Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261651
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1151 AM CDT Fri May 26 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 321 AM CDT Fri May 26 2017

Primary concern in the short term is convective trends.  The cluster
of storms over east central Kansas apparently hasn`t been looking at
the models this morning, because it shouldn`t be there.  Both the
NCEP and ESRL HRRR models have been trying to weaken the storms and
move the remnant area of showers east-northeast across northern
Missouri.  NSSL and NCEP WRF models on the other hand are doing
better moving precip east into western Missouri by 09Z and into
central Missouri between 11-12Z.  This looks like good timing based
on current radar trends.  Additionally, regardless of what the QPF
says, mass fields on the RAP, GFS, and NAM all support the continued
eastward movement and maintenance of this cluster.  Moderate to
strong 850mb moisture convergence on a 30-40kt low level jet
combined with 100-1500 J/Kg MUCAPE should should be more than enough
to sustain convection.  The jet does weaken and veer after sunrise
as the cluster moves into our area.  Would therefore expect come
weakening which the CAMs do support as well.

The low pressure currently over Oklahoma will drift northeast today
and drag a cold front into Missouri this afternoon.  Soundings are
showing CAPE in excess of 2500 J/Kg generally along and south of the
I-70 corridor with 3000 J/Kg or more along and south of I-44.  There
is a substantial cap though so it`s unclear how much of this
instability will be tapped.  Also, operational models appear
to be 3-5 degrees too high on dewpoint forecast this afternoon which
is likely affecting the CAPE forecast.  Again, CAMs aren`t
impressed, but the newest runs of the RAP are starting to hint at
convection breaking out along the front around 00Z.  Currently have
chance/slight chance going, but will have to keep a close eye on it
if the cap looks weaker than forecast.

Overnight forcing is pretty weak so do not expect any storms that do
form during the late afternoon/evening to continue into the
overnight hours.  Leaned heavily on warmest guidance for
temperatures through the period.

Carney

.LONG TERM...  (Saturday through Thursday)
Issued at 321 AM CDT Fri May 26 2017

...Severe weather likely on Saturday afternoon/evening...

It still appears that NAM/GFS will deepen and move a confluent
trough that will extend from the Dakotas into the central Rockies on
Saturday morning into the central CONUS by Sunday morning.  At the
surface at 12Z Saturday morning, a frontal boundary will be located
across southern Illinois and southern Missouri.  This front will
move slightly north during the day in response to the mid level
trough deepening and a surface low over Kansas moving eastward along
the front.  By afternoon, both the GFS/NAM are showing the front
moving about halfway north through the CWA with MLCAPES between 3000-
5000 J/kg along and south of the front.  Deep layer shear values are
also impressive with values of 50kts indicating the potential for
supercells and organized multicells. 00Z 3km NAM reflectivity
continues to depict a QLCS moving quickly through the CWA during the
late afternoon and early evening hours on Saturday which would have
the potential to produce damaging winds and a few tornadoes given
strong low level shear aligned along the boundary. Large hail will
also be a threat. Will continue to with messaging the severe weather
threat for Saturday afternoon and early evening given that many
people will likely be outdoors during the holiday weekend.

The GFS/ECMWF then shows the surface boundary remaining over the
area on Saturday night into early Sunday as another surface low
moves across Missouri and Illinois.  This will be in response to the
upper trough deepening over the central CONUS.  Chances overnight
into Sunday will scattered as models are not showing focused forcing
at this point.  Then mostly dry weather is expected through mid week
as the front moves to the south and the upper trough moves east.
There will be a chance of showers and thunderstorms returning by
Thursday along a warm front.

Temperatures tomorrow will likely climb well into the 80s ahead of
the QLCS, particularly along and south of the front.  Temperatures
next week will be mainly in the 70s to around 80 degrees given 850mb
temperatures in the 10-15C range.

Britt

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1110 AM CDT Fri May 26 2017

Scattered showers and storms in the UIN area should shift east of
UIN by 18Z this afternoon. Cold front across northwestern MO will
move southeastward through the taf sites late this afternoon and
tonight, becoming nearly stationary just south of COU and STL by
12Z Saturday, then slowly lifting back northward on Saturday.
Although the atmosphere will become unstable this afternoon it
will be capped, especially across southern portions of MO and IL.
Could not rule out isolated showers/storms this afternoon and
this evening mainly along the cold front, but for now may leave
the tafs dry as coverage should be sparse and timing is still uncertain.
Mainly just areas of VFR, low-mid level clouds this afternoon and
evening, then the MOS guidance indicates the development of
stratus and light fog late tonight as the surface wind becomes
light near the surface front with plenty of low level moisture,
high surface dew points near the boundary as well. It appears the
NAM MOS is overdone with its stratus and fog forecast, but will
forecast MVFR cigs and light fog at the taf sites late tonight and
early Saturday morning. Southwest surface wind will veer around to
a north-northwest direction after fropa.

SPECIFICS FOR KSTL: Cold front across northwestern MO will move
southeastward through the STL area by late tonight, becoming
nearly stationary just south of STL by 12Z Saturday, then slowly
lifting back northward on Saturday. Although the atmosphere will
become unstable this afternoon it will be capped, especially
across southern portions of MO and IL. Could not rule out isolated
showers/storms this afternoon and this evening mainly along the
cold front, but for now may leave the STL taf dry as coverage
should be sparse and timing is still uncertain. Mainly just areas
of VFR, low-mid level clouds this afternoon and evening, then the
MOS guidance indicates the development of stratus and light fog
late tonight as the surface wind becomes light near the surface
front with plenty of low level moisture, high surface dew points
near the boundary as well. It appears the NAM MOS is overdone with
its stratus and fog forecast, but will forecast MVFR cigs and
light fog at the STL taf site late tonight and early Saturday
morning. Southwest surface wind will veer around to a north-
northwest direction after fropa by late tonight, although it will
be light by this time. Organized convection may be approaching the
STL area by the end of the taf forecast period.

GKS

&&

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

$$

WFO LSX


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