Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 051018

Area Forecast Discussion
National Weather Service Saint Louis MO
418 AM CST Mon Dec 5 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 417 AM CST Mon Dec 5 2016

Have issued a dense fog advisory for this morning across central and
northeast Missouri as well as west central Illinois.  This area
cleared out last night allowing the right setup for fog to form.
Several observation sites have reported 1/4sm visibility for the
last few hours.  Outside of the advisory, there are a few locations
that have had visibilities close to advisory criteria but have
either been variable or not widespread.  Will monitor trends to see
if the advisory needs to be expanded this morning. Given the light
winds and that dewpoint depressions will not increase until late
morning given the time of year, will keep the advisory in effect
until Noon.

NAM, GFS, and ECMWF all now look like they are keeping the onset of
precipitation over our area with the upper low now over Old Mexico
until after 00Z.  There will be an increase of mid and high clouds
today in advance of this upper low.  The track of the upper low is
now slightly farther south which will keep most of the precipitation
and the likely/categorical PoPs over southeast Missouri and southern

With the fog this morning and the increase in clouds today, went
below MOS highs.


.LONG TERM...  (Tuesday through Sunday)
Issued at 417 AM CST Mon Dec 5 2016

Surface low pressure should be over Tennessee and Kentucky
Tuesday morning with strong cold advection setting up over the Mid
Mississippi Valley. Lingering precipitation in the deformation
zone in the northwest quadrant of the storm will be moving east
and out of the CWFA by 15-18Z, but guidance still has enough QPF
hanging back during the morning to continue to hang on to chance
PoPs generally along and east of the Mississippi. Doubt there will
be any frozen precip to worry about since the deep cold air won`t
arrive until the afternoon. Wouldn`t be surprised to see steady or
falling temperatures in the afternoon on Tuesday due to the
strength of the cold advection.

High pressure behind the system will be the first true Arctic
airmass to affect our region this season.  The 1050+ mb high with
origins in northwest Canada will build south across the Great Plains
and Midwest through the end of the week.  This will easily be the
coldest weather so of the 2016-2017 winter season so far.
Temperatures will be well below normal through at least
Saturday...with highs not evening climbing out of the 20s Thursday
and Friday and lows in the teens.  Most of the Wednesday through
Sunday period should be dry, but there is a shortwave moving through
Wednesday and Wednesday night which could produce some light snow.
Plenty of frontogenesis out ahead of the wave, but forecast
soundings show a significant low level dry layer to overcome. Not
saying it can`t be done...but think mid-high chance PoPs are still
appropriate rather than likely.

Temperatures should start to moderate on Saturday (although it will
still be below normal) as southerly flow develops in the low levels
and the flow aloft becomes more zonal.  The ECMWF and GFS are in
reasonable synoptic agreement for Sunday with a shortwave and
attendant cold front moving across the Midwest.  GFS spreads warm
advection precip up across the area ahead of the front while the
ECMWF is dry.  Think this may be the GFS`s typical wet bias in
warm/moist advection so dry to only slight chance PoPs across
southeast Missouri look appropriate at this time.



.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1143 PM CST Sun Dec 4 2016

Uncertainty in some of the specific details for the TAFS persists
for the overnight and early Monday morning period. The stratus
will have cleared east of all the TAF sites by 06Z. Areas of fog
have already begun developing across parts of central and western
MO where clouds cleared earlier. Fog should continue to develop
across the remainder of the region overnight and into early Monday
morning. Still unsure how low visibilities may drop as there is
disagreement between some of the MOS guidance and higher
resolution short term models. I think IFR flight conditions with
fog are likely at both KCOU and KUIN, and it is certainly possible
that dense fog with visibility AOB 1/4SM may occur at those
locations as well. My confidence remains too low to get overly
pessimistic. For the St. Louis region, think MVFR flight
conditions/visibilities are more likely, although KSUS could drop
to IFR. Present thinking is any fog should dissipate by mid
Monday morning with VFR thereafter.

Rain and lower clouds associated with the next storm system will
begin to spread to impact the terminals after 06Z Tuesday.


Stratus has cleared to the east. With light winds and clear skies, I
still expect fog to develop but there remains uncertainty in how
the low the visibility may drop. I have opted for MVFR flight
conditions/visibility developing overnight. Present thinking is
any fog should dissipate by mid Monday morning with VFR

The next storm system will begin spreading rain into the terminal
around 06Z Tuesday, with flight conditions lowering to MVFR
between 06-12Z Tuesday.



Saint Louis     46  36  41  27 /   5  40  20   5
Quincy          46  31  37  23 /   0  10   5   5
Columbia        49  33  40  24 /   5  30   5   5
Jefferson City  51  34  41  26 /   5  40   5   5
Salem           46  38  44  27 /   5  50  40   5
Farmington      46  37  43  26 /  10  70  30   5


MO...Dense Fog Advisory until noon CST today for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Gasconade MO-Knox MO-Lewis MO-Marion MO-
     Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls
     MO-Shelby MO.

IL...Dense Fog Advisory until noon CST today for Adams IL-Brown IL-
     Pike IL.



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