Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 251543

Area Forecast Discussion
National Weather Service Lincoln IL
1043 AM CDT Fri Jul 25 2014

ISSUED 1043 AM CDT Fri Jul 25 2014

Already did a major update to the forecast today for more cloud
cover along with much higher chances of showers along with
isolated thunderstorms for areas west of I-57 into early
afternoon. Continued a chance of showers and thunderstorms during
mid/late afternoon over central IL with just slight chances in
southeast IL while far southeast IL near Lawrence county to likely
stay dry today. Trimmed highs down a bit into upper 70s to around
80 degrees due to more showers and clouds over central IL.

MCS over eastern IA, northeast MO and nw half of IL has held
together longer than earlier expected as it tracked se to along
I-72 by late morning. Just isolated thunderstorms west of I-55
while more numerous thunderstorms and heavier rains have been on
western edge of MCS over ne MO and se IA. SPC has 5% risk of large
hail and damaging wind gusts nw of a Shelbville to Tuscola line
today so severe threat quite low today.

12Z ILX sounding showing a lot of dry air below 650 mb so this
should weaken the convection into this afternoon as it continues
moving se toward I-70. Think HRRR model is holding onto convection
too long as it moves it into southeast IL by mid afternoon. NAM
and RUC/RAP model on the other hand appears too quick diminishing
convection during early afternoon. Will blend between the wetter
HRRR and drier NAM/RUC models for the afternoon forecast.
Southerly winds increasing to 10-15 mph this afternoon between
1021 mb high pressure over WV and ridging into the ohio river
valley and 1006 mb low pressure in eastern SD/NE to bring in a bit
more humid air into central IL this afternoon. Dewpoints currently
in the upper 50s to around 60F rising to between 60-65F during the
afternoon and highest west of the IL river.



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.


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.




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