Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 070926
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
326 AM CST Thu Dec 7 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 325 AM CST Thu Dec 7 2017

Relatively quiet (but cold) weather expected today as a positively
tilted longwave trof digs southeast through the Midwest and southern
Great Plains today.  Strong low level cold advection combined with
morning low clouds are expected to limit the diurnal rise today.
Think operational MAV/MET guidance is a few degrees too warm given
the conditions so have undercut them by 3-5 degrees.  Cold advection
stratus/strato-cu may produce a few flurries along and east of the
Mississippi this morning (a few stations in southeast Iowa and west
central Illinois are reporting flurries at this time) before the
clouds thin out and dissipate this afternoon.

The ridge axis will shift southeast of the forecast area overnight
tonight which will turn the wind back around to the southwest. There
will be a period where the wind goes light and variable though...and
the sky should be clear or mostly clear.  This will result in
excellent radiational cooling conditions and temperatures are likely
to bottom out in the mid to upper teens tonight...probably colder in
low lying and other well sheltered areas.

Carney

.LONG TERM...  (Friday through Wednesday)
Issued at 325 AM CST Thu Dec 7 2017

The positively tilted digging trof looks like it will begin bottoming
out on Friday and take on a neutral tilt by Saturday afternoon.  The
longwave pattern then becomes more or less stationary with a trof
over the eastern CONUS and a strong ridge over the west.  This
leaves the Mississippi Valley in northwest flow aloft with at least
two vigorous shortwaves diving southeast across the area and
possibly another weaker one toward the end of the period.  While
the overall pattern should b e cooler than what we`ve experienced
over the past few weeks, the devil will be in the details with the
passage of each shortwave, the warm advection ahead of them, and the
cold fronts that are driven south by them.

Medium range guidance has been and continues to be fairly consistent
with the timing on these shortwaves.  The first wave is forecast to
move through the Midwest early Saturday with the cold front trailing
it slightly.  This would put Friday on the warm advection side of
the wave, and Saturday on the cold.  While MOS highs are only a few
degrees apart, I think this is deceiving as Saturday`s high
temperature may occur during the morning...or even during the
predawn hours depending on when the front actually passes through
the area.  The raw MOS tables tell the tale with non-diurnal
temperatures at many stations on Saturday.  Obviously trying to time
a cold front tied to a fast-moving shortwave is a low confidence
forecast, but will continue with the below normal temperatures on
Saturday as there seems to be decent run-to-run consistency and
between models.  The surface ridge moves quickly south of the area
bringing the low level flow back around to the southwest for Sunday
and Monday.  Think these days will warm up back toward normal or
even above normal.  Consensus MOS guidance is showing highs near 50
in central Missouri to the low to mid 40s further north and east.
GFS and ECMWF have the second shortwave moving into Missouri between
12-18Z Monday along with the associated cold front...so we may be
faced with another non-diurnal curve though I think it`s too far out
yet to get that specific.

GFS and ECMWF diverge slightly Tuesday into Wednesday with the EC
being faster moving the trailing ridge axis south of the region on
Tuesday.  This would tend to lead to a warmer forecast for Tuesday
than previously expected.  The slower GFS still keeps us in cold
advection for most of the day.  Will lean toward the colder solution
for consistency this morning, but this period will need to be
watched closely as bust potential is high.  Guidance has the third
wave moving through late Tuesday night which turns the low level
flow around to the northwest yet again.  However the longwave trof
is taking on a positive tilt by that time and is lifting out of the
eastern CONUS.  This reorients the low level baroclinic zone to a
more northwest-southeast direction which limits the cold advection
potential.  Therefore think Wednesday`s temperatures will be
somewhat warmer than Tuesday`s despite the low level northwest
flow.

Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1134 PM CST Wed Dec 6 2017

The secondary cold front has passed through all of the terminals
and northwest winds gusting 16-21kts will continue overnight and
through Thursday morning. Any additional precipitation in the area
will be negligible and consist of no more than very light
flurries.

Periods of MVFR flight conditions due to cigs of 2500-3000 feet
are expected into Thursday morning. A band of clouds with MVFR
cigs centered through the St. Louis region will continue to move
southeastward and it will be followed by another band of clouds
with MVFR cigs moving out of southern IA into northern MO. I have
been a bit pessimistic on the longevity of the clouds and lower
cigs and they may not persist as long as currently forecast. After
the clouds clear on Thursday morning, VFR flight conditions will
prevail.

SPECIFICS FOR KSTL:

The secondary cold front has passed through KSTL and northwest
winds gusting 16-20kts will continue overnight and through
Thursday morning. Any additional precipitation in the area will be
negligible and consist of no more than very light flurries.

Periods of MVFR flight conditions due to cigs of 2500-3000 feet
are expected into Thursday morning. A band of clouds with MVFR
cigs centered through the St. Louis region will continue to move
southeastward and it will be followed by another band of clouds
with MVFR cigs moving out of southern IA into northern MO. I have
been a bit pessimistic on the longevity of the clouds and lower
cigs and they may not persist as long as currently forecast at
KSTL. After the clouds clear on Thursday morning, VFR flight
conditions will prevail.

Glass

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     32  19  42  28 /   5   0   0   5
Quincy          30  15  38  22 /   5   0   5   5
Columbia        31  17  40  25 /   0   0   5   0
Jefferson City  32  17  42  24 /   0   0   5   0
Salem           33  16  39  26 /   5   0   0  10
Farmington      32  16  40  24 /   0   0   0   0

&&

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

$$

WFO LSX



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