Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181612
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1112 AM CDT Thu May 18 2017

.UPDATE...
Issued at 1109 AM CDT Thu May 18 2017

Cold front has moved into northeast Missouri and west central
Illinois late this morning. This front will become aligned west-
east to the north of I-70 by this evening and become quasi-
stationary. Regional radars are not showing any showers or
thunderstorms over the area at the moment. However, latest runs of
the RAP are showing a favorable environment for scattered
thunderstorms to redevelop along and south of the front this
afternoon. Recent runs of the HRRR have shown scattered storms
developing from St. Louis eastward from mid afternoon into this
evening. If storms do develop, a few storms could possibly be
severe given the amount of instability and marginal shear.
Otherwise, there will be a contrast in temperatures across the
front with temperatures climbing well into the 80s to the south,
particularly where there is the most sunshine.

Britt

&&

.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 12z TAFs through 12z Friday Morning)
Issued at 644 AM CDT Thu May 18 2017

VFR conditions, outside of any TSRA, will prevail at the TAF
sites until at least late evening. A cold front will stall near
the I-70 corridor this afternoon and continue to be nearly
stationary thru tonight. This will act as a focus for scattered
TSRA from later this afternoon into the evening. A more organized
complex of TSRA with steadier rain is then expected to develop to
our west this evening and affect the TAF sites late tonight and
into Friday morning. CIGs will notably drop into at least MVFR
category later tonight north of the front and possibly into IFR. Southwest
winds will continue for areas south of the front, while areas
where the front passes thru, mainly UIN, will see winds more from
the NE beginning this evening.

SPECIFICS FOR KSTL: VFR conditions, outside of any TSRA, will prevail
thru late evening. A cold front will stall near the terminal and
continue to be nearly stationary thru tonight. This will act as
a focus for scattered TSRA from later this afternoon into the
evening and will eventually justify a VCTS heading into tonight.
A more organized complex of TSRA with steadier rain is then
expected to develop to our west and will affect the terminal
towards daybreak Friday. Southwest winds will continue thru
daytime today and then become more variable and light as the front
stalls nearby.

TES

&&

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

$$

WFO LSX



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