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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
240 PM CDT Tue Jun 27 2017

Short term...
203 PM CDT

Through Wednesday...

Quiet weather in place the rest of today under the influence of
high pressure, but precip chances will increase through the day
Wednesday ahead of an area of low pressure working east across the
upper Midwest.

This afternoon, high pressure is centered over Indiana with
ridging extending into northern Illinois providing light flow
and fair conditions. A lake breeze is charging westward across the
Chicago Metro pushing temps back down a couple degrees into the
upper 60s to lower 70s. Lake breeze should wash out later in the
evening with southerly flow overspreading the County Warning Area behind the
departing ridge and ahead of low pressure over the plains.

Attention this evening and tonight will turn off to the west where
an mesoscale convective system is expected to develop around the mid and upper Missouri
River valley on the nose of a strong low level jet. Convection is
expected to spread east across Iowa and Minnesota overnight and
eventually reach Wisconsin and potentially northern Illinois
early tomorrow. Convection is expected to outrun any instability
and the better forcing associated with the low level jet is expected to
remain to our west and northwest with mid/upper level height
rises/ridging building overhead locally. Some of the models do try
to bring in a decaying area of precipitation across northern
Illinois while a number more are dry. Given the limiting factors
mentioned above, have lowered pops but do maintain some slight
chance/low chance pops mainly across the northern two tiers of
counties in Illinois to account for the uncertainty. If something
were able to persist into the cwa, there is not much of a severe
threat given the lack of instability.

Broad area of low pressure is expected to advance east across the
Dakotas and far upper Midwest through the day Wednesday with
strengthening southerly flow expected locally. Tightening pressure
gradient and deep mixing should be able to tap into strong mid
level winds which will result in gusty conditions from mid/late
morning through the afternoon. Expect a few wind gusts to top out
in excess of 30 mph with more frequent gusts of 25 to 30 mph. Dew
points are expected to gradually increase throughout the day,
eventually pushing above 60f, especially across the western County Warning Area
where modest MLCAPE will eventually develop mid to late in the
afternoon. Hard to pin down specifics on convection tomorrow
before 00z with no strong focus for ascent in the area, but with
broad upper difluence behind the departing upper ridge axis and
the potential for remnant outflow boundaries cannot completely
rule out isolated storms tomorrow afternoon. If any storms are
able to develop, there would be some threat for severe
thunderstorms given a strongly sheared environment with
cyclonically curved hodographs in place. Better chances arrive
tomorrow evening as discussed in the extended portion of the
forecast discussion.



Long term...
236 PM CDT

Wednesday night through Tuesday...

Wednesday night through Friday looks stormy with a break from storms
over the weekend. Active weather returns early next week.

Convection is expected to fire over Iowa Wednesday afternoon and
then shift east into the forecast area Wednesday evening and night.
Main concern is flooding as convection may be training along a
boundary. Pwats are forecast to be a healthy 1.75-2 inches. Cape
values are forecast to be around 500 j/kg with shear around 40 kt.
Therefore, a few storms may be strong to severe and capable of
mixing down stronger winds aloft, but the main threat will be

The low's cold front stalls just north of the forecast area
Thursday. The atmosphere should recover from any morning convection
despite cloud cover, but the main question is how long will the
recovery take? Southwest winds will continue to usher in warm, moist
air. Forecast cape values are 2500-3000 j/kg Thursday afternoon with
shear around 30 kt. Model soundings feature a weak cap that erodes
in the afternoon. Despite fat cape, thinking damaging winds would be
the greatest risk given dry air aloft. Pwats remain at 1.5-1.75
inches so heavy downpours and flooding due to training and slow
moving storms is also a concern. While storms are expected after
18z/1pm CDT, I think the best storm coverage will be after 00z/7pm
CDT as an upper level wave approaches.

A surface low is forecast to form over the Midwest Thursday
night/Friday morning and then pass over southern WI and Lake
Michigan. More showers and storms are expected with the low.
Guidance features cape but also a decent cap. So unsure about the
severe potential. Precipitable water values are still high at around 2 inches, and
given two periods of possible heavy rain days before, this system
could bring more rain that we do not need leading to additional

We dry out Friday night into at least Saturday morning. Long term
models indicate precip is possible Saturday as the upper level
trough swings through. However, have low confidence in the strength
of the trough and how much precip will occur. Kept precip chances at
slight chance. Sunday looks similar with a dry start thanks to high
pressure. However, guidance again wants to bring showers into far
northern Illinois in the afternoon. Have even lower confidence in showers
Sunday afternoon. Both days will have highs in the upper 70s to mid

Guidance shows a lot of disagreement in how early next week will pan
out. The European model (ecmwf) keeps the pattern busy with a couple of upper level
systems bring rounds of showers and storms through Monday and
Tuesday. The GFS, on the other hand, has high pressure over the
northern Midwest keeping a low and the majority of its precip to the
south. Given the uncertainty, kept precip chances as is.



for the 18z tafs...

Main forecast concerns will be westward extent of a lake breeze
this afternoon, convective chances overnight and Wednesday, and
very breezy conditions developing by mid morning Wednesday.

For the rest of this afternoon and evening, high pressure centered
over Illinois will continue to gradually slide east across the
region providing relatively light flow (around 10 kt or less) and
VFR conditions at the terminals. A lake breeze has already
developed this afternoon and visible on tord but has made little
inland progress thus far. Given the modest opposing westerly flow
in place, it will be one of those days that the boundary will get
close to both Ord and mdw and likely stall somewhere in the area.
Confidence in exactly how far west the boundary will progress and
if it is able to pass over the terminals turning winds easterly
remains low thus have not made any significant changes to going
forecast with respect to winds this afternoon. Still maintain a
wind shift at mdw which has the best chance to lake breeze passage
but keep out of the Ord taf in the meantime. Some of the hi-res
models do bring it through Ord late this afternoon/early evening.

Expect south-southwest flow to overspread the terminals again mid to late this
evening once the lake breeze washes out and continue through the
overnight hours. Low pressure advancing east across the plains
will result in a sharp increase in winds tomorrow morning, with
gusts at times pushing to near 30kt late morning through the

Meanwhile, thunderstorms are expected to develop to our west this
evening and overnight eventually spreading east into far northern
Illinois and Wisconsin Wednesday morning. Precipitation is
expected to outrun the instability and the better forcing and
support for sustained nocturnal convection is generally expected
to stay to our west and northwest with low but non-zero chances
for a decaying area of precip to overspread the terminals tomorrow
morning. Have introduced a prob30 for showers at rfd but will
call for dry conditions for the Chicago area terminals where there
is only a slight chance for precipitation. Instability doesn't
arrive in the area until beyond the current taf period and it does
currently appear that Wednesday evening and overnight could be
active with areas of thunderstorms for the northern Illinois and
northwest in terminals.



236 PM CDT

High pressure over the lake continues east through Wednesday
afternoon. Meanwhile the low over southern Saskatchewan will reach
Minnesota Wednesday evening. The pressure gradient over the lake
tightens in response to the high and low leading to gusty south
winds Wednesday. Gusts to 30 kt are likely, and occasional gale
force gusts are possible in the nearshore waters and over the
northern end of the lake. The lake looks too stable to support more
persistent gales, although winds will be around 50 kt 1500-2000 ft
above ground level. Will not be issuing a gale headline due to
stability concerns over the lake, but will have ocnl gales in the
Gulf and nsh. Will issue a Small Craft Advisory for all of the
nearshore waters Wednesday morning through most of Wednesday night.
Winds slowly diminish as the the low passes over the northern Great
Lakes Wednesday night, and the low's cold front passes over the lake

A second low forms over the Southern Plains early Thursday morning
and passes over the southern or central portion of the lake Friday.
Winds will vary from southwesterly south of the low to northeasterly
north of the low. Winds settle to westerly Friday night through
Sunday. I have low confidence in exact details early next week as
models disagree at how the pattern will evolve.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...9 am Wednesday to 6 am Thursday.



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