Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271144

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
644 AM CDT Tue Jun 27 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 304 AM CDT Tue Jun 27 2017

High pressure centered over Iowa will drift southeast into the Ohio
Valley today and continue east over the central Appalachian
Mountains tonight.  Wind will turn to the southeast today and then
the south tonight as the high moves east and a surface trof develops
over the Great Plains.  There should be less cloud cover today than
there was yesterday across the majority of the area.  Strong
insolation combined with a more southerly flow should allow
temperatures to rebound into the upper 70s and low 80s from this
morning`s lows.  There may be an isolated shower or storm over the
Ozarks this afternoon...but most guidance keeps precip well outside
of our area so have left today`s forecast dry.

The aforementioned trof over the Great Plains will gather strength
overnight and southerly flow will continue to increase over the
region.  Current indications are that there will be nocturnal
convection over the Plains tonight which could push into northwest
Missouri before sunrise Wednesday.  While it looks like we should
stay dry tonight, the storms over the Plains will likely produce
plenty of high clouds which will likely expand over our area late
tonight.  These high clouds combined with the southerly flow will
limit radiation tonight resulting in warmer lows in the 60s across
the majority of the area.


.LONG TERM...  (Wednesday through Monday)
Issued at 304 AM CDT Tue Jun 27 2017

As surface ridge moves to the east on Wednesday, upper level flow
becomes zonal with series of shortwaves tracking across region.

With zonal flow aloft, surface frontal boundary that is approaching
region will stall out along Iowa/Missouri border southwestward into
southwestern KS. So best chances of rain will be along and north of
Interstate 70 Wednesday through Thursday night. So with decent
moisture, PWs between 1.5 and 2 inches, could see training across
this region. Some models are indicating higher amounts in excess of
4 inches. For now could see between 1 and 2 inches of rain during
this period with locally higher amounts possible.

Another issue to deal with is the possibility of severe storms both
Wednesday afternoon/evening and Thursday afternoon/evening. Will see
CAPES in excess of 2500 J/kg Wednesday afternoon for portions of
northeast MO/west central Illinois, so SPC has this area in a
marginal/slight risk outlook. Then on Thursday, a boundary sinks a
bit further south, low level moisture on the increase with sfc
dewpts in the upper 60s to low 70s, so MU CAPEs between 2500 and
3500 J/kg can be expected with decent lapse rates and low level jet.
So could see severe storms along and north of I-70 corridor Thursday
afternoon/evening. The main threats both days will be large hail and
damaging winds.

By Friday, frontal boundary begins to move through rest of forecast
area with activity tapering off Saturday morning as boundary exits
forecast area. As front moves through on Friday, decent instability
and convergence along boundary, so could see strong to severe storms
once again.

Will see some scattered showers/storms across far southern portions
of forecast area Saturday/Saturday night. Otherwise, dry conditions
expected. Beyond that, models have some differences on timing as
boundary lifts back north as a warm front by Monday. For now just
kept chance pops most locations through Monday.

As for temperatures, warmer, more humid weather Wednesday through
Friday, then more comfortable conditions for the weekend.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 640 AM CDT Tue Jun 27 2017

Areas of IFR fog primarily in river valleys along and south of the
Missouri River will dissipate over the next hour or two. Elsewhere
and otherwise, expect VFR flight conditions and light east-
southeast flow today. Wind will turn more to the south tonight
with VFR conditions continuing.


VFR flight conditions are expected to continue at Lambert through
Wednesday morning. Wind will turn to the east-southeast today, and
continue to veer to the south tonight.





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