Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 262351
AFDLSX

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

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 342 PM CDT Fri May 26 2017

...Severe Weather Likely Saturday...

Convection this afternoon was moving across northern IL, northeast
of our forecast area and ahead of a vorticty maximum or mesoscale
convective vortex.  A cold front across northwestern MO will move
southeastward late this afternoon and tonight, dropping to just
south of the I-70 corridor by 12Z Saturday.  Although the
surface/boundary layer moisture has been increasing, the atmosphere
has been capped to convective initiation in our forecast area so far
this afternoon. Would not be surprised to see a few showers or
storms break the cap late this afternoon and tonight along and ahead
of the cold front in the moist and unstable atmosphere.  For now
will include slight chance pops mainly across east central and
southeast MO and southwest IL for tonight.  It will be warm and a
little muggy tonight, especially south of the cold front.  There may
also be patchy fog late tonight and early Saturday morning due to
the light surface wind and high surface dew points near the front.
Saturday morning there may be isolated convection across southeast
MO and southwest IL along and south of the front.  A much better
chance for convection will occur Saturday afternoon as the front
tries to lift back northward as a warm front as an upper level
trough moves eastward into the northern Plains. There is still
uncertainty as to the exact location, timing and convective mode
Saturday afternoon, but some of the models were depicting an MCS
dropping southeastward into northeast and central MO, north and west
of STL Saturday afternoon. With a very unstable atmosphere along and
south of the front as revealed by very high CAPEs, severe convection
is expected.

GKS

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 342 PM CDT Fri May 26 2017

The synoptic pattern at the start of the long term period will
feature a trough/trough axis swinging through portions of the
Midwest and Great Lakes region.  The passage of this trough will set
the stage for a northwest flow regime to persist through the
remainder of the long term period.

Severe weather appears likely Saturday evening.  While some
supercells are possible Saturday afternoon or early evening, it
appears the preferred storm mode will be a liner, forward-
propagating MCS given deep-layer shear vectors oriented parallel to
the initiating boundary.  This would make widespread damaging,
straight-line winds the primary threat, but isolated tornadoes and
large hail will also be possible.  The tornado threat will be
highest near the surface boundary which will likely be located from
west to east across central MO.  Here, locally backed winds will
enhance SRH values ahead of the MCS. Thunderstorms should taper off
from west to east through the late evening and overnight hours
Saturday night. A cold front will settle into southeast MO and
western IL by early Sunday afternoon. Guidance differs on position
of the front and how much instability will build along it, but
current thinking is that any additional severe threat along the
front will likely be just east/southeast of the LSX CWA. Sunday will
be cooler, with highs in the mid to upper 70s.

Another front will drop southeast on Monday.  While we cannot
completely rule out an isolated shower/storm along the front, think
it will come through mostly dry thus will continue with no
mentionable precip in the forecast.  Guidance then differs with
quick-hitting systems through the middle of next week, both in
timing in placement.  Therefore, have continued with an ensemble
approach to the forecast which puts slight chance to chance of
showers/storms in the forecast Wednesday through Friday of next
week.  Temperatures will be near normal through much of next week,
with highs in the mid/upper 70s to low 80s expected.

KD

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 639 PM CDT Fri May 26 2017

Weak cold front currently located through central IL across
northeast MO will continue to move southward and stall overnight
across southern MO and southern IL. VFR cigs will prevail ahead of
the front this evening along with a spot shower or thunderstorm.
Probilities are too low for -SHRA/-TSRA to mention any threat in
the TAFS. The guidance is rather insistent that during the
overnight hours a combination of stratus and fog will develop,
however vary greatly in the extent and how low the CIGs or VSBYs
will lower. At this time I have opted for primarily MVFR flight
conditions but the potential is certainly there for IFR as well.
Stratus and fog should dissipate to a rising diurnal cumulus deck
by mid morning. Present thinking is the main threat of showers and
thunderstorms will be during the later afternoon and evening, as
a large thunderstorm complex takes shape across west central MO
and races east with an accompanying threat of severe weather and
strong damaging winds. There is some uncertainty in timing and
exact location so the forecast is conservative at this time.

SPECIFICS FOR KSTL:

A weak cold front currently located through central IL across
northeast MO will continue to move southward and move through KSTL
later this evening, then stall overnight across southern MO and
southern IL. VFR cigs will prevail ahead of the front this evening
along with a spot shower or thunderstorm. Probilities are too low
for -SHRA/-TSRA to mention any threat in the TAF. The guidance is
rather insistent that during the overnight hours a combination of
stratus and fog will develop, however vary greatly in the extent
and how low the CIGs or VSBYs will lower. At this time I have
opted for primarily MVFR flight conditions but the potential is
certainly there for IFR as well. Stratus and fog should dissipate
to a rising diurnal cumulus deck by mid morning. Present thinking
is the main threat of showers and thunderstorms will be during the
evening at KSTL, as a large thunderstorm complex takes shape
across west central MO and races east with an accompanying threat
of severe weather and strong damaging winds. There is some
uncertainty in timing and exact location so the forecast is
conservative at this time.

Glass

&&

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

$$

WFO LSX



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