Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 252329

Area Forecast Discussion
National Weather Service Lincoln IL
629 PM CDT Fri Jul 25 2014

ISSUED 220 PM CDT Fri Jul 25 2014

SHORT TERM...Tonight through Monday

Main forecast challenge revolves around the threat for severe
thunderstorms tomorrow afternoon and evening as a rather vigorous
upper wave shifts southeast into the upper Great Lakes. A secondary
concern will be with the Heat Index values forecast around 100
tomorrow afternoon.

Warm front forecast to shift northeast across the area tonight
with the better chances for storms along and to its northeast
tonight. That would put the higher pops across the east with
areas in the warm sector seeing a fairly significant warm layer
push in from the west. That capping inversion is now forecast to
break down quicker than earlier forecast as the upper dynamics
associated with the shortwave arrives sooner into our area tomorrow
afternoon. Any morning storms in the east should push out with a
frontal boundary drapped across parts of central Illinois as the main
upper level system approaches. Storms are expected to quickly initiate
tomorrow afternoon and spread east-southeast over central through
east central and southeast Illinois thru the evening hours. Extreme
instability levels and strong shear (Sfc base capes around 4000 j/kg,
and effective shear values 40-50 kts) suggest that supercells may
develop initially before translating into a fast moving Quasi Linear
Convective System by evening. Primary threats will be damaging wind
gusts, large hail and torrential rainfall. Any persistent rotating
storms that interact with the boundary across central Illinois have
the potential to produce a tornado or two early, before translating
to a fast moving line of storms by late afternoon or evening. Models
indicate the low level jet will veer rather quickly into a west
southwest component early tomorrow evening suggesting the potential
for the storms in the evening to back-build, increasing our chances
for heavy rainfall as well.

What`s left of the storms Sunday morning will be affecting southeast
Illinois with the threat for some heavy rainfall. Models indicate
the initial front should be pushing off to our east by mid-afternoon
so there is still a narrow window of opportunity for redevelopment
over parts of southeast Illinois in the afternoon on Sunday, with
low chance pops remaining to the northwest during the afternoon
hours. The cooler and drier air mass will start to settle southeast
into our area Sunday night and Monday as a large trof deepens in north
of the Great Lakes for early next week.

LONG TERM...Tuesday through Friday

Ensembles in reasonable agreement with respect to the unseasonably
deep trof over the eastern portion the country thru at least the first
half of the work week. Only question during this period will be with
several shortwaves forecast to rotate southeast into the lower Great
Lakes coupled with some rather cool temperatures aloft which may bring
us some diurnally driven showers later in the week. That would start
on Wednesday across our far northeast, with the instability shower
threat spreading over the remainder of the forecast area on Thursday
and Friday. Based on the 850 mb temp forecasts during this period, the
coolest temps will be Tue and Wed, with some day to day moderation seen
for the remainder of the period.



ISSUED 629 PM CDT Fri Jul 25 2014

Appears clouds are moving into the area from the northwest, but
should only affect PIA and BMI, and maybe CMI. SPI and DEC should
see just scattered clouds overnight. Then more CU will be possible
at all sites tomorrow as the front drops into the area overnight.
This front will be the focus for showers and thunderstorms to
develop across the area in the afternoon. Looks like the greatest
risk for thunderstorms will be in the late afternoon through the
evening. So, will just have VCTS in the TAFs for now, starting at
around 20z...though it may be a little later than that. Winds will
be southerly most of the period, until the front arrives tomorrow.
Then winds will be variable.



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