Area Forecast Discussion
Issued by NWS St. Louis, MO

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726
FXUS63 KLSX 271654
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1054 AM CST Mon Feb 27 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 337 AM CST Mon Feb 27 2017

(Today)

A slow decrease in cloudiness is expected through the morning hours
as bulk of midlevel moisture pushes off to the east. This should
result in a partly to mostly sunny sky for the area into early this
afternoon with much warmer temperatures expected compared to 24
hours ago. Forecast highs range from the upper 50s across portions
of southwest Illinois to the low 60s for most of central Missouri.
Leaned at or above warmer MET guidance numbers due to recent cool
biases with respect to model guidance temperatures with at least
partial sunshine expected.

(Tonight)

Southwest flow aloft will become established this evening as
longwave trough deepens across the desert southwest. Latest guidance
suggests strong warm/moist advection to ramp up late this evening
and early overnight with elevated convection blossoming in this
timeframe. Added thunder wording as there should be plenty of
elevated instability for thunder with hail also a possibility in
stronger cores. Coverage of thunderstorms is a bit uncertain however
as there appears to be a displacement between the stronger forcing
for large-scale ascent associated with the UL jet streak and vort
max across far northern sections of the CWA and most intense low-
level warm/moist advection across southern half of the CWA. Due to
this uncertainty...leaned toward more isolated-scattered coverage
for now which matches up well with overnight CAM guidance. Lows
tonight will be on the mild side due to increasing low-level cloud
cover...increasing dewpoints...and sustained southerly winds at the
surface of 5-10 knots. Expect lows only to dip back down into the
upper 40s to mid 50s or some 20-25+ degrees above normal for late
February.

Gosselin

.LONG TERM...  (Tuesday through Sunday)
Issued at 337 AM CST Mon Feb 27 2017

All guidance continues to point to an active period on Tuesday and
Tuesday night. Present indications are that elevated thunderstorms
that occur tonight, should have moved east of the CWA by daybreak in
association with veering of the LLJ and the northeast trek of the
disturbance aloft. A broad warm sector will be present across the
region at daybreak with a surface low in the vicinity of the IA/NE
border and a southwest trailing cold front. Strengthening and
veering south to southwest low level flow within the warm sector
will result in strong WAA and sustained low level moisture return
and deepening throughout the day ahead of the advancing cold
front. The progged H85 temps of +10 to +14 degC continue to
support the forecast of unseasonably warm temps. Record high temps
look quite possible considering these values and the pattern, and
the only caveat is a good deal of high cloudiness expected. The
thought is that thinning of the clouds and the strong advective
component will win out. By mid-afternoon the combination of
steep mid level lapse rates, low level moisture, and diabatic
heating will result in good instability for late February with
SBCAPE of 800-1300+ J/KG and only a weak CAP. Deep layer shear
remains impressive and quite strong at 60-80 kts by 00Z/Wed owing
to the already strong and strengthening flow aloft, thus the
shear/instability is quite favorable for organized severe storms.
Unfortunately the convective evolution remains a bit unclear at
this point with several scenarios possible. Both the GFS and ECMWF
suggest convective initiation during the afternoon in response to
large scale forcing/height falls/cooling aloft associated with a
lead shortwave. The favored corridor would be well ahead of the
cold front from central through northeast MO into west central IL.
The orientation of the deep layer shear vectors would support a
mixed mode including supercells and all severe weather threats,
but primarily large hail. Another area bearing watching is in the
vicinity of northeast OK/southeast KS/southwest MO/northwest AR,
along and ahead of the dryline. There are signals that discrete or
semi-discrete supercells could initiate in this region during the
afternoon and move rapidly east-northeast and impact southeast MO
by early evening. Individual members of the NCAR hi-res CAM
ensemble support both scenarios. Convection is expected to become
widespread during the evening as the cold front advances and large
scale ascent continues to overspread the area, and this should
help promote upscale growth of the convection with a continued
severe weather threat into the night.

Sans the NAM, the ECMWF and GFS both support a faster and
accelerating cold front on Tuesday night with the front either
through the CWA at 12Z Wednesday or close to exiting the area. The
advancing upper trof and frontogenetical forcing would support a
threat of post-frontal precipitation during the morning, especially
across northeast MO and west central IL. Otherwise gusty northwest
winds and good CAA in the wake of the front will have temperatures
falling back to seasonable levels.

A broad longwave trof evolves across the eastern 2/3rds on the CONUS
by early Thursday with northwest flow aloft established in the wake
of Wednesday`s upper trof passage. A short wave within the northwest
flow will bring another "reinforcing" cold front into the area on
Thursday and this will result in near normal temperatures for Friday.
Heights aloft are on the rise heading into next weekend as the
flow aloft flattens and becomes quasi-zonal, and this will help
bring a return of a pronounced WAA regime. Temperatures next
weekend will once again return to above normal levels.

Glass

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1022 AM CST Mon Feb 27 2017

A warm front will lift northward through our area tonight. MVFR
stratus clouds will advect into the taf sites by late evening with
widely scattered showers and thunderstorms developing. The
convection should shift east of the taf sites early Tuesday
morning. A southwest low level jet will lead to LLWS conditions
tonight, but the magnitude does not appear to be quite strong
enough to include in the tafs. South-southeast surface winds will
continue this afternoon and tonight, becoming more south-
southwesterly on Tuesday.

SPECIFICS FOR KSTL: A warm front will lift northward through our
area tonight. MVFR stratus clouds will advect into the STL area by
late evening with widely scattered showers and thunderstorms
developing. The convection should shift east of the STL area
early Tuesday morning. A southwest low level jet will lead to
LLWS conditions tonight, but the magnitude does not appear to be
quite strong enough to include in the STL taf. South-southeast
surface winds will continue this afternoon and tonight, becoming
southwesterly and gusty Tuesday afternoon. The cloud ceiling will
become VFR by early Tuesday afternoon.

GKS

&&

.CLIMATE...
Record Temperatures For Tuesday 2/28

Record Highs    Record High Lows

STL 80/1972     STL 54/1895
COU 75/1972     COU 52/1895
UIN 68/1932     UIN 40/1987


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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