Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 211116

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

.SHORT TERM...  (Through Late Tonight)
Issued at 319 AM CST Tue Feb 21 2017

The synoptic pattern early this morning features an elongated
trough stretching from the northwest Gulf Coast all the way north
into the western Great Lakes region. The southern part of this
trough will cut off across the Southeast today as the northern
portion is quickly shunted eastward in the swift westerly flow
across the northern CONUS. This will cause the portion of the
trough over us to take on a strong positive tilt today as it
shears out and sags southeastward through the region by tonight.

Latest radar imagery depicts a much-needed soaking rain affecting
much of central and eastern MO. This band of rain is in response
to a broad area of moisture transport/isentropic upglide ahead of
the upper trough. Latest surface analysis depicts a weak surface
low over AR, which will slowly lift northward this morning. As it
does, enhanced convergence should help provide continued 70-90%
coverage of rain showers mainly to southeast MO/southwest IL
through the mid- morning hours. By noon, the best moisture
transport will have pushed off to the southeast. However, the
upper-level trough axis will not have passed through east-central
MO and IL yet, thus have kept chances for showers in through the
late afternoon hours. Given the clouds/precip expected to linger
for at least the first part of the day, high temperatures will be
cooler than past days with readings expected to top out in the
upper 60s to low 70s.

For tonight, drier air aloft will attempt to filter in which
should cause a decrease in cloud cover. However, copious moisture
will remain in place at the surface and winds will be light, thus
could see some fog development (possibly dense) overnight. Lows
will dip into the mid/upper 40s to low 50s.


.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 12z TAFs through 12z Wednesday Morning)
Issued at 514 AM CST Tue Feb 21 2017

Complex TAF forecast this morning with areas of dense fog and rain
showers plaguing sites. A weak cold front has slid into portions
of western and northern MO. Ahead of it, a very thin area of
dense fog has developed at UIN and SUS, with low stratus at STL.
Expect these IFR conditions to continue at these sites, with MVFR
conditions likely at COU/CPS. In addition to the fog, rain showers
passing through will likely cause fluctuations in vsbys through
the morning hours, before the showers begin to taper off this
afternoon. Clouds (some MVFR) will hang tough today, before
clearing tonight as winds go light. This clearing will set the
stage for more fog development late tonight at the end of the TAF

IFR conditions to persist this morning, both in low stratus and
perhaps in vsbys as the low stratus builds to the ground. Rain
showers will pass through this morning, tapering off through the
afternoon hours. As skies clear tonight, additional fog
development will be likely, some of which could bring IFR vsbys.



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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