Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 131138

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
538 AM CST Wed Dec 13 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 353 AM CST Wed Dec 13 2017

Main focus today is the fire danger threat and strong winds.

This clipper is expected to be similar to the system on Mon,
although, not quite as warm in some areas. Southwest winds will
increase late this morning and veer to become wly, then nwly this
afternoon as a cdfnt pushes thru the area. Am concerned that going
forecast for wind speed is not high enuf and an wind advisory may be
needed. Main areas of concern are nrn counties late this morning and
thru the afternoon, and ern portions of the CWA for this afternoon
and into the early evening hours. Have held off on an advisory for
now with mdls suggesting winds diminishing slightly as the low moves

Have trended aoa warmer guidance for the entire CWA given the very
strong WAA and later arrival of the cdfnt. Going forecast may still
not be warm enuf given some guidance, like the RAP, suggesting
warmer temps. However, with lots of questions among mdls regarding
cloud cover, did not want to go too warm attm.

With deeper mixing, dewpoints will remain cool today, especially
within the warm sector. This will result in RH values dropping below
30 percent for the srn half of the CWA. The low RH values coupled
with the strong winds and dry fuels will result in a heightened fire
danger thru this afternoon. If temps are too low as mentioned above,
these dangerous conditions may extend across much of the CWA. Will
also need to monitor conditions for a possible Red Flag Warning with
going forecast being near criteria across portions of SE MO.

Low chance of any precip with this system, but have kept a mention
of sprinkles along the cdfnt as it passes thru the area.


.LONG TERM...  (Thursday through Tuesday)
Issued at 353 AM CST Wed Dec 13 2017

Colder high temperatures can be expected on Thursday as a surface
ridge over the Plains builds east-southeastward into our area.  Low
level cloud cover will also hinder daytime heating.  A positively
tilted upper level trough will approach our region Thursday night
from the northern Plains.  Most of the models dry up the
precipitation before it gets into our forecast area, although the
operational GFS does have very light QPF late Thursday night and
Friday morning mainly across northeast and central MO.  Do not think
we will get any measurable precipitation, but we may get a few
flurries.  Will see the start of a warming trend Friday afternoon
with much warmer temperatures Saturday as upper level heights rise
as an upper level ridge approaches the region, and the surface wind
becomes south-southwesterly as the surface ridge shifts southeast of
our area.  Precipitation will spread northeastward into our forecast
area late Saturday night and Sunday morning ahead of a southern
stream shortwave coming out of the southern Plains.  The operational
GFS is a little slower bringing precipitation into our area than the
ECMWF model.  The bulk of the precipitation should shift east of our
forecast area already by Sunday afternoon, but there may be patchy
light precipitation Sunday afternoon and evening with the approach
of a deep northern stream upper level trough. There was still some
model uncertainty as to the possible phasing of the northern and
southern stream energy and the timing of the precipitation.  It
appears that the temperature profile will be warm enough that most
of the precipitation will fall as liquid rain, although there could
be some snow mixed in with the rain late Saturday night and early
Sunday morning.  Despite a return to northwest upper level flow by
Tuesday, high temperatures should continue above normal.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 523 AM CST Wed Dec 13 2017

Swly winds will increase with gusts to around 30 kts and become
wly this morning. Winds will become nwly this afternoon as a cold
front pushes thru the area. MVFR cigs are expected behind the
cdfnt. A fair amount of uncertainty exists regarding how long
these MVFR cigs will linger, but have kept these cigs a little
longer for now. No precip is currently anticipated other than a
chance of sprinkles at KUIN this afternoon.





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