Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180900
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
400 AM CDT Thu May 18 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 359 AM CDT Thu May 18 2017

Storm system number one is approaching the western Great Lakes
region early this morning while a stronger storm system is settling
into the central CONUS Rockies near Utah.  At the surface, a cold
front extended from eastern IA to northeastern MO and into
northwestern MO near Kansas City.  Temps were still quite mild for
this time of year with readings in the upper 60s and lower 70s for
most locations but were slipping back towards the 50s for areas
behind the front.  Skies were clear for much of our region, but
there was an area of active convection edging portions of southeast
MO and southern IL.

SW flow aloft will continue to prevail over our region but with an
absence of any upper level disturbances for much of today.  An old
boundary is what is helping to fuel the current area of TSRA over SE
MO and S IL, and this is expected to fade or weaken heading thru the
morning.  The cold front should continue to advance into our
forecast area this morning and will eventually become stationary
just N of the I-70 corridor by early this afternoon.  Current
guidance suggests the cap will weaken across southeast MO first, but
with a lack of any clear focus for convection, what does develop
should be isolated and almost random.  By mid-afternoon, the cap
near the front should weaken enough to eventually get a more
organized development to where convection fires and have boosted
PoPs into chance/scattered category.

A piece of energy is expected to break off this evening from the
main storm system still well to our west and move into our region
overnight, and should provide sufficient widespread lift to expand
greatly the TSRA coverage that will primarily be along the stalled
front near I-70 during the evening.

Severe potential looks pretty limited but could not rule out
isolated severe storms, especially where the most organized and
sustainable convection will be: near the front this afternoon and
early this evening.

Otherwise, a very warm day on tap again for areas south of the
front, with mid-upper 80s for max temps with upper 70s to lower 80s
for areas north of the front in northern MO and central IL.  Min
temps are almost summerlike for areas south of front with near 70,
and should slip into the 50s for areas to the north of the front.

TES

.LONG TERM...  (Friday through Wednesday)
Issued at 359 AM CDT Thu May 18 2017

East to west front should extend across central MO, near the I-70
corridor at 12Z Friday.  The front should be nearly stationary on
Friday, then shift northward as a warm front as a surface low
develops ahead of the upper level low moving into the northern
Plains and moves northeastward from eastern KS to southwestern IA by
12Z Saturday.  Widespread convection is expected along and north of
this front, while more scattered activity should occur in the very
unstable atmosphere south of the front.  There will be the potential
for severe convection Friday and Friday night with the models
indicating increasing 0-6 km deep layer shear as a mid level wind
max moves into our area southeast of the upper level low, with very
strong instability as denoted by high values of CAPE Friday
afternoon and evening along and south of the front.  The models
depict relatively strong upper level divergence across our area
ahead of the approaching upper level trough.  A strong southwesterly
low level jet will bring strong low level moisture convergence to
parts of our area late Friday night. Highs on Friday will be cooler
across northeast MO and west central IL north of the front. The
surface low will drag a trailing cold front southeastward through
our forecast area Saturday afternoon and night with convection
expected along and ahead of this front.  Prefer the slower cold
front progression of the operational GFS and ECMWF models over the
quicker solution of the NAM model.  There may also be some post
frontal showers Saturday night until the upper level trough shifts
east of our area.  Cooler temperatures can be expected Saturday
night and Sunday due to low level cold air advection behind the cold
front. The coolest lows will occur Sunday night due to radiational
cooling with the surface ridge over the region leading to a clear
sky, light surface winds, and relatively low surface dew points.
After a dry period Sunday through much of Monday, the chance of
showers and storms should return by Monday night as a deepening,
positively tilted upper level trough and associated cold front shift
east-southeastward into our area.  Most of the significant
precipitation should shift east of our forecast area by Tuesday
night, although there may be some lingering light rain into
Wednesday as the upper level trough/low continues to deepen over the
region. Below normal highs can be expected on Wednesday on the
backside of the deep upper level trough/low with low level cloud
cover limiting solar insolation.

GKS

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1155 PM CDT Wed May 17 2017

VFR conditions are expected through the TAF period. Skies will
stay mostly clear to partly cloudy overnight. While winds will
continue to die down from earlier values, still could see some
gusts through the overnight hours near 20 knots. A cold front will
sag towards the I-70 corridor Thursday afternoon. This front will
focus some increasing cumulus development and could even spark an
isolated storm, but coverage is expected to be too low to include
at this time. Otherwise, the nearby front will make for rather
variable winds at COU/SUS/STL/CPS late tomorrow afternoon through
tomorrow evening, but speeds near the boundary will be fairly
light.

SPECIFICS FOR KSTL:
VFR conditions expected through the TAF period. SSW gusts near 20
knots will continue to be possible overnight, but winds should go
lighter by Thursday afternoon as a cold front settles near the
terminal. Given the proximity of the front, wind direction may be
rather erratic Thursday afternoon into Thursday evening. Can`t
completely rule out an isolated storm along the boundary tomorrow
afternoon, but coverage will be too low to include any mention at
this time.

KD

&&

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

$$

WFO LSX



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