Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221025

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

.SHORT TERM...  (Through Late Tonight)
Issued at 339 AM CST Wed Nov 22 2017

An upper RIDGE prevailed over the western CONUS and a TROF over the
eastern CONUS, leaving our region in NW flow aloft.  An upper level
disturbance existed over the eastern Dakotas and another one was
further upstream over southern British Columbia province.  Surface
Canadian high pressure was centered over eastern Nebraska, resulting
in northerly flow for our region.  This much cooler airmass has
pushed temperatures into the 20s for most locales.  A small area of
cirrus clouds were moving overhead, otherwise skies were clear for
most surrounding areas.  The atmospheric column was very dry, with
precipitable water (PW) values less than a quarter inch.

The NW flow aloft will continue thru tonight and will provide a
fast means for upper level disturbances to race thru.  The first
upper level disturbance over the Dakotas will move overhead during
the morning and early afternoon hours but will pass thru essentially
unnoticed as the column will remain very dry thru today, with the
surface high pressure RIDGE setting up overhead by late this
afternoon.  The only clouds that will affect our region will be
another round of high cirrus clouds moving in during the afternoon.
After a chilly start to the day in the upper teens and lower 20s,
somewhat decent sunshine should allow temps to rebound well into the
40-45 range and is handled well by 00z MOS values.

The second upper level disturbance over British Columbia will zip
down here by late tonight just ahead of a surface front.  Only a
modest increase in moisture is expected but likely not enough to do
anything more than a period of mostly cloudy skies from mid and high
level cloudiness.  At the surface, southerly flow will develop this
evening as the RIDGE moves to the south, and modestly strengthen by
12z/Thu.  While temps should drop into the 20s again tonight, the
southerly flow will ensure they will be higher than persistence,
with the highest values in the northern CWA, with upper 20s, and
near persistence in the eastern Ozarks of southeast MO where the
RIDGE will pass south of there last and late.


.LONG TERM...  (Thursday through Tuesday)
Issued at 339 AM CST Wed Nov 22 2017

A weakening, dry cold front will drop southeastward into our area
with little affect except a shift in the wind direction on Thursday.
Actually the high temperatures will be warmer on Thanksgiving with
plenty of sunshine due to decreasing cloudiness during the morning.
The warming trend will continue through Friday as the surface wind
becomes southwesterly.  Highs on Friday will be 10 to 15 degrees
above normal for late November.  An upper level trough will move
eastward through the northern Plains and Great Lakes region and send
a cold front southeastward through our forecast area Friday
afternoon and night. It appears that this will be a dry frontal
passage with precipitation north-northeast of our forecast area
where there will be more favorable upper level forcing.  Could not
rule out a few sprinkles across northeast MO and west central IL
Friday afternoon and evening. Temperatures will be colder this
weekend with northwesterly mid-upper level flow, and as a surface
ridge moves slowly east-southeastward through the region.  Warmer
temperatures will return Monday as an upper level ridge shifts
eastward into our area, while the surface winds become south-
southwesterly as the surface ridge moves southeast of the region. An
upper level trough/low will move eastward into the northern Plains
on Tuesday with an associated surface low and trailing cold front
approaching the forecast area. Showers along and just ahead of the
front may move into northeast and central MO Tuesday afternoon.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 425 AM CST Wed Nov 22 2017

VFR conditions and dry weather will prevail at the TAF sites thru
the valid period. Light northerly surface winds will continue thru
the morning hours, then becoming light and variable later this
afternoon, then becoming light southerly tonight.





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