Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 260206
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
906 PM CDT Mon Aug 25 2014
Issued at 853 PM CDT Mon Aug 25 2014
A cluster of storms currently extending from just south of Decatur,
IL to south of Charleston, IL continues to slowly edge south-
southwest toward far eastern portions of the CWFA this evening.
There is some uncertainty as to how much longer the storms will
survive given the loss of diurnal heating and the fact that the
associated outflow boundary has surged well ahead of the
convection, though a couple showers/thunderstorms cannot be ruled
out over the next few hours for portions of southwest Illinois.
Thus, have introduced some slight chance PoPs for the next few
hours for that area, and will continue to monitor trends and
update as needed this evening. Otherwise, the remainder of the
forecast for tonight looks to be on track, and only minor
adjustments have been made to account for near term trends. If
precipitation manages to edge into the eastern CWFA this evening,
low temperatures would likely need adjusted. But for now, lows in
the low to mid 70s area-wide with upper 70s in the St. Louis metro
area still look to be on target.
.SHORT TERM: (Through Late Tonight)
Issued at 320 PM CDT Mon Aug 25 2014
Diurnal convection has once again developed over the higher terrain
of the eastern Ozarks in response to orography and SBCAPE exceeding
3000 j/kg. If these storms exhibit the same trends as those of the
last 5-6 days, they should drift westward and dissipate by sunset.
Otherwise the only other apparent threat of storms should be late
tonight as the cold front currently extending from southeast Iowa
into northwest Missouri slides southeastward into the northern
portion of the CWA. It will remain on the mild-warm side with lows
in the 70s.
.LONG TERM: (Tuesday through Next Monday)
Issued at 256 PM CDT Mon Aug 25 2014
The expansive upper ridge will remain firm through Tuesday with the
aforementioned surface front oozing only a bit southward from its
position across the northern CWA. Cooler H85 temps and mid level
temps suggest the magnitude of heat will be a tad less than
previous days, however still quite hot and well within advisory
and warning criteria. These cooler temperatures aloft and the
strong heating will contribute to very unstable conditions and
what appears to be a larger region susceptible to diurnal thunderstorms,
enhanced a bit along the front where there is some focus.
Changes in the pattern aloft begin to take place beginning late Tuesday
night-Wednesday as a short wave trof moves along the northern tier
states and across the Great Lakes region into the northeast U.S.
This should give the front some impetus and it is expected to
gradually move southward on Wednesday, with the front anticipated
across southern MO and southern IL by 00z Thursday. Increasing
clouds and a threat of scattered showers and thunderstorms should
accompany the front along and to its north. Present indications
are cooling aloft and the front should put an end to the magnitude
of heat that we have experienced since late week. That said there
could be a few locations along and south of I-70, especially metro
St. Louis, where highs push into the lower 90s and afternoon heat
index values top 100. These locations would warrant an extension
of advisories or warnings through Wednesday, however given some
uncertainty in the coverage of these heat index values due to more
clouds, the front and convection, I will hold off and let later
shifts get a fresher look.
Eventually as we head into the period from Thursday into the weekend
the impacts of a slow-moving upper trof origination in the western
U.S. will come into play. The models have displayed variability
with the speed and magnitude of this system. It looks like as the
upper trof and associated surface system push into the Plains on
Thursday, it should lead to a northward retreat of our initial
east-west front and then sometime late in the weekend the Plains
system will pass through. In between that time it appears there
will be a prolonged/daily threat of showers and thunderstorms with
highs in the upper 80s to lower 90s.
.AVIATION: (For the 00z TAFs through 00z Tuesday Evening)
Issued at 635 PM CDT Mon Aug 25 2014
A frontal bndry currently extends from NE KS...thru extrm NW MO
and on into s cntrl and NE IA. The aftn convection has begun to
diminish and should continue to do so thru sunset. Overnight...
expect lght/vrb winds with some rvr vly fg possible once again.
The bndry is fcst to slowly sag into the area tomorrow allowing
winds to remain lght/vrb thru much of the day...eventually
becoming NE by either late aftn or evng. The threat for convection
will accompany the front as well. The front is fcst to make it
near the I70 corridor by evng. While it is certainly possible to
have convection near KCOU and the STL metro area tomorrow
aftn...opted to leave mention of it out for now based on
uncertainty in frontal position during the aftn.
Specifics for KSTL:
Clear skies and winds aob 7 kts overnight. Tomorrow...diurnal cu
with winds becoming wrly as a weak frontal bndry approaches the
terminal. There is the potential for convection tomorrow
aftn...but not confident enough with placement or coverage to
MO...HEAT ADVISORY until 8 PM CDT Tuesday FOR Audrain MO-Boone MO-
Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
Iron MO-Knox MO-Lewis MO-Madison MO-Marion MO-Moniteau MO-
Monroe MO-Montgomery MO-Osage MO-Ralls MO-Reynolds MO-
Shelby MO-St. Francois MO-Ste. Genevieve MO-Warren MO-
EXCESSIVE HEAT WARNING until 8 PM CDT Tuesday FOR Jefferson MO-
Lincoln MO-Pike MO-St. Charles MO-St. Louis City MO-St.
IL...HEAT ADVISORY until 8 PM CDT Tuesday FOR Adams IL-Brown IL.
EXCESSIVE HEAT WARNING until 8 PM CDT Tuesday FOR Bond IL-
Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
Pike IL-Randolph IL-St. Clair IL-Washington IL.