Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212351

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
551 PM CST Sun Jan 21 2018

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 323 PM CST Sun Jan 21 2018

A very well developed storm system will move across the central
Plains and into the Midwest over the next 24 hours.  The mid and
upper level cut off low is currently over eastern Colorado and will
be moving east tonight across the low level baroclinic zone over
Kansas.  This will cause the surface low currently over Kansas to
deepen and begin moving northeast.  The vertically stacked system
will then move east-northeast into southeast Iowa and central
Illinois on Monday.  Warm and moist air continues to flow into the
Mid Mississippi Valley ahead of the system and dew point
temperatures across our forecast area are in the low to mid 50s...up
10 to 15 degrees from 24 hours ago.

Thunderstorms are already forming over eastern Oklahoma ahead of the
system and these storms are expected to increase in coverage and
intensity this evening as the system intensifies and the dryline
rushes east-northeast out of the Plains.  Current thinking is that
the storms will form into a broken line this evening as they move
eastward and the line should affect most if not all of the forecast
area.  MOS PoPs are in the categorical range tonight, and I see no
reason to dispute them. There may be a few showers out ahead of the
line early this evening but the best chance of rain should be from
03Z and later as the storms move across central into eastern Missouri
and then southern and central Illinois.  Convection allowing models
show areas of simulated reflectivity in excess of 50dBZ. There
should be more than enough deep-layer shear 50kts+ with this wound
up system to make severe thunderstorms a real possibility.  However,
GFS/NAM really put a damper on the instability after about 03Z,
dropping MUCAPE down to 200 J/Kg or less as the line would be .
moving into eastern Missouri.  The RAP is more stout, keeping as
much as 300-400 J/Kg even after 06Z into Illinois.  Given the
differences, it still looks like the SPC marginal risk is in the
right spot over our central Missouri counties tonight.

Thunderstorms will continue to be possible on Monday morning as
boundary layer heating ahead of the cold front increases instability
along and east of the Mississippi River.  Again, the RAP is more
robust with this instability, pushing MUCAPE above 500 J/Kg by 17Z.
This may be too high as the GFS and NAM only around 200 to 350 J/Kg.
Still, it bears watching and there could be a quick threat for
severe storms before the convection exits the eastern portions of
the forecast area early Monday afternoon.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 323 PM CST Sun Jan 21 2018

Global models are still in good agreement that the upper low that
will be located near Quincy on Monday evening will move
northeastward into the eastern Great Lakes by midday Tuesday. Colder
air will already be moving into the area behind the front on Monday
night, and the rain is still expected to changeover to snow as the
low level thermal profiles fall below freezing.  There may also be
some light freezing rain or drizzle after midnight over central and
northeast Missouri as the upper low lifts out of the area and the
mid level ascent weakens.  Little if any snow/ice accumulation is
expected on Monday night as QPF amounts are less than a 0.10" on all
of the global models.

Then mainly dry weather is expected from Tuesday afternoon into
Friday as subsidence sets in behind the upper low and an upper ridge
builds over the central CONUS mid-late week.  There will be a weak
shortwave trough move across the area on Wednesday, but it will have
little moisture to work with, so will keep the forecast dry for now.
Temperatures will stay above normal this week, with highs near 50 on
Tuesday and Wednesday.  By late in the week, a surface high will
move off into the southeast CONUS allowing the low level flow to
turn out of the southwest and warmer air to move into the area.
Temperatures will be 10-20 degrees above normal by late in the week.

While the global models agree that a longwave trough will dig into
the central CONUS by next weekend, there are differences in the
details.  The GFS is showing a is showing a more progressive trough
with a slightly quicker frontal passage next Friday night into
Saturday which would mean more rain followed by a cooler, drier
weekend.  The ECMWF is showing a slower trough with a cold front
moving through on Saturday, with rain changing to snow with the cold
frontal passage, and snow lingering Saturday night as the upper
trough deepens.  Have gone with rain Friday night and Saturday, with
a changeover to snow on Saturday night. Temperatures will return
close to normal by Sunday as 850mb temperatures fall below -10C.



.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 550 PM CST Sun Jan 21 2018

Area of IFR CIGs over southeast MO and southern IL is heading
north now that the sun has gone down and should impact STL metro
sites for a period this evening. CIGs have already deteriorated
per latest obs in the past few mins. Lower MVFR CIGs should also
make a return to UIN by 03z or so. In the meantime, SHRA with a
few TSRA will increase the next few hours over the region, exiting
from west (COU) to east (STL Metro) from 09z to 12z Monday. While
conditions should improve to VFR after the rain exits with even
some cloud clearing, another round of MVFR CIGs looks likely for
Monday afternoon-night with the wraparound and colder air return.
S winds at most sites will occasionally gust to 15-20kts tonight,
increasing on Monday and continuing into Monday night with gusts
to 25-30kts at times. Another round of light pcpn is possible late
Monday afternoon-night with it changing from rain to snow as the
colder air deepens. Probs look low enough at this time to focus on
the front-end of the TAF with weather and will address in later

SPECIFICS FOR KSTL: Period of IFR conditions looks likely this
evening and may continue at times overnight with heavier rain in
SHRA or TSRA. VFR then looks likely for Monday morning before a
return of marginal VFR conditions late in the day.





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