Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210921

Area Forecast Discussion
National Weather Service Saint Louis MO
321 AM CST Tue Nov 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 315 AM CST Tue Nov 21 2017

A  upper TROF was located over the Mississippi River Valley early
this morning, with several imbedded shortwave disturbances.  Three
are of note:  the first was directly over our region, helping to
create an area of mid and hi level cloudiness to its east in IL; a
second, stronger disturbance was over northern MN and eastern ND;
and a third was over western MT.  At the surface, a cold front
extended from western WI thru central IA and into southeast
Nebraska.  SW surface winds existed ahead of the front across our
region and have kept temperatures in the 40s for much of the area.
The atmospheric column was much too dry and lifting mechanisms too
weak to produce any pcpn anywhere near our region.

The first shortwave disturbance over our region will exit before
12z/Tue.  The strong second disturbance over MN/ND will be the main
driver in deepening the upper TROF and its southern periphery will
come close to edging our northern and northeastern CWA during the
afternoon hours, but much of the lift from it will remain outside
the CWA.  The third disturbance over western MT should track well
south of our region during late this afternoon and early evening.
Focusing on the second disturbance to edge our N and NE CWA this
afternoon, this will also track behind the timing of the passage of
the surface cold front which should make it thru areas N and W of
STL metro this morning, STL metro during midday, and areas S and E
of STL metro during early-mid afternoon.  Both low level moisture
will be limited due to presence of a blocking RIDGE over the Gulf of
Mexico and aloft due to a very dry initial column and the main lift
missing our area and it tracking too fast to saturate anything below
the mid levels.  Taking all of this into account, despite a cold
FROPA, it should be dry today but with plenty of cloud cover.  The
strength of the cold front today will result in falling afternoon
temps, especially for areas near and north of Interstate 70, along
with gusty NW winds of 25-30mph.  Max temps will be slightly above
average ranging from the upper 40s in northern MO and central IL, to
the 50s elsewhere.

A cold Canadian high pressure will build in and dominate our region
tonight with clear skies and below average temps, as readings bottom
out in the lower 20s for most areas, or about 10 degrees below


.LONG TERM...  (Wednesday through Monday)
Issued at 315 AM CST Tue Nov 21 2017

The tranquil, dry weather pattern will continue through the
forecast period with dry, cold frontal passages.  Despite full
sunshine it will be colder on Wednesday with northwesterly mid-upper
level flow, and a surface ridge moving through the forecast area.
Warm advection, mid level cloudiness will drop southeastward through
our area Wednesday night with a warming trend beginning on Thursday.
More pronounced warming can be expected on Friday as surface/low
level winds strengthen from a southwesterly direction.  Highs on
Friday will be around 10 degrees above normal for late November.  A
cold front will drop southeastward through our forecast area Friday
afternoon and night as an upper level trough moves eastward through
the Great Lakes region.  It appears that any precipitation will be
northeast of the forecast area closer to the better upper level
forcing, although could not totally rule out a few sprinkles in
northeast MO and west central IL Friday afternoon and evening. Will
see cooler temperatures beginning Saturday and continuing through
the weekend as a large surface ridge moves slowly through our area
with continued northwesterly upper level flow, although the coldest
air should remain northeast of the forecast area.  Will see a
warming tend beginning on Monday as the surface winds become
southerly as the surface ridge shifts east of the region.



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1050 PM CST Mon Nov 20 2017

Dry and VFR conditions are expected through the period. LLWS will
continue through mid morning on Tuesday as a south southwesterly
low level jet has developed over the area. A cold front will move
southeast across the area on Tuesday causing the wind to shift
from southwest to northwest.


Dry and VFR conditions are expected through the period. LLWS will
continue through 15Z. A cold front will move through the terminal
around 19Z causing winds to shift out of the northwest.





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