Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 121145

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
545 AM CST Tue Dec 12 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 345 AM CST Tue Dec 12 2017

Clouds currently across the area are expected to exit the sern
portions of the CWA by mid morning. Some CU shud form along the nrn
flank delaying the clearing after sunrise. As the low system that
brought the strong gusty winds yesterday pulls ewd out of the
region, winds will diminish thru the morning and gradually become
wly. With few clouds expected today, have trended twd the warmer
guidance despite the nwly winds.

Winds become fairly light this evening with few clouds expected.
This shud allow for temps to drop quickly with sunset. Winds are
expected to become sly to swly and increase late tonight ahead of
the next clipper. This shud cause temps to become steady overnight
and slowly rise late tonight into Wed morning. Still, with the dry
air expected across the region, have trended twd the cooler guidance
expecting temps to drop quickly this evening especially in sheltered

With the very dry air in place and strong winds with gusts to 20 to
25 mph, much of the area will see heightened fire danger today.
While relative humidity values will drop into the lower 20 percent
or even into the teens, conditions are not as extreme as yesterday
with much lighter winds anticipated today.


.LONG TERM...  (Wednesday through Monday)
Issued at 345 AM CST Tue Dec 12 2017

Warmer temperatures on Wednesday due to low-mid level warm air
advection and southwest surface winds during the morning.  A
shortwave and associated cold front will move southeastward through
our area on Wednesday, but it appears that any precipitation will be
north and east of our forecast area along and left of the track of a
southeastward moving surface low where better low level moisture
will exist.  Colder conditions can be expected on Thursday well
after the passage of the cold front and as a strong surface ridge
builds into our area from the Plains. The operational GFS model, in
particular is now showing the potential for at least light QPF
across mainly central and southeast MO Thursday night, ahead of a
positively tilted upper level trough with embedded shortwaves and a
weak surface low reflection. The NAM model has only very light QPF
for our area, and the ECMWF is weaker at the surface and does not
have any precipitation for our area. For now will just include
slight chance pops across central into parts of southeast MO
Thursday night.  The temperature profile supports a precipitation
type of snow.  A warming trend will begin Friday afternoon due to
rising upper level heights, and surface winds backing around to a
southwest direction. Highs on Saturday should be at least 10 degrees
above normal for mid December.  The latest models were showing more
favorable conditions for precipitation on Sunday as another
positively tilted upper level trough approaches with a weak surface
wave moving along an associated cold front.  The best chance of
precipitation should be across southeast MO and southwest IL, just
north of the surface wave.  The ECMWF model solution is less
progressive than the operational GFS as it develops a southern
stream upper level low and keeps precipitation going into Monday
morning.  At this time this solution appears to be an outlier and
will lean toward the more progressive solution and end pops by
Sunday night.  It appears that the temperature profile will be warm
enough that this precipitation will be liquid rain.



.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 536 AM CST Tue Dec 12 2017

Strong and gusty nwly winds will diminish late this afternoon and
into the evening. As the winds diminish, winds will back to become
wly, then swly overnight and increase slightly. A strong LLJ will
move into the area late tonight and into Wed morning. With surface
winds being light, have add LLWS at all terminals.





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