Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180256
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
956 PM CDT Fri Mar 17 2017

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 353 PM CDT Fri Mar 17 2017

A short wave is fcst to track from near DLH this evng to NW PA by
Sat evng. A cold front assoc with the short wave currently extends
from N cntrl IL to the STL metro area to SW MO. Low clouds continue
to clear from NW to SE with the back edge roughly along I44 in MO
and I70 in IL. This is just in advance of the bndry which should
clear the sthrn zones this evng. Most guidance indicates FROPA will
be dry this aftn/evng but can`t rule out a couple light SHRAs across
the ern Ozarks...but the bulk of the activity should remain S of the
CWA overnight. 1030mb SFC ridge builds into the region tonight and
should keep conditions quiet thru Sat. CAA thru the day Sat will
keep temps in check. After some areas that cleared out by early aftn
today reached into the lower 70s...most locations will remain in the
50s tomorrow. The exceptions are portions of cntrl and SE MO which
could rebound into the lower 60s after a chilly start in the mid 30s
to near 40.

BM

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 353 PM CDT Fri Mar 17 2017

Saturday night will be relatively cold across west central and
southwest IL near the surface ridge axis where the best radiational
cooling will occur.  Rain may move into central MO as early as
Sunday morning as low-mid level warm air advection increases over
the area and low level moisture moves into the western portion of
the forecast area, although it appears that the GFS model may be a
little overdone on its pops and qpf Sunday and Sunday night.  Highs
on Sunday will be warmer as surface winds veer around to a southerly
direction on the backside of the surface ridge.  Elevated convection
is expected across northeast MO and west central IL late Sunday
night as 850 mb theta-e advection and moisture convergence increases
over this area on the nose of a west-southwesterly low level jet.
This convection will likely weaken Monday morning as it shifts
southeastward, but then redevelop across southeast MO and southwest
IL Monday afternoon along a southeastward moving cold front. Despite
cloud cover and at least scattered showers and a few storms, the
highs on Monday will be unseasonably warm, around 15 degrees above
normal.  The GFS model is now keeping the chance of post frontal
rain going Monday night through Tuesday evening, especially across
southeast MO and southwest IL with low-mid level warm air advection
over the cold front and weak northwest flow shortwaves moving
through the area.  The precipitation should finally shift south of
the entire forecast area by late Tuesday night as a surface ridge
over the northern Plains continues to build southward into the
region.  Will see a gradual cooling trend from Monday night through
Wednesday, although temperatures will never get much below normal.
Will likely see a break in the precipitation threat late Tuesday
night through Wednesday evening as an upper level ridge builds over
the region ahead of a developing storm system in the lee of the
Rockies.  Will also see the start of a warming trend beginning
Wednesday night and Thursday.  Convection will return to the area
late Wednesday night and Thursday as a warm front lifts north-
northeastward through the area.

GKS

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 955 PM CDT Fri Mar 17 2017

VFR conditions and dry wx will prevail at the TAF sites for much
of the valid period. The only item of concern continues to be an
area of MVFR CIGs to our north, now reaching the quad cities area.
Some new model guidance has backed off on low cloud extent, while
other model guidance has become more aggressive with it.
Considering its current position and flow continues to present a
threat to terminals near the MS river for early Saturday morning,
will continue the course of a period of MVFR conditions at UIN and
a SCT cloud layer around 2500ft for STL metro sites until a
clearer direction can be determined. See no strong reason to back
down from current forecast. Otherwise, NW surface winds will
persist for much of the valid period, only veering NE and
becoming light around sundown Saturday night.

SPECIFICS FOR KSTL: VFR conditions and dry wx to prevail.
Continuing to watch region of MVFR CIGs to the north as they drop
southward overnight and into early Saturday morning. Latest model
guidance continues to be split on how much impact it will have on
STL and with it still several hours out, will continue the SCT
layer between 14-18z with the understanding it may need to be
upgraded to a CIG later tonight. Otherwise NW surface winds
will persist until Saturday night when they become light and
variable as a RIDGE of high pressure builds in.

TES

&&

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

$$

WFO LSX



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