Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261056

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
456 AM CST Sun Feb 26 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 354 AM CST Sun Feb 26 2017

Center of high pressure is quickly moving east/southeast of the
area this morning with return flow already developing to our west.
Weak upper level disturbance noted across Kansas will trek east
today and bring the southern half of the CWA a good chance of light
rain showers late this afternoon and this evening as a weak inverted
trof develops across the Ozarks.  After a cold start this morning,
temperatures are expected to rise into the upper 40s and lower 50s
this afternoon on an increasing southerly wind.


.LONG TERM...  (Monday through Saturday)
Issued at 354 AM CST Sun Feb 26 2017

The latest model guidance continues to show that the first part of
the upcoming week will be rather active. Southwest flow aloft will
be present across the region early Monday with a broad and
relatively low amplitude long wave trof centered in the far western
U.S.. A postively tilted low-amplitude short wave will stretch from
IL into SW MO at 12Z Monday. The northern extent of weak large scale
forcing with this short-wave could help support some lingering
showers across far southeast MO and far southwest IL during the
first part of the morning. I think the remainder of the day looks
largely dry for most of the CWA, with an increasing southwesterly
LLJ promoting good low level WAA and northward moisture transport.
Temperatures should once again rise to above normal levels. Another
weak short-wave in the southwest flow aloft will traverse the area
on Monday night. Forcing with this short-wave as well as continued
WAA and moisture transport/lift associated with a veering
southwesterly LLJ should result in scattered showers to a broad
region. The greatest coverage should be during the evening, with
the threat shifting to the east-northeast overnight as the LLJ

The most active and noteworthy period during this forecast will be
centered on Tuesday afternoon and Tuesday night. Strong southwest
flow aloft will be present across the region ahead of the
deepening and advancing western upper trof. Veering south-
southwesterly low level flow will be well established ahead of an
advancing cold front. This flow regime will promote strong low
level WAA and impressive low level moisture return (sfc dew points
in the low 60s into southern MO and southern IL by afternoon)
throughout the expansive warm sector ahead of the front. Some
record high temperatures will be possible. The combination of
steep mid level lapse rates, low level moisture, and diabatic
heating will result in a broad unstable warm sector. Deep layer
shear will be quite strong owing to the strong flow aloft, and
combined with the instability, will be favorable for severe
storms. The convective evolution at this point is a bit unclear
with the GFS suggesting a weakening cap in the afternoon and
convective development across central/northeast MO into central
IL. The orientation of the deep layer shear vectors would support
a mixed mode with a severe hail threat, with upscale growth into
the evening. The NAM and ECWMF delay development until Tuesday
night as a lead short wave ahead of the main trof spreads large
scale ascent into the area. It would appear a more favorable
environment for supercells would exist on Tuesday evening within
the higher quality moisture across southern MO and where better
orientation of the deep layer shear vectors might help for
discrete or semi-discrete modes, especially for any long-lived
storms initiating across southwest MO or eastern OK.

The ECMWF and GFS are closer in the latest model runs and favored
with the cold front position near or just southeast of St. Louis at
12Z Wedneday. There will remain a threat of showers ahead of the
front, and the upper trof and attendant forcing along with
frontogenetic forcing will support a threat of post-frontal

Northwest flow aloft becomes established in the wake of Wednesday`s
upper trof passage and as a broad longwave trof evolves through the
end of the week in the eastern U.S.. This will allow for another
reinforcing cold front passage on Thursday.



.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 452 AM CST Sun Feb 26 2017

Weak upper level disturbance will bring an increase in cloud cover
today as well as a chance of showers. VFR conditions will continue
as winds turn around to the south and increase.

VFR conditions are expected today as mid level clouds increase.
There is a chance of a few showers this afternoon and this
evening. South wind will increase today.



Issued at 354 AM CST Sun Feb 26 2017

Record High Temperatures For Tuesday 2/28

STL 80/1972
COU 75/1972
UIN 68/1932




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