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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
912 am CDT Thursday Jul 24 2014

Discussion...
207 am CDT

Today through tonight...
early this morning skies are generally clear...with only a few
high based clouds. Temperatures have been able to radiate into the low
50s...and just before daybreak a few points could dip to the upper
40s.

Broad anti-cyclonic flow will continue to dominate the region
today...providing abundant sunshine and unseasonably cool/dry air
for a second day. Temperatures should be able to hold generally in the middle
70s...however with a northeast flow off of Lake Michigan the cooler
marine air may keep temperatures along the shoreline in the upper 60s.

By this afternoon the surface ridge axis will be drifting east across the
Ohio Valley...with flow slowly turning southerly by late tonight.
Depending on the timing...temperatures could radiate in the late evening
before holding steady overnight in the middle/upper 50s. A weak wave will
be sliding east from the Dakotas late tonight...with some isentropic
moistening on the Lee-side allowing for some spotty showers and
perhaps an isolated thunderstorms and rain just before daybreak Friday in the western County warning forecast area.

Confidence through today...high.
Confidence in precipitation overnight/temps...medium.

Friday through Sunday...
500mb ridge over the Southern Plains will begin to
flatten...allowing a quasi-zonal flow along the northern periphery
with a series of weak middle-level vorts sliding east into the upper
Midwest/Great Lakes. Guidance has a weak surface wave moving east from
the Central Plains into the Great Lakes Friday before weakening. The
stronger middle-level vorticity maxima remains confined along the High
Plains/Canadian prairies Fri/Sat...which will allow a few rounds of
showers/thunderstorms to push across the forecast area starting
Fri-sun. At this time though it does appear the best chance for
precipitation Friday will be in the evening hours.

A boundary lays out over the middle-Missouri Valley stretching east
through Ohio Sat aftn/eve. For the second half of the weekend the
pattern will once again be starting to change towards a western
ridge and Great Lakes trough. The semi-stationary middle-level vorticity over
the High Plains will slide east early sun into the northern Great
Lakes...with a series of weak surface waves consolidating over lower
Michigan Sun morning.

Temperatures will start to moderate over the weekend into the upper 70s/low
80s...and with the increased low level moisture dew points will creep back
into the middle/upper 60s Sat evening. The far west/southwest forecast area
could see dew points back into the low 70s.

Then for sun a frontal boundary will be sliding southeast across the
County warning forecast area...however temperatures should still be able to reach 80 to the low 80s.

Confidence...medium/high.

Monday through Thursday...
ensembles for several cycles have prognosticated a North Atlantic ridge
coupled with a high amplitude ridge developing across western
Canada. The resulting feature will likely be a trough digging south
across the Great Lakes/Hudson Bay early next week and likely
persisting through late next week. Unseasonably cool air will bleed
south with ensembles demonstrating a high confidence in the 850mb
isotherms over the forecast area generally between 8-10 degree c for
Mon/Tue...then very slow to moderate back to a 10-12 degree c range
late week. So to kick-off next week temperatures in the low 70s look likely
with a northwest flow and broad surface ridge. As the week progresses
temperatures should push back into the middle/upper 70s...but currently the
northwest flow would suggest dry conditions to persist.

Confidence...high.

Beachler

&&

Aviation...

//Ord and mdw concerns...updated 14z...

* northeasterly winds around 10kt.

Krein

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

High pressure is centered across the upper Midwest and western Great
Lakes region this morning resulting in light northerly flow across
the terminals and VFR conditions which are expected to persist
through the day. Winds will veer to the northeast today then to the
south this evening as the high moves across the area. Tonight middle
and high clouds will begin to increase as middle level warm advection
Wing lifts across the region. Models suggest that a convective
complex may form to our northwest on the nose of the low level
jet...then move southeast along the instability gradient. The
instability doesnt quite reach as far east as the terminals though
and dry low/middle levels should make it difficult for precipitation to
persist into the area.

Bmd

//Ord and mdw confidence...updated 14z...

* high confidence in all forecast elements.

Krein

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

Friday night through Sunday...periodic chances for thunderstorms and rain.
Monday through Wednesday...VFR.

Bmd

&&

Marine...
307 am CDT

Conditions will continue to gradually improve across Lake Michigan
as high pressure builds over the area today. Winds have already
abated across much of the lake with some lingering winds and waves
mainly confined to the far south end of the lake early this morning.
As winds continue to dissipate today...expect lake breezes to
develop with onshore flow this afternoon and evening. Southwest flow
will freshen up across the lake tonight through the weekend as a
pair of low pressure systems develop to our west. While the southern
low is prognosticated to weaken and dissipate...a slightly deeper low over
Canada will move across the Great Lakes Sunday dragging a cold front
across Lake Michigan with north/northwest winds once again
approaching 30 knots Sunday night into Monday behind the front.

Bmd

&&

Lot watches/warnings/advisories...
Illinois...beach hazards statement...ilz006-ilz014 until 10 am Thursday.

In...beach hazards statement...inz001-inz002 until 10 am Thursday.

Lm...Small Craft Advisory...nearshore waters
until 10 am Thursday.

&&

$$

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