Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161054
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
554 AM CDT Mon Jul 16 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 340 AM CDT Mon Jul 16 2018

With light surface winds, high surface dew points and recent
rainfall fog has developed early this morning across parts of
southwest IL from ALN and BLV to 3LF and SLO.  This fog will
dissipate shortly after sunrise.  A cold front currently extending
from Wisconsin southwest through southeastern Nebraska will move
southeastward through our forecast area today.  At least isolated
showers and thunderstorms are expected along and just ahead of this
front late this morning and this afternoon as the atmosphere becomes
unstable ahead of the front with daytime heating.  It appears that
the best coverage of storms should be across southwest IL where the
models were depicting the highest instability.  Highs today will be
slightly above normal with humid conditions across southwest IL due
to the high surface dew points.  Convection should shift southeast
of our forecast area this evening as surface ridging builds
southward into our area behind the cold front with slightly cooler
air advecting into our area along with lower surface and 850 mb dew
points. Cooler lows can be expected tonight, especially across
northeast MO and west central IL with less humid conditions.

GKS

.LONG TERM...  (Tuesday through Sunday)
Issued at 340 AM CDT Mon Jul 16 2018

The synoptic pattern at the start of the long term period will
feature amplifying flow aloft, as an upper-level trough builds into
the Northeast states. This will place our region within a northwest
flow regime through much of the period.

Tuesday will begin with a cold front to the south of the region.
This will allow surface ridging centered across the northern Plains
to briefly build into the area. The influence of the high and
northerly surface flow will bring a much more comfortable airmass
into the area, as dewpoints fall into the low 60s and possibly even
upper 50s. This airmass will remain in place through at least
Wednesday, providing a nice little break in the heat we`ve seen of
late. While it`s possible portions of central MO may get clipped
with some shower/storm activity late Tuesday night into Wednesday,
suspect much of the area will remain dry given the instability and
moisture convergence will be focused off to the southwest of the
area.

However, Wednesday night through Friday will mark the return of
shower/storm chances back into the forecast. A low-level jet will
focus into an approaching warm front, which should lead to the
development of a couple rounds of showers/storms Wednesday night
through Friday. We will have to keep an eye on the potential for
some severe thunderstorms during this timeframe as well. It appears
there will be much more deep-layer flow to work with than we have
seen in recent weeks (40-50 knots of deep-layer shear) which will
help to support the organization of thunderstorms. There`s still
plenty of time to dial in specifics, but we will keep an eye on this
time period in the coming days.

Deitsch

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 529 AM CDT Mon Jul 16 2018

The fog at UIN, SUS and CPS should dissipate by 14Z this morning.
Isolated convection is expected this afternoon in the St Louis
metro area where the atmosphere will become quite unstable ahead
of an approaching cold front. Scattered to broken diurnal cumulus
clouds can also be expected late this morning and this afternoon
with daytime heating. The sky will clear out this evening. The
light surface wind will become northwesterly later this morning,
and mainly northerly tonight as a surface ridge builds southward
into the area behind the cold front.

SPECIFICS FOR KSTL: Isolated convection is expected this
afternoon in the STL area where the atmosphere will become quite
unstable ahead of an approaching cold front. Scattered to broken
diurnal cumulus clouds can also be expected late this morning and
this afternoon with daytime heating. The sky will clear out this
evening. The light surface wind will become northwesterly by
afternoon, then northerly tonight and northeasterly Tuesday
morning as a surface ridge builds southward into the area behind
the cold front.

GKS

&&

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

$$

WFO LSX



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