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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1008 PM CDT Thursday Aug 28 2014

Discussion...
714 PM CDT

Pair of mcvs noted on radar imagery continue to move eastward
driving slowly decaying convective complex eastward into our County Warning Area
early this evening despite the much more stable and somewhat drier
air mass over the region. Have already updated precipitation trends a
couple time to account for the rain this evening. Anticipate
continued weakening trend...particularly from the north...in this
area of left over rain and even some lingering thunder. The threat
of thunder with the northern mesoscale convective vortex tracking close to I-88 looks to end
very soon...while the convection with the southern most mesoscale convective vortex lies
close enough to more unstable air to potentially maintain
thunderstorms across the southern tier of our County Warning Area the next few
hours. In the wake of these mcvs...which really havent been handled
well by numerical guidance...should see a period of dry conditions
however models do break out additional convection overnight which
isn't that unreasonable given the northward advancing warm front.

Izzi

//prev discussion...
230 PM CDT

Visible imagery this afternoon shows a middle/upper level cloud deck
lifting northeast across northern Illinois. Remnants from a morning
complex of thunderstorms resides over Iowa...with some convective
look to the clouds lingering into this afternoon. Surface temperatures across the
forecast area have been able to warm into the middle/upper 70s. A few
points south of Pontiac to Kankakee have pushed to around 80 degree...but
with the light easterly flow at the surface some weak subsidence has
been noted with dew points being pushed into the low 50s along the
Lakeshore and middle/upper 50s inland.

The main challenge for this evening will focus on convective
chances/coverage...which will continue through daybreak Friday.
Guidance has continued to back off on the arrival of precipitation into the
forecast area...which seems to line up relatively well with the
current trends. The frontal boundary will slowly lift north this
evening...but not expecting much in the way of precipitation until perhaps
after 04z with a gradual upglide of parcels allows for moistening.
Some elevated convection could begin to develop...although not
expecting much coverage overnight. The focus for slightly better
precipitation coverage will remain along the northern row of counties in
northern Illinois...however closer to daybreak Friday it appears some
precipitation/convection could sag southeast and clip northeast
Illinois/northwest in before dissipating.

With increasing cloud cover tonight...it should limit how cool temperatures
become. So have made only minor adjustments to the temperatures with
generally middle/upper 60s for lows.

Beachler

Friday and Friday night...
closed upper low currently near the Colorado Front Range will slowly
traverse east towards the middle-Missouri Valley Friday becoming an
open wave. At the surface...broad and weak surface low will lift
into the upper Midwest while southwest flow ramps up through the
column as the warm front lifts into the central Great Lakes. Heat
and humidity will make a return as this occurs. 850 mb temperatures warming to
around 18c will help support middle/upper 80s during the day
Friday...and surface dewpoints are prognosticated to push back into the low
possibly middle 70s. This will result in moderate instability
developing during the day...and only weakly capped during the
afternoon as broad height falls associated with the upper wave start
to overspread the area. There doesnt seem to be much focus in the
lower levels so would expect only scattered thunderstorm development
across the region Friday afternoon/evening. Upper wave begins to
move into the upper Midwest tomorrow night and broad warm advection
out ahead during the evening and overnight hours will help sustain
some convection late but would expect some nocturnal downtick in
activity overnight with the low level jet focused well to our north.
GFS/NAM seem to be overdoing probability of precipitation much of the day and night
Friday...and opted to lean closer to European model (ecmwf) these periods.

This weekend...
cold front will sweep across the forecast area starting very early
on Saturday. While not a favorable diurnal time...we should still
have some very modest instability in place that will support some
early day thunderstorms along the frontal boundary...which would be
able to expand in coverage as the day progresses and the boundary
layer warms. In addition...guidance suggests a second shortwave
will wrap into the area that will provide additional broad scale
ascent. The European model (ecmwf) is about six hours slower with this feature
compared to the NAM/GFS...with the Gem a compromise...so there
remains some uncertainty in timing. High pressure will build in
behind the front Saturday night into Sunday and will provide a
brief respite from the active weather. The atmosphere looks to
quickly reload however with thermal ridge once again starting to
tilt back over the area with 850 mb temperatures around 20c by later in the
day Sunday.

Next week...
an active pattern looks to persist through at least the middle of
next week with a series of waves pushing across the region bringing
continued periodic chances for showers and thunderstorms. Another
cold front will push across the area later on Monday with the
frontal boundary stalling just to our south. Thereafter...there is
considerable uncertainty on how far north the boundary may lift
with the best precipitation chances focused in the vicinity of the
front as a series of upper level disturbances push across the
area. Precipitable waters remain juicy along the front...around two inches...and
guidance suggests that there could be several convective complexes
next week with a concern for heavy rain and flooding should this
pan out.

Deubelbeiss

&&

Aviation...

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

* isolated rain showers/thunderstorms and rain late tonight into early Friday morning.

* Scattered thunderstorms and rain Friday afternoon/early evening.

* South wind with gusts developing to 20kt midday Friday.

Kmd

//discussion...updated 00z...
challenging forecast for the next 24-30 hours. Area of showers
will continue to push eastward and weaken some this evening.
Thunder activity looks to remain south of the Interstate 80
corridor along a line from kvys to klot to kgyy. Ceilings and visibilities
should generally remain VFR though brief MVFR visbys are possible
mainly west of the main Chicago terminals.

Expecting a lull this evening...then as a warm front lifts through
tonight...additional convection is expected in northwest Iowa and
spreading into Wisconsin overnight. As an upper ridge buckles some
late tonight into early Friday...some of this activity may graze
northeast Illinois in the 12z-15z time frame. Instability may
support rain showers with isolated ts.

Instability then increases very quickly on Friday as we get in the
warm sector and dewpoints rise. Upper height falls combined with
peak heating suggests afternoon air mass thunderstorms though
difficult on timing. Could be thunderstorms at any time Friday
afternoon early evening with no obvious surface focus. With high
precipitable water values...heavy downpours are possible.

Kmd

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

* low-medium in precipitation coverage late tonight into early Friday.

* Low in thunder chances/coverage and specific timing Friday afternoon.

* High in wind direction/speed and trends

Kmd

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

* Saturday...thunderstorms and rain likely.
* Sunday...slight chance of thunderstorms and rain.
* Monday and Tuesday...chance of thunderstorms and rain.
* Wednesday and Thursday...slight chance of thunderstorms and rain

Kmd

&&

Marine...
123 PM CDT

Surface high pressure over the Great Lakes region this afternoon
will drift to the east tonight in the wake of a departing upper
trough over New England/Quebec...and ahead of another surface trough
and weak cold front stretching from far northwest Ontario to the
Central Plains. Winds on Lake Michigan will shift to the southeast
and eventually south heading into Friday...as an area of surface low
pressure develops across the plains and lifts across the upper
Mississippi River valley. Tightening of the pressure gradient
southeast of the low track will result in S-southeast winds increasing into
the 15-25 knots range tonight and Friday...gradually weakening Saturday
as the low moves off to the northeast and a weak cold front trails
across the lake followed by weak high pressure.

This cold front appears to dissipate or stall across far Southern
Lake Michigan or northern Illinois/ind late Saturday night or early Sunday
before developing back to the north as a warm front Sunday night as
another area of low pressure develops over the plains and lifts
northeast into the western/northern lakes late Sunday night into
Monday. Again...a tightening of the pressure gradient along the
southeast periphery of the low track is expected with winds again
becoming southerly into the 15-25 knots range on Monday. Another
trailing cold front will pass across the lake Monday night...with
weak high pressure and lighter winds developing Tuesday.

Ratzer

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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