Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 280456

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1156 PM CDT Tue Jun 27 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 320 PM CDT Tue Jun 27 2017

Warmer temperatures are expected tonight and Wednesday as a surface
ridge centered over IL and IN shifts eastward and a south-southeast
surface wind increases due to a tightening surface pressure gradient
over our area.  Surface dew points will also be gradually rising
leading to more humid conditions.  Although warmer, lows tonight
will continue to be below normal for late June, especially across
southwest IL. Highs on Wednesday will be close to seasonal normals.
Nocturnal convection is expected across IA into northwest MO tonight
associated with a strong southwesterly low level jet ahead of an
approaching shortwave.  It appears that any showers/storms should
remain north and west of our forecast area tonight. Weakening
overnight convection may move into parts of northeast MO Wednesday
morning, with additional development possible across northeast MO
and west central IL along any outflow boundaries from late
night/early morning convection as the atmosphere gets unstable in
this area during the afternoon.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 320 PM CDT Tue Jun 27 2017

Surface ridge continues to move east and will to be centered over
the southeast US by Wednesday night. A frontal boundary will be
located from southeast Nebraska down to northeast Kansas into the
Southern Plains. Ahead of the cold front,warm and moist air will
continue to stream northward ahead of the system. A look at model
soundings throughout the area show PW values of 1.5 to 2 inches in
the area during the Thursday through Saturday morning time frame.
Aloft the flow will be quite weak with a series of shortwaves moving
across the area through early next week. The combination of light
flow aloft, a slow moving front, and ample instability over the area
will allow for potential training of storms over the same areas.

There is potential for locally heavy rainfall amounts of over 4
inches in the northern tier counties with less amounts possible
further south. Wednesday night forecast CAPE values are in range of
2500 to 3000J/KG with mid lapse rates around 7C/KM range. Thursday
and Friday both have forecast CAPE values in the range of 1500 to
2500J/kg with lapse rates in the range of 7 to 8 C/km. The bulk
shear also steadily increases in the Wednesday night through Friday
period from as little as 35 knots Wednesday night to near 60 knots
Friday. A limiting factor for Friday would be extent of cloudiness
over the area from previous convection. The primary threats for
Wednesday and Thursday will be damaging winds and large hail.

The front is forecast to move just south of the forecast area by
Saturday morning, then move up to the I-70 corridor by Sunday
morning. Areas south of I-70 could see an isolated shower or
thunderstorm while area north of I-70 will likely be dry. Sunday
should be dry for most areas until Sunday afternoon and evening as
the warm front moves north. Monday and Tuesday will have atleast a
chance for scattered showers and thunderstorms with the warm front
close to the Missouri and Iowa state line and a couple of
shortwaves coming across the region.



.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1149 PM CDT Tue Jun 27 2017

Present thinking is that showers and thunderstorms will remain
well to the north and northwest of the terminals overnight and we
should see VFR flight conditions with increasing high clouds. A
bit more uncertainty on Wednesday across northern MO and west
central IL, and the potential for any residual boundaries to be
the focus for new shower and thunderstorm development. Confidence
is low for any showers and storms during the day, and think gusty
southerly winds and VFR flight conditions consisting of diurnal
instability cu and high clouds will be the rule. Instead my
thoughts are that new storms will develop in IA during the late
afternoon and evening and settle southeast Wednesday evening,
potentially impacting KUIN.


Still thinking that KSTL should experience VFR flight conditions
tonight through Wednesday evening. We will see increasing high
high clouds tonight and then an increase in diurnal cu and
convective debris clouds (mid/high clouds) on Wednesday. Gusty
southerly winds are expected from mid-late Wednesday morning into
the afternoon.





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