Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170106

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
806 PM CDT Mon Oct 16 2017

Issued at 742 PM CDT Mon Oct 16 2017

The elongated surface high pressure centered in southwestern
Missouri will slide eastward tonight and will be in control over
the region. A few high clouds are possible overnight over parts of
central Missouri and western Illinois. A combination of light
winds and good radiational cooling will lead to some steam fog
over the rivers tonight particularly in southeast Missouri and
southwestern Illinois. There also could be some patchy frost
overnight, particularly in parts of southeast Missouri and
southwestern Illinois. The winds will likely be light or calm
along with the development of a strong surface based inversion
which will allow temperatures to fall into the mid to upper 30s
over those areas.



.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 309 PM CDT Mon Oct 16 2017

The elongated surface high currently centered near Joplin will
slide eastward tonight and will be the controlling weather
feature. As is typical with retreating surface highs in this
scenario, the surface winds will attain a light southerly
component in central and northeast MO this evening. This wind
flow along with weak low level WAA and an increase in high clouds
late will be somewhat balanced by low surface dew points leading
to mins in the lower to mid 40s. Elsewhere and more specifically
across southeast MO and southern IL, the wind will be light to
calm, and there should be decoupling and the development of a
strong surface based inversion. This should allow for most areas
to fall nicely and it will be a cool night with mins in the mid-
upper 30s. Attendant with these cool temps across southeast
MO/southern IL will be fog in the river valleys and even some
patchy frost. Note some patchy frost was reported this morning.

Temps should moderate nicely on Tuesday. The pattern aloft will be
somewhat progressive and flattening in general with the eastern
U.S. trof scooting into the western Atlantic. The lower tropospheric
high will continue to slowly retreat with south-southwesterly low
level flow and pronounced WAA. The dry air mass should respond
nicely with highs in the upper 60s - lower 70s.


.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 309 PM CDT Mon Oct 16 2017

The remainder of the weak should be tranquil with moderating,
above average temperatures. The flow aloft generally remains
quasi-zonal with periodic bouts of high clouds. The low level WAA
regime remains intact with south-southwest low level flow. A cold
front will encroach into the northern CWA on Thursday morning,
but its presence will be short-lived as it bounces north and
undergoes frontolysis with heights on the rise aloft.

These height rises aloft will be part of significant
amplification/change in the large scale pattern beginning during
the later part of the week and continuing through the weekend.
Initially an upper trof will deepen across the western U.S. with
an upper ridge building from the Dixie region into the OH Valley.
This will bring a return of southwest flow aloft to the mid MS
Valley and strengthening low level south-southwest flow on Friday.
The presence of the expansive surface/low level anticyclone in the
eastern U.S. will slow the return of moisture return despite this
flow regime. Present indications are that the combo of the mobile
western upper trof and eastward progession of the anticyclone
will allow for moisture return into the area beginning on
Saturday, increasing Saturday night into Sunday ahead of an
advancing cold front. Steep lapse rates will be present thanks to
the southwest flow ahead of the progressive upper trof, contributing
to sufficient instability for showers/thunderstorms with the cold
front and ahead it.



.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 552 PM CDT Mon Oct 16 2017

VFR flight conditions expected through Tuesday evening. An area of
high pressure will slide south of the area with a few high
clouds possible tonight over central Missouri and western
Illinois. A combination of light winds and good radiational
cooling will likely lead to some steam fog tonight particularly
at KSUS and KCPS. The timing of the steam fog development will
roughly be in the 9z to 15z time frame with visibilities of 1 to
2SM possible within the vicinity of rivers. Winds will pickup by
Tuesday morning and be out of the south.

SPECIFICS FOR KSTL: VFR flight conditions are expected through
Tuesday evening. A few high clouds are possible late tonight into
early Tuesday morning over the terminal. Winds will be light
overnight, but pick up Tuesday morning out of the south.



Saint Louis     43  72  48  75 /   0   0   0   0
Quincy          44  70  47  72 /   0   0   0   0
Columbia        44  72  45  74 /   0   0   0   0
Jefferson City  42  72  45  75 /   0   0   0   0
Salem           38  70  45  73 /   0   0   0   0
Farmington      35  69  43  73 /   0   0   0   0




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