Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 242114
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
314 PM CST Tue Jan 24 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 313 PM CST Tue Jan 24 2017

A maturing low pressure system over the plains will move into MO
tonight and continue northeastward towards the Great Lakes on Wed.
Rain showers are possible across northeastern MO and west central IL
until the upper trough moves east of the region.

Early afternoon METARs over the central CONUS clearly depicted a
surface circulation over western KS in addition to a surface
baroclinic zone which extended from southeastern CO through KS and
into southwestern MO and northern AR. The warm front will pivot and
lift into MO tonight in tandem with the lifting surface low. The
initially narrow warm sector does not lift completely through the
CWA overnight, and the trailing cold front then quickly moves
through the CWA on Wed morning. This will lead to an initially non-
diurnal temperature trend featuring rising temperatures across the
southeastern CWA tonight and falling temperatures across the
northwestern half of the CWA on Wed morning. A modest rebound is
expected during the afternoon. The tightening surface pressure
gradient along with strong winds aloft will also lead to gusty W/WSW
winds on Wed.

42

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 313 PM CST Tue Jan 24 2017

The synoptic pattern at the start of the long term period will
feature a rather strong PV anomaly sliding into portions of the
Midwest and Great Lakes region.  In response to this feature, a
surface low will deepen and push east of the region on Thursday,
with northwest upper-level flow prevailing in its wake through the
remainder of the period.

By 00Z Thursday, the surface low (mentioned in the short term
section above) will be centered across southern Lake Michigan, with
a cold front having pushed east of the LSX CWA.  Despite the surface
low and PV anomaly pushing east, a weak area of mid-level
deformation on the northwest side of the cyclone will attempt to
pivot east/southeast Wednesday night into Thursday morning. Forecast
soundings suggest this may bring just enough lift/saturation to
squeeze out a period of light snow (possibly mixed with rain
initially) to portions of western IL and far northeast MO.  Given
the fairly weak forcing and short duration of any precipitation, no
snowfall accumulations are expected.

Guidance is then suggestive that additional snow showers will be
possible on Thursday.  However, a thorough examination of
soundings/cross-sections show moisture will be fairly shallow.
Additionally, in the wake of the passing trough, mid/upper-level
heights will be on the rise inferring some mid-level subsidence. May
need to include mention of flurries Thursday morning into early
Thursday afternoon in coming forecasts, but will leave any
measurable precipitation out of the forecast for now given the
reasons above.

Otherwise, the pattern late week into the upcoming weekend will
continue to feature northwest flow aloft.  As mentioned by the
previous forecaster, these regimes are inherently difficult to
predict given fast-moving/compact waves within the flow that
typically aren`t sampled well.  It appears the best signal for the
next chance of measurable precipitation is late Saturday night into
Sunday.  Guidance has trended a bit quicker/weaker with this system,
but enough agreement exists to keep slight chance pops in the
forecast.  Otherwise, given the northwest flow regime, high
temperatures through the period will be cooler than temperatures as
of late, with readings generally in the 30s Thursday through early
next week.

KD

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1134 AM CST Tue Jan 24 2017

SPECIFICS FOR KCOU, KUIN: MVFR stratus will continue moving away
from the terminals early this afternoon, but additional stratus is
expected to reach KUIN after 25/03z. It is unclear attm whether
the overnight stratus will extend far enough south and west to
reach KCOU. LLWS is expected at both terminals overnight due to a
strong LLJ ahead of a developing low pressure system. Winds will
turn westerly after 25/09z after a cold front moves across the
region. Wind gusts will increase again by mid-morning.

SPECIFICS FOR KSTL, KSUS, KCPS: Gusty winds will persist through
the afternoon and into the evening hours. LLWS is expected
overnight due to a strong LLJ ahead of a developing low pressure
system. Winds will turn westerly after 25/12z after a cold front
moves across the region. Wind gusts will increase again by mid-
morning.

42

&&

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

$$

WFO LSX


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