Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 120840
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
340 AM CDT Fri Sep 12 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 340 AM CDT Fri Sep 12 2014

Could not rule out a little drizzle this morning from the persistent
stratus cloud deck across most of our forecast area.  An upper level
trough, strong shortwave will move eastward through the northern
Plains today and bring measurable rain to at least northeast MO this
afternoon.  Unseasonably cool weather will continue today as the
strong surface ridge centered across the northern Rockies and
northern Plains continues to build southeastward into MO.  The
850 mb temperatures will continue to fall, ranging from only about
2 degrees C across northeast MO to around 11 degrees C across
southeast MO by 00z Saturday.

GKS

.LONG TERM:  (Tonight through Thursday)
Issued at 340 AM CDT Fri Sep 12 2014

An interesting scenario for tonight, which is a carryover from
today.  A powerful shortwave TROF will track just north of our
region this evening and early overnight with much of the good
broadscale lift also passing to our north.  But...a lobe of
vorticity will trail to the south thru our region and maintain some
presence of this broadscale lift.  This combined with similar values
for moisture and deep lift thru the column currently found over
western Kansas where there is decent coverage of rain should result
in something more than what is currently depicted by many of the 00z
run models.  Needless to say, strongly favor the wetter MET MOS PoPs
for this evening, and in some cases, going even higher.  PoPs should
rapidly taper off after midnight as much drier air and clear skies
expand.

A strong Canadian surface high pressure--the same one that delivered
the cold air that helped blanket parts of western South Dakota with
snow 24 hours ago--will then build in for Saturday and continue to
dominate our region into Sunday.  This will maintain the well below
normal temp regime and promote dry weather after tonight.  Nighttime
min temps may dip into the upper 30s in spots--first across areas in
northeast MO early Saturday morning--and then again across the
eastern Ozarks of southeast MO early Sunday morning.  Strong
sunshine which can still be seen in September, especially before the
equinox, will result in large diurnal swings.  Prefer the warmer MAV
MOS for max temps this weekend.

The chance for rain will begin to return late Sunday night and
Monday as a cold front moves thru.  The airmass behind this front is
not quite as cool as the one we`re going to get this weekend and so
the resultant daytime max temps are only slightly below normal.
Models diverge on handling of precipitation chances after this, but
with decent agreement on northwest upper flow, prefer a dry solution
at this time.

TES

&&

.AVIATION:  (For the 06z TAFs through 06z Friday Night)
Issued at 1110 PM CDT Thu Sep 11 2014

Stratus remains widespread late this evening producing MVFR flight
conditions at all the terminals. There is however some drier air
slowly advecting southwestward due to northeasterly low level
winds. This has brought a jagged clearing line into far northern
MO which then curves back into far northern IL. There is potential
that KUIN could clear out for a time only to have clouds move back
in - confidence is low on the trends. I have also added a prob30
for light rain in the afternoon as an upper trof digs across the
upper MS valley. Elsewhere the aforementioned dry air has lessened
my confidence in the previously anticipated lower cigs overnight
and threat of drizzle. I have maintained it in the new TAFS
although with higher cigs and only a TEMPO group. Clearing should
commence at all sites Friday night, some of which is beyond the
valid forecast period.

Specifics for KSTL:

Stratus remains widespread late this evening producing MVFR flight
conditions. There is however some drier air slowly advecting
southwestward due to northeasterly low level winds. This could
could initially result in the cig height bouncing from 1800-2000
ft. The dry air has also lessened my confidence in the previously
anticipated lower cigs overnight and threat of drizzle. I have
maintained it in the new TAFS although with higher cigs and only a
TEMPO group. Otherwise MVFR flight conditions will persist on
Friday with with clearing late on Friday night.

Glass

&&

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

$$

WFO LSX




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