Area Forecast Discussion
Issued by NWS St. Louis, MO

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154
FXUS63 KLSX 191201
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
701 AM CDT Sat May 19 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 406 AM CDT Sat May 19 2018

Dense fog has developed over parts of central and northeast Missouri
this morning.  Visibility at or below 1/4 mile have been reported.
Therefore have issued a dense fog advisory effective until 900 AM.

The forecast area will be sandwiched between two upper level lows
today.  The first low which brought much of the area rain yesterday
is continuing to move east-northeast into the Ohio Valley.  Some
lingering showers continue over south central Illinois early this
morning, but radar trends and latest short range guidance is
indicating these showers should be east of the area into southeast
Illinois by 11-12Z. A shortwave ridge which is currently over
western Missouri will build east to near the Mississippi River this
afternoon as the second wave moves across the central and northern
Plains.  Forecast soundings are showing a decent inversion under
this ridge today, and operational models have little or no QPF over
our forecast area today, with the exception of the NAM which
develops thunderstorms over west central Missouri during the
afternoon and moves them east down the I-70 corridor.  Cannot
totally rule out anything at this time since the large convective
complex over Nebraska will likely produce an MCV which will drift
east today.  This could be enough to break the cap and allow
thunderstorms to develop during max heating.

All models have some kind of convective complex developing over
northeast Nebraska into Iowa this evening on the leading edge of a
cold front.  The convective trends are unclear at this time as the
storms will eventually run into the aforementioned shortwave ridge
which will be drifting from eastern Missouri into Illinois this
evening.  Don`t have much confidence in the 4km NAM`s solution for
what will happen as it carries the storms currently over eastern
Nebraska all the way east across Missouri into Illinois in the face
of the shortwave ridge. Would like to see some of the other CAMs
catch up to the NAM before introducing higher PoPs
tonight...therefore will stay with chance to high chance and let
the day shift reevaluate as more updated short range guidance
rolls in.

Carney

.LONG TERM...  (Sunday through Friday)
Issued at 406 AM CDT Sat May 19 2018

Will continue to see the potential for a few strong to possibly
severe storms on Sunday and Sunday evening.  Both the GFS and NAM
are showing that a stationary front will lie across the CWA, most
likely to the north of I-70 during the day on Sunday. The
atmosphere will likely become very unstable during the day across
much of the CWA with MLCAPES between 1500-3000 J/kg and deep layer
shear between 25-40kts. Thunderstorms may develop along the
aforementioned front, or a weak cold front that will move
southeast across the area during the area late in the afternoon
and the evening. The main limiting factors with this event will be
lack of mid-level ascent and some capping seen on the forecast
soundings. The main threat from any strong storms will be large
hail and damaging winds.

The front will move south of the CWA on Sunday night into Monday
limiting shower and thunderstorm chances to southeast Missouri and
southwest Illinois.  This front will remain south of the area
through midweek keeping rain chances over southeast Missouri.  Rain
chances will increase across the CWA by late in the week as an upper
trough moves across the Midwest.

Temperatures are still expected to stay above normal through next
week as 850mb temperatures stay in the middle to upper teens Celsius
and the GEFS mean temperatures are 5-10F above normal.

Britt

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 640 AM CDT Sat May 19 2018

Areas of IFR fog and stratus extending to the Mississippi River
and further northeast into west central Illinois will prevail for
2-3 hours this morning. VFR conditions to prevail thereafter with
a chance of afternoon thunderstorms generally south of I-70.
Another round of thunderstorms is possible late this
evening/overnight...though confidence low at this time as to
timing and coverage of storms.

SPECIFICS FOR KSTL:

IFR flight conditions at Lambert are expected to improve over the
next 2 hours. Conditions are already improving at KSUS, so
Lambert shouldn`t be too far behind. VFR conditions expected for
the late morning through the evening. There is an outside chance
of a thunderstorm this afternoon...but think the best chance of
storms will stay south/southwest of the terminal today. Another
round of storms is possible overnight tonight, but timing and
coverage of these storms is very uncertain at this time.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Gasconade MO-Knox
     MO-Lewis MO-Lincoln MO-Marion MO-Moniteau MO-Monroe MO-
     Montgomery MO-Osage MO-Pike MO-Ralls MO-Shelby MO-Warren MO.

IL...Dense Fog Advisory until 9 AM CDT this morning for Adams IL-Pike
     IL.

&&

$$

WFO LSX



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