Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 251157

Area Forecast Discussion
National Weather Service Lincoln IL
657 AM CDT Fri Jul 25 2014

ISSUED 247 AM CDT Fri Jul 25 2014

Illinois remains on the periphery of high pressure centered over
Ohio early this morning. To our northwest, convection has been
ramping up quickly over the last couple hours in central and
northwest Iowa along a warm front, as the low level jet increases
from the central Plains. The initial concern for this forecast
package involves how much of this convection will affect our area
today. However, the main forecast issue is with severe weather
potential Saturday afternoon and evening.

SHORT TERM...Today through Saturday Night:

The track of this developing thunderstorm cluster has necessitated
some noticeable changes to the going forecast for today. Earlier
grids mainly had the north half affected by the overnight
convection. Latest high-res models are all indicating a
southeast track, which would favor the western parts of the CWA.
However, very dry conditions above 850 mb were observed on our
evening sounding, and LAPS soundings for Peoria and Springfield at
1 am still showed a sizable dry wedge around 800 mb. Model
soundings show this eroding somewhat by mid morning but not
completely going away, so eastward extent is still a bit in
question. Will have the highest PoP`s from around Rushville to
Jacksonville, with lower chances spreading eastward as far as
I-57. Rain chances should be diminishing during the afternoon as
the atmosphere becomes more capped.

Evening models in general agreement with a shortwave tracking
along the periphery of the upper ridge to our southwest, but
convective alignment is more variable. The general consensus is
for a convective complex to develop to our north by late evening.
The ECMWF and GEM track this southeast and just brush by the
forecast area, while the NAM and GFS show some development
extending southwest along a frontal boundary into Kansas. Have not
made much change here at this point, since leftover boundaries
from the daytime activity may also contribute and are not
adequately handled by the synoptic models.

The frontal boundary will likely hang up somewhere in the I-72
vicinity on Saturday. Have gone with only slight chance PoP`s
during the morning, then increased them to around 30-40% in the
afternoon as the cap starts to erode. Very unstable conditions
will be developing, as dew points rise into the mid 70s and yield
CAPE values well over 3000 J/kg. 0-6km shear around 35 knots is
progged by evening, when the best storm chances in our area are
likely. Latest SPC Day2 outlook shows 30% severe probabilities
over the entire forecast area, with damaging winds favored.
Precipitable waters rising to just over 2 inches will also result
in locally heavy rains, and the ECMWF and GFS indicate some
training echoes could also be a concern.

Some secondary concerns also exist on Saturday for heat advisory
potential. With dew points rising into the mid 70s, am expecting
heat index values of 100-102 over most areas except perhaps along
and north of I-74. This is just shy of the 105F criteria for this

LONG TERM...Sunday through Thursday:

Any lingering rain on Sunday will likely be in the morning, before
a cold front passes. This will be ahead of an upper low, currently
entering southwest Saskatchewan, that will be opening up as it
drops southeast through the Great Lakes region. This will evolve
into a broad upper trough that will dominate the eastern U.S.
throughout the week, bringing another period of cooler conditions.
Monday is likely to be the coolest day, with highs mainly in the
mid 70s, and comfortable humidity levels to persist much of the
work week.



ISSUED 657 AM CDT Fri Jul 25 2014

Mainly VFR conditions are expected over central and southeast
Illinois today and tonight. A weakening thunderstorm system will
move across central AZ today bringing increasing low and mid-
level cloud cover along with scattered showers and isolated
thunderstorms. The best chances for showers and thunderstorms
until 18Z will be from KSPI-KBMI northwestward. Have placed
temporary MVFR conditions in the KPIA TAF, but left out of other
area TAFS due to low probabilities and coverage of MVFR
cigs/vsbys. Another disturbance tracking into the area this
evening is expected to bring more showers and thunderstorms
overnight. Winds generally southerly around 10 KTS from 15Z through
12Z Saturday.




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