Area Forecast Discussion
Issued by NWS St. Louis, MO

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747
FXUS63 KLSX 161221
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
721 AM CDT Sat May 16 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 410 AM CDT Sat May 16 2015

Somewhat tricky forecast for today regarding expected precipitation
trends.  Early on...showers and a few thunderstorms are expected
over at least the southeast half of the CWA as shortwave lifts
northeast from AR...and have used a combination of the latest HRRR
and the explicit WRF solutions for an estimate of the northern edge
of this precip shield. Primary question is what will happen in the
wake of this feature during the afternoon hours.  Both GFS and NAM
indicate much less instability than that of Friday afternoon (blend
of NAM/GFS output suggests MUCAPES 500-1500 J/kg), with no obvious
upper level feature to help fuel the convection. Because of these
two parameters, have trended PoPs to reflect more of a
scattered-type coverage during the afternoon.

Truett

.LONG TERM:  (Tonight through Friday)
Issued at 410 AM CDT Sat May 16 2015

Any diurnally-driven convection that does form this afternoon
should weaken rapidly this evening. Attention will then turn to
the severe convection that will be occurring over the Plains
during the evening, and monitoring its movement towards the CWA
during the late overnight and predawn hours. It`s interesting that
all model output...including the hi-res as well as the 00z
synoptic solutions...all suggest that the convection will weaken considerably
as it attempts to enter western sections of the CWA during the
predawn hours as instability becomes much more limited by this
time and as the expected line outruns much of the upper level
dynamics.

AMS should then try to reset for another round of convection on
Sunday afternoon. Although convergence along weakening Pacific
front and dry line will be increasingly feeble, dynamics with
shortwave rotating around the upper low during max heating/instability
should be more than enough to generate at least scattered
convection. There is a decent amount of deep layer shear progged over
the area during the afternoon (35-45 kts), so severe weather
potential will likely hinge on amount of instability that can be
attained before convection develops.

Haven`t really made too many changes to the going forecasts for
Sunday night and Monday. As upper low drifts from the Red River
into the upper Great Lakes, aforementioned cold front will slowly
work southeast into the Ohio Valley, gradually lowering PoPs from
NW to SE.  Moist air ahead of the front will become unstable once
again on Monday afternoon, and there are some hints in the 00z
runs of a weak shortwave passing over the region during this time.
Because of this I have maintained some high chance PoPs over our
far southeast counties on Monday afternoon. Forecast frontal
position was determined using ensemble of short-range guidance,
but if slower NAM is correct it may be that we are ending PoPs too
fast over our NW counties on Monday.

Medium range solutions are all forecasting some cool May weather
for the middle of next week. After a dry Tuesday, guidance is
suggesting an overruning rain-type event for Wednesday into
Thursday as surface ridge will dominate the mid and upper
Mississippi Valley and low pressure tracks from the southern
Plains into the lower Mississippi Valley.

Truett

&&

.AVIATION:  (For the 12z TAFs through 12z Sunday Morning)
Issued at 631 AM CDT Sat May 16 2015

Specifics for KCOU, KUIN: VFR outside of scattered thunderstorms
this afternoon at KUIN and KCOU. Scattered rain showers may affect
KCOU before 18z depending on how far north and west the
precipitation extends. Additional convection is possible at both
terminals late tonight, but confidence regarding timing was too
low to introduce into the TAFs attm. Winds remain southerly to
southeasterly through the period.

Specifics for KSTL, KSUS, KCPS: Expect rain showers and possibly a
thunderstorm before 16z because of an upper level disturbance
moving through the region. Scattered thunderstorms may also
redevelop this afternoon. Additional convection is possible late
tonight, but confidence regarding timing was too low to introduce
into the TAFs attm. Brief periods of MVFR cigs are possible this
morning due to increasing moisture ahead of the rain showers.
Winds remain southerly to southeasterly through the period.

Kanofsky

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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