Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 152353

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
653 PM CDT Thu Mar 15 2018

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 321 PM CDT Thu Mar 15 2018

Present thinking is that at least the first part of this evening
if not all of the evening should be void of precipitation across
the region. A weak west-east oriented cold was located pretty
close to I-70 and will continue to sag southward tonight and
should move through the southern CWA overnight. This will not be
a catalyst for any activity as the air mass across the region is
currently quite dry and hostile to surface-based development. For
the later part of the evening, we may need to watch the potential
for some mid level based WAA showers developing in a NW-SE zone
centered through northeast MO. Otherwise the big thrust of
precipitation should be well overnight. A negatively tilted short
wave trof presently extending from the Great Basin into the
southwest states will deepen and move into the Plains. Associated
with this deepening system will be the development of a LLJ to the
west of the region, which will result in moisture transport and
lift into central MO in the predawn hours. Present thinking is a
warm conveyor belt of showers and some thunder will evolve on the
very leading edge of large scale ascent within the region of
deeper moisture and warm thermal advection with the aforementioned
southwesterly LLJ.

The deeply stacked system will then progress eastward through the
Plains on Friday and into the western MS Valley by early evening.
In concert with the eastward motion, this initial warm conveyor
belt of showers will shift eastward across the region on Friday
morning becoming less organized with time as it reaches the MS
River. Large scale ascent will really ramp-up again in the
afternoon as the Plains system pushes into the MS Valley, and
current thinking is another N/S oriented swath of showers and
isolated-scattered thunderstorms will develop and move across
central and eastern MO in the afternoon. I am not overly excited
about the severe weather prospects with this system at this point
due to the overall lack of instability, but it could improve in


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 321 PM CDT Thu Mar 15 2018

The system will continue to move across the area on Friday night
with the highest precipitation potential along and east of the MS
River in the evening in association with the strongest large scale
forcing. This would include a continued chance for isolated-
scattered thunderstorms in the overall band of showers. Present
thinking is the surface low will be located across southern IL at
12Z Saturday with extensive stratus in its wake and maybe some
lingering showers during the morning in northeast MO and west
central IL. The system will then weaken and move east through the
day on Saturday into the OH Valley region.

A ridge will then build aloft into the area late Saturday into
Sunday with high pressure dominating at the surface. Dry and
tranquil should persist during this period. Another upper low/trof
will then impact the area late Sunday night into Monday with yet
another weaker wave in its wake for Monday night into Tuesday. The
first of these 2 systems will bring another good round of rain, and
some thunder to southern sections of the area. There could be
some rain/snow mix with the second system late Monday night into
Tuesday morning across northern sections of the CWA. Below average
temps are expected Monday-Thursday, especially Tuesday and beyond
as the upper air pattern ampflies and a deep trof takes hold
across the eastern CONUS.



.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 625 PM CDT Thu Mar 15 2018

A front has pushed south of the terminals this afternoon and will
remain in place thru the TAF period. Believe the evening will be
dry, tho some SHRA are possible near Midnight. Otherwise, believe
SHRA will develop/move east into the area during the pre-dawn
hours and continue east thru Fri morning. After a brief lull, a
second round shud develop as the low system approaches the region.
Cigs for the first round of precip are more uncertain, but may
drop into the MVFR range, especially after the precip has ended.
Confidence is higher for MVFR cigs with the second round of
precip. Have also added VCTS for this round of precip, expect for
UIN where chances are lower.

SPECIFICS FOR KSTL: Nely winds shud become ely overnight as the
front continues to sink south. Believe precip will hold off until
mid Fri morning, but may be as early as sunrise. As mentioned
above, held off on lower cigs for the first round of precip. Went
with MVFR cigs for the second round, as well as mention of TS,
with higher confidence.





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