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fxus63 klsx 270829 
afdlsx

Area forecast discussion
National Weather Service Saint Louis MO
329 am CDT Sat may 27 2017

Short term... (through late tonight)
issued at 327 am CDT Sat may 27 2017

Primary concern for the short term is potential for severe weather
this afternoon and evening. Atmosphere will be primed by mid to
late afternoon with 3500-4000+ j/kg of MLCAPE generally along and
south of I-70 according to the rap and GFS with somewhat lower
values further north. These very high cape values are paired
with with 40+kts of 0-6km shear. Forecast soundings are showing a
deep elevated mixed layer with 7-9c/km lapse rates from 700m to
500- 450mb which is helping to produce these very high cape
values. These values are well into the range to produce some
significant severe weather Saturday afternoon and evening.
Hodographs show some decent directional shear in the lowest
kilometer of the atmosphere, but quickly straighten out with speed
shear dominating from 2-6km. This suggests that the initial storm
Mode may be discrete cells or clusters, but it should grow
upscale into a qlcs. All convection allowing models (cams) are
showing this scenario now, with the only real differences between
them being timing and exact position of the qlcs. For the pop
forecast, I stuck pretty close to the 3km NAM which as been pretty
consistent with the development, Mode, and timing of the system
as it moves across our area. Expect the leading edges to be
approaching our central Missouri counties between 21-23z, with the
system approaching the Mississippi River in east central Missouri
around between 01-02z...and through the eastern portion of the
County warning forecast area by around 05z. With the severe weather parameters on the high
end of the scale, damaging wind in excess of of 80 mph, Golf Ball
size hail or larger, and a few tornadoes are possible. As
mentioned before...the primary threat area will be along and south
of I-70.

Carney

Long term... (sunday through friday)
issued at 327 am CDT Sat may 27 2017

While the primary mesoscale convective system will have moved well to the southeast of the
County Warning Area by 12z on Sunday morning, the GFS/NAM are still showing a
surface low over northern Illinois with the main cold front trailing
southwest through east central and south central Missouri. This
front will move slowly east during the morning hours and should
clear the County Warning Area around midday. A mid-level trough that will start the
day over the Central Plains will move east through the day and will
provide at least some some ascent through mid afternoon across
southeast Missouri and south central Illinois. Will continue to go
with a chance of showers and thunderstorms in the morning over the
southern and eastern parts of the County Warning Area when the instability will be
the highest ahead of the main cold front, and back down to a slight
chance in the afternoon. Will also continue mention the potential
for a few strong to severe thunderstorms on Sunday in the marginal
risk area during the day on Sunday.

Then it still looks like a mainly dry forecast Sunday night into
Monday night as the cold front will move well south of the area and
most the Great Lakes upper low pivots to the north. By Tuesday and
Wednesday, the front will move back north into southern Missouri and
Illinois and showers and thunderstorms will develop along it as the
GFS/European model (ecmwf) show varying timing of shortwave troughs moving along the
front. The chance of showers and thunderstorms will continue into
late in the week as both the GFS/European model (ecmwf) continue to show the front
over Missouri and southern Illinois.

Highs most of next week will be in the mid 70s to around 80 degrees
given that 850mb temperatures will be around 10-12c. Sref mean
temperatures reflect this thinking.

Britt

&&

Aviation... (for the 06z tafs through 06z Saturday night)
issued at 1207 am CDT Sat may 27 2017

A weak cold front located from central MO through south central Illinois
continues to ooze southward, and will stall overnight across
southern MO and southern Illinois. The guidance is rather insistent
that during the overnight hours a combination of stratus and fog
will develop, however vary greatly in the extent and how low the
cigs or vsbys will lower. At this time I have opted for primarily
MVFR flight conditions but the potential is certainly there for
IFR as well. Stratus and fog should dissipate to a rising diurnal
cumulus deck by mid morning. Present thinking is the main threat
of showers and thunderstorms will be during the late afternoon
and evening, as a large thunderstorm complex takes shape across
west central MO and races east across east central and southeast
MO with an accompanying threat of severe weather and strong
damaging winds. I have a tempo group highlighting the time frame
that thunderstorms are most likely, and conditions may be much
worse for a brief period than indicated.

Specifics for kstl:

A weak cold front located from central MO through south central Illinois
continues to ooze southward and will pass through kstl shortly
then stall overnight across southern MO and southern Illinois. The
guidance is rather insistent that during the overnight hours a
combination of stratus and fog will develop, however vary greatly
in the extent and how low the cigs or vsbys will lower. At this
time I have opted for primarily MVFR flight conditions. Stratus
and fog should dissipate to a rising diurnal cumulus deck by mid
morning. Present thinking is the main threat of showers and
thunderstorms will be during the evening, as a large thunderstorm
complex takes shape across west central MO and races east across
east central and southeast MO with an accompanying threat of
severe weather and strong damaging winds. I have a tempo group
highlighting the time frame that thunderstorms are most likely,
and conditions may be much worse for a brief period than
indicated.

Glass

&&

Lsx watches/warnings/advisories...
MO...none.
Illinois...none.

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