Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 260444

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1144 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 06z TAFs through 06z Tuesday Night)
Issued at 1136 PM CDT Mon Jul 25 2016

While isolated showers/thunderstorms are possible for mainly KCPS
tonight...main concern continues to be on fog/stratus potential.
Did not make too many changes to previous set of TAFs. Did move up
timing of MVFR ceilings a couple of hours at KCPS and hit fog a
bit harder at KSUS as T/Td spread is almost nil already. Cannot
rule out fog and stratus at KCOU and KUIN but better chances for
the metro terminals. Best chances of showers/storms late tonight
and during the day on Tuesday continue to look to be just to the
SE of the terminals...but could be close for the metro TAFs. Winds
will remain largely light/variable tonight and early on Tuesday
before becoming light out of the northeast by Tuesday afternoon.

Specifics for KSTL:

Some MVFR ceilings and possibly visbys with fog still appear
likely later on tonight. Conditions should improve by mid morning.
Winds will remain light/variable tonight and early on Tuesday but
become more northeasterly by afternoon. Chances of showers/storms
continue to look too low attm to mention in TAF but are possible
throughout the 30-hr forecast.





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