Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 121722
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1222 PM CDT Thu Oct 12 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 336 AM CDT Thu Oct 12 2017

Kudos to the NAM and RAP 925 mb RH fields from model runs since last
night which have done a great job depicting the extent of stratus.
The latest observations and GOES-16 data depict the stratus has
actually spread westward since late evening in response to east-
southeast flow on the western periphery of the surface/low level
ridge. This ridge was centered between KCOU and KSTL early this
morning and is creeping eastward. The latest guidance suggests a
continued slow eastward retreat through mid-morning and then better
progression of the ridge axis into the afternoon and tonight. The
ramifications are the entire CWA will be locked in stratus thru mid-
morning with some patchy drizzle continuing. The onset of
increasing south-southwest low level flow/WAA on the backside of
the retreating ridge will then lead to slow but somewhat steady
east-northeastward clearing. This should commence late morning-
midday in central MO, far eastern MO to near the MS River from
21-00Z, and in the far eastern CWA in south central IL near 03Z.
Temperatures today will accordingly be cooler than previously
forecast from eastern MO into western IL with highs late in the
day. The continued retreat of the low level ridge late evening and
overnight will result in the increased extent and intensity of
the low level WAA regime. The late clearing in southwest and south
central IL along with lighter southeast surface winds would
suggest that fog development is a good possibility overnight and
have mentioned some patchy fog at this time.

Glass

.LONG TERM...  (Friday through Wednesday)
Issued at 336 AM CDT Thu Oct 12 2017

Upper level ridge will shift a bit further east by Friday, so
forecast area will be in between this ridge and digging upper level
trof to our west. So with decent southerly winds at surface, will
see temperatures warm up well above normal Friday through Saturday.
Could see near record highs on Saturday, especially along and south
of I-70, depending on cloud cover and precipitation.

In the meantime, next system to approach region by Friday as first
in a series of shortwaves slides northeast along stalled frontal
boundary extending from south central KS northeastward through
northwest MO and into southeastern IA. Chances of showers and storms
will be on the increase Friday night through Saturday in this area.
Models have the best instability, moisture return and strengthening
low level jet Saturday afternoon through evening. So could see
strong to severe storms. Some concern that despite loss of daytime
heating, there are decent dynamics with this system so could see
severe chances persist through the rest of Saturday night into
Sunday morning, mainly over central/northeast MO and west central
IL.

By Sunday, main upper level shortwave to lift out of northern plains
into the Great Lakes, dragging cold front through region, exiting
forecast area by midday. So will precipitation taper off by 00z
Monday.

Much colder air to filter in behind this front with highs on Sunday
occurring by midday, then remain steady or slowly fall from there.
Depending on where front is, temps on Sunday will range from the low
60s far north to the mid 70s far south. Temperatures through rest of
forecast period to remain near normal for this time of year.

Byrd

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1210 PM CDT Thu Oct 12 2017

Main TAF impact will continue to be stratus this afternoon. The
stratus has quickly eroded across portions of southwest and
central Missouri, and expect this clearing line to spread through
all TAF sites this afternoon. The expected clearing should help
mitigate fog concerns tonight, but can`t completely rule it out
especially at UIN if the clearing is slower than expected.
Otherwise, VFR conditions are expected tonight into Friday as
winds turn southerly.

SPECIFICS FOR KSTL:
MVFR stratus should clear out of STL later this afternoon. VFR
conditions are expected thereafter tonight into Friday. May see
some periods of VFR cigs on Friday with winds out of the south
around 10 knots.

KD

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     65  55  83  64 /   0   0   0   5
Quincy          63  53  81  64 /   0   0   5  40
Columbia        68  55  84  65 /   0   0   5  20
Jefferson City  69  54  85  65 /   0   0   5  10
Salem           63  52  79  59 /   0   0   0   5
Farmington      67  52  81  61 /   0   0   0   5

&&

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

$$

WFO LSX



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