Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 150505

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1205 AM CDT Sun Apr 15 2018

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 334 PM CDT Sat Apr 14 2018

Strong, stacked low pressure system centered over northern
Missouri/northeast Kansas will continue moving east through Sunday.
The dry slot has temporarily scattered the clouds out over
parts of central and northeast Missouri.  The wrap around moisture
on the cold side of the cyclone is inching its way into the central
Missouri as the system shifts east.  Think the clouds will surge
further east after the sun sets this evening...and the deformation
zone precip will drift into the area after 06-09Z.  Forecast
soundings indicate temperatures in the column will be cold enough
for some snow in northeast Missouri and west central Illinois by
early Sunday morning...but QPF is very light so do not expect much
more than a dusting to perhaps 1/2 inch on grassy areas.
Expect lows tonight range from about 33 in northeast Missouri
to near 40 in south central Illinois.  Clouds and
potentially some light rain along with northwest flow on Sunday will
suppress the diurnal rise in temperatures to only 5 to 7 degrees.
This will make it feel more like February than April with highs in
the upper 30s to mid 40s.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 334 PM CDT Sat Apr 14 2018

(Sunday Night)

Deep mid/upper level low will located near the Quad Cities early
Sunday evening and slowly depart to the northeast. Should see some
lingering precipitation mainly in the form of light snow showers or
snow flurries across parts of the area, especially across west-
central Illinois. No accumulation is expected however due to the
very light intensity of the snowfall and warm ground temperatures.

Main story for Sunday night will be the well below normal
temperatures with a hard freeze for many locations likely. Not sure
how cold to go given that stratus will be stubborn to depart the
area. However, the strength of the airmass is very impressive with
850-hPa temperatures bottoming out below -10C Sunday night (about 3
sigma below normal). Have high confidence that most areas will at
least dip slightly below the freezing mark, especially for areas
within central and northeastern Missouri as well as west-central
Illinois. How hard the freeze will be is the main question and
leaned at or above the warmer MAV guidance for low temperatures due
to expected low stratus hanging around. If sky conditions clear out
quicker, would expect lows to be at least a couple of degrees cooler
than currently forecast. Regardless, it appears increasingly likely
that a freeze warning will be needed down the road for those areas
where the growing season has already begun.

(Monday - Next Saturday)

Well-below normal temperatures will continue to start off the new
work week with highs on Monday only reaching the upper 40s for most
locations. One more subfreezing night is on the table for at least
some locations Monday night into Tuesday morning as surface ridge
passes across the mid-Mississippi Valley. Went a bit cooler than
guidance due to the favorable conditions for radiational cooling and
may still be cold enough. Additional frost/freeze products may be
necessary for this period of time as well.

The rest of the extended forecast looks fairly quiet for a change
with quasi-zonal flow aloft. A strong system will pass across the
Upper Midwest midweek but weak moisture return and little to no
convergence along the trailing boundary suggest a dry frontal

The next chance of any widespread rainfall appears to be Friday
night/next Saturday as a closed low slowly migrates out of the
central Plains. However, would not be surprised if this storm system
trends slower with time as models typically eject closed lows out a
bit too quick in the medium range, especially out of the desert

All in all, a tranquil period of weather looks to be in store for
next week with little to no chance of any measurable rainfall and
temperatures remaining slightly below normal.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1204 AM CDT Sun Apr 15 2018

VFR conditions will likely hold through early morning for most
TAF sites with bkn/ovc cigs varying from 5-10k ft. IR satellite
imagery continue to show a north/south oriented dry slot running
just about parallel with the Mississippi River. Meanwhile, MVFR
cigs are just beginning to move into KCOU as a result of wrap-
around moisture funneling in behind the system. Kept westerly
winds a bit higher as it has been reluctant to drop below 10 kts
this evening. If trend continue with the dry slot, we may see more
clearing for locations further east from KCOU for a few hours

Main concern continues to be gusty westerly winds Sunday, along
with IFR cigs for points north of KSTL through the mid-morning
hours. Any precip should remain light with snow mainly affecting
KCOU and briefly mixing in at KUIN.

VFR condition persist overnight with westerly winds around
floating around 10 kts. We may have a brief lull in winds after
06z, but this won`t last long as it kick up again Sunday. Westerly
gusts will reach into the 20-25 kt range much of the day Sunday.
Best chances for light rain and MVFR cigs will come in after
12-13z with the moisture wrapping in around the area of low
pressure. Expected gradual improvement Sunday evening, but west
and northwesterly gusts could extend well into the evening until
the system departs to the northeast. Cigs may return to VFR late
in the TAF period.





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