Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201103

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
503 AM CST Mon Nov 20 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 318 AM CST Mon Nov 20 2017

A longwave upper TROF was over eastern North America early this
morning, resulting in NW flow aloft over our region.  A shortwave
disturbance was based over the central High Plains and was helping
to create a large area of high cirrus clouds that is moving into our
region.  At the surface, high pressure has moved off into the Mid
and Deep South regions, resulting in a southerly flow for our
region.  Temperatures were in the 20s and 30s and slowly rising from
having attained minimums earlier in the night.

The upper shortwave disturbance over the High Plains currently will
track thru our region by early afternoon, driving an area of semi-
thick high cirrus clouds with it, and should result in partly sunny
skies becoming more clear during the afternoon.  At the same time, a
tightening pressure gradient will result in strengthening southerly
winds to 15-25mph with gusts to 35mph, with the highest wind speeds
in northeast MO.  The strong southerly flow and clearing skies
should allow temperatures to surge well into the 50s, with 60-65
maxes being attained at many locations, especially in a corridor
from central MO to STL metro.  The atmospheric column is forecast to
be much too dry for any chance at pcpn or clouds lower than 25kft.

Another upper shortwave TROF will approach our region later tonight
and will be nearly coincident with an approaching surface cold front
from the northwest that is slated to begin moving thru just beyond
12z/Tue.  Thanks to a blocking surface RIDGE to our south, moisture
will effectively be held in check from increasing sufficiently to
support much in the way of lower cloud bases or any real chance at
pcpn despite an approaching front.  The decent southerly flow will
keep temps from dropping too much, with readings in many areas
staying in the 40s, with some 30s possible in southern IL or
southeastern MO.


.LONG TERM...  (Tuesday through Sunday)
Issued at 318 AM CST Mon Nov 20 2017

An upper level trough will move eastward through the Great Lakes
region and send a cold front southeastward through our forecast area
Tuesday.  With limited low level moisture across our region it
appears that this will be a dry frontal passage with any
precipitation northeast of the forecast area where there will be
more favorable upper level forcing.  Colder conditions can be
expected across northeast and central MO and west central IL Tuesday
and the entire area Tuesday night and Wednesday due to cold air
advection behind the cold front. Lows Tuesday night will be about 10
degrees below normal. Warmer temperatures will occur Thursday
through Friday night due to rising upper level heights and
southwesterly surface winds. Highs by Friday will be about 10
degrees above normal. Another cold front will move southeastward
through our forecast area Friday night as an upper level trough
moves eastward through the Great Lakes region. It appears that this
will be another dry frontal passage with the precipitation northeast
of the forecast area. Colder temperatures can be expected across
northeast and central MO and west central IL Saturday and the entire
area Saturday night and Sunday due to northwest mid-upper level
flow, and as a large surface ridge builds into our area from the
northern Plains.



.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 503 AM CST Mon Nov 20 2017

VFR conditions, dry weather, and southerly surface winds will
prevail at the TAF sites thru the valid period. A low level jet
whose base is only about 800-1000 feet AGL will support LLWS at
most TAF sites thru about mid-morning before it can mix down as
gusty winds. The strong gusty S winds of 15-30kts should then
continue until around sunset but only diminish to 10-12kts
sustained for most sites while veering slightly from the SW.





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