Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241759

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1159 AM CST Sat Feb 24 2018

Issued at 1129 AM CST Sat Feb 24 2018

After a brief lull in the rain, another round of showers and
embedded thunderstorms is moving toward the CWA late this morning.
This round will likely bring additional moderate to heavy rainfall
across the flood watch area this afternoon into this evening.
There also remains a threat for a few severe storms late this
afternoon into this evening across southeast Missouri into south
central Illinois. Latest RAP runs show a weak surface low moving
northeast from near Farmington to Salem during the early evening
hours in a weak instability/high shear environment. Any organized
thunderstorms that can develop will have the potential to produce
damaging winds.



.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

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.


.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.



.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1129 AM CST Sat Feb 24 2018

Low ceilings and visiblities will persist through the afternoon
into the evening as an area of showers and scattered thunderstorms
move into the terminals. This area of rain will move out of the
terminals as a front moves east of the area this evening. Easterly
winds will shift northwesterly this evening and turn gusty tonight
before winds diminish tomorrow morning. Dry and VFR conditions are
expected after 06Z tonight as the storm moves out of the area.


The currently VLIFR conditions will improve once the rain moves
into the area, though IFR ceilings will persist into the evening
hours. This area of rain will move out of the terminal as a front
moves east of the KSTL this evening. Easterly winds will shift
northwesterly this evening and turn gusty tonight before winds
diminish tomorrow morning. Dry and VFR conditions are expected
after 07Z tonight as the storm moves out of the area.



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.

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.



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