Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252019
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
319 PM CDT Thu May 25 2017

.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.

GKS

.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
expected.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1101 AM CDT Thu May 25 2017

Surface ridge axis will move eastward through the taf sites this
afternoon causing the northwest surface winds in UIN and the St
Louis metro area to weaken. Only few to scattered diurnal cumulus
clouds are expected. Could not rule out a few showers/storms late
tonight and Friday morning in the UIN and COU areas due to weak
upper level disturbances and as a southwest low level jet over
southwestern MO brings increasing low level moisture into this
area, but the probability or possible coverage is too low to
warrant including in the tafs. Should see increasing mid-high
level cloudiness late tonight, with low level cloudiness Friday
morning though still likely VFR. The wind will gradually
stengthen from a southeast direction tonight, then southerly
Friday morning as the surface ridge shifts east of the region.

SPECIFICS FOR KSTL: Surface ridge axis will move eastward through
the STL area this afternoon causing the northwest surface wind to
weaken. Only scattered diurnal cumulus clouds are expected. Could
not rule out a few showers/storms Friday afternoon due to weak
upper level disturbances and as a cold front approaches, but the
probability or possible coverage looks too low to warrant
including in the STL taf for now. Should see increasing mid-high
level cloudiness late tonight, with low level cloudiness Friday
though still likely VFR. The wind will gradually stengthen from a
southeast direction tonight, then veer around to a southwest
direction Friday afternoon as the surface ridge shifts east of the
region.

GKS

&&

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

$$

WFO LSX



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