Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211804

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1204 PM CST Tue Feb 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 319 AM CST Tue Feb 21 2017
Forecast adjusted to account for clouds and rain that will impact
current temperature forecast. The weak cold front should clear the
metro by 0z and forecast area by 6z. High pressure will build into
the area slowly from the west. The atmosphere will be relatively
moist and this coupled with light winds will set the stage for
widespread fog overnight, which could be dense in places.


.LONG TERM...  (Wednesday through Monday)
Issued at 319 AM CST Tue Feb 21 2017

The primary items of interest during the extended forecast period
are temperature trends and precipitation chances.

Daily high temperatures through Friday will be more similar to
the average high temperatures for the months of April and May
(upper 60s to mid 70s) than February (low-mid 40s) across most of
the area. Several record highs and record warm lows have already
been tied or broken at our three climate sites during the recent
period of warm weather, and additional records will probably fall
before the end of the week. That said, batches of thick cirrus
could easily limit insolation and keep highs a few degrees shy of
the records on any given day...but most residents will probably
focus less on the daily records and more on the subjective sense
that it`s warm and feels like spring.

The stretch of unseasonably warm weather will end this weekend,
starting on Friday night when temperatures plummet into the 20s
and 30s behind a strong cold front. Near-normal temperatures are
expected on Saturday and Saturday night before another warming
trend begins by the early or middle part of next week.

Precipitation chances on Thursday/Thursday night are tied to the
initial location and subsequent movement of a quasistationary
boundary which will have settled across the CWA on Wednesday night.
A developing surface low over CO/OK/KS will start to lift the
boundary northward as a warm front on Thursday, and LLJ interactions
with the boundary could bring periods of SHRA/TSRA to areas north of
the front on Thursday/Thursday night. The southern extent of
precipitation chances will depend on how far south the boundary
settles and on how quickly it lifts northward on Thursday. There
will probably be a lull in precipitation on Friday morning due to
capping within the warm sector, but additional SHRA/TSRA should
develop across the eastern CWA during the afternoon ahead of a
strong cold front.

The tightening pressure gradient ahead of the approaching surface
low will also create breezy conditions on Friday.

Precipitation chances on Saturday are associated with instability
showers in the wake of Friday`s low pressure system. Models depict
steep lapse rates across the area by 18z Saturday in addition to
lift from either a vort lobe rotating around the primary vort max
or a secondary disturbance which approaches from the west and then
phases with the primary area of vorticity.

Another developing low pressure system could bring a chance of
precipitation to the area early next week, but there is poor
agreement between models on the 21/00z runs.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1141 AM CST Tue Feb 21 2017

A weak cold front is pushing through the area currently and has
already cleared the UIN and COU terminals. The front should clear
remaining terminals by 0z tonight.Winds expected by remain at
10kts or less through period. Ceilings will rise quickly after
frontal passage with high pressure building into the area from the
west. LIFR conditions in fog will impact terminals starting after
9z through 12z.

SPECIFICS FOR KSTL: Rain will clear terminal by 21z and
cigs will raise quickly afterwards. The front should clear
terminal by 0z and high pressure will slowly build into the area.
The winds are expected to stay at 10kts or less through the
period. LIFR conditions in fog expected to develop after 9z.



Record Highs

      2/21    2/22    2/23

STL: 79/1935 78/1995 77/1996
COU: 76/1935 76/1995 73/1933
UIN: 69/1930 70/1922 70/1922




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