Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 171936

Area Forecast Discussion
236 PM CDT Wed Sep 17 2014

.SHORT TERM...(Tonight)

A small MCS over southern MO into far northern AR associated with a
vort max with the short wave tracking ESE into mid MS river valley
into this evening and staying south of the forecast area. However
some weak isentropic lift on northern fringe of system affecting our
sw counties into mid afternoon with isolated showers and even one
or two thunderstorms. The models show this isentropic lift
diminishing later this afternoon and early evening as it shifts
southward, so going to keep a dry forecast tonight. Broken to
overcast mid level clouds should also decrease from the north while
scattered to broken cumulus clouds of 4-6k ft also dissipate late
today. Weak high pressure of 1018 mb over central IN and southeast
IL will weaken as a stronger 1024 mb Canadian high pressure over
Manitoba settles sse into the upper Great Lakes by Thu morning. This
will provide the fair weather tonight into Thu along with turning
SSE winds more ENE and staying fairly light. With some drier air
advecting in from the ne, do not think fog will be as prevalant
overnight where we had widespread dense fog in southeast IL, but can
not rule out a few patches of shallow ground fog over southern and
western areas where dewpoints will be a few degrees higher and also
had some light rain sw of Lincoln today along with extensive cloud
cover. Lows in the upper 40s and lower 50s with coolest readings
from Lincoln northeast.


.LONG TERM...(Thursday through Wednesday)

Weak low-level cold advection behind shortwave moving through the
area today will keep temps from warming much compared to today`s
highs despite more sunshine and increasing 500mb heights.  By Friday
afternoon 850mb temps respond to the building ridge and begin to
climb back toward seasonal normals.

Ridge begins to flatten as weekend approaches. Mid-level heights
build over western North America. Energy now off west coast is
expected to split into a weak cutoff along the California coast and
a series of shortwaves topping the developing western North America
ridge. Although the pattern shift is similar between the operational
models and NAEFS mean, the relative strength of the individual
shortwaves have implications on the timing of precip chances through
the weekend. Latest ECMWF is weaker with initial wave expected to
move across the area Saturday afternoon, while 17/12z GFS is strong
enough to produce sufficient lift and cold advection aloft for the
development of thunderstorms. 17/00z is weaker with this initial
wave and produces more spotty precip.

Despite increasing clouds, breezy south to southwest winds will
allow temps to climb into the 80s Saturday as models are consistent
in enabling 850mb temps to reach 16-18C by 12z.

Clouds may have a hard time clearing on Sunday and even some precip
is possible during the day in southeast Illinois as second wave in
the trough moves across the area.  Low-level cold advection kicks in
Sunday Night following this second wave, but mid-level heights begin
climbing again by Monday Night. Although, temps will likely be below
normal through midweek, the models no longer reflect the very cold
temperatures that were progged by the EMCWF several days ago.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)

Carried chance of rain showers at SPI until mid afternoon with
vsbys 5-7 miles as isolated showers drift ESE. May need to carry
VCSH at DEC but think brunt of showers will pass sw of Decatur.
Showers and thunderstorms with small MCS over southern MO to track
ESE into mid MS river valley during afternoon and we will be on ne
fringe of this weather system today with some isentropic lift
supporting the isolated showers in our sw counties. Scattered to
broken clouds of 3-5k ft to occur this afternoon along with broken
to mid level clouds around 9k ft and scattered at times along I-74
TAF sites of PIA...BMI and CMI. Clouds should decrease later this
afternoon and early evening leaving fair skies tonight. SSW winds
5-10 kts this afternoon to become light ENE this evening and then
east around 6 kts Thu morning. Have light fog with vsbys 4-6 miles
from 08Z-14Z.




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