Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250459

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1159 PM CDT Wed May 24 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 325 PM CDT Wed May 24 2017

Upper level low over southeast MO will continue dropping
southeastward, while surface low over KY moves northeastward into
the OH Valley region.  Scattered showers will continue to rotate
through our forecast area late this afternoon.  The shower activity
should lessen this evening due to the loss of daytime heating, and
as the upper level low shifts southeast of the forecast area, while
the surface low shifts well northeast of the region.  May have a few
lingering showers over southwest IL until midnight.  The sky should
gradually clear out from west to east tonight as a weak surface
ridge shifts eastward through MO later tonight into Thursday
morning.  Low temperatures tonight will be cool, especially across
northeast and central MO and into the Ozarks where the clouds will
clear out, the surface wind will become light, and the surface dew
points will only be in the middle to upper 40s.  The lows tonight
will be around 5 degrees below normal. Warmer temperatures can be
expected for Thursday due to upper level ridging building into the
region, the surface wind becoming southerly over parts of our area
Thursday afternoon as the surface ridge shifts east of the region,
and also due to plenty of solar insolation because of little cloud


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 325 PM CDT Wed May 24 2017

An upper LOW, currently spinning over southern Alberta province,
will play a dominant role over setting up our wx pattern during the
next seven days and the wx pattern will be--what else--an active one
to close out the typically active month of May.

After a brief quiet period of being under the influence of a weak
upper RIDGE late Thursday, the flow aloft will shift from the west-
southwest where it will remain in place from early Friday thru early
Sunday.  A series of upper level disturbances will track thru during
this time, and along with increasing moisture thru the column and
the presence of a surface front nearby throughout this time, will
lead to a persistent chance for thunderstorms.

This persistent chance for storms looks to peak on Saturday
afternoon, where a notable severe wx threat also exists.  Models
continue to advertise strong 0-1km shear and high instability which
could favor a prolonged severe wx outbreak much of the day of
Saturday and into Saturday evening, with hail, damaging straight
line winds and also isolated tornadoes.

A lesser chance for severe wx exists during peak heating on Friday,
and appears more focused on hail and damaging straight-line winds,
but this also appears conditional given rather low probs for storms
as there is some questionability on their ability to break the cap.

A chance for lingering showers looks to persist into Sunday with
another chance for showers and storms late on Memorial Day as a cold
front swiftly moves thru, thanks to NW flow at the mid levels
by this time which should preclude any stalling.

Quieter wx should try to establish itself shortly after as NW flow
aloft persists and promote some drying.

Daytime max temps should be near average for late May for much of
this period, with the best potential for warm days being on Friday
and Saturday, but will dependent on where the front sets up and when
storms begin to fire.



.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1156 PM CDT Wed May 24 2017

Radiational fog is possible across portions of central and
northeast MO and west central IL overnight impacting KUIN and
KCOU. Otherwise VFR flight conditions are expected with bouts of
cumulus clouds impacting far eastern MO and western IL through
Thursday morning.


VFR flight conditions are expected with bouts of cumulus clouds
through Thursday morning.





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