Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 190005
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
605 PM CST Sat Feb 18 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 307 PM CST Sat Feb 18 2017

Upper level low to track east of region tonight. So any lingering
light rain/drizzle over southeast MO, southwestern IL should taper
off by 00z Sunday. Otherwise, main forecast issue for tonight will
be cloud cover. Central/northeast MO and west central IL have had
quite a bit of sunshine today, while rest of area was cloudy due to
upper low. As the low tracks east, should see clouds scatter out
everywhere by midnight. With less cloud cover and light winds, lows
will be a bit lower tonight, in the 40s.

Upper level ridge to build in on Sunday with southerly surface winds
returning. So with warm south winds and plenty of sunshine, highs
will warm up into the upper 60s to mid 70s.

Byrd

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 307 PM CST Sat Feb 18 2017

Parts of southeast MO have been fortunate to see some nice steady
light rain today. However for most of the region, we are quite dry
and experiencing a signifcant rainfall deficit. We are anxiously
eyeing a system for Monday into early Tuesday that could bring us
some widespread precipitation.

The elongated upper trof currently across the far western U.S. is
forecast to move from the Rockies into the Plains Sunday night into
Monday, and then into the MS Valley Monday night, before
advancing quickly eastward on Tuesday. The latest model runs are
all a bit slower with the evolution/onset of precipitation, and it
appears we may not see any rainfall in the CWA before 12Z Monday.
Current thinking is a north-south oriented band of rain/showers
will move slowly eastward across the area Monday-Monday night
ahead of the progressive upper trof and attendant cold front.
Increasing south-southwesterly low level flow will transport
deeper moisture back into the region ahead of the trof/front -
moisture we haven`t seen in awhile. This should support both
widespread coverage and longevity of the rain band as it moves
east given ample large scale forcing. The moisture return will
also promote weak elevated instabily and potential for a few
thunderstorms. Any model differences that exist are associated
with speed and timing. The ECMWF is exceedingly slow with the
eastward spread of moisture and rain on Monday. It along with the
NAM also suggests that precipitation may linger across parts of
southwest IL on Tuesday morning, owing to a slower progression of
the weaking and shearing upper trof and attendant cold front. As
far as the front, the air behind this front is not appreciably
cooler and quickly modifies thus temperatures will remain well
above average on Tuesday.

There is a brief and general deamplification of the flow aloft
Tuesday night into early Wednesday, and then the next Pacific upper
trof moves inland thru the western U.S. bringing west-southwest flow
aloft and the onset of a pronounced WAA regime. Wednesday looks to
be another day of well above average temps. Model differences
exist Thursday into early in the weekend with the southern exent
of a west-east boundary and potential impacts on temperatures/precipitation
on Thursday, and then the track/speed of the ejecting western
upper trof and associated deepening surface low and cold front.
There should be at least some locations seeing unseasonably warm
temperatures ahead of the system on Thuesday, as well as a threat
of thunderstorms. A changeover from rain to snow on the backside
of the system for parts of northeast MO/west central IL is something
we will have to monitor late Friday/Friday night.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 549 PM CST Sat Feb 18 2017

Mid-high level cloudiness will advect out of the St Louis metro
area this evening as an upper level disturbance moves well
southeast of our region. The surface wind will become light this
evening with weak surface ridging over the area. May have light
fog developing late tonight in COU and the St Louis metro area due
to a clear sky, light surface wind and relatively high surface dew
points. The CPS area may have the best potential for fog due to
limited diurnal heating, mixing and drying this afternoon. The
fog will dissipate by late Sunday morning with just some high
level clouds expected. The light surface wind will become south-
southeasterly by Sunday afternoon as the surface ridge shifts east
of the region.

SPECIFICS FOR KSTL: Mid-high level cloudiness will advect out of
the STL area this evening as an upper level disturbance moves
well southeast of our region. The surface wind will become light
this evening with weak surface ridging over the area. May have
light fog developing late tonight due to a clear sky, light
surface wind and relatively high surface dew points. The fog will
dissipate by late Sunday morning with just some high level clouds
expected. The light surface wind will become southeasterly Sunday
afternoon as the surface ridge shifts east of the region.

GKS

&&

.CLIMATE...
Record Highs

      2/19    2/20    2/21    2/22    2/23

STL: 77/2016 78/2016 79/1935 78/1995 77/1996
COU: 76/1930 76/2016 76/1935 76/1995 73/1933
UIN: 71/1930 72/2016 69/1930 70/1922 70/1922


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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