Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 150019
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
719 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 00z TAFs through 00z Monday Evening)
Issued at 719 PM CDT Sun Sep 14 2014

Mid cloud deck to continue to develop and spread east across
forecast area this eveing ahead of next weather system. Then will
see showers and thunderstorms develop on nose of low level jet and
spread east into KUIN by 08z. As low level jet veers to the east
towards daybreak, activity to slide southeast towards rest of taf
sites. Coverage to become more scattered in nature for sites along
I-70 corridor, so just kept vcnty shower/thunderstorm mention
through the afternoon hours, even after front moves through
region. Could see MVFR cigs behind frontal boundary, especially
taf sites along I-70, so added mention. Begin to dry things out by
late afternoon. As for winds, to persist from the southeast to
south ahead of front, then veer to the west, then to the north by
Monday afternoon.

Specifics for KSTL:
Mid cloud deck to continue to develop and spread east across
forecast area this eveing ahead of next weather system. Then will
see showers and thunderstorms develop on nose of low level jet and
spread east into northeast MO. As low level jet veers to the east
towards daybreak, activity to slide southeast towards metro area.
Coverage to become more scattered in nature, so just kept vcnty
shower/thunderstorm mention through the afternoon hours, even
after front moves through region around 20z Monday. Could see MVFR cigs
behind frontal boundary, so added mention after 01z Tuesday as
metro area dries out. As for winds, to persist from the southeast to
south ahead of front, then veer to the west by 20z Monday, then to the
north by 01z Tuesday.

Byrd
&&

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

$$
WFO LSX




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