Area Forecast Discussion
Issued by NWS St. Louis, MO

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198
FXUS63 KLSX 220507
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1107 PM CST Tue Feb 21 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 311 PM CST Tue Feb 21 2017

A weak cold front will continues to push through region and should
clear the area by 9z tonight. This will keep overnight lows well
above seasonal normals.  An area of high pressure will slowly build
into the area from the west. The combination of light winds and wet
ground will allow for areas of fog tonight after midnight. There
could be a patchy dense fog but likely not widespread as warm cirrus
overhead will impede chances of a more widespread event. The fog is
likely to mix out by mid morning to allow for ample warming. It is
likely that many records will either be tied or broken tomorrow.

Kelly

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 311 PM CST Tue Feb 21 2017

(Wednesday Night - Thursday Night)

Frontal boundary is expected to stall across the center of the CWA
late Wednesday night as leeside cyclogenesis commences well to the
west of the area. An increase in cloud cover with light/variable sfc
flow is expected during the night along with low temperatures some
20-25 degrees above normal.

Aforementioned surface low will deepen during the day on Thursday
and transverse into north-central Kansas by Thursday evening.
Attendant frontal boundary will begin to lift northward as a warm
front though its movement northward may be abated a bit by the
possibility of showers and a few thunderstorms. Timing and placement
of this boundary will be key for sensible weather impacts including
high temperatures on Thursday. Could be a fairly significant
temperature gradient from north to south across the area with highs
in the 70s south of the boundary to 50s/60s north of the front.
Consensus placement however of the warm front does take it north of
the CWA by early evening Thursday.

Best chances for shower/thunderstorm activity on Thursday night will
continue to be across far northern sections of the forecast area in
closer proximity to the warm front. Most of the CWA should stay dry
as warm sector looks capped. Lows Thursday night will continue to be
incredibly warm for mid/late February with lows ranging from the
upper 40s to upper 50s.

(Friday)

Weak dryline should be across far western sections of the CWA early
Friday morning. This boundary will try to be the focus of isolated
to scattered convection but question will be if and when the cap is
able to be eroded. Best chance for storms appear to be over far
eastern portions of the CWA where combination of slightly stronger
forcing for mid/upper air ascent and better moisture return will
exist Friday afternoon.

Highs on Friday will once again be tricky and be heavily dependent
on warm sector cloudiness and timing of trailing cold front. Leaned
toward warmer guidance for eastern 2/3 of CWA as actual cold front
doesn`t look to clear CWA until early Friday evening. NAM model
timing of cold front was discounted for now as it was some 9-15
hours faster than operational GFS/ECMWF/CMC.

(Friday Night - Next Tuesday)

Much cooler weather is on tap for this upcoming weekend. While it
will be near normal...temperatures on Saturday and Saturday night
will be some 30 degrees colder than much of this week and last
weekend. Forecast lows on Saturday night are in the mid 20s.

A slight moderation in temperatures is expected Sunday into Monday.
Weak disturbance will quickly move across the mid-Mississippi Valley
on Sunday and bring a chance at a quick rain/snow mix changing over
to rain Sunday afternoon. Another disturbance looks to bring another
round of primarily rain during the day on Monday.

Warm front will move north of area Monday night yielding much above
normal temperatures for the end of the extended forecast along with
a more unstable atmosphere with showers and thunderstorms likely
Tuesday afternoon ahead of an approaching cold front.

Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1055 PM CST Tue Feb 21 2017

With a light surface wind, relatively high surface dew points and
surface/ground moisture from the recent rainfall, the fog and
stratus clouds will continue to develop late tonight and early
Wednesday morning with visibilities dropping to 1/4 mile or less
at the taf sites. The fog should gradually dissipate and stratus
clouds will lift by late Wednesday morning as a south-southwest
wind strengthens.

SPECIFICS FOR KSTL: With a light surface wind, relatively high
surface dew points and surface/ground moisture from the recent
rainfall, the fog and stratus clouds will continue to develop late
tonight and early Wednesday morning with visibilities dropping to
1/4 mile or less. The fog should gradually dissipate and stratus
clouds will lift by late Wednesday morning as a south-southwest
wind strengthens.

GKS

&&

.CLIMATE...
Record Highs

       2/22    2/23    2/24

STL: 78/1995 77/1996 81/1930
COU: 76/1995 73/1933 81/1930
UIN: 70/1922 70/1922 73/1930

Record High Minimums

       2/22    2/23    2/24

STL: 55/1985 55/1930 58/2000
COU: 53/1922 53/2000 61/1930
UIN: 57/1922 49/2000 55/1930


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CST Wednesday for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Moniteau MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Louis City MO-Saint Louis MO-Warren MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CST Wednesday for Adams IL-Bond IL-
     Brown IL-Calhoun IL-Greene IL-Jersey IL-Macoupin IL-Madison
     IL-Monroe IL-Montgomery IL-Pike IL-Saint Clair IL.

&&

$$

WFO LSX



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