Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 230453

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1153 PM CDT Mon May 22 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 345 PM CDT Mon May 22 2017

Southwest-northeast oriented area of showers and a few thunderstorms
will continue to shift mainly northeastward over the next several
hours. This area of precipitation looks like it is mainly forced by
strong upper-level jet dynamics in conjunction with weak low-level
moisture convergence. These forcing mechanisms begin to shift slowly
southeastward this evening into the overnight hours so have PoPs
also raising across portions of central Missouri into west-central
Illinois during these hours. Very limited instability as of 1900 UTC
so thinking this will be mainly showers, especially after mid
evening when the limited instability we currently have is forecast
to wane nocturnally. Lows tonight look to be near normal for most
locations in the 50s to near 60 degrees.

Cold front will very slowly progress southeastward through the
day on Tuesday with a continued threat for precipitation. Chances
of thunder will increase by midday as midlevel low and its
associated cold pocket aloft move toward the area helping to
significantly increase midlevel lapse rates. Highest chance of
showers and storms because of the timing of the cold front will be
across central portions of the forecast area. Highs on Tuesday
will be dependent on frontal timing as well as
prevalence/thickness of cloud cover and precipitation chances.
Coolest locations are expected to be across northwestern sections
of the forecast area with highs in the low 60s and warmest areas
including portions of southwest Illinois and southeast Missouri
topping out in the low 70s.


.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 345 PM CDT Mon May 22 2017

Track of the upper low is a little further west today, with it
moving down along the Mississippi River on Wednesday.  Still looks
like Tuesday night into Wednesday night will have a chance of
showers under the upper low before drier weather is expected as
shortwave ridge moves into the area from the west.  Then it looks
like it may be active Friday into Sunday as a frontal boundary will
become quasi-stationary over the area and several shortwave troughs
move across Missouri and Illinois in west southwesterly flow aloft.
The GFS is showing the atmosphere becoming quite unstable with quite
a bit of deep layer shear early this weekend, so will have to watch
out for a few strong to severe thunderstorms.  ECMWF and GFS begin
to diverge in solutions by Sunday into Monday which brings
uncertainty in the forecast by early next week.

Still have temperatures below normal under the upper low Wednesday
and Thursday, but they will climb back to near normal late week into
the holiday weekend.



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1150 PM CDT Mon May 22 2017

Main TAF concerns will be on precipitation trends along with
lowering cigs. An area of showers continues to push through
north- central MO this evening, toward COU and UIN. A few of these
showers may just be heavy enough at UIN to bring brief MVFR vsbys.
Showers will persist overnight at COU/UIN, but much of the
activity should stay just northwest of the St. Louis metro sites.
Showers will move east towards the St. Louis sites by Tuesday
morning, with a few thunderstorms possible across the area Tuesday
afternoon as a cold front works through the region. Along and
behind the front, some MVFR cigs look likely Tuesday morning
mainly at UIN/COU. The passage of the front will bring a wind
shift to the WNW Tuesday afternoon into Tuesday evening.

VFR conditions expected overnight, with showers and a few storms
possible on Tuesday. Current shower activity over northern MO
should remain just northwest of the terminal through much of the
overnight hours, but a few showers may begin to creep into the
region by Tuesday morning. A better chance for showers and a few
thunderstorms will come Tuesday afternoon as a cold front swings
through. This front will bring a wind shift to the WNW as it
passes by.





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