Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1104 am CDT Tuesday Jul 7 2015

Short term...
330 am CDT

Through Thursday...

Main forecast concerns/challenges in the near term are with
precipitation trends through midday with precipitation likely lingering across
the southeast half of the County Warning Area...and then with much cooler temperatures
expected today.

Latest radar imagery depicting precipitation still persisting over much
of the area this morning with heavier showers and isolated thunder
confined primarily to the southeast third of the County Warning Area...along and
south of Interstate 80. This better development is occurring south
of the front early this morning where high moisture content air
continues to be transported northeastward. Although instability is
limited with the highest instability just south of the County Warning Area...high
precipitable water air in excess of two inches remains situated over much of the
region. This is allowing for continued heavy downpours with the
stronger precipitation...with high instantaneous precipitation rates still noted
on dual pol. However...these heavier showers remain fairly
progressive and continue to steadily lift northeast through the
region. Looking would appear that precipitation will likely
move over much of the County Warning Area through the 12-14z time frame with short
term guidance supporting this...but with the heavier rain and
isolated thunder pushing further south throughout this time.
Conditions should be dry for much of the County Warning Area by midday...and
especially by early afternoon.

This will be supported with much drier and cooler air advecting
across the region Post frontal passage early this morning. Early morning
surface analysis would put the front across a Mendota to Lake Forest line across northern Illinois. Over
the next two to three hours...this front will continue through
remaining areas in northern Illinois with remaining areas in east
central Illinois and Northwest Indiana not observing frontal passage until
closer to middle morning. Behind this front...northerly winds are
ramping up rather quickly while temperatures fall to the lower 60s. This
will set the stage for a rather cool and breezy day today with
well below normal temperatures expected for all areas. Areas across north
central Illinois may observe somewhat of a recovery through clouds begin to scatter out.

By this afternoon and especially this evening and overnight...dry
conditions likely in place with front well south of the County Warning Area and
with more zonal flow in place across the region. Tonight through
early Wednesday morning will be dry but then turn attention
towards shortwave energy lifting northeast through the Central
Plains. This energy and associated surface reflection will lift to
the middle Mississippi Valley on Wednesday...with associated precipitation
spreading northward out ahead of it. Do bring chance probability of precipitation back
over the southern half of the County Warning Area on Wednesday and did keep
mention of slight chance thunder as well.
instability looks to be situated to the south of the County Warning Area during
this time and am not overly excited about thunder during this
time. Best chances for precipitation for the County Warning Area will arrive Wednesday
night into early Thursday morning with the approach of this
system. However...guidance does vary with placement/track of this
system...but with areas south of Interstate 88 having the better
chances of observing precipitation. Increasing moisture/instability out
ahead of a strengthening low level jet should allow for better chances for



Long term...
330 am CDT

Friday through Monday...

The extended period continues to look warmer and more
active...with chances for thunder throughout much of the period.
This will begin on Friday with fairly energetic flow riding along
rising heights across the region with a good warm/moist advection
push...with at least scattered thunderstorm development likely.
Thunderstorm development appears to be likely again Friday night
into Saturday...while strong low level jet pumps higher moisture content air
over the region. Thunderstorms may linger Saturday morning...but
should push north of the County Warning Area through early afternoon. With this
likely occurring...above normal temperatures are expected Saturday...with
similar temperatures expected Sunday.




//Ord and mdw concerns...updated 16z...

* northeast winds gusting to 25 knots through early afternoon...with
sustained speeds above 10 knots through the early evening.


//discussion...updated 12z...

Still have a few more hours of rain to go...especially south of
Interstate 80. Thunder is confined well to the south at this point
as well. Visibilities in the rain/drizzle behind the front have been IFR
at times as well...and expect this to be the case until the precipitation
clears the terminals. There may also be lingering visibilities in the
MVFR category through the morning ceilings are also holding at low
end MVFR...with some pockets of IFR as well. Ceilings should generally
hold where they are or briefly dip before settling at MVFR for a
few hours behind the precipitation as has been occurring upstream. Time
height guidance then suggests a lifting of the lower clouds...but
still expect to see some lower VFR clouds drifting southward in
the Post frontal cold advection airmass.

The other concern today will be the gusty north-northeast winds.
Expect sustained speeds in the middle teens with gusts to the middle
20s...with a drift towards a slightly more northeast direction
closer to the lake as the day wears on. The gradient relaxes in
the afternoon so expect some slackening of the winds later
today...dropping back under 10 knots this evening.


//Ord and mdw confidence...updated 16z...

* high in wind direction...medium high in speed.


//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...

Wednesday night...chance rain showers...schc thunderstorms and rain.

Thursday...VFR. East-northeast winds.

Friday through Monday...periods of showers and thunderstorms.



250 am CDT

Low pressure passing over Southern Lake Michigan will continue off
to the northeast this morning...allowing a cold front to clear the
southern tip of the lake just after daybreak. Waves will quickly
build on the southern half/third of the lake as this
occurs...reaching 7-8 feet on the Indiana shoreline...maybe a touch
lower on the Illinois shoreline. Expansive high pressure over the
upper Midwest will build east across the central Great
Lakes...becoming centered over Northern Lake Michigan Wednesday.
Several lows will continue to progress south and east of the lake
through Friday.



Lot watches/warnings/advisories...
Illinois...beach hazards statement...ilz014 until 4 am Wednesday.

Beach hazards statement...ilz006 until 7 PM Tuesday.

In...beach hazards statement...inz001-inz002 until 4 am Wednesday.

Lm...Small Craft Advisory...nearshore Winthrop to Wilmette harbors until 7 PM Tuesday.

Small Craft Advisory...nearshore Wilmette Harbor to Michigan City in until
4 am Wednesday.



Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations