Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200831

Area Forecast Discussion
National Weather Service Saint Louis MO
331 AM CDT FRI MAY 20 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 308 AM CDT Fri May 20 2016

An upper low was located over northeast KS early this morning with
the attendant upper trof extending southward into the southern
Plains. Several vort maxes were rotating northeast through the
trof and contributing to a sizeable area of rain/showers stretching
across southeast and south central MO into central MO. The vast
majority of the guidance, with emphasis on the CAMS, have this
precipitation lifting to the north/northeast this morning
gradually diminishing in areal coverage as the upper low/trof
lifts east/northeast. The greatest coverage this afternoon, which
should be scattered in nature, is expected to arc from northeast
MO into south central IL, with a more isolated shower threat
elsewhere. Present indications are the air mass will remain too
stable to support any thunder. Extensive clouds will limit heating
with highs in the 60s, still below average.


.LONG TERM...  (Tonight through Thursday)
Issued at 308 AM CDT Fri May 20 2016

The upper low will continue to move to the east northeast tonight
and actually will gradually deepen as the large scale pattern
amplifies. The upper trof axis will be located over the far
eastern CWA early this evening supporting at least a slight chance
for showers in SC IL. Otherwise surface high pressure will nose
back into the area tonight. There will be potential for fog
tonight into early Saturday morning across parts of eastern and
central MO given the extensive clouds today, late decrease in
clouds this afternoon/evening and light winds. The SREF
probabilities hit this region rather hard.

The large scale amplification will result in a prominent omega
block over the CONUS and the eastern lobe/trof will maintain
surface high pressure Saturday into Saturday Night. Slow progression
of the block will allow the upper ridge to shift from the Plains
into the MS River Valley on Sunday, and slow eastward retreat of
the surface high pressure system. This will result in continued
moderation of temperatures that will begin on Saturday.

By early next week a positively tilted long-wave trof evolves over
the western U.S. and the upper ridge weakens and shifts east into
the TN/OH Valley region. On Monday/Monday Night this will lead to
continued retreat of the surface high, improving low level WAA and
moisture return, and a chance of showers and thunderstorms. By
Tuesday a broad and unstable to very unstable warm sector will be
in place. Weak impulses within the southwest flow aloft and a
southwesterly LLJ will result in continued thunderstorm chances
through midweek and seasonably warm temperatures.



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1136 PM CDT Thu May 19 2016

Scattered showers are now moving into central MO which will affect
KCOU. These showers will spread eastward the rest of the night
into tomorrow morning affecting KUIN and the St. Louis area
terminals between 09-13Z. The showers will last into Friday
afternoon before exiting the area to the east. While conditions
are expected to be mainly VFR in the showers, some MVFR ceilings
and visibilities will be possible. Winds will remain out of the
east at less than 10kts.

Specifics for KSTL: Scattered light showers currently over south
central MO will move into the terminal after 09Z. While conditions
are expected to be mainly VFR in the showers, some MVFR ceilings
and visibilities will be possible. Winds will remain out of the
east at less than 10kts. The rain will move out of the area by 00Z
Friday evening.





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