075 
FXUS63 KLSX 120846
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
346 AM CDT Mon May 12 2014

.SHORT TERM: (Today and Tonight)
Issued at 330 AM CDT Mon May 12 2014

Main forecast issues for today are the probability of thunderstorms
and any severe potental, as well as just how warm it may get.
Current thinking is there should be a gradual upswing in convection
as the day progresses, primarily during the later half of the
afternoon. It appears that northeast Missouri will be the area we
will have to watch most closely through mid morning. Two things to
key an eye on - an outflow boundary pushing southeastward across
eastern Iowa as a focus for additional thunderstorms, and also a
cluster of thunderstorms across eastern Kansas just southwest of
Kansas City and if they will hold together as they move northeastward.
Otherwise I think that there will be a general decrease in the
convective cloud debris during the morning, which given the mild
early morning temperatures should allow for a quick warm-up by
early afternoon. The convection allowing models and more
specifically the HRRR is suggesting that by mid morning, isolated
showers or thunderstorms could develop just about anywhere within
the CWA, but most favored across southeast Missouri and southern
Illinois. The main action still looks like the later half of the
afternoon. The warm sector air mass ahead of the cold front will
become quite unstable by mid-late afternoon with SBCAPE in excess
of 2000 J/KG and little CIN. Expecting numerous thunderstorms to
develop across western Missouri into central and northeast
Missouri. Deep layer shear of 20-30 kts and the anticipated CAPE
would support a few severe storms capable of large hail and
damaging winds from the afternoon into about mid-evening. The
front moves through all but the far eastern sections of the CWA
tonight accompanied by widespread showers and thunderstorms and
ushering much cooler air into the region in its wake. It also
appears that there will be an increasing amount of post-frontal
showers overnight immediately in the cool air in response to large
scale forcing.

Glass

.LONG TERM: (Tuesday through Sunday)
Issued at 330 AM CDT Mon May 12 2014

This marks the beginning of a cooler than normal period with a
slow-moving and deep upper trof dominating the region through the
week. The front will quickly exit the remainder of the CWA early
Tuesday morning with post-frontal showers impacting eastern
Missouri and southwest Illinois in the morning and exiting by
early-mid afternoon. Highs on Tuesday will be around 20 degrees
cooler than today. A strong short-wave and possibly upper low will
then migrate through the base of the deep trof from the southern
Plains and into the mid MS valley Tuesday night-Wednesday night.
More reminiscent of Fall or early Spring, large scale ascent with this
system should produce a large area of showers or rain spreading
into the area late Tuesday night into Wednesday. Its northward
extent and longevity is all dependent on the strength of the
system, and NAM featuring a closed low and large deformation zone, has
the greatest northward push and keeps precipitation well into
Wednesday night. Regardless the precipitation and accompanying
overcast low clouds will keep temperatures well below average on
Wednesday. We could continue to see a periodic shower threat
through the end of the week as we get into deeper cyclonic flow on
the backside of the slow-moving deep upper trof. Temperatures will
remain below average into at least Friday.

Glass

&&

.AVIATION:  (For the 06z TAFs through 06z Monday Night)
Issued at 1136 PM CDT Sun May 11 2014

Expecting VFR flight conditions to prevail across most of the area
for the rest of the night. Low level wind shear conditions will
spread further than previously thought; probably as far east as
southwest Illinois. Have included LLWS in all terminal forecasts.
Thunderstorms over Iowa are trying to spread a bit further south
into northern Missouri, however most guidance agrees that storms
will be weakening as they get far enough east to threaten our
area. Some widely scattered showers or storms may develop as
diurnal heating increases during the late morning or early
afternoon, but the primary threat for thunderstorms will be during
the late afternoon and evening ahead of the cold front. Guidance
seems to be in good agreement that there will be a line of storms
marching eastward from western Missouri in the afternoon, reaching
eastern Missouri by early evening. With these high precipitation
chances in guidance, feel it's ok to go ahead and star TEMPOs in
the terminal forecasts.

Specifics for KSTL:

VFR flight conditions are expected to continue at Lambert tonight.
Low level wind shear looks to spread further east than previously
forecast with 45kt or higher winds around 1500-2000ft, so have
included it until 12Z. The guidance suite seems to be in good
agreement that the storms over Iowa won't spread south to affect
Lambert through the morning, but there may be some widely
scattered showers or storms developing during the late morning or
early afternoon. Think coverage will be pretty low though so have
not mentioned it in this update. The primary threat for
thunderstorms will come during the early evening ahead of a cold
front. While timing is still uncertain, it looks very likely that
Lambert will receive a period of thunderstorms in the early
evening followed by lighter rain later in the evening/overnight.

Carney

&&

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

$$

WFO LSX