Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251121

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
521 AM CST Sat Feb 25 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 352 AM CST Sat Feb 25 2017

Cold advection and cyclonic flow in the wake of the winter storm
system that tracked across the Midwest and into the Great Lakes
yesterday is responsible for more typical February temperatures, a
brisk northwest wind and a few flurries this morning. High pressure
expected to build into the region this afternoon with clearing skies
and a relaxing pressure gradient. Should be ideal conditions for
radiational cooling tonight and have temperatures forecast in the
lower to middle 20s areawide.


.LONG TERM...  (Sunday through Friday)
Issued at 352 AM CST Sat Feb 25 2017

A rather active pattern will take shape from Sunday through midweek.
Rather fast quasi-zonal flow aloft will be present across the
Nation`s midsection at the beginning of this period. A low-amplitude
and positively tilted short wave trof will move across the region
within this fast flow late Sunday into early Monday. An established
low-level WAA regime will be present across the area and lift
associated with it, along with forcing/lift with the short-wave
and within the entrance region of the ULJ is expected to
contribute to an area of precipitation. The precip will primarily
impact the southern 2/3rds of the area on Sunday afternoon and
evening. The low levels are initially quite dry, however there
appears to be sufficient lift and mid level moisture for
measurable precipitation and at least higher chance pops. Diabatic
cooling could result in some brief mixed precipitation at the
onset in central MO, especially if this occurs a bit faster during
the morning hours.

While we have some slight chance pops on Monday across southern
portions of the CWA, I think it will be largely dry. Continued WAA
will once again return temperatures to above average levels. The
threat of showers will ramp-up on Monday Night as moisture
transport increases via a southwesterly LLJ, and as another fast-
moving low-amplitude short-wave traverses the area. The flow
aloft will become increasingly southwesterly heading thru Tuesday
as a broad upper trof over the western U.S. migrates east. This
will bring a cold front into the MS Valley and the evolution of
the front is where there is some disagreement amongst the models.
The GFS is a bit faster with the advancing NE-SW cold front into
NE MO by early Tuesday evening, then moving through the entire CWA
on Tuesday night/early Wednesday morning. Alternatively the ECMWF
and CMC GEM are slower with the front west of the CWA at 00Z
Wednesday, and a position near or just north of St. Louis at 12Z
Wed. There is good agreement that the air mass in advance of the
front will become increasingly unstable beginning on Tuesday, and
supportive of thunderstorms. Depending on how this whole system
evolves, there might eventually be a chance of snow across the
northern CWA on Wednesday well in the wake of the cold front
associated with frontogenetic forcing and forcing via the
migrating upper trof.



.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 520 AM CST Sat Feb 25 2017

MVFR stratus clearing from the northwest this morning with gusty
northwest wind subsiding by this evening as high pressure builds
into the region.

Few more hours of VFR stratus and flurries before clearing. Gusty
northwest wind will subside this evening as high pressure builds
over the area.





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