Area Forecast Discussion
Issued by NWS St. Louis, MO

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758
FXUS63 KLSX 281641
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1041 AM CST Tue Feb 28 2017

.UPDATE...
Issued at 1041 AM CST Tue Feb 28 2017

...Confidence Increasing on Severe Weather Outbreak Late This
Afternoon into the Overnight Hours...

The latest data, including the E-HRRR/HRRR/WRF-ARW/WRF-NMM are in
general agreement of supercell thunderstorms developing late this
afternoon into early this evening. The 12Z sounding out of SGF
showed a deep low-level moisture profile, with saturation up to
around 880mb. However, it did show a substantial cap above the
moist low-levels (16C around 875mb), with steep mid-level lapse
rates above the cap. With time, this cap is expected to be
overcome due to increasing solar insolation as low-level stratus
mixes out, and with continued low-level moisture advection. While
the focus for initial supercell development is a bit nebulous,
the latest guidance suggests development is likely within the
left-exit region of a developing 925mb low-level jet axis, which
will shift toward the MS River Valley by 00Z this evening. The
timing of this feature could develop supercell thunderstorms
across east-central MO and western IL (including the St. Louis
metro) near rush hour. Forecast soundings during this time suggest
a large to very large hail threat, damaging winds, and the
potential for tornadoes given the strongly-sheared low-levels.

This initial round of semi-discrete supercells will push east by
late this evening, but attention will then turn to the northwest
along the approaching cold front. A PV anomaly will eject out atop
this cold front, leading to additional supercell development
initially, but likely evolving into a line overnight given
orientation of the 0-6km shear vectors to the slab-like forcing
along the approaching front. Once again, all hazards will be
possible with this round of thunderstorms including large hail,
damaging winds, and tornadoes.

KD

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 356 AM CST Tue Feb 28 2017

...Severe Thunderstorms Likely Across Portions of the Area Late This
Afternoon and Especially Tonight...

The latest model guidance continues to hone in on a volatile period
beginning possibly as early as the later part of this afternoon, but
especially tonight. Clusters of elevated thunderstorms that are
ongoing across south central and southeast MO into southern IL are
in response to moisture convergence/lift associated with a stout
southwesterly LLJ, and an airmass featuring steep mid level lapse
rates and MUCAPE of 1000+ J/KG. This activity could persist within
this corridor just past daybreak, but is eventually expected to move
east-northeast into the OH Valley region by mid-morning.

From mid-morning into the early afternoon I think the weather will
be largely quiet. Increasing and veering south-southwesterly low
level flow ahead of the approaching cold front will result in strong
WAA and significant northward moisture transport within the broad
warm sector. Despite a good deal of cloudiness, I think that
temperatures will warm well into the 70s by mid-afternoon. The
progged H85 temps of +10 to +14 degC continue to support the
potential for record or near record high temps. The moisture
transport today is impressive and there is relatively good
agreement that lower 60s will advect much further north than
previously indicated, into parts of northeast MO and central IL.
The combination of diabatic heating, steep mid level lapse rates,
and aforementioned low level moisture will result in seasonably
strong instability for late February with SBCAPE of 1000-1800+
J/KG and only a weak CAP by mid-afternoon. The deep layer shear
remains quite strong throughout the warm sector at 50-70 kts owing
to the strong mid-upper level flow which will continue to
increase tonight.

Convective evolution still remains unclear and there continue to be
several scenarios suggested by the models and multiple CAMS.
Consensus of the guidance suggests that the convective initiation on
a widespread basis may be delayed a little later that earlier
expected in the pre-frontal corridor across northern MO. However
further south and well ahead of the advancing cold front, the zone
from northeast OK/northwest AR northeastward across southeast MO
into the St. Louis region continues to be highlighted for
development in the mid-afternoon, and remains an active zone into
the evening. The westerly orientation of deep layer shear vectors
and only weak forcing in this later region would suggest that
discrete or semi-discrete supercells would be favored. Any storms
that can remain relatively isolated in the free-warm sector would
pose the greatest tornado threat, including possibly significant
tornadoes, as low level shear and helicity will be ramping up within
this zone into the evening with LCL heights lowering.

During the evening hours, large scale forcing/height falls/cooling
aloft associated with the approaching upper trof should lead to
severe storm development across northern MO into west central IL
ahead of the advancing cold front. This activity would include
supercells with all severe weather threats. As the evening
progresses and cold front advances southeast, the activity should
grow upscale eventually becoming a severe QLCS moving to the
southeast with a severe threat well into the overnight hours. The
cold front is expected to accelerate overnight as the upper trof
moves across the Plains and into the MS Valley and thunderstorms
should have cleared the CWA near or just after 12Z.

It should be emphasized that instability and shear will be favorable
for severe weather well into the night, this includes potential for
significant severe. Partners should emphasize that much of the
severe threat will be after dark and the populous needs to be
prepared and review their safety plans.

Glass

.LONG TERM...  (Wednesday through Monday)
Issued at 356 AM CST Tue Feb 28 2017

Cold front position early Wednesday morning should be near or just
southeast of the CWA. Threat of severe weather should have exited
the CWA by this time with only lingering showers possible mainly for
southern and eastern sections of the area. Much cooler (yet still
above normal) conditions are anticipated for Wednesday with strong
CAA depicted at low levels as high temperatures top out in the upper
40s to low 50s. Bulk of stratus clouds should move off by early
Wednesday afternoon to the east of the CWA with decreasing clouds
expected for western counties.

A reinforcing shot of cooler air will come in behind a cold front
associated with a disturbance within NW flow aloft during the day on
Thursday. The bi-state area will reside within the warm sector of
this clipper-like disturbance so leaned above warmer MAV guidance
for high temperatures. In the wake of this disturbance, surface high
pressure will quickly move to the southeast and into central
Missouri by 1200 UTC Friday. Exact timing of this sfc high will go a
long way in determining exactly how cold it gets Thursday
night/Friday morning, but regardless, temperatures should dip well
below freezing in most areas. Coldest readings of low to mid 20s are
forecast for portions of northeastern Missouri where best
radiational cooling conditions are expected given close proximity to
sfc anticyclone late Thursday night and into early Friday morning.

Aforementioned surface high pressure will maintain control of the
sensible weather to begin the weekend with moderating temperatures
back well above normal as the high slides into the Ohio Valley. A
warm and mostly dry weekend looks to be on tap across the mid-
Mississippi Valley with highs predominantly in the 60s. There is a
chance of showers on Sunday however as low-level warm advection
strengthens.

An area of low pressure is forecast to track near the US-Canadian
border on Monday and drag a weak cold front through the area late
Monday. Showers are possible with this feature though PoPs are only
in the schc-chc range not due to poor model continuity with the
timing of the front but look to be attributed to a lack of forcing
for large-scale ascent at middle and upper levels of the atmosphere.
Strong dynamics aloft reside well to the north across the Upper
Mississippi Valley and Great Lakes region.


Gosselin

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 614 AM CST Tue Feb 28 2017

MVFR flight conditions are expected across the area this morning
with cigs improving late morning into early afternoon to VFR.
Southerly winds will also become gusty this morning, continuing
this afternoon into the evening. The main uncertainty with the
TAFS is the timing of thunderstorm chances. There are indications
that the first wave of thunderstorms will form across south
central MO and impact the St. Louis region early in the evening.
Otherwise a cold front will push southeast across the region
tonight preceded by a line of thunderstorms. Some of the storms
could be severe. The TAFS reflect the main time frames where the
showers and thunderstorms are expected, however the flight
conditions may be as low as IFR at times. Following the cold
front, gusty west-northwest winds are expected and a return of
MVFR cigs/flight conditions.

SPECIFICS FOR KSTL:

MVFR flight conditions are expected this morning with cigs
improving to VFR by late morning. Southerly winds will also
become gusty this morning, continuing this afternoon into the
evening. The main uncertainty with the TAF is the timing of
thunderstorm chances. There are indications that the first wave of
thunderstorms will impact the St. Louis region early in the
evening. Otherwise a cold front will push southeast across the
region tonight preceded by a line of thunderstorms. This later
line of storms would occur during the overnight hours. Some of
the storms could be severe both this evening and overnight. The
TAF reflects the main time frame where the showers and
thunderstorms are expected, however the flight conditions may be
as low as IFR at times. Following the cold front, gusty west-
northwest winds are expected and a return of MVFR cigs/flight
conditions.

Glass

&&

.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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