Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 160912

412 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.


.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

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.



.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1103 PM CDT Fri May 15 2015

Isolated ongoing showers/storms between COU and UIN may move
through UIN from 05-06Z tonight. An upper level disturbance will
bring more showers and storms to southeast MO and southwest IL
late tonight and Saturday morning. Most of this activity should be
s-se of the St Louis metro area, but will include VCSH for the St
Louis metro area late tonight/early Saturday morning. Scattered
diurnal convection is expected again for Saturday afternoon as the
atmosphere destabilizes, so will include VCTS at the taf sites
Saturday afternoon into the early evening. Prevailing VFR
conditions can be expected at the taf sites outside of the
showers/storms. The NAM MOS cloud guidance appears too low on its
ceiling height forecasts. S-sely surface winds will continue
through the period.

Specifics for KSTL: Another batch of showers and storms is
expected across southeast MO and southwest IL late tonight and
Saturday morning. It appears that most of this activity will
remain just south/southeast of STL. Typical scattered diurnal
convection is expected for Saturday afternoon into the evening so
will include VCTS for the STL taf during this time period. S-sely
surface winds will continue through the forecast period.



Saint Louis     81  69  84  70 /  60  30  50  30
Quincy          80  67  83  63 /  50  40  50  20
Columbia        79  67  85  63 /  50  60  50  20
Jefferson City  80  68  85  64 /  50  60  50  20
Salem           79  69  82  68 /  70  30  50  40
Farmington      77  66  80  65 /  70  30  50  40




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