Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 162047
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
347 PM CDT Sat Aug 16 2014
.SHORT TERM: (Through Late Tonight)
Issued at 347 PM CDT Sat Aug 16 2014
We have entered a quiet period of weather early this afternoon as
the atmosphere re-organizes itself. Just entering extreme northeast
MO was a powerful 50-something MCV, and with the very moist
atmospheric column in place where PWs were in excess of two inches,
it is having no problem in perpetually generating rain around it.
Extending to the southwest of this important upper level feature was
a surface cold front: from just W of Moberly to just N of Sedalia.
This front was intersecting a very moist atmosphere as well, where
surface dewpoints were in the low 70s and PWs near two inches, and
an unstable atmosphere as well, with MLCAPES up to 3000 J/kg. This
front was just beginning to ignite some TSRA along it. Further
downstream into eastern MO and southern IL, temps were just
beginning to recover into the upper 70s and low 80s where clouds and
rain had previously dominated.
The atmosphere is essentially primed and merely needs a trigger to
get the next round of rain going. Moisture convergence should be at
sufficient levels for TSRA to get going along the front, but have
also noted that convergence was also beginning to increase over STL
metro and southeast MO where a window of opportunity for additional
development will present itself between now and early this evening.
Otherwise, primary foci will be surface front and MCV as they both
very slowly progress southeastward thru our region tonight and
continue on Sunday morning...finally expected to leave and pull away
Sunday afternoon. As stated earlier, PWs will be in excess of two
inches, which is greater than two standard deviations from the mean
for mid-August, and models are also forecasting rather deep warm
cloud layers over 4km. Greatest threat for excessive rainfall will
be an area sandwiched between the MCV, the broadscale lift from the
mid level shortwave, and the trailing surface front. This all seems
to point to an area from east-central MO and throughout southwest
IL, including STL metro. The problem is that 6hr FFG values are
mainly above 3 inches, with a small area just north of STL metro
where it is as low as 2.5 inches. Given this, will pass these
concerns on to evening shift and let them see how convection
develops before issuance of any Flood Watch.
Categorical PoPs were forecasted thru this system`s conclusion on
Sunday with a warm and muggy night ahead tonight and a cool day on
Sunday with substantial lo clouds in the system`s wake expected.
.LONG TERM: (Sunday through Next Saturday)
Issued at 347 PM CDT Sat Aug 16 2014
A somewhat drier atmosphere is in place for Monday and with a lack
of upper level support and a decent lo level cap in place, should
see one day of dry wx.
A weak cold front drops down on late Monday night and lingers thru
late Wednesday and will result in a threat for thunderstorms.
Depending on where the front settles, it will separate seasonable
warmth to the north from summertime heat in the south. This
summertime heat will be coupled with humidity that may result in 100
heat index values for parts of the area Tuesday and Wednesday
By late next week, there trend will be for drier and hotter wx as an
upper ridge builds overhead and may result in the summer we nearly
missed. Looks like a period of a few more days of max temps in the
90s with heat index values between 100-105. May need a heat
headline at some point for quite a few areas for next week as a
.AVIATION: (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1213 PM CDT Sat Aug 16 2014
Upper level vort max/mcv/upper low over NE MO SE IA will move
slowly southeast overnight. Looks like a cloudy, rainy, foggy
night. Associated surface low over north central mo will mirror
the upper system and also move se. Trailing cold front may be the
triger for thunderstorms late this afternoon into the evening. It
will depend on how much instability can form. If clouds break,
things may take off. Looks like IFR conditions overnight into
about midday Sunday before the system pulls out.
Specifics for KSTL: Current rain will be east of the terminal at
issuance so will go with VSCH for the bulk of the afternoon. Will
bring in 6sm and showers at 00z with a VCTS. Will drop visibity to
marginal by 05z with fog and then drop to IFR at 09z with fog and
low ceilings. Guidance and conditions currently over southern
Iowa support this. An optimistic forecast will improve conditions
to marginal by 18z.