Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261822

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1222 PM 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 positively 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 veers.

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 Wednesday. 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 18z TAFs through 18z Monday Afternoon)
Issued at 1200 PM CST Sun Feb 26 2017

VFR flight conditions with south-southwest flow will prevail
across the area this afternoon. Some light rain or sprinkles may
fall this afternoon...generally along and south of the I-70
corridor, but rainfall is not expected to be heavy enough to
impact visibility. Expect ceilings to lower overnight across
southeast Missouri to MVFR/IFR range with these lower ceilings
potentially building north into east central Missouri and
southwest Illinois. Not sure about timing and heights on ceilings
into east central Missouri so was conservative in the St. Metro
TAFs. Think most overnight cloud cover will break up during the
late morning/early afternoon except over eastern sections of the
area where ceilings could linger.


VFR flight conditions with south-southwest flow is expected to
continue to prevail through this evening. There may be some light
rain or sprinkles this afternoon, but any rain that does get to
the terminal is not expected to be heavy enough to impact
visibility. Low ceilings are likely to move up into the vicinity
of the terminal early Monday morning. Timing and height of the
ceiling is not certain at this time, so was conservative with the
first introduction of the MVFR. May have to revise this downward
this evening. Ceiling should break up Monday morning...though
timing on that is also uncertain.



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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