Area Forecast Discussion
Issued by NWS St. Louis, MO

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712
FXUS63 KLSX 181132
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
632 AM CDT Mon May 18 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 301 AM CDT Mon May 18 2015

A trailing cold front will slowly progress through the CWA today.
The slow southeastward motion of the front means that the cooler
post-frontal air mass will only affect daytime high temperatures for
approximately the northwestern one-third of the CWA. The rest of the
forecast area should see temperatures similar to yesterday with
highs reaching the low to mid 80s.

Although the surface convergence with the front is not very strong,
it may be sufficient to focus isolated to scattered SH/TS between
15-00z, especially when aided by diurnal destabilization along with
H7-H5 lapse rates of 6 to 6.5 deg C/km as depicted by several
models. Any convection which develops along or south of the front
should quickly dissipate after 00z due to the loss of diurnal
heating, the increased capping of the frontal inversion, and lack of
upper support.

Kanofsky

.LONG TERM:  (Tonight through Sunday)
Issued at 301 AM CDT Mon May 18 2015

(Tonight-Wednesday)

Any convection that does manage to fire along the southward
pressing cold front should exit our far southern counties very
early this evening. Large Canadian high building into the upper
Midwest will then assume control of our weather for the remainder
of the night and into Tuesday. This should mean dry but chilly May
weather, with lows dipping into the lower 40s to lower 50s by
Tuesday morning, and highs primarily in the 60s on Tuesday
afternoon.

Over this past weekend the medium range solutions were pointing to a
very chilly and wet Wednesday across the region as broad upper trof
pushes across the Plains and into the mid-Mississippi Valley, and
tonight`s short term solutions are continuing to paint a very
similar scenario for our FA. While the Canadian high will push the
surface cold front well south of our CWA, NWP solutions have
consistently forecast the primary 850mb front to be draped from west
to east across the CWA on Wednesday. This should result in an
excellent zone of isentropic ascent once southery low level flow in
advance of the upper level trof interacts with the thermal
gradient.  Models do vary on intensity of the southerly flow, with
NAM, GEM, and ECMWF forecasting a compact 850mb low to ride along
the thermal gradient while GFS 850mb low is not as well defined, but
given the intensity of the thermal gradient even the weaker solution
should lead to an area of showers that will work its way across the
CWA during the day.

Surface wave is also forecast to form somewhere over the southern
Plains on Wednesday with some solutions placing the low as far north
as SW MO.  However, even the more northward solutions then take the
low into northeast AR during the day; therefore it would appear that
the CWA will stay in the cool Canadian airmass as the aforementioned
precipitation works its way across the area.  This combination of
cool air and rainfall should make for a very chilly Wednesday, and
the consensus of guidance is that northwest sections of the
CWA...where the rain will form first...will not get out of the 50s.
Some areas may be near their record low max temps for May 20th...see
climate section for specifics.

(Thursday-Saturday)

Passage of the upper trof will allow the Canadian high to sag back
south a bit on Thursday, with the high maintaining control over the
region as it begins to retreat to the eastern U.S. on Friday.  Temps
for the last two days of the week should remain below normal,
although not as chilly as the rain-cooled temps on Wednesday.

Medium range guidance indicates that bits of energy will be ejected
from large upper level low over the southwest U.S. during the start
of the weekend, with the main upper system then lifting into the
Plains by Sunday.  For our CWA, this should mean an increasing
threat of showers and thunderstorms as we head into next Saturday
and Sunday, with the upper level pattern looking eerily similar to
that of the past two weekends.

Truett

&&

.AVIATION:  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 627 AM CDT Mon May 18 2015

Patchy morning fog will dissipate within the next few hours. VFR
conditions are then expected for the majority of the TAF period,
except for any isolated thunderstorms which might develop this
afternoon near a slow-moving cold front. The expected sparse
coverage of convection precludes a mention in the TAFs attm.
Winds will briefly become westerly just ahead of the front then
turn northwesterly after fropa. Winds will gradually veer and
become northerly towards the end of the TAF period.

Kanofsky

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX






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