Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 260450

Area Forecast Discussion
National Weather Service Lincoln IL
1150 PM CDT Fri Jul 25 2014

ISSUED 900 PM CDT Fri Jul 25 2014

Clouds advecting into the area from the northwest and looks like
they will hang around til sometime in the overnight hours. This
will likely hold temps from falling too much, but some sites
already near forecasted lows. So will be updating to adjust lows
in some areas for overnight. Update will be out shortly.



ISSUED 1150 PM CDT Fri Jul 25 2014

A mid level cig around 7kft has overspread the sites this evening,
but based on satellite trends, the southwest edge of the clouds is
becoming more scattered. So for SPI and DEC, will have them at
scattered with a TEMPO group for broken. PIA/BMI/CMI should stay
broken to overcast overnight but then begin to improve toward
morning and become scattered. Then scattered mid clouds will
continue, but at a higher level during the day tomorrow. As the
warm front slowly sinks into the area tomorrow afternoon, this
will become the focus for the development of thunderstorms late
afternoon and into the evening. Will start each site with VCTS and
then have a 2hr TEMPO group for when best chance of thunder on
station. This could get adjusted as the features become better
defined tomorrow afternoon. Will also keep VCTS in TAFs for
remainder forecast period, due to some uncertainty of timing.
Winds will be southeasterly overnight, but then become
southwesterly after the warm front lifts north through the area.
As the front gets pulled back south into the area during the
evening, winds will become variable. With the threat of severe
weather possible tomorrow evening, will have strong wind mentioned
with the TEMPO group.



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 draped 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.




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