Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241207
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
607 AM CST Sat Feb 24 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 348 AM CST Sat Feb 24 2018

Ongoing heavy rain event is obviously the first concern.  The
forecast looks to be working out pretty well with a large area of
rain forced by low level moisture convergence over a quasi-
stationary front stretching across southern Missouri.  Rainfall
amounts since midnight range from about 0.20-0.60 inch stretching in
a band from near Vichy up into the St. Louis Metro area with the
higher amounts to the southwest of the STL Metro.  This wave of rain
should end from southwest to northeast this morning from about 12Z
to 16Z as the 850mb flow veers and becomes more parallel to the
baroclinic zone.  The axis of heaviest rain is within the
area of the flood watch so no changes to the watch are
anticipated at this time

Meanwhile, up across northeast Missouri, temperatures have fallen to
right around freezing, and freezing rain is currently being reported
in Kirksville.  Still think some light ice accumulation on elevated
surfaces is likely this morning before temperatures climb back above
32, so the winter weather advisory still looks good.

A strong shortwave will drop into the Great Plains today forcing
cyclogenesis over western Arkansas or eastern Oklahoma.  The
resulting low will move northeast as the positively tilted wave lifts
into the Midwest.  Warm moist air in the warm sector of the system
will bring another wave of rain to our forecast area along with an
increased chance for thunderstorms.  Highest QPF and chance for
flooding is south of the I-44 corridor in Missouri late this
afternoon and this evening ahead of the cold front.  Additionally,
the NAM shows MUCAPE values between 500-1000 J/Kg in southeast
Missouri this evening.  The GFS is not quite as aggressive with the
instability, but there will be more than enough shear to produce
severe thunderstorms if the higher instability can be realized.
Convective mode will be highly dependent on the amount of
instability which develops, but could range from a severe squall
line to low topped supercells.  Regardless...the threat for severe
weather should be localized to southeast Missouri and southern
Illinois.

The cold front will move through quickly between 00Z and 05Z
tonight, bringing the wet weather to an end for the time being.
Increasing pressure gradient behind the front will likely produce
strong and gusty winds after FROPA with gusts to around 35 mph
likely...particularly in northeast Missouri into central Illinois.

Carney

.LONG TERM...  (Sunday through Friday)
Issued at 348 AM CST Sat Feb 24 2018

There is fairly good agreement among the models that the period from
Sunday through Monday Night will be rather tranquil with above
normal and slowly moderating temperatures. Southwest flow aloft will
persist in the wake of the upper trof exiting the Great Lakes into
southeast Canada while weak high pressure to the south of the CWA
dominates. A fast-moving low-amplitude short wave trof will traverse
the region on Sunday night. Moisture appears to be quite limited
with this wave and present indications are it will be just
accompanied by some mid clouds.

Temperatures will continue to moderate Monday as 1) heights rise
aloft associated with modest ridging occurring in response to
upstream amplification featuring a digging/deepening west coast
upper low/trof, and 2) southerly flow returns bringing weak WAA. A
better boost in the warming appears on tap for Tuesday as flow aloft
become more southwesterly and WAA increases in response to
strengthening south-southwest low level flow.

From Tuesday Night through the end of the week the GFS and ECMWF
begin to diverge in the details of the forecast. This is most
prominent beginning late Wednesday night when they depart
significantly with the handling of the ejecting west coast upper
trof. First on Tuesday Night the ECWMF is rather stingy with the
amount and northern extent of a light precipitation event as a weak
impulse aloft traverse the area. The wetter GFS solution appears to
be a reflection of better low level moisture return. The GFS then
has the west coast trof ejecting across the Plains and through the
MS Valley Wednesday Night into Thursday bringing strong large scale
ascent and accompanied by a a well developed surface low and
warm/cold frontal systems. Alternatively the ECWMF has a cold
frontal passage due to a weak northern stream short wave with the
weakening west coast trof lagging back into the southern Rockies and
southern Plains. Overall the GFS has slightly better continuity.
While the GEFS shows a good deal of spread, it has more members than
not closer to the operational GFS solution, suggesting then another
wet period from Wednesday afternoon into Thursday morning.

Glass

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 549 AM CST Sat Feb 24 2018

Flight categories are going to be a challenge again this morning.
A persistent band of light to occasionally moderate and heavy rain
is affecting parts of southeast and east central Missouri as well
as southern Illinois. The rain has washed out some of the low
ceilings in its path, with many observations reporting 3,500 ft or
higher. Not sure how long these VFR ceilings will persist this
morning, but low MVFR and IFR clouds should build back from the
south through the morning as the rain ends. Elsewhere, IFR flight
conditions will continue to prevail this morning...possibly
lifting to low end MVFR early this afternoon. A strong storm
system will move through this afternoon and evening, bringing more
rain and thunderstorms, particularly to areas along and south of
I-44 in Missouri and I-64 in Illinois. The cold front associated
with this system will push through during the evening and wind
will pick up from the west-northwest, potentially gusting to 30kts
behind the front.

SPECIFICS FOR KSTL:

Low confidence forecast for ceilings at Lambert this morning.
Current VFR clouds could go IFR at almost any moment, but there is
enough of a hole over the Metro area and points to the southwest
where it`s been raining for several hours tonight. Expect the hole
to fill at some point in the next 2 to 4 hours...perhaps sooner.
On the reverse side, visibility should improve after the rain
ends. The next wave of rain will move into the vicinity of the
terminal this afternoon. Think there will be more thunder with
this batch before it ends during the early evening. Cold front
is expected to pass through the terminal at about 04Z and we could
see wind gusts from the west-northwest up to around 30kts for a
few hours.

Carney

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     55  34  57  34 / 100  80   0   5
Quincy          48  29  50  29 /  80  80   0   0
Columbia        53  30  54  30 /  80  80   0   5
Jefferson City  55  31  56  31 /  90  80   0   0
Salem           57  37  53  34 / 100  90   0   0
Farmington      60  35  57  33 / 100  80   0   5

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through this evening for Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Madison MO-Osage MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

     Winter Weather Advisory until 9 AM CST this morning for Knox MO-
     Lewis MO.

IL...Flood Watch through this evening for Bond IL-Clinton IL-Fayette
     IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint Clair
     IL-Washington IL.

&&

$$

WFO LSX



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