Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221112

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
512 AM CST Wed Feb 22 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 424 AM CST Wed Feb 22 2017

The synoptic pattern early this morning features split flow
aloft, with a cutoff low meandering through the northern Gulf of
Mexico and generally zonal flow over the Midwest. The upper-level
pattern over our region will transition to more southwesterly
through the period as a potent trough and associated PV anomaly
slide onto the Northwest Coast.

Main forecast issues will be dense fog this morning and just how
warm temperatures get today. As mid/high clouds cleared out last
evening, dense fog rapidly developed over the region. Surface flow
has slightly backed through the overnight hours in response to a
low sliding across the northern Plains. This has allowed the dense
fog to slowly advect a bit more to the northwest, thus have
expanded the Advisory northwest as well.

For today, the aforementioned surface low will slide across
southern MN and northern IA. This will cause an uptick in the
surface pressure gradient, helping winds to increase and
turbulent mixing to deepen. This should allow for the fog to mix
out fairly rapidly across central and northeast MO, thus expect
temperatures to be warmest in these areas this afternoon (upper
70s, maybe even an isolated 80). The fog/stratus may be a bit more
stubborn across eastern MO and western IL where mixing won`t be
as robust initially. Therefore, have actually cooled temperatures
a couple degrees in these areas (low to mid 70s). Certainly a
tricky temp forecast as quicker/slower mixing out of the fog may
result in much higher/lower temps, with the highest bust
potential being over eastern MO and IL.

A weak cold front associated with the low passing well to the
north will slide into northern MO this evening, likely stalling
somewhere near the I-70 corridor overnight as another surface low
deepens in the Central Plains. This will allow for a gradient of
low temperatures tonight, with low 50s to the south of the
boundary and low 40s to the north.


.LONG TERM...  (Thursday through Tuesday)
Issued at 424 AM CST Wed Feb 22 2017

The aforementioned deepening surface low over CO/TX/OK will lift
the stalled frontal boundary northward as a warm front on
Thursday. The front should be slightly south of the MO/IA border
by 00z Friday. The front`s position will affect high temperatures
on Thursday as well as the southern extent of precipitation
chances on Thursday night.

Temperatures will remain well above normal on Thursday across the
area, but there will be a temperature gradient across the front.
Highs for NE MO and west central IL will be cooler compared to
southern MO and southwest IL, but "cooler" is relative since it
will still be around 20 degrees above average on the cool side of
the boundary.

A strengthening S/SSW LLJ will impinge upon the boundary on
Thursday night, and this interaction will support SHRA/TSRA to
the north of the surface boundary. The best precipitation chances
will be well to the north of the CWA.

A dryline will move through the CWA on Friday followed by a cold
front on Friday night. The warm sector ahead of the dryline still
looks capped based on BUFKIT soundings except across the far
eastern CWA during the afternoon hours. Unless the entire low
pressure system`s eastward progress slows down, the greatest
potential for severe weather will be located east of the CWA.
However, strong to severe thunderstorms are still possible on
Friday afternoon across the far eastern CWA where BUFKIT
soundings suggest that the cap will have weakened by the time the
dryline arrives. Favorable elements for severe weather across the
far eastern CWA include around a day of low-level moistening ahead
of the boundary, 50-70 kts of 0-6 km shear, excellent H7-H5 lapse
rates (several models depict lapse rates in excess of 8 deg C/km),
and hints of an additional impulse rounding the base of the trough
on Friday afternoon. The shear vectors are oriented nearly
parallel to the H85 boundary which suggests that storms will
quickly develop into a line. However, if local weakening of the
cap allows convection to develop a few hours earlier than
anticipated, there could be some discrete storms because the
shear vectors will have slightly more of a perpendicular
component at that time.

The stretch of unseasonably warm weather and record-breaking heat
will end on Friday night when temperatures plummet into the 20s
and 30s behind a strong cold front. Although temperatures will be
near normal over the weekend, it will feel quite chilly compared
to the recent unseasonable warmth.

A weak disturbance could bring a light rain/snow mix to the
northern CWA on Sunday. There is a potential for rain/snow during
the early or middle part of next week, but models have diverged
regarding the evolution of the low pressure system which would
bring precipitation to the area.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 512 AM CST Wed Feb 22 2017

Dense fog to bring LIFR vsbys to all sites this morning. This fog
will slowly mix out from west to east through the day today, with
conditions returning to VFR at all sites by early this afternoon.
Winds will increase from the WSW this afternoon, with a few gusts
around 20 knots possible mainly at COU/UIN. Could potentially see
some localized fog again tonight as a weak front drops toward the
region, but not enough confidence to include at this time.

STL has been stubborn to drop to LIFR vsbys this morning, but
still think over the next hour or two vsbys could go below 1SM.
Conditions will improve through the late morning hours as mixing
commences, with VFR conditions expected by this afternoon. Winds
will increase out of the SSW this afternoon with gusts of 15-20
knots possible.



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


MO...Dense Fog Advisory until 9 AM CST this morning for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CST this morning for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-Washington



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