Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212130

Area Forecast Discussion
National Weather Service Saint Louis MO
330 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 18z TAFs through 18z Monday Afternoon)
Issued at 1149 AM CST Sun Jan 21 2018

First concern today is MVFR and IFR conditions over parts of
northeast Missouri and south central Illinois. Guidance has not
been handling the stratus and fog very well over the past 24 to 48 have little confidence in it now. Current thinking is
that the VFR conditions across central and eastern Missouri will
continue to spread north-northeast this afternoon. The low clouds
and fog over the eastern Ozarks into south central Illinois will
likely erode and lift...but may not entirely clear out. Once
flight conditions get to VFR, expect VFR to prevail until the
evening. A broken line of thunderstorms is expected to develop by
early to mid evening over southwestern Missouri and move east.
MVFR ceilings and visibilities will likely prevail as this area of
rain moves through tonight...and there may well be pockets of IFR
with the heavier storms. Storms will also be capable of locally
damaging wind gusts to around 50kts. Expect flight conditions to
improve Monday morning after the rain ends.


Primary concern for this afternoon is the possibility that MVFR
ceilings could return earlier than forecast in the TAF. The short
range model guidance has not been handling the ceilings very well
over the past couple of days, and in fact most of the guidance
says that Lambert should still be MVFR. Seeing some redevelopment
of low level clouds right now...but it looks mostly scattered
right now. Current thinking is that VFR conditions will prevail
until sometime this evening. A broken line of thunderstorms is
expected to move into the vicinity of the terminal between 05Z and
07Z tonight, bringing the a return of lower ceilings, moderate and
possibly brief periods of heavy rain, and potentially strong wind
gusts up to 35-45kts. MVFR conditions are expected to prevail
until the rain ends toward sunrise.





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