Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 160312

912 PM CST Mon Dec 15 2014

Issued at 910 PM CST Mon Dec 15 2014

As of 03z band of pre-frontal showers has pushed into south central
IL, and extrapolation of radar trends over the last several hours
indicates that this band of showers will exit our far eastern
counties by around 10 pm.  In the cooler air in the wake of the
front, cyclonic circulation around the intense upper low over IA
is producing spotty showers over roughly the northern half of MO,
and this should continue some threat of precip during the
overnight hours.  Greatest threat should be over ne MO and west
central IL, but some lesser threat should extend as far south as
the northern Ozarks and the I70 corridor in IL.

Update will be issued as soon as 03z obs can be incorporated into



.SHORT TERM:  (Through Late Tonight)
Issued at 250 PM CST Mon Dec 15 2014

Occluded low pressure system across the center of the CONUS will
continue to move east/northeast over the next 24 hours with best
chance of rain showers being confined to the northern CWA, closer
to the track of the upper level low circulation. Temperatures will
remain above average tonight as the colder air will lag some behind
the system/cold front.


.LONG TERM:  (Tuesday through Next Monday)
Issued at 250 PM CST Mon Dec 15 2014

That colder air will build into the area on Tuesday with
temperatures remaining nearly steady with overcast conditions.
Some question as to how much clearing will occur Tuesday night as
NAM model guidance indicates some remaining low level moisture
between 925 and 950 mb. This will have big implications on just
how cold we will get Wednesday morning. Have increased cloud
cover, especially across the northern CWA and bumped temperatures
up a few degrees.

The forecast after Wednesday is really still up in the air. There is
certainly some indication that a lead shortwave will eject into the
Plains Wednesday night out of the larger trof that will be settling
into the southwest CONUS. Precipitation develops in response to warm
advection aloft and it should be in the form of light snow for the
majority of the area that sees it. Big question is how far east will
the precipitation make it into the CWA. Majority of the guidance
keeps the highest QPF across portions of central Missouri and the
eastern Ozarks with a fading further east toward the Mississippi
River (likely due to the antecedent dry conditions from the surface
ridge in the models). The last few "light" QPF events have been
underestimated by the model guidance and the notorious "dry air"
didn`t stop the precipitation from moving east across the CWA.

Medium range model guidance still battling on how to handle the
"main event" this coming weekend with a southward shift in the 12Z
guidance noted.  Have kept chance pops in the forecast to avoid any
flip-flopping in the forecast. Precipitation, if it occurs this far
north would appear to be all snow and therefore this system
certainly bears watching over the next few days as it makes its way
toward the western coast. The storm system should be onshore by 12Z
Wednesday and hopefully well sampled by 00Z Thursday.



.AVIATION:  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 540 PM CST Mon Dec 15 2014

Cold front is moving through the area at this time, and should be
east of the Mississippi River by 01Z. IFR flight conditions ahead
of the front will briefly give way to MVFR and even VFR as the
front moves through with scattered showers. VFR ceilings will
follow the front by about 30 minutes with heights between
1000-2000ft. Patchy ceilings at or below 900ft will also be
possible through the night, with the greatest chance of IFR across
northeast Missouri and west central Illinois. Wind will increase
behind the front and become gusty up to 25-30kts from the
southwest. Current indications are that these gusts will continue
through much of the day on Tuesday as the wind continues to veer
to the west and finally northwest. Low end MVFR ceilings should
rise slowly through the day.

Specifics for KSTL:

Cold front should be passing through the STL terminal at this
time. Expect a brief period of high end MVFR or even VFR
conditions behind the front, but another batch of stratus is
moving in from the west and ceilings should drop again to between
1000 and 1500ft between 01-02Z. Wind will turn to the southwest
and become gusty, topping out between 20-25kts. This will produce
some decent crosswinds on the main runways tonight. Wind will
continue to veer slowly overnight becoming westerly by Tuesday
morning then northwesterly by Tuesday afternoon. Ceilings will
likely stay low through Tuesday morning, but should be rising
above 2000ft Tuesday afternoon.





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