Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 222359

Area Forecast Discussion
National Weather Service Lincoln IL
659 PM CDT Tue Apr 22 2014

ISSUED 259 PM CDT Tue Apr 22 2014

Breezy conditions persist across central and southeast Illinois
this afternoon between yesterday`s departing cold front and an
approaching ridge of high pressure. The ridge will continue to
dominate the weather for Wednesday, keeping quiet conditions in
place. The next storm system is still on track for Thursday, with
an associated risk of showers/thunderstorms. The precipitation
threat for the end of the weekend into early next week remains
murky as a slow moving system, part of a developing Rex block,
moves through our vicinity. Main forecast problems today remain
trying to pin point precipitation chances/timing with these two

SHORT TERM...Tonight through Friday:

The ridge of high pressure to our west will push into the forecast
area overnight. The main concern with the arrival of the ridge is
if temperatures will fall enough to pose a threat of frost. At
this point, forecast lows are only marginal for frost in most
areas. Also, winds appear as though they may stay up enough to
mitigate the frost threat. If winds die off quickly this evening,
the frost threat will need to be looked at more closely.

Return flow on the back side of the ridge increases on Wednesday,
with a warm frontal zone setting up to the east of a developing
storm system in the plains. Many model solutions persist in trying
to produce some precipitation in this warm advection regime,
Wednesday afternoon/evening, mainly across northern portions of
the forecast area. Still feel there is likely to be too much dry
air to overcome initially and that models are a little too
bullish. With this thinking in mind, plan to keep forecast dry
until overnight Wednesday night when some precipitation may begin
to slip in ahead of Thursday`s system.

Showers/storms still expected to spread from west to east on
Thursday as storm system crosses the forecast area. Severe threat
expected to be low overall due to low instability (CAPE values
below 1000 j/kg for the most part), and modest shear profiles
(bulk shear values no better than 35-40 kts). Drier conditions
quickly filter in behind the cold front associated with the system
overnight Thursday into Friday, although the post frontal airmass
is not especially cool.

LONG TERM...Saturday through Tuesday:

Attention turns to the Rex block expected to develop during the
period as we head into the extended range of the forecast. The
ECMWF has been the most aggressive by far with the return of
precipitation chances over the weekend, but it has backed off a
bit in its 12Z run. In any event, once the upper low with the Rex
block is in the vicinity, period of showers and some thunderstorms
are likely for at least a few days. However, due to the model
spread, run-to-run inconsistency, and intra-model ensemble spread,
confidence remains quite low in the details. Unfortunately, this
results in several periods of chance PoPs, starting Saturday
night. Hopefully, a clear consensus with the ultimate forecast
solution will emerge soon. However, considering the trouble models
have entering (and leaving) blocky patterns, we may be waiting a



ISSUED 659 PM CDT Tue Apr 22 2014

High pressure at the surface will advance into IL tonight,
bringing dry air and mostly clear skies. Some thin cirrus will
progress over the top of the ridge in the northern horizon, but
little to no mid to low clouds are expected over the next 24
hours. Gust NW winds to 18-20kt will diminish with sunset. Winds
will also diminish in response to a weakening pressure gradient as the
surface ridge axis approaches from the west.

Winds will shift to the southeast Wed morning as the surface high
passes to the east and return flow develops.



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