Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210835
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
335 AM CDT Fri Jul 21 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 335 AM CDT Fri Jul 21 2017

Upper ridge remains over region, so hot and humid conditions to
persist today. Will see highs in the upper 90s to low 100s once
again. With dewpoints in the upper 60s to mid 70s, heat index values
will be between 105 and 114 today. One fly in ointment is stalled
frontal boundary over southern Iowa, where storms are developing and
sliding eastward along it. Some of the cloud cover from complexes
are skirting portions of northeast MO/west central IL. So depending
on how long activity lingers, clouds could affect temps in these
areas.

HRRR indicating a few isolated storms not out of question for
portions of west central and southwestern IL this afternoon, so kept
slight chance pops going there.

Hot and humid conditions to persist tonight with temps only dipping
down into the mid 70s to low 80s.

Byrd

.LONG TERM...  (Saturday through Thursday)
Issued at 335 AM CDT Fri Jul 21 2017

The extreme heat is set to continue thru at least Saturday and
perhaps into Sunday.

Upper RIDGE will easily dominate our region thru Saturday and
combined with a surface front remaining to the north and a W-SW
surface flow over our region and mostly clear skies, will yield yet
another very hot day over the region.  Realized temps have been
running about 2F higher than the highest MOS in most areas, and
around 4F higher in parts of STL metro.  In addition, forecast H850
temps will be about 1C higher than persistence.  This all points to
max temps on Saturday in the 100-105 range for most locations with
max heat index values 105-115.  All heat headlines look valid for
this period.

By Sunday, a transition to a NW flow aloft is expected in response
to a strong upper level storm system dropping southeastward from the
Canadian Plains.  The NW flow aloft will allow a pair of surface
cold fronts to move thru our area.  The first is expected on
Saturday night and Sunday morning and should result in scattered
showers and storms along with cooler temps, but with the
understanding that the temps will still be above average.  In
addition, most of the rain chances are expected to be thru early on
Sunday leaving a dry and potentially partially sunny period in the
afternoon for many areas.  Continued to favor forecasting higher
than the highest MOS for max temps, with low-mid 90s.  This yields
some max heat index values in the low 100s, mainly for areas south
of I-70.  This fits in well with heat headlines in southeast MO that
continue into Sunday evening but may eventually mandate an extension
of the heat headlines into Sunday for STL metro and possibly a few
additional areas.  The overall situation is questionable enough that
we can afford to wait at least another package or two before
deciding on this.

The second, and stronger, cold front will drop down thru our area on
Sunday night and should result in another round of isolated
to scattered showers and storms along it.

Temps are then expected to drop to seasonal averages Monday and
Tuesday, thus bringing an end to any lingering heat headlines.

Already by late Tuesday, however, heights aloft are building as the
RIDGE seeks to re-establish itself for late next week.  And combined
with surface flow becoming southerly, do the daytime max temps climb
back thru the 90s once again and so we could very well be looking at
a new episode of high heat by this time.

One interesting item that was mentioned in the previous discussion
was the outlier EC bringing a Pacific system and then cutting it off
in the building RIDGE regime by Tues-Wed.  It continues to do this
with the 00z run and does have some minor support from the GFS now.
Trended towards including PoPs for Tues-Wed, but kept them low for
now.  Something to watch, especially if the EC ends up being
correct, with a slow moving rain maker.

TES

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1124 PM CDT Thu Jul 20 2017

Dry and VFR conditions are expected through the period with light
and variable winds.

SPECIFICS FOR KSTL:

Dry and VFR conditions are expected through the period with light
and variable winds.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Sunday for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

     Excessive Heat Warning until 8 PM CDT Saturday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-
     Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Marion 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.

IL...Heat Advisory until 8 PM CDT Sunday for Randolph IL.

     Excessive Heat Warning until 8 PM CDT Saturday for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX



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