Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270000

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
700 PM CDT Wed Jul 26 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 323 PM CDT Wed Jul 26 2017

Primary concern will transition this evening from heat to localized
heavy rainfall. Will be trimming far northwestern portion of heat
advisory off as thicker midlevel clouds have kept temperatures in
the low 90s preventing heat index values from approaching 105F.
Rest of advisory still looks good and will continue until 8PM.

Still lots of uncertainty with respect to convective trends for
overnight tonight. Retreating warm front is currently across the
Mississippi River in Illinois and continues to push northeastward.
Believe heaviest axis of rainfall may set up roughly along that
boundary which is forecast to also be roughly collocated with
strong upper-level forcing for ascent via vorticity maxima and
low-level moisture convergence. This axis however will likely stay
east/northeast of the CWA. Showers and thunderstorms will likely
fill in behind that area along trailing cold front. Due to copious
amounts of cloud cover and more stable air across the lower-mid
Missouri Valley, convection will likely not be as rigorous as
thought 24 hours ago. Not seeing too much of a severe threat as
freezing levels are very high with only modest midlevel lapse
rates. Cannot rule out a wet microburst however in a stronger

Main concern with convection tonight will continue to be on
localized heavy rainfall. There are quite a few mitigating factors
for widespread flash flooding tonight including dry antecedent
conditions (and low soil moisture percentiles), more stable air
upstream than forecast 12-24 hours ago mentioned above, and very
high flash flood guidance (2-3" in one hour). That being said,
will be leaving the flash flood watch alone for now as still
expecting a round of showers and thunderstorms within the watch
area in a very supportive environment (deep warm cloud depths and
very high PWAT values) for efficient precipitation production.
Training would need to occur though to produce problems in local

Scattered-numerous storms will continue along and ahead of the cold
front through the overnight hours into early Thursday morning
bringing for most areas some very welcome rainfall. Believe there
will be a lull in activity during the late morning hours before
mostly scattered storms fire along the cold front in the early
afternoon. Highest PoPs for Thursday afternoon look to be across
portions of southeast Missouri and southwest Illinois where cold
front is expected to be located. Localized heavy rainfall again may
be possible if storms train across the same areas.

High temperatures tomorrow will be quite a bit cooler than that of
today due to an increase in cloud cover and convection. Highs in the
mid 80s are likely for most, which is near to slightly below normal
for late July.


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 323 PM CDT Wed Jul 26 2017

High pressure will build over the region by Friday night. Northwest
flow will dominate the pattern throughout the forecast period.
The area will see temperatures below climatological normals for
both afternoon highs and morning lows along with dry weather. High
pressure aloft will move west in response to a series of short
waves diving south out of the high plains starting Monday morning.
A weak boundary will setup in southern Iowa ahead of a cold front
that will impact the area later in the week.



.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 642 PM CDT Wed Jul 26 2017

Primary concern for tonight is thunderstorm trends. Ongoing
scattered thunderstorms south and east of St. Louis should
diminish and finally dissipate after 00-01Z. Shower activity
across central and northern Missouri will likely continue to
migrate eastward through the evening, but don`t expect much impact
other than a little light rain and possible minor visibility
reductions with VFR ceilings.

Expect thunderstorms over northeast Kansas and southeast Nebraska
to grow in coverage and intensity this evening and spread eastward
across northern Missouri into central Illinois. The storms will
drift south through the overnight hours producing pockets of MVFR
and IFR conditions in moderate to occasionally heavy rain.
Current indications are that the storms will probably get as far
south as the I-70 corridor before 12Z. Don`t have particularly
high confidence in what will happen after 12Z as this depends
highly on thunderstorm trends, but much of the model guidance is
indicating that MVFR ceilings...possibly IFR...and visibility
will move in across northern portions of the forecast area and
persist until at least early afternoon.


An outflow boundary is approaching the terminal from the southeast
and will likely turn the wind to the southeast for a few hours
this morning. Thunderstorm chances should be diminishing this
evening as we lose daytime heating, but an area of showers over
central Missouri will likely drift into the vicinity of the
terminal later this evening. Expect little impact from these
showers other than a brief period of light rain or sprinkles with
VFR conditions. Thunderstorms over the eastern Plains are expected
to continue to develop and intensify this evening and spread
across northern Missouri overnight. The storms will likely drift
southward and be in the vicinity of the terminal between 08Z and
10Z. Think a period of heavy downpours is likely during the pre-
dawn hours which will bring cigs/vsbys down to MVFR and possibly
IFR conditions. Forecast beyond 12Z is low confidence as it will
depend entirely on thunderstorm trends. Current thinking is that
VFR conditions will prevail with scattered showers in the
vicinity. Could see additional thunderstorm development in the
afternoon...but higher probabilities are south of the terminal.



MO...Heat Advisory until 8 PM CDT this evening for Callaway MO-Cole
     MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson
     MO-Lincoln MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
     Washington MO.

     Flash Flood Watch through Thursday morning for Knox MO-Lewis MO-
     Marion MO-Shelby MO.

IL...Heat Advisory until 8 PM CDT this evening for Bond IL-Calhoun IL-
     Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-Randolph IL-
     Saint Clair IL-Washington IL.

     Flash Flood Watch through Thursday morning for Adams IL-Brown IL-
     Pike IL.



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