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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
955 PM CDT Sat Apr 18 2015

Short term...
302 PM CDT

Through Sunday night...

Scattered showers and storms will continue to fire along a
Bradford Illinois to Rensselaer Indiana line this afternoon. The storms
formed along a back door cold front sinking south through the
region. While current conditions support isolated to scattered
thunderstorm development...they do not support severe storms. The
storms quickly dissipate late this afternoon as the little
instability out there weakens and the front continues south.

Looking higher up in the atmosphere...a closed low is over the
Central Plains and it opens to a wave Sunday afternoon as another
closed low sinks south over southern Manitoba. A surface low forms
over the southern MS River Valley this evening and pushes north
through northern Indiana by Sunday evening before continuing over
the southern Michigan peninsula Sunday night.

For the forecast tonight...conditions are rather dry with dew point
depressions around 30 degrees. Thinking it will take longer than
the models suggest for precipitation to develop over the region. As such
have a dry forecast going through tonight. For Sunday...thinking
precipitation chances will increase from south to north throughout the
morning with rain likely along and south of I-80 by noon. Some of
the latest guidance suggests the low may be slower which would lead
to a slower progression of rain. I am expecting widespread
widespread rain coverage Sunday afternoon and evening with isolated
embedded thunder and pockets of heavy rain.

For the thunder...conditions are not overly ripe for thunderstorms.
Forecast soundings feature only a couple of hundred j/kg of elevated
cape throughout the afternoon. Thinking the low will provide enough
lift to force a few embedded thunderstorms and expecting the storms
to form close to the low. Not expecting thunder after 00z as the
low pulls away over Michigan.

For rainfall...precipitable water values will be around an inch with up to 1.5
inches over eastern Illinois and Indiana. 1 inch is above average while
1.5 inches is in the 99th percentile...Aka very very above normal.
Therefore pockets of heavier rain are possible tomorrow afternoon
especially east of I-55. Expecting 0.5-0.75 inches of rain west of I-
55 Sunday and Sunday evening...and 0.75-1.00 along and east of I-55.
0.5-0.75 inches of rain could fall in a 6 hour period along and east
of I-55 Sunday afternoon which could lead to ponding on roads and
rises on local creeks and rivers.

Overall have high confidence in widespread rain Sunday and isolated
thunderstorms. Have high confidence in heavy rainfall...but only
medium-high confidence in the exact location as the heavier rainfall
will be tied close to the low.

Temperatures on Sunday will only be in the 60s with 50s expected along the
lake. There is a chance the low may shift east and we get in the
warm sector which would lead to warmer highs. However guidance seems
very consistent on the low traveling to our east so kept a cooler
forecast going. Rain continues Sunday night with lows around 50.

Jee

&&

Long term...
302 PM CDT

Monday through Saturday...

The upper level trough remains over the upper Midwest through middle
next week before slowly shifting east late next week. Cooler air
associated with the low moves in Monday with highs in the upper 50s.
Monday begins with rain over the majority of the region and the
area of rain pushes east over eastern Indiana Monday night.

For early next week looking at below normal temperatures for most of
next week with highs ranging from the upper 50s south of I-80 to the
low to middle 50s north of I-80. Highs along the lake will be around 50
with flow off the lake middle and late next week. The really
unfortunate part is overnight lows. Looking at lows in the middle 30s
to around 40 Monday night and Tuesday night with lows dropping
further into the low to middle 30s middle to late next week. Its too far
out to put frost into the grids but will keep an eye on conditions
since the growing season has begun.

Have low confidence in precipitation chances in the long term. Could see
showers Tuesday and Wednesday as vorticity streamers create forcing
over the region. However with limited confidence went with a slight
chance Tuesday and kept Wednesday dry. The rest of the week looks
dry as well with surface high pressure. A surface low passes over
the Gulf region late next week and thinking the precipitation with that
will stay over southern Illinois and Indiana.

Jee

&&

Aviation...

//Ord and mdw concerns...updated 03z...

* east-northeast winds around 10 knots.

* MVFR ceilings likely by middle to late Sunday afternoon...with
IFR/LIFR possible late Sunday afternoon and evening.

* Showers likely late Sunday afternoon into the evening.

Kjb

//discussion...updated 00z...

East-northeasterly winds will continue through much of the period. A
weather disturbance is expected to lift northward from the western
Gulf Coast region tonight and to near the southern tip of Lake
Michigan by late Sunday afternoon. This disturbance will transport
deeper moisture northward over the area...likely resulting in a
period of rain showers and low ceilings later Sunday afternoon and
evening...especially across the eastern terminals. I have pushed off
the timing of the rain showers and lower ceilings by a few hours as it appears
that it will take some additional time for the moisture and forcing
for precipitation and lower ceilings to reach northeastern Illinois/northwestern in.
It appears that IFR ceilings and some lingering showers will continue
Sunday evening.

Kjb

//Ord and mdw confidence...updated 03z...

* high with wind speed/direction.

* Medium with ceiling trends and rain showers timing Sunday afternoon and
Sunday evening.

Kjb

//outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...

Monday...chance of shra/MVFR. Gusty west-northwesterly winds.

Tuesday...slight chance PM rain showers...but mainly VFR. Gusty west winds.

Wednesday...slight chance rain showers. Mainly VFR. West-northwest winds.

Thursday...VFR. North-northeast winds.

Friday...mainly VFR. Chance PM rain showers.

Kjb

&&

Marine...
239 PM CDT

Gusty northerly winds in nearshore areas this afternoon have been
driven by a tight pressure gradient that will only slowly ease into
tonight. The wind direction should reorient more easterly by late
evening while gusts are likely to fall below small craft criteria
along the Illinois shore...and possibly as early as Sundown or so. Feel
most comfortable extending the Small Craft Advisory into early
evening given also the persistent fetch today having built the
waves.

Sunday will see a low pressure trough over the plains in the morning
basically split into separate low pressures during the day. The
strongest and more impacting one will be the southern low that
will track from the lower Ohio Valley midday Sunday to just east
of Lake Michigan Sunday night. This will drive north to northwest
winds behind it...with a noteworthy increase in the Indiana
nearshore area. It is possible wind gusts end up higher than
forecast...warranting a Small Craft Advisory during the late day
Sunday and Sunday evening. As the low pressure phases and pivots
in eastern Ontario...northwest to west winds will continue over
the lake into midweek.

Mtf

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...Illinois nearshore waters until 10 PM Saturday.

&&

$$

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