Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 061112
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
612 AM CDT Thu Aug 6 2020

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 431 AM CDT Thu Aug 6 2020

A surface ridge extended from MI southwest into eastern MO early
this morning.  Unseasonably cool conditions were being experienced
again this morning, mainly across east central and southeast MO
and southwest IL near the surface ridge axis where the sky was
clear and the wind calm. Fog can be expected early this morning,
mainly across southwest IL and along and near river valleys. There
were scattered light showers across parts of IA early this morning
in a region of low-mid level warm air advection ahead of a weak
northwest flow shortwave. Most of this activity should weaken and
dissipate before it drops southeastward into our forecast area
this morning, although there may be a few sprinkles across
northeast MO and west central IL from mid level cloudiness. A
gradual warming trend can be expected as the surface ridge shifts
east of the region and surface winds become southeasterly. Upper
level heights will also rise on Friday as an upper level ridge
over the southern Plains builds northeastward into our area. Highs
on Friday should be no more than 5 degrees below normal. There
will be a chance of elevated convection late tonight and Friday
morning, mainly across central MO as a southwesterly low level
jet brings increasing low level temperature and moisture advection
to western MO. Showers and storms will develop across western MO
by late tonight and gradually weaken as they shift eastward into
central MO towards morning.

GKS

.LONG TERM...  (Friday Night through Wednesday)
Issued at 431 AM CDT Thu Aug 6 2020

There will be another and better threat for nocturnal convection,
particularly across northeast and central MO and west central IL
late Friday night and Saturday morning on the nose of a veering
low level jet and ahead of a weak shortwave or MCV. The models are
also depicting increasing mid level instability across this area.
This convection should dissipate by Saturday afternoon with a
clearing sky resulting in high temperatures at or just above
seasonal normals. This along with humid conditions will make it
feel much more like typical summertime weather in August. Even
warmer temperatures are likely on Sunday as the surface wind
becomes southwesterly after passage of a weak warm front. A more
active and wet pattern is expected to begin the next work week as
a low amplitude upper level trough moving eastward through the
Northern Plains and Great Lakes region causes the upper level
ridge to be suppressed south of our area.  Rounds of
showers/storms are likely as several shortwaves move through our
area in nearly zonal upper level flow. A cold front will also try
to sag southward into our area.  Both the operational GFS and
ECMWF deterministic models try to drop the cold front southward
into our forecast area Monday night. The ensemble means are not
quite as progressive, only dropping the front to near the IA and
MO border. Favor the less progressive solution of the ensemble
guidance which would result in the highest pops and most QPF
mainly across northeast MO and west central IL.

GKS

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 556 AM CDT Thu Aug 6 2020

Fog in the river valleys will impact SUS and CPS until 14Z
Thursday. Light showers may move through UIN early this morning.
Mainly just some mid-high level cloudiness today and tonight.
There may also be scattered diurnal cumulus clouds late this
morning and this afternoon. Showers and possibly storms may move
into COU late tonight after 09Z Friday. East-southeasterly winds
today and tonight as a surface ridge extending from MI southwest
into eastern MO shifts slowly eastward.

SPECIFICS FOR KSTL: Mainly just some mid-high level cloudiness
today through Friday morning. There may also be scattered diurnal
cumulus clouds late this morning and this afternoon. Relatively
weak east-southeast winds today through Friday morning as a surface
ridge extending from MI southwest into eastern MO shifts slowly
eastward.

GKS

&&

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

$$

WFO LSX


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