Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 260452

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1152 PM CDT Thu May 25 2017

Issued at 624 PM CDT Thu May 25 2017

I have made a few changes to the overnight forecast into Friday
morning. The guidance is in good agreement with the surface ridge
continuing to depart east tonight and warm frontogenesis from
southeast KS across southern MO overnight. A short wave trof
currently moving into the high plains will migrate eastward
tonight and is forecast to be located across eastern MO by
Friday morning. In advance of this short wave, a southwesterly LLJ
is forecast to ramp-up overnight with intensifying low level WAA
and moisture transport. The combination of these advective
processes and quite steep mid level lapse rates will result an
increasingly unstable air mass aloft with elevated CAPE/MUCAPE of
at least 500-1200+ J/KG by daybreak. The presence of the migrating
short wave and lift/mcon attendant with the veering southwesterly
LLJ suggests potential for elevated showers and thunderstorms. I
have introduced pops from west to east in the CWA from 09-15z.
Several of the CAMS support this scenario and especially the NCAR
MPAS which shows a fairly impressive convective cluster moving
into this area from KS as the LLJ veers. While confidence is not
great, the synoptic pattern at the very least supports some low
pops. I will continue to monitor through the evening and could
increase pops as newer data becomes available.



.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 317 PM CDT Thu May 25 2017

A warming trend will continue tonight and Friday along with slowly
rising humidity as the surface ridge over the forecast area shifts
eastward allowing the surface wind to become southeasterly tonight,
then south-southwesterly on Friday.  While the NAM model keeps us
dry tonight and Friday, the operational GFS and ECMWF models do
generate some convection across parts of our area.  It appears that
the best threat for convection will be across northeast MO and west
central IL with a more capped atmosphere across southern portions of
MO and IL.  For now will just include slight chance pops across
northeast MO and west central IL late tonight/early Friday morning
as the upper level ridge shifts east of the region and a shortwave
moves eastward through southern IA.  Could not rule out isolated
showers/storms across central into southeast MO in a region of low
level moisture convergence on the nose of a southwesterly low level
jet over southwestern MO, but for now will keep it dry in this area
late tonight.  Convection may also develop across northeast MO and
west central IL Friday afternoon ahead of an approaching cold front
dropping southeastward through northwestern MO.  If this convection
does develop it may be severe owing to steep lapse rates and strong
instability.  Although highs Friday will only be about 5 degrees
above normal for late May it will feel noticeably warmer compared to
the past couple of days, plus dew points will rise into the 60s
making it feel more humid.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 317 PM CDT Thu May 25 2017

...Severe Storms Likely Saturday...

The synoptic pattern at the start of the long term period will
feature a fairly low amplitude trough over the southwest CONUS, with
an upstream ridge across the Southeast states into the Ohio Valley.
This will place the Midwest within an active, southwesterly flow
regime through the first part of the weekend, before northwest flow
takes over late this weekend into next week.

The main focus of the forecast continues to center around severe
potential Saturday afternoon into Saturday evening.  While a few
storms may be possible Saturday morning mainly across southeast MO
in response to fairly weak isentropic ascent, the thinking is much
of the region will start the day off dry.  This dry start will allow
for impressive thermodynamics to build by the early afternoon as
MLCAPEs climb in excess of 3500-4500+ J/kg. A quick-moving
shortwave within the southwesterly flow aloft along with the exit
region of an 80-knot upper-level jet will contribute to increasing
upper-level forcing for ascent atop a nearly stationary front.
Cooling mid-level temperatures in association with the approaching
wave coupled with increasing surface convergence along the front
will lead to explosive thunderstorm development across
western/central MO Saturday afternoon. Given plenty of deep-layer
shear (45-55 knots of 0-6km bulk shear), these initial storms will
likely be supercellular with all modes of severe weather
possible. However, the general signal in guidance is that this
activity may quickly grow upscale into a large forward-propagating
MCS (which makes sense given the parallel orientation of the
deep-layer shear vectors to the boundary), capable of damaging
winds, locally large hail, and isolated tornadoes. The tornado
threat will be maximized along the surface boundary, which at this
time looks to be draped from west to east across central MO into
west-central IL. Given this is a holiday weekend with many people
expected to be outdoors in vulnerable areas, we urge people to
keep up to date with the latest forecasts/warnings and have ways
to receive warnings even if outside.

The expected MCS will push to the east by late Saturday evening,
with many areas becoming dry by Sunday morning.  Another front will
drop down on Monday bringing a slim chance of a few showers/storms
for Memorial Day.  Despite a few minor chances of storms midweek, it
appears most locations will stay dry through Wednesday night, before
better chances of showers/storms return by the end of next week.
Temperatures next week will be near average, with highs in the 70s



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1142 PM CDT Thu May 25 2017

The main question overnight and into mid morning on Friday
continues to be if we will see showers or storms. The LLJ is
supporting an area of showers and storms in central KS at this
time. Some of this activity in some form or another could make it
into the area after 09z as the LLJ veers to more southwesterly.
There is also potential for some scattered showers or storms to
develop across central MO as well. At the least there will be an
increase in VFR clouds of 5,000 to 8,000 ft. Confidence is not
high enough at this point to include any precipitation in the
TAFS but there seems to be a window of potential from 09-15Z.
Other scattered thunderstorms could develop along a cold front
encroaching upon northeast/central MO and west central IL late
Friday afternoon. Again confidence is too low to mention at this
time in the forecast. Outside of any precipitation, VFR flight
conditions will prevail.


Still need to watch a window near 10-15Z for potential showers
and thunderstorms. Confidence is not high enough to include in the
TAF at this time. Outside of any precipitation, VFR flight
conditions will prevail. Late Friday night into early Saturday
morning we could see MVFR flight conditions prevail due to





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