see a need for any watch products
right now.  Temperatures will be cooler than in recent days in the
upper 50s to low 60s today and in the 40s tonight.

Carney

.LONG TERM...  (Saturday through Thursday)
Issued at 355 AM CDT Fri Apr 21 2017

Still appears that rain will continue through the first half of the
weekend as the GFS/NAM/ECWMF all show the upper low moving from
southwest Missouri at 12Z Saturday morning into western Tennessee by
12Z Saturday.  Models continue to show concentrated low level
moisture convergence underneath strong mid-level ascent along and
south of I-70 on Saturday morning warranting likely PoPs.
Thereafter, the low level jet will veer eastward and the upper
level ascent will begin to wane causing the precipitation to
slowly end from west to east on Saturday night.

Sunday and Monday both still look dry as a ridge moves in quickly
behind the upper trough.   Then the upper flow becomes more
progressive by the middle of next week with the GFS/ECMWF/Canadian
all showing a shortwave trough moving through the upper Midwest.
This brings an attendant cold front across Missouri and Illinois
by Tuesday night and Wednesday night. The global models show a
couple of additional shortwaves moving through at midweek which
will help hold up the front over the area before a deeper trough
builds into the western CONUS by late in the week.

Temperatures will begin to warm back up on Sunday as the upper low
moves off to the east. Temperatures will be above normal most of
next week as 850mb temperatures hover mainly between 10-15C.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1111 AM CDT Fri Apr 21 2017

Surface ridge extends from the northern Plains southeast into MO
and IL. A developing surface low over eastern OK will move east-
northeastward along a nearly stationary front and bring periods of
showers or rain to COU and the St Louis metro area this forecast
period. Current activity on radar staying mainly south of the St
Louis metro area with only a few sprinkles, but another batch of
showers across southwest MO should spread into COU and the St
Louis metro area later this afternoon and this evening. Cloud
ceilings will lower, dropping into the MVFR catagory this evening
in COU and the St Louis metro area, and possibly down to around
1000 feet late tonight and early Saturday morning. Northeast
surface wind will strengthen this afternoon and tonight as the
surface pressure gradient tightens.

SPECIFICS FOR KSTL: Surface ridge extends from the northern Plains
southeast into MO and IL. A developing surface low over eastern
OK will move east-northeastward along a nearly stationary front
and bring periods of showers or rain to the STL area this
forecast period. Current activity on radar staying mainly south of
STL with only a few sprinkles, but another batch of showers
across southwest MO should spread into STL late this afternoon
and this evening. Cloud ceilings will lower, dropping into the
MVFR catagory this evening, and possibly down to around 1000 feet
late tonight and early Saturday morning. Northeast surface wind
will strengthen this afternoon and tonight as the surface pressure
gradient tightens. The cloud ceiling should gradually improve
Saturday afternoon with the rain tapering off.

GKS

&&

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

$$

WFO LSX



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