Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 031740
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1140 AM CST Tue Mar 3 2015

.UPDATE:
Issued at 725 AM CST Tue Mar 3 2015

Surface temperatures have begun to rise in earnest over central
and parts of northeast MO during the past hour as winds have
turned more from the due south. Further east, winds remain from
the SE and any temp rises, if any, have been much less. The
location of the 32F isotherm extends from just south of Kirksville
(IRK) southeastward to the northern STL Metro area and east
towards Vandalia, IL. We anticipate some acceleration in the
warming to expand areawide over the next hour or two as winds
continue to veer more S and eventually SW.

Couple that with pcpn...models have done a good job with
forecasting our brief light rain/freezing rain event at sunrise
and with little in the way of echo to the west behind it, lots of
very low clouds in its wake filling in behind it, and some weak
low level convergence from warm front slowly lifting north, should
see patchy drizzle for pcpn from time to time thru about early
afternoon.

Putting it all together, currently anticipate being able to allow
the Advisory to expire everywhere at 8am this morning with limited
areas still below freezing at that time and only very light pcpn
if any left.

TES
&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 403 AM CST Tue Mar 3 2015

At 09z, the critical 32F isotherm was draped near I-70 in MO and
then curved southeastward near the MS river with much of IL at or
below freezing.  There remains a decent dewpoint depression of
around 10 degrees in place for many locations, giving some room to
spare when pcpn does get going, to prevent too great of a temp rise
too soon.  SE surface winds are already in place, though, resulting
in some weak WAA.

The models are in decent agreement on a batch of light pcpn
developing over central MO in the next couple hours and then
propagating eastward down I-70 into STL metro and into southwestern
IL, developing and growing as it does so.  Given the decent dewpoint
depressions, the pcpn development should act to counter or severely
retard any temp rises for the next several hours until it passes
thru by mid-late morning.  This could be a key point, with the 32F
isotherm not really expected to budge much thru about 12-13z/06-07am
and should result in light FZRA/FZDZ for many areas near and north
of I-70 with the potential for a light glazing still in place.
While ice accums look to be pretty spotty for areas outside of
northeast MO and west-central IL, will maintain current alignment of
the Winter Wx Advisory since brief FZRA periods are still expected
throughout these areas.

Once we get past mid-late morning, the midday and afternoon hours
look pretty quiet from a sensible wx perspective with spotty drizzle
perhaps the only pcpn expected.  Pcpn for the event heading into
tonight will try to get going very late this afternoon but should
largely hold off until after sundown.

Cloudy skies will limit the warmup we could have, but gusty S-SW
winds will do their part, with temp readings ranging from the 40s
north of I-70, to the lower 50s for many areas south.

TES

.LONG TERM:  (Tonight through Monday)
Issued at 403 AM CST Tue Mar 3 2015

Cold front will drop southeast of our forecast area this evening.
Rain will continue this evening across southeast MO and southwest
IL, mainly southeast of STL due to low-mid level warm air advection
ahead of weak southwest flow shortwaves along with mid level
frontogenesis behind the surface front.  As the initially shallow
cold layer progresses southeastward the rain will change to freezing
rain, then briefly to sleet and finally to snow late tonight.  There
will likely be some minor ice and snow/sleet accumulations late
tonight southeast of STL.  Precipitation should ramp up on Wednesday
as a positively tilted upper level trough approaches with increasing
upper level divergence across our area in the right entrance region
of an upper level jet streak over the Great Lakes region.  The
strongest low-mid level warm air advection along with the best low
level moisture should continue to be across southeast MO and
southwest IL.  The NAM model appears a little too strong with this
low level forcing and also stronger with a southern stream shortwave
and hence has more qpf as well as its precipitation extending a
little further north than the other models.  Went closer to the
ECMWF model solution for Wednesday and Wednesday night as its a
compromise between the NAM and the GFS which has the precipitation
shield further south and is also lighter on the amounts.  There will
continue to be sleet and freezing rain Wednesday morning across
southeast MO and southwest IL, but as the cold layer deepens and the
elevated warm layer shrinks the precipitation should change over to
all snow by Wednesday afternoon.  The snow will taper off by late
Wednesday night and shift southeast of our forecast area.  With
still plenty of model uncertainty with regards to precipitation
amounts and how far north the significant snow will extend will just
issue a winter storm watch for Wednesday and Wednesday evening for a
small portion of southeast MO and southwest IL where there will be
at least a light glazing of ice Wednesday morning followed by the
potential of a total snow accumulation of around 4 to 7 inches.
Little diurnal temperature rise is expected on Wednesday with cold
air advection along with cloud cover and precipitation.  Wednesday
night and Thursday will be unseasonably cold as the strong surface
high moves southeastward through the region with the models dropping
the -16 degree C 850 mb isotherm southward to just south of STL by
Thursday morning.  Warmer temperatures are expected by Friday as the
surface wind comes around to a southerly direction as the surface
ridge shifts east of our area.  A weak cold front will move
southeastward through the forecast area on Saturday and Saturday
evening, but it appears that any precipitation will remain northeast
of our forecast area ahead of a northern stream shortwave and in the
vicinity of an associated surface low.  Only slight cooling is
expected for the remainder of the weekend after passage of the cold
front, mainly across northeast MO and west central IL. Temperatures
for the weekend into early next week should be close to seasonal
normals for early March.

GKS
&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1130 AM CST Tue Mar 3 2015

IFR/low end MVFR to persist across forecast area ahead of cold
front with winds picking up from the southwest. Winds to veer to
the west with frontal passage between 21z Tuesday and 00z
Wednesday, then to veer to the north after 03z-05z Wednesday.
Through forecast period, best chances of precipitation will be
just south of taf sites, so kept mention out for now. Cigs to lift
to VFR between 10z-16z Wednesday.

Specifics for KSTL:
IFR/low end MVFR to persist across metro area ahead of cold
front with winds picking up from the southwest. Winds to veer to
the west with frontal passage after 00z Wednesday, then to veer
to the north after 05z Wednesday. Through forecast period, best
chances of precipitation will be just south of metro area, so kept
mention out for now. Cigs to lift to VFR by 16z Wednesday.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...WINTER STORM WATCH from Wednesday morning through Wednesday
     evening FOR Iron MO-Madison MO-Reynolds MO-St. Francois MO-
     Ste. Genevieve MO.

IL...WINTER STORM WATCH from Wednesday morning through Wednesday
     evening FOR Randolph IL.

&&

$$
WFO LSX




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