Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180836
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
336 AM CDT Fri May 18 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 332 AM CDT Fri May 18 2018

Upper level cut off low centered over northeast Arkansas is forecast
to gradually fill in and get absorbed into the mean flow over the
next 24 hours.  All models print out widespread QPF today across
the forecast area as the shear axis passes over the Mid Mississippi
Valley.  However, the models look quite a bit overdone both with
areal coverage and intensity when compared to the current
observations and radar.  Have toned down PoPs a bit from what
guidance is showing, but still think widespread showers are likely
today, with diurnal strengthening into thunderstorms this afternoon.
 Should be pretty cloudy today under the low with intermittent
precip.  This should keep temperatures down in the mid to upper 70s
today, unless there are substantial breaks in the clouds.  Diurnal
precip should wind down by mid evening with only a few lingering
showers after midnight.  The upper low will be moving northeast
tonight which should bring some clearing toward morning.  May have
to watch for fog tonight, but will wait to see how much precip we
get today before adding fog to the forecast.

Carney

.LONG TERM...  (Saturday through Thursday)
Issued at 332 AM CDT Fri May 18 2018

A shortwave ridge will move across Missouri and Illinois on Saturday
as the upper trough lifts off to the east.  There will still be a
slight chance of showers and thunderstorms as a warm front moves
back north and becomes stalled over the region by late on Saturday.
While this front remains stalled over the region, MCS activity that
develops over the Plains and moves along the front into CWA Saturday
night into Sunday night or the front will act as a focus for
thunderstorms to develop along it.  A few strong or severe storms
will be possible Sunday and Sunday evening given the amount of
instability and deep layer shear, mainly along and north of I-70.
The main threats will be large hail and damaging winds.

It still looks like the upper flow will become southwesterly behind
the shortwave ridge and a series of upper level disturbances will
move over the area in the southwesterly upper flow.  The upper flow
will eventually become zonal by the middle of next week in response
to a large upper trough dropping into the western CONUS and closing
off. There is some disagreement between the global models with
whether the front will move back south of the area or if it will
remain stalled across Missouri and Illinois. Will keep a low chance
of showers and thunderstorms over much of the area into Wednesday.
The best chance at this point look like Wednesday when the most
pronounced shortwave trough will be moving through the area.

Temperatures are still expected to stay above normal over the
weekend into next week as 850mb temperatures stay in the 15-20C
range and the GEFS mean temperatures are 5-10F above normal.

Britt

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1029 PM CDT Thu May 17 2018

Convection across southern IL should eventually spread into the
St Louis metro area early Friday morning, associated with an
upper level disturbance. Will include VCTS for this time period.
At least MVFR cloud ceilings are also likely at the taf sites
Friday morning, with ceilings possibly as low as 1000 feet in the
St Louis metro area. The cloud ceiling should gradually rise
during the afternoon, with a redevelopment of convection possible
in the St Louis metro area. A persistent northeasterly surface
wind can be expected through the period with surface ridging
extending from the Great Lakes region southwest into northeastern
MO.

SPECIFICS FOR KSTL: Convection across southern IL should
eventually spread into the STL area early Friday morning,
associated with an upper level disturbance. Will include VCTS for
this time period. At least MVFR cloud ceilings are also likely at
STL Friday morning, with ceilings possibly as low as 1000 feet.
The cloud ceiling should gradually rise during the afternoon, with
a redevelopment of convection possible. This convection should
shift east of STL by early Friday evening. A persistent
northeasterly surface wind can be expected through the period with
surface ridging extending from the Great Lakes region southwest
into northeastern MO.

GKS

&&

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

$$

WFO LSX



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