Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271155
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
655 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
CWFA 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 MCS will have moved well to the southeast of the
CWA 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 CWA 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 CWA 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/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/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 12z TAFs through 12z Sunday Morning)
Issued at 641 AM CDT Sat May 27 2017

Areas of fog and stratus over parts of of the area should be
lifting and scattering over the next couple of hours. Attention
then turns to thunderstorm probabilities for the afternoon and
evening. The line of thunderstorms over northeast Kansas is not
being handled well by the models this morning. Becoming concerned
that this may be the start of the severe thunderstorms forecast
for today. If this is the case, storms will enter the area much
sooner than earlier expected. Will have to keep an eye on this
line of storms this morning.

For the 12Z TAFs, have stuck with the idea that a line of storms
will move into the area just before 00Z and proceed rapidly east-
southeast through the area during the evening. Wind gusts to 70
kts, and 1 3/4 to 2 inch hail looks likely as the line of storms
moves through. Additionally, there could be scattered discrete
storms or clusters of storms ahead of the line.

Confidence is low on ceiling/visibility forecast behind the line
as some guidance drops conditions into MVFR/IFR range while other
guidance stays VFR.

SPECIFICS FOR KSTL:

Looks like Lambert will avoid any fog this morning and VFR
conditions are expected to prevail into the afternoon.

The line of thunderstorms over northeast Kansas is not
being handled well by the models this morning. Becoming concerned
that this may be the start of the severe thunderstorms forecast
for today. If this is the case, Lambert could see storms as early
as 18-19Z. However, for the 12Z TAF, have stuck with the idea
that a line of storms will affect the terminal after 00Z this
evening. Wind gusts to 70 kts, and 1 3/4 to 2 inch hail will be
possible...though I think it`s too early to go that strong in the
TAF. Confidence is low on ceiling/visibility forecast behind the
line as some guidance drops conditions into MVFR/IFR range while
other guidance stays VFR.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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