Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170455

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1155 PM CDT Sun Jul 16 2017

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 345 PM CDT Sun Jul 16 2017

Cold front over southern Iowa will continue to drift south-southwest
tonight and stall near a line from Chillicothe to Poplar Bluff.
Already have 2000-3000 J/Kg of MLCAPE along and north of the I-70
corridor across Missouri and Illinois and expect instability to
increase through the afternoon as temperatures continue to rise and
dew points pool ahead of the front.  Already have widely scattered
storms over southeast Iowa along the front, and would expect
coverage to increase this afternoon as the atmosphere continues to
destabilize.  Additionally, effective bulk shear increases to 30-
35kts.  Would expect some degree of organization to the storms with
this amount of instability and shear.  Freezing levels are pretty
high around 14.5-15Kft so large hail might be hard to come by, but
dry mid-levels would indicate a better possibility for strong
downdrafts and high winds.

Instability will gradually decrease through the evening hours and
thunderstorm coverage should decrease.  Model guidance continues to
show convergence along the front through the night with MUCAPE
holding around 1000-1500 J/Kg so can`t rule out isolated showers and
storms in the vicinity of the front for most of the
night...primarily across east central and southeast Missouri.
Isolated convection is possible again Monday afternoon in these same
areas as weak convergence continues along the stalled front.

Temperatures will be tricky, especially on Monday.  An area of
stratus/stratocumulus is expected to develop behind the front on
Monday morning, and possibly persist into late morning or early
afternoon, much as is occurring at this time over northern Illinois.
Went a bit above guidance for highs on Monday as the models have
been a bit cool of late.  However, if the clouds don`t break up as
expected this may be too warm.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 345 PM CDT Sun Jul 16 2017

Upper level RIDGE will begin to build eastward Tuesday and early
Wednesday, reaching more of an overhead position by Wednesday
afternoon.  This represents a bit of a delay to the onset of this
pattern as depicted from a couple of days ago.  Down at the surface,
the backdoor front will also be delayed in exiting our region and
will still be lingering over our area thru Tuesday evening, although
it will become increasingly ill defined and the areas on the "cool"
side of it to the east being protected by an increasingly shallower
layer of cool air.  While we may get one final day of isolated
convection in association with this old frontal boundary, the main
story will be the high heat which has been building out west being
allowed to inexorably ooze eastward deeper into Missouri.  If any
lesson has been learned with these early-mid summer heat events so
far, it is if the heat is given any chance/opportunity to reach
here, it does and so despite what looks to be waning influence of
the backdoor front over the area, have decided to maintain the Heat
Watch for STL metro which begins Tues aftn and, in fact, expand it
into much of central/northeast MO which will be more susceptible to
the excessive heat building from the west and under little-no
influence from the front at this point. Max temps are forecast
1-2F above the highest MOS west of the anticipated frontal
position and lean towards the higher MOS east of the frontal

The surface front is expected to completely break down Tuesday night
and with the upper RIDGE firmly in place overhead by Wednesday
afternoon, dangerous levels of heat and humidity look set for most
if not all of the CWA and this will continue easily into Thursday.
Max temps from the mid 90s to the low 100s are expected with heat
index values from 100-110.

By Friday and continuing into most of next weekend, the upper RIDGE
will weaken some, but still retain enough influence over our region.
Perhaps most importantly, any relief from a surface front looks to
be a low prob concern except for the far northern CWA and this is
the primary reason for not having in the Heat Watch at this time.
Extended the Heat Watch to Saturday as it looks like most of the CWA
will remain well south of any relief from a front with further
extension into next Sunday ultimately possible.

For those areas not in the Heat Watch (far northern and southern
CWA), a Heat Headline still looks probable, but more strongly favors
a smaller time period or lesser magnitude than the other areas.


.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1140 PM CDT Sun Jul 16 2017

Isolated storms to diminish by 06z over STL metro area, so kept
tafs dry. Otherwise, mvfr cigs to develop and sink south on
backside of weak cold front. MVFR cigs to lift and scatter out
from north to south between 12z and 15z Monday. As for winds, to
remain light and variable.

Isolated storms to diminish by 06z over STL metro area, so kept
taf dry. Otherwise, mvfr cigs to develop and sink south on
backside of weak cold front by 10z. MVFR cigs to lift and scatter
out by 15z. As for winds, to remain light and variable.


MO...Excessive Heat Watch from Tuesday afternoon through Saturday
     evening for Audrain MO-Boone MO-Callaway MO-Cole MO-
     Franklin MO-Gasconade MO-Jefferson 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...Excessive Heat Watch from Tuesday afternoon through Saturday
     evening for Madison IL-Monroe IL-Saint Clair IL.

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Bond IL-Calhoun IL-Clinton IL-Fayette IL-Greene
     IL-Jersey IL-Macoupin IL-Marion IL-Montgomery IL-Pike IL-
     Washington IL.



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