Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 260225

Area Forecast Discussion
925 PM CDT Mon Aug 25 2014


Outflow boundary from this afternoon and early evening storms has
shifted into south central thru southeast Illinois. Cold pool
has brought some gusty east to northeast winds along and just
behind the outflow boundary with a marked decrease in temps
and dew points. Quick look at the latest Nam-Wrf model indicates
the cool front will be located from near Danville southwest thru
Quincy by mid-morning Tuesday. With winds shifting into the
north behind the boundary coupled with more cloud cover and
possible showers and thunderstorms, it appears the apparent
temperatures across the north and northeast will not meet heat
advisory criteria. As a result, will go ahead and cancel the
heat advisory for our far west, north and northeast counties with
only a small portion of the heat advisory remaining near and south
of Champaign. No changes were made to the Excessive Heat warning
at this time. Will have an updated NPW out highlighting the areas
that will be dropped and will update the ZFP once again this



Storms continue to slowly diminish early this evening and expect
that trend to continue over the next few hours as we lose daytime
heating and the storms track further south into the upper ridge.
Hi-res short term models providing mixed signals as to when
additional thunderstorms develop/move into our area late tonight
as a cold front edges slowly into western Illinois by morning.
Not very confident on any one solution at this point but it appears
the low level jet and 850 mb moisture convergence will be more
supportive of storms developing over Iowa late tonight and then
track into our west during the early morning hours. Series of
shortwaves expected to slowly break down the upper level ridge
and drag a frontal boundary south across our area early this
week, before the front heads back into our area late in the week
and into next week providing daily chances for showers and storms.
One more day of high heat and humidity levels on Tuesday before we
see a gradual downward trend in temps for the remainder of the week.

Zones have already been updated to reflect the cancellation of the
watch and timing trends with the showers and storms over east
central Illinois.


.SHORT TERM...(This evening through Tuesday)

Hot and very humid conditions are the rule across central Illinois
this afternoon, with 19z/2pm temperatures in the lower to middle 90s
across the board.  Further north, clusters of thunderstorms have
developed along outflow boundaries from eastern Iowa across
northern Illinois.  This convection is generally tracking eastward,
with a slight southeastward drift.  High-res models are once again
handling the convection quite poorly, with the 2-hr HRRR forecast
completely missing the storms currently ongoing south of Chicago.
The Chicago storm complex will continue developing E/SE into
northwest Indiana over the next couple of hours.  Meanwhile, we will
have to keep an eye on additional storms just north of Moline.
These cells will likely track E/SE along an outflow boundary evident
on satellite/radar imagery and should remain just north of the KILX
CWA.  Have included a slight chance PoP along/northeast of a Henry
to Danville late this afternoon into the early evening in case they
develop a bit further southward.

A cold front currently analyzed across central Iowa will gradually
push eastward over the next 12 to 24 hours, reaching the northern
KILX CWA by Tuesday morning.  Many of the 12z models show little or
no convection with the boundary overnight, but have included low
chance PoPs along/northwest of a Rushville to Bloomington line as
the front encounters a very moist airmass.  Aside from any scattered
convection that may develop, the potential for fog will once again
be an issue overnight.  Light/variable winds along with dewpoint
pooling along the boundary will favor fog, but increased mid/high
clouds may tend to mitigate it somewhat.  Based on an unchanged
airmass and persistence from the past several nights, have included
patchy fog in the forecast after midnight.

Any fog will dissipate early Tuesday morning, followed by partly to
mostly sunny conditions across the southern half of the CWA.
Further north in closer proximity to the nearly stationary frontal
boundary, mostly cloudy skies and scattered convection will keep
temps slightly cooler than they have been recently.  High temps on
Tuesday will range from the upper 80s north to the middle 90s
along/south of I-70.  Due to the slightly cooler temps, heat index
readings across the north may not reach advisory criteria.  May need
to drop the advisory for the Peoria area northward, as heat index
values will likely remain in the 95 to 100 degree range on Tuesday.


.LONG TERM...(Tuesday night through Monday)

A frontal boundary will become somewhat stationary over the area and
is forecast to remain there through the rest of the week and into
the weekend. This will become the focus for thunderstorms throughout
the period. Thunderstorms will not be continuous throughout the
period, but trying to time when the dry, or drier, periods is going
to be is the main forecast challenge. Due to the difficulty of
timing the precip periods and the dry periods during the medium and
long ranges of the forecast, best to just have chance pcpn through
the period over the whole area. Will have likely pops in the north
third of the CWA for Tue night, but remainder of the forecast will
just be chance across the area.

With the frontal boundary getting into the area, relatively cooler
air will advect into the area. Temperatures will gradually drop back
to around normal, which is middle to upper 80s, by the weekend.
However, with all the moisture forecasted over the area, muggy/humid
conditions will continue through the period.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)

Convection affecting areas from just east of Peoria to north of
Bloomington southeast towards Champaign. The cells have decreased
in intensity over the past hour and unless we have a merger/collision
with an outflow boundary or another cell, expect this gradual weakening
trend to continue. After the storms move out, the main threat overnight
looks to be with fog again with some low vsbys possible in the 09z-12z
time frame. A cold front currently out over central Iowa will drift
east and approach the region by Tuesday morning. Most of the short term
models suggest we may see an increasing threat for scattered showers
and storms with the front, especially across the northwest Tuesday
morning. Once we lose any fog or low cloudiness in the morning,
forecast soundings still suggest mainly VFR conditions into the
afternoon hours. Surface winds this evening near the storms will
be variable in direction but eventually most areas should see light
winds out of the southeast tonight and then switch into a northwest
direction in the morning across the north, and by afternoon in the
south. However, wind speeds will still be averaging less than 10 kts.


EXCESSIVE HEAT WARNING until 7 PM CDT Tuesday FOR ILZ042-047>054-

HEAT ADVISORY until 7 PM CDT Tuesday FOR ILZ027>031-036>038-040-



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