Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 222046

Area Forecast Discussion
National Weather Service Saint Louis MO
346 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 southweastward 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 18z TAFs through 18z Tuesday Afternoon)
Issued at 1247 PM CDT Mon May 22 2017

Watching a band of showers moving toward KUIN early this
afternoon. Believe they will be affected by this area of showers
with a few thunderstorms also possible through late afternoon.
Elsewhere, believe terminals should stay dry through the afternoon
and early evening. Chances for showers will increase very slowly
from northwest to southeast overnight tonight, with scattered
showers and storms possible everywhere on Monday afternoon. Cold
front will move through during the day with winds switching to the
north/northwest along with MVFR ceilings.


Should be dry and VFR through the evening and early overnight.
Chance of showers will increase slowly overnight tonight and early
Tuesday morning. Thunderstorms will also be possible ahead of a
cold front on Tuesday afternoon. Behind the front, look for winds
to become predominantly northwesterly along with ceilings possibly
becoming MVFR.





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