Area Forecast Discussion
Issued by NWS St. Louis, MO
FXUS63 KLSX 180051
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
651 PM CST Tue Jan 17 2017
.SHORT TERM... (Through Late Wednesday Afternoon)
Issued at 324 PM CST Tue Jan 17 2017
Skies have remained cloudy this afternoon as low level moisture
remains trapped under an inversion. Latest regional satellite does
show that the clearing line is currently over far eastern Kansas.
RAP low level progs suggest that drier air will work into the area
during the late evening and overnight hours allowing low clouds to
erode. This will be about the same time a surface ridge will move
into the area causing the winds to turn light allowing some patchy
fog to form, particularly across central Missouri where the clearing
will occur first. This fog will remain over the area into the
morning hours before it burns off by mid morning.
Should see more sun on Wednesday as a shortwave ridge moves across
the area. More clouds will move into southeast Missouri late in the
day as mid-high level moisture streams northeastward ahead of an
upper low in the Southern High Plains.
Lows tonight will not fall much with the clouds staying in most of
the night. Went with a compromise of MOS guidance for low tonight.
With sunshine tomorrow and winds turning out of the south, highs
will once again be close to the agreeable MOS highs.
.LONG TERM... (Wednesday Night through Next Tuesday)
Issued at 324 PM CST Tue Jan 17 2017
The models remain in relatively good agreement with the large
scale mass fields this cycle depicting a stout ULJ emanating from
the Pacific and contributing to an energetic lower latitude storm
The upper low currently over the southwestern U.S. will lift
northeast into the central Plains Wednesday afternoon/night. Then
the main upper low takes a more northerly track as a strong
negatively-tilted short wave rotates around it and thru the Lower
and Mid MS Valley region Thursday afternoon/night. The models while
still showing some differences with respect to QPF with this system,
have trended closer and generally all show more backed low and mid
level flow. This results in a faster and more westward shift of the
warm conveyor belt and associated moisture transport, beginning on
Wednesday night and increasing on Thursday. The combination of large
scale ascent and the moisture transport on Thursday will
correspondingly lead to greater coverage and northward/westward
extent of precipitation. Moderately steep mid level lapse rates will
also support a threat of at least isolated thunderstorms.
Friday and Friday night look dry in the wake of this departing first
system. Another upper low and trof will lift north/northeastward
from the southern/central Rockies and through the central U.S.
Friday night and Saturday morning bringing deepening south/southwest
flow aloft. The are some decent height falls and weak large scale
forcing for ascent on Saturday. Moisture however seems pretty
minimal in the wake of the previous system and a bit low for much
in the way of any decent precipitation threat.
The eye-catching system is still on track to impact the region late
Saturday night-Monday as a potent upper low tracks across the
southern Plains and across the lower/mid MS Valley. The guidance has
all shifted south with this system and present indications the main
impact would be for the southern/southeast 2/3rds of the CWA with
a good rain event.
Mild, above average to well above average, temperatures will
prevail thru the forecast period.
.AVIATION... (For the 00z TAFs through 00z Wednesday Evening)
Issued at 558 PM CST Tue Jan 17 2017
Ridge of high pressure over western MO will move eastward through
our area later tonight and Wednesday morning. Stratus cloud deck
around 1000 feet in height will continue this evening, but
clearing line over the KS MO border was advancing slowly eastward.
Model guidance continues to shift the clearing line eastward
tonight and Wednesday, although there is still some uncertainty
with the timing of the clearing, especially during the overnight
hours. For now will scatter out the low level cloud ceiling in COU
at 05Z, UIN at 11Z, and in the St Louis metro area at 10-11Z. At
least light fog will develop late tonight, especially where it
clears out well before sunrise. Westerly surface winds will back
around to a south-southwest direction Wednesday morning as the
surface ridge shifts east of our area.
SPECIFICS FOR KSTL: Ridge of high pressure over western MO will
move eastward through our area later tonight and Wednesday
morning. Stratus cloud deck around 1000 feet in height will
continue this evening, but clearing line over the KS MO border was
advancing slowly eastward. Model guidance continues to shift the
clearing line eastward tonight and Wednesday, although there is
still some uncertainty with the timing of the clearing, especially
during the overnight hours. For now will scatter out the low level
cloud ceiling in STL at 10Z. Some light fog will develop shortly
after it scatters out. Westerly surface winds will back around to
a south-southwest direction Wednesday morning as the surface
ridge shifts east of our area.