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Area forecast discussion...updated 
National Weather Service Chicago/Romeoville Illinois
558 am CDT Monday Oct 20 2014

Discussion...
557 am CDT

We are seeing scattered showers spark near Chicago as of 555 am
ahead of the upper disturbance mentioned in the previous
discussion. This disturbance has a pocket of steep lapse rates
aloft /500mb-750mb/ of 8c/km and greater and that is aiding some
of these shower updrafts. This is indicated by a couple lightning
strikes over Lake Michigan and the observers at Ord and mdw reporting
intra-cloud and cloud-to-air lightning. The good news is this
upper wave and its forcing is moving about 35-40 miles per hour so not
expecting these to stick around the Chicago area after 7 am.
Continue the shower chances this morning as is in Northwest
Indiana and have added a slight chance of thunder for areas near
the I-80/I-90 corridor.

Mtf

&&

Previous discussion...
319 am CDT

A quiet weather pattern will remain in place in terms of lack of
precipitation chances and any high impact weather through next
weekend...though still several little interests to tackle. These
included how widespread clouds will reach later today through
Tuesday as well as lake effect shower potential during that time.
Then looking at when and how quickly of a warm-up occurs late in
the week into the weekend.

Today through Tuesday...
the continued amplified but progressive flow over Canada and the
northern Continental U.S. Continues for a little longer with a Pacific/clipper
hybrid system moving southeastward from the central Great Lakes.
Around this a well-defined short wave on GOES water vapor imagery is
moving into northern Illinois early this morning. This is fairly moisture
starved but cannot rule out a few showers this morning mainly in
Northwest Indiana. This short wave also begins to break down the
low-level thermal ridge over the area...with clouds likely to
advect/develop southward into the forecast area this afternoon. In
addition...temperatures may also peak early in the afternoon and
then experience a slow drop. Temperatures should still peak around
climatological normal /60 to 65/ with the Morning Sun. The cyclonic
flow over the region and rapidly cooling 850-925mb temperatures look
to support clouds over much of the area by early this evening and
upstream observational trends support this. Have gone well above
most guidance in sky cover from late today all the way through
Tuesday morning as high-resolution model condensate /such as the
4km Storm Prediction Center WRF/ seems to work better in such a pattern with thin but
probably broken-overcast cloud cover. The clouds and north-
northeast winds on Tuesday morning should limit warming by quite a
bit with some locations possibly not reaching 50.

As the cooler air at or around 0c at 850mb expands southward down
the lake later today foresee deepening lake effect clouds and
some showers developing. These showers look to be not particularly
concentrated with limited convergence and mainly light in
intensity with lake-induced equilibrium levels forecast by the
NAM of around 5000 feet. The boundary layer flow fetch should favor
Northwest Indiana for these showers tonight into early
Tuesday...though some veering toward the Illinois/in state line and even
potentially Chicago Tuesday morning could support some light
shower activity Tuesday morning in those places. The hopwrf-ts
supports such a general evolution with its recently added extended
high-res output.

Tuesday night through Thursday...
surface high pressure will build over the area from the north during
this time as the overall upper pattern slows thanks to amplification
of a trough along the West Coast of North America. Overall benign
weather looks to be the rule with temperatures slightly below normal though
gradually modifying. Model guidance does indicate an elongated short
wave to move into the upper Mississippi Valley during or around
Thursday night though this will be moving into an unfavorable
upper level and moisture supportive pattern. So continue the dry
forecast.

Friday into next weekend...
the only change with this portion of the forecast was to increase
temperatures...mainly on Saturday. If cloud cover can remain limited
with the pattern forecast...the 850mb-925mb temperatures would
support temperatures potentially in the 70s with a thermal ridge
steered in from the west-southwest.

Mtf

&&

Aviation...

//Ord and mdw concerns...updated 09z...

* southwest winds becoming west this morning...northwest this
afternoon then north/northeast late this afternoon. Cms

//discussion...updated 06z...

A cold front will move across the terminals later this morning...
shifting southwesterly winds to the west/northwest by middle morning
and then to the northwest by early afternoon. Speeds 10-12kts with
a few higher gusts to start will diminish some by sunrise then
increase again to 10-12kts this afternoon. As the front continues
east this afternoon...the gradient becomes a bit weaker over the
southern end of Lake Michigan. This will likely allow winds to
shift to the north/northeast by late afternoon. Confidence in
speeds is a bit lower...with prevailing speeds likely to be just
under 10kts.

Middle clouds will shift east early this morning with lower VFR...
3-5kft ceilings over the northern lakes moving south across the area
later this morning and continuing into this evening with some
potential for high MVFR ceilings...especially south of the lake
affecting gyy. Cms

//Ord and mdw confidence...updated 09z...

* high for winds through middle morning...then medium for direction and
timing changes this afternoon/evening. Cms

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

Tuesday...VFR. Breezy northeast winds.

Wednesday...VFR. East winds.

Thursday through Saturday...VFR. Light west to southwest winds.

Mdb

&&

Marine...

309 am...low pressure over the Upper Peninsula of Michigan will
move southeast today and tonight reaching the middle Atlantic on
Tuesday. A trailing cold front will move across Lake Michigan this
morning. Southwest winds will turn northwest this morning then
north this evening. The gradient remains fairly tight across the
southern third or so of the lake...with gusts to 30kt possible for
a few more hours. High pressure will build across Ontario today
and tonight as it strengthens. While the center of this high will
move east into Quebec Tuesday night...a ridge will extend south
across the western lakes tonight and Tuesday...then move across
the lake Wednesday. The gradient will tighten between this
approaching ridge and the departing low with northerly winds
increasing to 30 knots this evening and remaining there until Tuesday
afternoon...as the ridge slowly works east. Cms

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...nearshore waters
until 7 am Monday.

Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 PM Monday to 7 am Wednesday.

&&

$$

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