Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 160542
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1142 PM CST Mon Dec 15 2014

.UPDATE:
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
database.

Truett

&&

.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.

CVKING

.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.

CVKING

&&

.AVIATION:  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1140 PM CST Mon Dec 15 2014

Gusty southwest flow behind the cold front and mostly low end
MVFR ceilings will prevail for the rest of the night. IFR flight
conditions will prevail across northern Missouri and west central
Illinois. Additionally, isolated to widely scattered showers will
pop on and off primarily north of I-70 for much of the remainder
of the night. Should see ceilings improve slowly through the
morning into early afternoon, though current thinking is that
heights will not get above 3000FT. MVFR ceilings may dissipate
and/or move east of the region Tuesday night according to some
guidance, but others keep MVFR ceilings through Tuesday night.
Think the conservative approach is warranted at this time so will
keep ceilings between 2000-3000FT in terminal forecasts for
Tuesday evening. Gusty southwest flow will veer to the west by
early Tuesday morning and continue to veer to the northwest by
Tuesday afternoon.

Specifics for KSTL:

Ceilings between 1000-1500 ft are expected to prevail throughout
the perdawn hours with an isolated shower or two possible, tho
expect most of the precipitation to fall north of the terminal.
Some guidance suggests that ceilings could drop below 1000FT
before 12Z Tuesday morning. While I cannot rule this out present
indications are that the lower ceilings should stay north of the
terminal. Should see ceilings improve slowly through the morning
into early afternoon, though current thinking is that heights will
not get above 3000FT. MVFR ceilings may dissipate and/or move east
of the region Tuesday night according to some guidance, but others
keep MVFR ceilings through Tuesday night. Think the conservative
approach is warranted at this time so will keep ceilings between
2000-3000FT in terminal forecasts for Tuesday evening. Gusty
southwest flow will veer to the west by early Tuesday morning and
continue to veer to the northwest by Tuesday afternoon.

Carney/Truett

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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