Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272021
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
321 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.

GKS




.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.



Kelly




&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1135 AM CDT Tue Jun 27 2017

Just few-scattered diurnal cumulus clouds this afternoon which
will dissipate early this evening due to the loss of daytime
heating. There will be some mid-high level cloudiness late tonight
and Wednesday morning, especially at UIN, but any convection
should remain north and west of the taf sites. Southeasterly
surface winds will strengthen through the period as the surface
ridge centered over IL and IN shifts eastward and the surface
pressure gradient tightens over our area with the surface wind
becoming south-southeasterly and gusty by late Wednesday morning.

SPECIFICS FOR KSTL: Just few-scattered diurnal cumulus clouds this
afternoon which will dissipate early this evening due to the loss
of daytime heating. There will be some high level cloudiness late
tonight and Wednesday, but any convection will remain well north
and west of the STL area. Southeasterly surface winds will
strengthen through the period as the surface ridge centered over
IL and IN shifts eastward and the surface pressure gradient
tightens over our area with the surface wind becoming southerly
and gusty by Wednesday afternoon.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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