Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161805

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
105 PM CDT Thu Aug 16 2018

Issued at 625 AM CDT Thu Aug 16 2018

Visibilities continue to bounce around this morning, ranging from
0 to P6SM. Fortunately the dense fog has not settled anywhere
other than the usual locations, so no headline will be needed.



.SHORT TERM...  (Through Late Tonight)
Issued at 329 AM CDT Thu Aug 16 2018

With very light winds and dewpoint depressions approaching 0, the
initial concern with this morning`s forecast is the potential for
fog. Several locations across central IL and NE MO are already
reporting reduced visibilities, a few of which are below 1SM. With
the surface low just now moving into the area, conditions will
likely only worsen over the next few hours. Fortunately most high
res guidance keeps the worst of the fog to our NE and visbys near
1SM across our N. With all this, I`m not planning a Dense Fog
Advisory at this moment, but it`s possible visibilities tank so I`ll
be keeping a close eye on this.

Once the fog clears after sunrise, attention will then turn to the
potential for storms. By this afternoon, an upper level trough will
begin to swing through the Upper Mississippi valley, deepening the
profusely mentioned surface cyclone and spurring it to the east.
This will also serve to tighten the thermal gradient along the
trailing cold front before it sweeps through the CWA tonight. This
forcing, coupled with precipitable water exceeding 1.5" across much
of the CWA, should be more than enough to support at least scattered
showers as the front move through. MUCAPE should remain in excess of
500 j/kg ahead of the front and deep layer shear will be in the 20-
30 kt range. With that, at least scattered thunder seems likely and
the marginal risk for severe weather from SPC seems appropriate.
Damaging winds and hail are the most likely hazard from any of the
stronger storms, as most storms will likely be elevated when the
move through the CWA.


.LONG TERM...  (Friday through Wednesday)
Issued at 329 AM CDT Thu Aug 16 2018

As a cold front pushes south of the CWA on Friday, pops will end
northwest to southeast through the day into the evening. The area
will then experience a quiet stretch from late Friday through most
of Sunday prior to the next system.

Late in the day Sunday, a trough will move out of the Plains into
the Midwest and bring with it our next chance of rain Sunday night
through Monday. The GFS and ECMWF continue to differ in their
handling of the surface low on Monday. The GFS is slower and keeps
the low to the south, prolonging our chance of rain into Tuesday
morning while the ECMWF wants to track the low across northern MO,
sweeping a cold front across the CWA on Monday and ending rain
chances Monday night. Both models have been consistent with their
solutions, so will go with a blend between the two for now.

After this trough, the CWA will continue to be under northwest flow,
in which a weak disturbance will pass through the area Tuesday.
Given the weaker nature of this disturbance and differences in the
guidance, have stuck with low pops over the area Tuesday afternoon.

From late Tuesday through the end of the period Thursday morning,
northwest flow aloft and ridging at the surface will keep pops down
and temperatures mild.



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1241 PM CDT Thu Aug 16 2018

A cold front over northeast and central Missouri will slowly move
east this afternoon and evening. An isolated shower or
thunderstorm is possible this afternoon, but confidence is too low
to include in the TAFs. The front will continue to move through
the area in the overnight and morning hours with more widespread
coverage of showers and thunderstorms with MVFR conditions
possible. Winds will be light out of the south-southwest ahead of
the front, then veer to the northwest behind the front and remain
light. Otherwise, VFR conditions will persist through Friday


A cold front over northeast and central Missouri will continue to
move east toward the terminal tonight. The front will be close
enough by around 03z to introduce VCTS to the terminal.Expect
frontal passage around 06z. MVFR conditions are possible at the
terminal from 03z till after frontal passage before ceilings lift
back up to VFR. Light south- southwest winds ahead of the front
will veer to the northwest but remain light through the rest of
the period.





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