Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 182118
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
318 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 18z TAFs through 18z Sunday Afternoon)
Issued at 1148 AM CST Sat Feb 18 2017

Upper low over southeast Missouri will continue to slowly slide to
the east. So with taf sites to the north and northwest of system,
VFR conditions to prevail. Main question is precipitation
development. HRRR model indicating some increased shower activity
over STL metro area, so added VCSH mention through mid to late
afternoon today. As for winds, west to southwest winds to prevail
before diminishing to light and variable early this evening.

SPECIFICS FOR KSTL:
Upper low over southeast Missouri will continue to slowly slide to
the east. So with metro area to the north of system, VFR
conditions to prevail. Main question is precipitation development.
HRRR model indicating some increased shower activity over STL
metro area, so added VCSH mention through 21z today. As for
winds, southwest winds to prevail through tonight before backing
to the southeast on Sunday. So that means a cross wind at KSTL
through at least early this evening.

Byrd

&&

.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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