Area Forecast Discussion
Issued by NWS St. Louis, MO

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060
FXUS63 KLSX 250806
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
306 AM CDT Tue Jul 25 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 302 AM CDT Tue Jul 25 2017

A light northwest flow aloft prevails over our region early this
morning with an upper level RIDGE centered over the southern Plains.
A stalled surface front extended over far southern MO and into
western KY giving our region a light easterly flow at the surface.
Skies were clear for many areas, although cloudiness has been trying
to increase for portions of southern IL and southeast MO and a few
TSRA have even managed to persist over southeast MO.  Temps ranged
from the mid 60s to mid 70s generally from north to south across our
region.

Upper level RIDGE will extend eastward towards our region today and
tonight.  If it weren`t for the setup at the surface, where the
stalled front will remain in place thru today before buckling
northeastward as a warm front later tonight, would be aggressive on
max temp forecast this afternoon.  However, easterly surface flow
expected to prevail and this will limit our heating potential.  Once
again stayed within the realm of the MOS forecasts, but overall
preferred the higher MOS values.

Speaking of which, the stalled front over southern MO should once
again as a focus for TSRA development with the heating of the day,
from roughly the midday hours into early this evening.  Do not
expect TSRA to persist as late into the night as it is doing now
with what should be a more ill-defined frontal convergence as what
is left of it lifts northeastward as a warm front to set up a
hot/humid day for Wednesday.

Mostly clear skies are anticipated for many areas tonight, but with
a warm front edging thru, something along the lines of a couple degs
higher than persistence seemed best with min temps from the upper
60s to mid 70s.

TES

.LONG TERM...  (Wednesday through Monday)
Issued at 302 AM CDT Tue Jul 25 2017

An active period of weather is on tap for Wednesday through Thursday
with a short-lived espisode of dangerous heat followed by
thunderstorms, and potential severe weather. This time frame will
feature amplification and retrogression of the upper air pattern.
The upper high initially centered in the southern Plains on
Wednesday morning will shift west to the southern Rockies by
Thursday morning, as a broad upper trof develops from southeast
Canada into the northeast quarter of the Nation. A quasi warm front
will stretch across eastern MO into far southern IL at daybreak on
Wednesday. This front will shift to the east/northeast during the
morning, well in advance of a short wave migrating east from the
north central Plains and its associated cold front. Southwesterly
lower tropospheric flow in the wake of the warm front, good WAA, and
diurnal heating should allow temps to rise into the 90s in the
afternoon. Confidence is fairly high that the St. Louis metro region
and immediate surrounding counties in east central MO and southwest
IL will have highs in the 95-99 degree range with peak afternoon
heat index values of 105-110. I will be issuing a heat advisory
for this area with this forecast for Wednesday afternoon/early
evening. Confidence is slightly lower elsewhere and will let later
shifts assess, with some high clouds coming into play in northeast
and central MO.

An elongated MCS is expected to evolve to our north across IA late
Wednesday afternoon and evening in association with the migratory
short wave trof and attendant, advancing cold front. This MCS should
move east-southeast into northern MO and western IL on Wednesday
night with the advancing cold front/surface low, intensifying
west-southwesterly LLJ, and increasing northwesterly deep layer
shear. There is a fair amount of uncertainty in how far southeast
the MCS or additional outflow generated convection will progress.
The NAM has been consistently very agressive with the southeast
push and speed of this system. My initial thoughts are it is too
far southeast and this forecast leans heavily on the ECMWF and GFS
with likely POPS almost to I-70 in Missouri late Wednesday night.
Increasing, moderate deep layer shear and ample CAPE should
support a severe threat on Wednesday night with damaging wind the
primary threat. We will also need to keep an eye on the heavy rain
potential across northeast MO and west central IL where very high
PWS`s of 2+ inches will exist and training could set-up.

The convective trends on Thursday will to at least some degree be
highly dependent on the extent of showers and thundestorms on
Wednesday night. Somewhere over the northern half of the CWA there
will likely be ongoing activity during the morning which should
weaken with time. New showers and storms should then develop by
early afternoon either with the advancing cold front or a residual
outflow boundary. Depending on the amount of instability, the
afternoon activity could pose a severe threat. All of the showers
and thunderstorms will push southeast during the afternoon and on
Thursday night. By daybreak Friday the cold front should be well
south of the CWA, with surface high pressure and cooler, drier air
enveloping the area.

Amplification and some retrogression of the upper air pattern will
continue into the weekend with an upper level ridge established in
the western U.S. and mean long wave trof in the eastern U.S.. This
pattern aloft will maintain surface high pressure across the mid-
upper MS Valley with generally low humidity and below average temps
into early next week.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1132 PM CDT Mon Jul 24 2017

Scattered thunderstorms over the eastern Ozarks are expected to
dissipate overnight and stay well south of the terminals.
Otherwise...dry and VFR conditions are expected through the
period. Light winds will veer from north to east as high pressure
moves through the Great Lakes.

SPECIFICS FOR KSTL:

Dry and VFR conditions are expected through the period. Light
winds will veer from north to east as high pressure moves through
the Great Lakes.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon to 8 PM CDT Wednesday for Franklin MO-
     Jefferson MO-Lincoln MO-Saint Charles MO-Saint Louis City
     MO-Saint Louis MO-Warren MO.

IL...Heat Advisory from noon to 8 PM CDT Wednesday for Calhoun IL-
     Jersey IL-Madison IL-Monroe IL-Saint Clair IL.

&&

$$

WFO LSX



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