Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181646
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1146 AM CDT Wed Apr 18 2018

.UPDATE...
Issued at 1036 AM CDT Wed Apr 18 2018

Issued a Wind Advisory for late this morning through the
afternoon and into the early evening hours for central and east
central MO and southwest IL. Am getting peak winds gusts at COU
and JEF around 45 mph just after fropa. Will likely continue to
get wind gusts peaking near 45 mph at least for a brief period of
time after the passage of the cold front along I-70 into the St
Louis metro and eventually into southwest IL this afternoon.

GKS

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 300 AM CDT Wed Apr 18 2018

Surface low pressure was located near Kansas City early this
morning with a warm front extending east southeastward toward
Cape Girardeau. This front will move east across Missouri into
south central Illinois by early this afternoon. The warm front
will move north ahead of the low, allowing temperatures to climb
into the 60s to near 70 degrees over southeast Missouri and south
central Illinois by early this afternoon. Behind the low, the
pressure gradient will quickly tighten causing west to northwest
winds to pick up during the late morning and afternoon. Sustained
winds will be 20-25+ mph with gusts up to 40 mph which are not
quite high enough for an advisory at this time.

Will keep a chance of showers going across northeast Missouri and
west central Illinois this morning and early this afternoon as
there will be modest convergence along the inverted trough north
of the surface low. Combine this with mid level ascent ahead of
the upper trough that will be moving through the area, this will
be enough forcing to producing a few showers over the northern
part of the CWA despite limited Gulf moisture.

Winds will decrease through the night tonight. With cold air
advection, lows will fall into the lower 30s. It appears that
there will be enough clouds and wind to prevent widespread
development of frost tonight.

Britt

.LONG TERM...  (Thursday through Tuesday)
Issued at 300 AM CDT Wed Apr 18 2018

Quiet weather is expected across the area from Thursday through
Saturday due to the influences of a surface high pressure center
as well as an upper ridge which builds across the region and then
shifts eastward. Frost is expected across the area on Thu
night/Fri morning. A closed upper low develops over the Four
Corners region by Friday and then slowly drifts E/ESE over the
weekend while decreasing in intensity. This system is expected to
have limited impact on the forecast area.

Conditions continue to look favorable for widespread frost on
late Thu night and early Fri morning. A 1030+ hPa surface high is
forecast to be over IA/MN at 12z Fri with the ridge axis
extending southward through MO. The combination of light winds,
little cloud cover, overnight lows in the low/mid 30s, and dew
point depressions of approximately 5 degrees are favorable for
frost formation. If these forecast trends hold, then a Frost
Advisory and/or a Freeze Warning would be needed for parts of the
forecast area where the growing season has already begun.

Model guidance has been trending farther southward with the track
of the surface low associated with the aforementioned low
pressure system which will be moving from the Four Corners region
towards the southeastern CONUS over the weekend. Precipitation is
expected to be limited to the southern part of the CWA from Sat
night through Sunday evening. H7-H5 Q-vector divergence shows
broad lift coincident with favorable LER jet dynamics as a H25 jet
rounds the base of the closed upper low in addition to H85
moisture convergence across southern and southeastern MO. This is
sufficient to support SChc-Chc PoPs across the southern part of
the CWA even though the chance for SHRA/TSRA is much higher
farther to the south. After this system moves away from the
region, the next opportunity for widespread rain appears to be on
Tuesday night, although the ECMWF/GFS/GEM disagree on the upper
air pattern.

Temperatures will remain on the cool side on Thu/Thu night due to
CAA in the wake of Wed`s system during the day followed by good
radiational cooling conditions on Thu night (see frost discussion
above). Temperatures then gradually moderate from Fri through Tue.
Highs on Mon/Tue are expected to be in the upper 60s to around 70,
which is close to normal for this time of year.

Kanofsky

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1119 AM CDT Wed Apr 18 2018

Surface low just northeast of STL with a cold front extending
southwest through the St Lous metro area late this morning. Strong
and gusty west-northwesterly surface winds will occur after
fropa. These winds will gradually diminish later this evening and
overnight as the surface pressure gradient relaxes. MVFR cigs have
already advected into UIN and post frontal low level cloudiness
will move into COU and the St Louis metro area this afternoon,
with the cloud ceiling dropping into the MVFR catagory tonight.

SPECIFICS FOR KSTL: Surface low just northeast of STL with a cold
front extending southwest through the St Lous metro area late
this morning. Strong and gusty west-northwesterly surface winds
will occur after fropa. These winds will gradually diminish later
this evening and overnight as the surface pressure gradient
relaxes. MVFR cigs have already advected into UIN and post frontal
low level cloudiness will move into STL afternoon, with the cloud
ceiling dropping into the MVFR catagory tonight. These clouds will
scatter out by early Thursday afternoon.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Wind Advisory until 7 PM CDT this evening for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson
     MO-Lincoln MO-Moniteau MO-Montgomery MO-Osage MO-Pike MO-
     Saint Charles MO-Saint Louis City MO-Saint Louis MO-Warren
     MO.

IL...Wind Advisory until 7 PM CDT this evening for Bond IL-Calhoun IL-
     Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-Saint Clair IL-
     Washington IL.

&&

$$

WFO LSX



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