Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 202301
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
601 PM CDT Wed Jun 20 2018

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 230 PM CDT Wed Jun 20 2018

Atypical June setup with upper low currently over nern NEB.
Thunderstorms have developed ahead of a cdfnt across north central
MO stretching swwd to SW MO. This front will move east slightly
this evening before stalling until a reinforcing cdfnt pushes into
the area. As the nearly stacked low approaches, the cdfnt will
slowly push thru the area. The cdfnt is expected to be roughly
from KIJX to KSTL to KFAM around 18z Thurs and continue to push
thru the CWA during the afternoon.

Expect the broken line of TSRA to continue to slowly move east
into the area, as well as area of SHRA further SE. These storms
are expected to diminish late this evening, before redeveloping
and moving out of the area during the pre-dawn hours.

Thunderstorm development on Thurs will largely depend on the
location of the cdfnt. Believe the best coverage during the late
morning and into early afternoon will be across ern portions of
the CWA, ahead of the cdfnt. Another area of showers with isod to
sct thunder is expected with the low as it approaches the area.
With the setup expected, believe there will be a threat of low-
topped supercells on Thurs afternoon. Looks like two areas
possible, one across SW IL, but another area will be possible
across nrn MO into west central IL along the occluded front. Mdls
are suggesting to around 1000 J/kg of MUCAPE associated with the
occluded fnt. This setup will need to be monitored for future
updates as the system evolves and details of frontal location
become more confident.

As for temps, have trended twd the warmer guidance ahead of the
expected front location, with much cooler temps behind given the
expected precip and cloud cover.

Tilly

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 346 PM CDT Wed Jun 20 2018

The threat for showers and storms will continue Thursday night and
Friday as the upper level low and associated surface low move east-
southeastward through northeast MO and central and southern IL.  The
surface low will drag a cold front southeastward through the
forecast area Thursday night.  Cooler and less humid air will filter
southeastward into our forecast area behind the low and trailing
cold front Thursday night.  The best chance of rain and best
coverage of thunderstorms should be across southwest IL Thursday
night and Friday where better instability will exist. Temperatures
will be below normal on Friday due to the low level cold air
advection, low mid-upper level heights underneath and behind the
upper level low, plus low level cloud cover which will inhibit
diurnal solar insolation.  The precipitation should shift east of
the forecast area by Friday evening with the coolest temperatures
expected late Friday night and early Saturday morning along with
relatively low surface dew points and much less humid conditions.
Convection may move back into parts of our area already on Saturday
as an upper level trough moves through the region and a weak surface
low develops ahead of the upper level trough and moves eastward
through southern portions of MO and IL.  The operational GFS model
has the strongest solution and highest QPF Saturday afternoon and
night.  The ECMWF model has most of its QPF south of our forecast
area during this time period.  A surface ridge will build southward
through the Great Lakes region Sunday night and may bring slightly
cooler and less air into northeast MO and west central IL.
Convection may impact northeast MO and west central IL Monday night
through Wednesday as an upper level trough moves eastward through
the northern Plains and Great Lakes region.  The GFS model is a
little further south with its QPF versus the ECMWF which has most of
the precipitation just north of our forecast area.  A gradual
warming trend can be expected due to persistent south-southwesterly
surface winds with highs on Wednesday back above normal.

GKS

&&

.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 545 PM CDT Wed Jun 20 2018

A line of thunderstorms, ahead of an approaching cold front,
extending from UIN southwest to COU will continue to shift
eastward and impact the St Louis metro area early this evening.
This line should shift east of the taf sites by 03Z late this
evening as it gradually weakens as the atmosphere slowly
stabilizes. The model guidance was hinting at the possibility of
MVFR cloud ceilings Thursday morning which should rise into the
VFR catagory by Thursday afternoon. Another batch of showers and
storms may move through the taf sites Thursday afternoon along a
cold front and ahead of a slow moving upper level disturbance.
Southwesterly surface winds tonight will become more west-
southwesterly on Thursday.

SPECIFICS FOR KSTL: A line of thunderstorms, ahead of an
approaching cold front, extending from UIN southwest to COU will
continue to shift eastward and impact the STL area early this
evening. This line should shift east of STL by 03Z late this
evening as it gradually weakens as the atmosphere slowly
stabilizes. The model guidance was hinting at the possibility of
MVFR cloud ceilings early Thursday morning which should rise into
the VFR catagory by Thursday afternoon. Another batch of showers
and storms may move through STL Thursday afternoon along a cold
front and ahead of a slow moving upper level disturbance.
Southwesterly surface winds tonight will become more west-
southwesterly on Thursday.

GKS

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     72  85  65  78 /  70  60  50  60
Quincy          67  77  63  75 / 100  60  40  50
Columbia        67  77  61  75 /  80  60  40  40
Jefferson City  68  78  62  76 /  90  60  40  40
Salem           69  84  64  78 /  60  70  60  60
Farmington      68  83  62  77 /  50  60  40  50

&&

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

$$

WFO LSX



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