Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 180458
AFDILX

Area Forecast Discussion
NATIONAL WEATHER SERVICE LINCOLN IL
1158 PM CDT Sun Aug 17 2014

.UPDATE...
ISSUED AT 902 PM CDT SUN AUG 17 2014

Only a few remaining showers in SE Illinois this evening as lift
associated with the recent storm system diminishes as the low
moves off to the east. The air mass left behind in central/SE
Illinois remains quite moist at the surface with dewpoints in the
upper 60s to around 70 and near saturated conditions up to around
750 mb. Skies clearing above will allow for good radiational
cooling which will bring temperatures rapidly down to the
dewpoint. Fog will become prevalent overnight as a result.
Forecasts reflect this well, and will be keeping an eye on the
potential for dense fog formation. The mitigating factors are the
steady light northeast winds which will keep the surface layer
mixed, and the depth of the fog layer may preclude dense fog
formation later in the night. So far, updates have been sent to
account for trends on shower activity, temperature, humidity, and
winds and later updates may be sent as fog trends become more
definite.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 259 PM CDT SUN AUG 17 2014

Surface low has dropped into far southern Illinois early this
afternoon with remnant MCV still evident on radar imagery over the
far southeast CWA. This is where most of the lingering showers are
continuing, with a few as far north as Taylorville and Tuscola. Some
thinning of the low clouds has been developing but skies still
mainly cloudy.

Main concern for this part of the forecast is with fog potential
tonight. Cloudiness across far northern Illinois and southeast Iowa
is more cellular in nature and should diminish with loss of daytime
heating. Latest RAP guidance showing the northern CWA becoming
partly cloudy this evening. With northeast flow continuing, not a
lot of drying will take place in the lower levels with dew points
upstream still in the mid 60s in many areas. This should allow
visibilities to start dropping by late evening across the north.
Model guidance showing a fair range as to how extensive or thick the
fog gets, so for now will only mention patchy or areas of fog in the
grids for late tonight and early Monday. Skies expected to become
partly to mostly sunny by midday as forecast soundings are rather
dry.

In terms of the lingering showers, most of what`s left in our area
is along a weak trough axis dropping into southern Illinois, aided
by the upper low which is currently located in far southwest
Indiana. Am expecting the showers to fade early this evening as the
low pulls away. On Monday, have maintained some isolated showers/
storms across the eastern CWA during the afternoon as a weak wave
passes through the area.

&&

.LONG TERM...(Monday night through Sunday)
ISSUED AT 259 PM CDT SUN AUG 17 2014

Monday night will feature an upper trough and weak surface low
tracking from the upper midwest into the western Great Lakes, with a
cold front extending south into northern Illinois. Rain chances will
return to areas NW of the I-55 later Monday evening, with chances
expanding to Indiana along and north of I-72 by 12z/7am Tuesday.
Better forcing from the 500mb vort max should remain in northern IL,
but moisture convergence along the front and a weak 300mb jet max
overhead could provide just enough lift for a few storms with the
front. No severe weather is expected, especially with limited
instability overnight.

Tuesday should see a decreasing trend in rain/storm chances in the
afternoon as the bulk of the mid and upper level energy departs to
the east. The break in the rain should continue Tues night for all
but the far southwest KILX CWA, from Jacksonville to Effingham,
where a warm front will quickly approach the area. Rain/storm
chances will remain mainly southwest of Havana to Lincoln to
Shelbyville on Wednesday as the front slowly pushes into IL.

That warm front is expected to usher in a progressively warmer
airmass  the remainder of the week and into the weekend. Wednesday
night, an MCV is projected to move from N Iowa across N Illinois,
with enough support from strong mid level warm air advection and a
low level jet across central IL to trigger showers/storms across most
of our forecast area.

Low surface pressure and a stalled frontal boundary are still
expected across northern Illinois for Thursday, keeping rain chances
going into Thursday night. The front is expected wash out Thursday
night as warming continues through a deep layer of the atmosphere.
The warming at mid levels will eventually work to cap the atmosphere
and limit storm potential, despite ample low level moisture. A
shortwave is forecast by the ECMWF to move from Nebraska across N
Illinois Saturday, which could affect our northern counties with a
few storms, depending on the extent of elevated CAPE above the
capping inversion.

The ECMWF is also showing a cold frontal passage following the
shortwave on Saturday night, bringing a period of storms and
slightly cooler air for Sunday. The GFS keeps the upper level ridge
entrenched over IL, limiting much storm potential by keeping
shortwave tracks north of central IL. It also has much warmer
conditions continuing into Sunday. Confidence remains low on the
scenario for next weekend due to the differences.

Heat and humidity levels will climb the last half of the week, as
highs reach into the lower 90s south of Lincoln to Mattoon Thurs
through Saturday. Dewpoints in the low to mid 70s will help push
heat index readings into the upper 90s to around 100 in at least the
south half of the KILX CWA.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
ISSUED AT 1158 PM CDT SUN AUG 17 2014

MVFR vsby has become predominant with local IFR/LIFR
cigs/vsby over central and SE Illinois as fog begins to develop
this evening. Overnight, fog will develop, with potential for
LIFR or VLIFR conditions...mainly 07Z-15Z. So far, steady light NE
winds 5-10 kts has kept fog diffuse through mixing of the surface
layer, but winds should decrease overnight as pressure gradients
relax across the region. Improving conditions after 15Z with
ceilings becoming sct-bkn030 for most of the afternoon. A weak
trough and surface low moving in from the NW Monday evening will
bring a chance of rain back to areas KPIA-KBMI northward after
00Z, but chances for significant vsby/cig reduction in precipitation
remain too low for mention in central IL TAFs at this time.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...ONTON
SHORT TERM...GEELHART
LONG TERM...SHIMON
AVIATION...ONTON






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