Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 111958

Area Forecast Discussion
National Weather Service Lincoln IL
258 PM CDT Fri Jul 11 2014

ISSUED 258 PM CDT Fri Jul 11 2014

High clouds spreading across the forecast area from the northwest
this afternoon, from the remnants of convection across Iowa. Still
have some dry air across the eastern CWA with dew points in the
50s, but southerly winds advecting moisture into the area, and dew
points across the far west had reached the upper 60s. Surface map
shows the high pressure to our east finally losing its influence,
with the gradient starting to tighten ahead of a frontal boundary
from western Minnesota to a low in eastern Colorado.

Initial forecast concern involves severe weather potential through
the weekend, then focus shifts to the unseasonably cool pattern
for much of next week.

SHORT TERM...Tonight through Monday:

Convection expected to develop across Iowa later this evening,
then track southeast into Illinois. Models showing some spread
with the exact timing of arrival around here, with the GFS way
ahead of the others with widespread activity already over the
north half of Illinois by midnight. Think this is too aggressive
and have limited evening PoP`s to northwest of the Illinois River,
but continued with the existing likely PoP`s across the northern
parts of the forecast area after midnight, spreading across the
east into Saturday morning. There may be a bit of a lull Saturday
afternoon in the precipitation, especially across the southwest,
but several of the models ramp up CAPE`s to over 2500 J/kg by late
afternoon. Best shear will be arriving after sunset and just to
our north, coinciding with the incoming front to favor MCS
development. SPC Day2 slight risk basically from around I-72
northward. Precipitable water values of 2 to 2.3" favor some very
heavy rain with the storms, with overall highest totals across the
far northern CWA closer to any MCS activity.

Followed the general model guidance of sweeping the front to
around I-70 by midday Sunday. Severe weather threat will persist
until the front clears the area. Have reduced PoP`s to slight
chances (around 20%) west of I-55 by afternoon. Have some
reservations about completely removing them, as the arrival of an
upper trough late afternoon may be enough to trigger a few more
showers/storms. Additional showers/storms possible Monday ahead of
the stronger surge of cooler air.

LONG TERM...Tuesday through Friday:

Very strong upper low for this time of year will be dropping into
the upper Great Lakes on Monday and dominating our weather for
much of the remainder of the week. GFS ensemble temperatures at
850 mb running around 3 standard deviations below normal by
Tuesday and continuing for a few days, reaching as low as 6
degrees C by late Tuesday afternoon. May be a little conservative
with highs reaching the lower 70s Tuesday, as some of the guidance
suggests some upper 60s with the strong surge of cool air. Record
cool highs for Peoria, Springfield and Champaign on Tuesday are
only in the lower 70s, so these could be in danger of falling,
along with some potential record lows Wednesday morning. Some
rise in temperatures expected during the latter part of the
week as the air mass moderates, but will remain about 10 degrees
below normal. Dry conditions currently expected through the period
as surface high pressure dominates, but may later need to add
some low PoP`s on Tuesday for cold-core shower potential.



ISSUED 1224 PM CDT Fri Jul 11 2014

A lot of differences between models with the handling of the next
system and the frontal boundary expected to form/move through the
CWA tonight. Overall, models have differed on timing and have
trended the upper air disturbance further to the north. With that
in mind, keeping mention of thunder to the north for the overnight
hours, slowly pushing it south, per the 4km WRF solution, as it is
so far the most accurate reflection of current conditions. Also
staying with VC until better coverage is anticipated.
Showers/thunderstorms expected, but will withhold predominant
mention until confidence increases. Both GFS and NAM falling
behind in current conditions with convection in central Iowa.
South southeasterly winds dominate.




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