Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 240915
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
315 AM CST Sat Jan 24 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 315 AM CST Sat Jan 24 2015

The upper level TROF axis of a storm system extended from near
Louisville, KY to Jackson, MS and represented the backedge of a
large shield of precipitation as well.  In this storm system`s wake
over our region, we have NW flow aloft and mostly clear skies.  SW
surface winds were keeping temps from dropping too much, in fact,
with some rises since early evening.  Readings were close to the
freezing mark at 08z.

A quiet wx day is expected today with persistent NW flow aloft and
westerly winds at the surface.  An Alberta clipper system will surge
towards our region this afternoon, with its outer lobes of vorticity
already edging into northern MO by dusk.  But the atmospheric column
will be too dry for these outer lobes to be able to produce any
pcpn, yet.

Plenty of sunshine is expected for much of the day--until late
afternoon--and so should allow the anticipated somewhat deeper
mixing levels from the westerly winds to push temps into the 50s for
much of our region.  MOS has been struggling to keep up with temps
in these types of situations and went above MOS at all locations.

TES

.LONG TERM:  (Tonight through Friday)
Issued at 315 AM CST Sat Jan 24 2015

The Alberta clipper system is still on track for later tonight and
Sunday, but the latest model trends are a bit more southwesterly in
track, and depict a deeper system overall--especially as the system
tracks thru our region.  If it comes to fruition, a deeper system
will be able to tap into more cold air as well as be able to keep
more precipitation on its backside, leading to more snow (at least
the appearance of) than previously anticipated.

While the system is expected to have a lot of forward speed thru
most of tonight, there is a good model consensus on the forward
speed then slowing down significantly as it deepens--and even closes
off--just before entering our region and then proceeding at a much
slower rate of speed across our region during the daylight hours of
Sunday.  Timing-wise, then, it first looks like we will have a more
prolonged event:  beginning it with what will probably be a
quick-hitting band of light rain that will develop right over our
region--perhaps as early as the late evening hours but more probable
after midnight--with scattered coverage precipitation heading into
Sunday daytime for most locales, with a more solid area of
precipitation as one heads towards central IL, especially northeast
of a UIN-SLO axis.  This axis is where the new model consensus has
the surface low track which is usually pretty critical delineator
between solid pcpn coverage on the left and much more scattered
coverage on the right.  Deep cyclonic curvature should maintain some
presence of precipitation into the evening, especially for areas SE
and E of STL.

That said, cold air looks to be a little more stout later in the
event as well.  Temps will begin the event rather warm thanks to
increasing clouds and SW winds ahead of the system--in the 35 to 40
range--but with early onset of some CAA and it strengthening from
late morning on, should see falling afternoon temps at all
locations.  The air aloft will support snow and it will all boil
down to boundary layer and surface temps.  At that point, it will
merely be a race on how fast surface temps can reach the lo-mid 30s
before the precipitation pulls out.  The deeper system depicted over
the last two runs favors increased mention of snow as a result, but
other than areas in west-central IL extending to the lower Wabash
valley, snow is currently only anticipated to make a brief
appearance, if that, before the precipitation ends.  Areas from UIN
to the lower Wabash may get a dusting of snow and are the only areas
depicted with snow accums with this package.

The system will be something to watch, with the trends heading
towards an increasingly snowier forecast.

Otherwise, a weak front late Monday is still expected to pass thru
dry with seasonable temps holding steady for early next week.  A
shortwave TROF may deliver some pcpn on Wednesday night, but the
track currently is a bit too far north and so only have slight
chances in for now.

An even milder period looks on tap for Wednesday and Thursday next
week with an upper level ridge building overhead and S-SW flow at
the surface.

Lurking just beyond the extended is an Arctic boundary for next
weekend with perhaps a winter storm.

TES

&&

.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1137 PM CST Fri Jan 23 2015

Weak cold front/trof has moved into northern Missouri and will
continue to slide southeast through forecast area through the
overnight hours. Will just see mid and high clouds with it.
Otherwise, light and variable winds to become southwesterly then
veer to the west behind the boundary. By this evening, the winds
will back to the southwest ahead of next weather system.

Specifics for KSTL:
Weak cold front/trof has moved into northern Missouri and will
continue to slide southeast moving through the metro area by 12z
Saturday. Will just see mid and high clouds with it. Otherwise,
light and variable winds to become southwesterly then veer to the
west behind the boundary. By 02z Sunday, the winds will back to
the southwest ahead of next weather system.

Byrd

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX




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