Area Forecast Discussion
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FXUS63 KILX 231736

Area Forecast Discussion
1236 PM CDT Sat Aug 23 2014


15z/10am radar mosaic shows large cluster of thunderstorms
tracking E/NE across eastern Iowa into northwest Illinois. The
storms extend as far south as Knox and northern Peoria counties,
where frequent lightning and torrential rainfall is currently
occurring. These particular cells will continue to push eastward
across the far northern KILX CWA over the next 2-3 hours,
primarily staying north of an Elmwood to Minonk line. Meanwhile,
isolated storms have developed along an outflow boundary from this
complex southward along a Canton to Springfield line. These cells
will also track eastward and may become more numerous across
east-central Illinois this afternoon. Have made some adjustments
to PoPs today, mainly to go categorical across the far north this
morning. Aside from the widely scattered convection, the main
weather story today will be the hot and humid conditions.
Afternoon highs are still on track to reach the upper 80s and
lower 90s, with heat index readings exceeding 100 degrees.


.SHORT TERM...(Today and tonight)

Short-term concerns focus on convective development and heat today.
Differing solutions among the high-res models give lower confidence
on timing an coverage of storms today. Outflow boundaries lingering
from yesterday`s storms will likely play a role in today`s
development. The most noticeable boundary extending from SW to NE
across our counties.

One storm complex has remained active just to the SE of LWV, but is
drifting SE. A couple of localized storm complexes to our west may
evolve eastward this morning, as low level jet dynamics expand
toward Illinois. That could help those storms affect areas NW of the
IL river this morning. Current thinking is that expansion of the
storm development along the outflow boundary generally keep storms
NE of a line from Rushville to Effingham. Coverage may not be as
extensive as the GFS and ECMWF are indicating, so we will continue
to Chance PoPs mainly NE of a line from Peoria to Lawrenceville,
with slight chances back west to Rushville to Effingham. By this
evening, storms are expected to be mainly confined to areas east of
I-57, then ending by midnight.

The very moist air mass will remain in place today, with precipitable
water values of over 2 inches. Therefore, locally heavy rainfall
will be possible again today from any storms. Rainfall rates could
reach as high as 2 to 3 inches per hour.  Depending on the timing of
convection and amount of sunshine during the afternoon, high
temperatures today should reach the upper 80s and lower 90s. These
readings, combined with dew points in the middle 70s, will produce
heat index values of 100 to 105.  Have continued the current Heat
Advisory with no changes in coverage. Lows tonight will be held in
the low to middle 70s due to very moist dew points. Patchy fog will
likely develop under light winds and clearing skies.


.LONG TERM...(Sunday through Friday)

Expansive upper ridge remains in place through Tuesday. Main frontal
boundary will be well to our north during this time, allowing for
the tropical conditions to continue. Will need to watch for
potential need to expand the heat advisory northward toward at least
the I-74 corridor, as heat index values there should be approaching
105F from Sunday through Tuesday. However, heat index values today
will be low enough that the 4-day criteria of 100-105F will probably
not be met.

With main frontal boundary to our northwest through Tuesday, any
convection further south within the ridge will be more isolated and
will struggle to break through the cap. Will have some 30-40% PoP`s
across the northeast half of the forecast area Tuesday afternoon
along the fringes of a potential MCS. Will concentrate PoP`s Tuesday
night northwest of I-55, but main focus will likely be more
northwest of the Illinois River.

Ridge starts to break down on Wednesday as a broad trough heads
eastward into the Plains. The GFS is more progressive with a broad
wave reaching the Midwest on Thursday and ridging building across
the Rockies, while the ECMWF cuts off a piece of energy from the
main jet and drifts it eastward the remainder of the week. The
former solution focuses the rain on the Wednesday/Thursday time
frame, while the latter would linger the rain all the way into the
weekend. A bit of a wild card will be the track of the tropical
system lifting north from the Bahamas toward midweek, and how fast
it ejects northeast (i.e. slowing down systems moving through the
CONUS). The GFS ensemble spread is fairly wide by Thursday
afternoon, indicating a lower confidence. The forecast right now
will lean a bit more toward the progressive GFS, but will not clear
it out as sharply or quickly behind the main front, with slight
chances continuing Thursday night into Friday in many areas.

Unfortunately, the varying solutions result in a wide temperature
range toward the end of the week. The GFS would suggest highs only
in the lower 70s by Friday, while the ECMWF keeps the very warm and
humid conditions ongoing. Will favor the MEX MOS guidance for now
with temperatures a couple degrees either side of 80.


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

Southern flank of storm complex tracking across northern Illinois
is skirting along the I-74 corridor early this afternoon. Storms
have already impacted KPIA with heavy rain and gusty winds up to
around 25kt and will push through KBMI over the next hour. Still
some question whether they will hold together and reach far enough
south to impact KCMI, so have only gone with VCTS there. Elsewhere
around the area, will maintain dry conditions through the
afternoon. Once convection exits into Indiana, skies will
gradually clear by this evening. Due to light winds and very high
dewpoints, think fog will once again develop across the area
overnight. Have therefore included restricted visbys down to
between 2 and 4 miles between 09z and 14z.


HEAT ADVISORY until 7 PM CDT Monday FOR ILZ042-047>054-056-



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