Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 260230

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
930 PM CDT MON JUL 25 2016

Issued at 924 PM CDT Mon Jul 25 2016

Isolated-scattered showers and thunderstorms have been persistent
across portions of the southeastern 1/3 of the CWA. Expect a
gradual weakening of this activity through the evening hours but
kept at least schc PoPs through the night as secondary boundary
lingers across the area.



.SHORT TERM...  (Through Late Tonight)
Issued at 225 PM CDT Mon Jul 25 2016

The threat of additional showers and thunderstorms is the main
forecast issue tonight. The primary cold front appears to be
located across southern IL and southeast MO early this afternoon
with the majority of convection occurring along/ahead of it. There
however is a secondary wind shift and more defined dew point
gradient across northern MO and northern IL. This boundary is
forecast to sag southward tonight as weak high pressure continues
to settle into the mid-upper MS Valley. Aloft there are at least 3
impules upstream moving slowly east-southeast. While there is
nothing jumping out of the data, there appears to be at least a
threat of showers and thunderstorms generally across the southeast
half of the CWA due to these factors.


.LONG TERM...  (Tuesday through Next Monday)
Issued at 211 PM CDT Mon Jul 25 2016

Through the end of the week, upper level high pressure will
retrograde slightly and remained anchored in the western U.S. with
broad troffing aloft continuing and modulating in intensity across
the northeast quarter of the CONUS. Temperatures will generally be
near or slight below average. From Tuesday into Wednesday the entire
atmosphere across the region is dominated by weak flow, especially
in the lowest 2 km. The lack of a well-developed LLJ and the
parade of weak upper level impulses within the weak west-southwest
flow aloft makes for a challenging forecast for showers and
thunderstorms. Convergence is weak and subtle and model QPFs are
quiet varied. It appears the effect front will be well south
however there are hints of a subtle convergence area extending
ENE-WSW from the OH Valley to near or just south of St. Louis into
southwest MO, possibly the aforementioned secondary front. Thus
the southeast half of the CWA and especially southeast MO and
southwest IL appear to have the greatest pcpn threat.

Wednesday afternoon into Thursday night appears to be the period
with the most identifiable forcing and thus highest pops. The front
will first lift northward late Wed/Thurs and then an advancing
cold front and surface low will traverse the area Thurs night
into Friday.

A wavering frontal boundary will then be drapped across the area
late Friday into late Sunday as upper troffing persists across
the northeast quarter of the Nation. Changes in the upper air
regime are forecast early next week as there is progression in the
flow aloft, especially in the higher latitudes from the Pacific NW
into south-central Canada. This will allow the western U.S. upper
high/ridge to shift east and build into the central U.S. by
Monday, and the front to lift northward opening the doors to a
return of summer heat and humidity.



.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 623 PM CDT Mon Jul 25 2016

Main frontal boundary is to the south of the terminals where most
of the shower and thunderstorm activity resides. However...there
remains a secondary boundary near the southeastern portion of the
St. Louis metro area which will have to be watched through early
evening. Main concern for tonight will be on fog and low stratus.
Believe MVFR fog and stratus likely at the metro
terminals...particularly at KSUS and KCPS. IFR visbys and/or
ceilings also possible at these locations. Ceilings and visbys
should improve on Tuesday with a slightly higher theat of showers
and thunderstorms. Best bet for these still looks to remain south
of the terminals however so no mention in the TAFs for now.

Specifics for KSTL:

Main concern for tonight will be low stratus and potentially fog.
Maintained the TEMPO group for MVFR fog but also added some
stratus as well. Conditions should improve by mid Tuesday morning.
Chances of showers and thunderstorms are possible throughout the
period...but too low to mention in the TAF.





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