Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 142023
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
323 PM CDT Sun Sep 14 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 322 PM CDT Sun Sep 14 2014

Low level ridge extending from the Ohio Valley into the lower and
middle MS Valley will drift eastward tonight, but maintain an
influence on southeast MO and southern IL promoting overnight lows
in the upper 40s. Meanwhile the main concern will be the threat of
showers and thunderstorms tonight. Westerly flow aloft will advect a
region of relatively steep mid level lapse rates presently across
the Plains across Missouri and into IL tonight. Beneath this, a
rather broad low level jet will ramp up this evening across the
central Plains and veer to more westerly overnight.  The
differential advections will result in elevated instability with the
LLJ providing good moisture transport/warm advection and lift.
Current thoughts are elevated showers and thunderstorms will develop
near the terminus of LLJ with showers and thunderstorms spreading
across northern MO and into west central IL during the late night
hours. Given the progressive nature of the flow aloft, I have
favored the more agressive eastward and southern extent of the NAM
and local WRF.

Glass

.LONG TERM:  (Monday through Next Sunday)
Issued at 322 PM CDT Sun Sep 14 2014

(Monday through Wednesday)

Main concern deals with precipitation chances through Monday night.

The GFS and NAM are in decent agreement with the speed and depth of
the shortwave trough currently that is dropping south into Montana
that is expected to deepen and move southeast across Missouri and
Illinois late on Monday.   Expect the greatest concentration of
showers and thunderstorms to be across central and northeast
Missouri and west central Illinois on Monday morning where the GFS
and NAM is showing the strongest low level moisture convergence
ahead of the approaching trough.  This morning`s 12Z NSSL WRF
depicts this idea, showing showers and thunderstorms over northern
Missouri on Monday morning.  Will keep categorical PoPs going during
morning, and then keep likely PoPs going into the afternoon over
central parts of the CWA as the cold pushes south through the CWA.
There will be some lingering showers over southeast Missouri into
southern Illinois into Monday night as the cold front exits the area.

Tuesday and Tuesday night then will be dry as heights rise behind
the trough.  I did add some slight chances of thunderstorms across
central Missouri on Wednesday and Wednesday night as the GFS has
been consistent in showing some light QPF with a weak shortwave
moving southeast across the area in northwesterly flow aloft.

For temperatures, I used a blend of MOS guidance tomorrow, favoring
the cooler NAM values where I expect there to be precipitation and
earlier frontal passage in the north earlier in the day.   Used a
blend of MOS guidance for highs on Tuesday which works well in
mixing down 900mb temperatures.  Lows are the the agreeable MOS
guidance.

(Thursday through next Sunday)

Still looks like the end of the work week should stay dry as surface
high moves across the area.  The chance of rain will increase next
weekend as both the ECMWF and GFS show a trough approaching from the
west on Saturday with the attendant cold front crossing the area
Saturday night and Sunday.  Temperatures will climb back to near
normal by late in the week as 850mb temperature climbs into the mid
teens Celsius.

Britt

&&

.AVIATION:  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1228 PM CDT Sun Sep 14 2014

VFR flight conditions are expected through at least 06z tonight
with some batches of sct-broken clouds from 6000-10000 ft. The
brunt of these through this afternoon will impact mid MO including
KCOU. Overnight a west-southwest low level jet will develop and
this should lead to a growing area of showers and some
thunderstorms across northern MO into central IL that should
impact KUIN late tonight and into Monday morning. This region of
showers and thunderstorms should sind southward with time on
Monday but the coverage and intensity are difficult to gage.

Specifics for KSTL:

VFR flight conditions are expected throught daybreak on Monday.
From Monday morning into the afternoon, an area of showers and
thunderstorms originating in northern MO will be settling
southward. The coverage and intensity are difficult to gage the
potential impact to the terminal should ramp up from mid morning
into the afternoon.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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