Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212037

Area Forecast Discussion
National Weather Service Saint Louis MO
337 PM CDT TUE JUN 21 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 307 PM CDT Tue Jun 21 2016

Focus tonight will be on convective trends across the mid-upper Mississippi
River Valley. Current expectation is for scattered showers and thunderstorms
to develop near the mid-Missouri Valley by early this evening. Storms will
increase in coverage and push southeastward in time. Best chances of showers
and thunderstorms overnight tonight will be roughly along and east of the
Mississippi River. Concerned that retreating warm front may be a bit slower
to push as far northeast as anticipated ~24 hours ago as early morning MCS
has pushed effective boundary further to the southwest. However...model
guidance between 0600 and 1200 UTC in terms of warm frontal position is fairly
clustered with the boundary lifting northeastward through the CWA by 0900 UTC. terms of timing of storms best chances will be generally be
between 0500 and 0900 UTC with the best chances lifting northeast of area
very late tonight.

Temperatures tonight will be quite mild once again due to the warm/moist
atmosphere in place along with light southerly surface flow. Lows in the low
to mid 70s are forecast areawide.


.LONG TERM...  (Wednesday through Next Tuesday)
Issued at 307 PM CDT Tue Jun 21 2016

The lifting warm front will stretch from western IA across far
eastern MO and into southern IL at daybreak on Wednesday. Any
ongoing convection should be located on the cool side of the
boundary in IL or points northeast. The front will lift quickly to
the northeast on Wednesday morning as a surface low moves east
across IA, taking the precipitation threat with it out of the CWA.
In the wake of the front, a broad hot and humid warm sector will
dominate the CWA. This air mass will be characterized by a stout
H85 thermal ridge with afternoon readings of +22 to +24 degC,
southwesterly low level flow, and mixed afternoon dew points in
the upper 60s to lower 70s. The result should be the hottest day
experienced thus far this year with most locations in the mid-
upper 90s and parts of metro St. Louis topping 100 degrees. I have
issued a heat advisory for Wednesday afternoon and evening to
account for the dangerous heat levels forecast with peak heat
index values in the 104-108 range. Mid level temps are expected to
warm to 14+ degC and this should effectively CAP any surface-based
convection within the warm sector.

A cold front will then drop gradually into northern MO and west
central IL late on Wednesday night. Some slight cooling aloft
along with the front may be enough to support some spotty showers
or storms, and these could be rather high-based. All in all the
precipitation prospects on Wednesday night are not very exciting.
The east-west cold front will sag southward on Thursday passing
through St. Louis near midday and moving into southern MO and
southern IL in the afternoon. The precipitation threat in the
morning will remain low and rather spotty, with increasing chances
in the afternoon as the front moves southward. It will be a warm
start to the day for those locations south of the front and this
will lead to a quick rise in temperatures. A heat advisory may
ultimately be needed for southern sections of the CWA.

The front eventually moves south of the CWA on Thursday night with
weak high pressure settling into the area. While I have some low
pops in southern and central MO, I think it will largely be dry.

The upper air regime as we head into the weekend and early next
week is rather typical of summer with the main band of westerlies
situated across the northern tier of the CONUS. Present indications
are a migratory upper trof will move from the northern Rockies
into the Great Lakes region while lower latitude high pressure
intensifies and gradually retrogrades into the southwestern U.S..
This will allow the front which came through on Thursday to
retreat back northward in the Friday-Saturday time frame and then
a cold front to drop back south into the area on Monday.
Temperatures will be seasonably hot, with Sunday likely the
hottest day.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1230 PM CDT Tue Jun 21 2016

Overnight MCS has weakened considerably over the past couple of
hours with just a few showers/storms remaining. Believe much of
the rest of the afternoon and early evening will remain quiet with
additional showers and storms developing along and to the
northeast of a retreating warm front. Believe best chance will be
along/east of the Mississippi River overnight tonight with best
chance at KUIN so did prevail thunderstorms at that site. KCOU is
believed to stay dry through the valid TAF period with the metro
TAF sites likely on the western edge of activity so have a VCTS
group in for those sites. Light/variable winds tonight will become
southerly and increase to 10-15 knots by midday Wednesday.

Specifics for KSTL:

Believe much of the rest of the afternoon and early evening will
remain quiet though will watch a small cluster entering
southwestern Pike County in Missouri as of 1730 UTC. Additional
showers and storms should develop along and to the northeast of a
retreating warm front near the MO/IA border by mid evening and
move southeastward in time. Better chances of showers/storms will
be to the northeast of terminal but concerned frontal boundary may
be a bit further southeast than previously predicted due to the
earlier effects of the MCS. Have therefore added a VCTS group in
for now. Light/variable winds tonight will become southerly and
increase to 10-15 knots by midday Wednesday.



MO...Heat Advisory from noon to 9 PM CDT Wednesday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Jefferson MO-Knox MO-Lincoln MO-Moniteau MO-
     Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint
     Charles MO-Saint Louis City MO-Saint Louis MO-Shelby MO-
     Warren MO-Washington MO.

IL...Heat Advisory from noon to 9 PM CDT Wednesday for Calhoun IL-
     Jersey IL-Madison IL-Monroe IL-Saint Clair IL.



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