Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221125

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
625 AM CDT Sat Apr 22 2017

251 AM CDT

Through Sunday...

The active weather for the weekend will remain focused on an area
of low pressure across the central and southern plains, with a
secondary feature/cold front across the Upper Midwest along the
Canadian border. Extending from the southern low is a cold front
stretched across the Ohio Valley. North of the the closed low
across the plains is a pronounced area of drying/subsidence in a
ridge/block pattern aloft, and a sprawling area of surface high
pressure stretch from the nation`s heartland northeast to the
Great Lakes region. Cool northeast winds are found in our region,
satellite imagery depicting the cirrus shield from the southern
low beginning to thin in spots as upper level flow shifts more
northerly, and forecast soundings confirming very dry low levels.

With the blocking pattern in place aloft, the closed low will only
slowly migrate eastward into southern Illinois late today and then
into the central Ohio/Tennessee valleys Sunday, with the upper
ridge holding across northeast Illinois and northwest Indiana
through the weekend. The ridge will also keep the front to the
north away as well. Therefore the surface high will remain across
the Great Lakes. From a sensible weather perspective, this will
maintain a cool though dry northeast wind today, and then with the
high more overhead on Sunday, light winds with an afternoon lake
breeze. Highs today will reach farther into the 50s with the
cirrus shield continuing to thin, but plentiful sunshine Sunday
and some slight low level warming will result in more seasonal
conditions for late April with 60s more common. Much cooler
conditions can expected at the lake.



251 AM CDT

Sunday night through Friday...

Monday will be a transition day as the surface high says goodbye
and departs eastward. A series of lows will move through the
center of the country toward the midwest during the middle of the
week, followed by a parade of potentially stronger systems for the
latter half of the week and weekend.

Ahead of the first low Monday surface winds will shift to
southeasterly. Lower level southerly flow and dry conditions will
allow temps to inch closer to or above the 70 degree mark, with a
continued onshore component keeping northeast Illinois lake
adjacent ares cooler, but still warmer then the weekend. The core
of this system will pass through Minnesota, giving us only very
small chances for precipitation in the warm advection ahead of it.
But a southern stream and stronger low will be quick on the heels
of the first one. Hints at some small chances for precipitation
Tuesday afternoon, but better chances arrive Tuesday night into
Wednesday as an increasing jet segment aloft in the southwest flow
will allow for a strengthening surface low to pass northeast
through the area during this time frame. Still some model
disagreement on if this low will pass right over the area or not
and how quickly it and the associated cold front will translate
through the region. This has impacts on how much the initial
chance of precip ahead of the system later Tuesday night will have
and areas that stand the better chance for rain with the cold
front itself. All guidance depicts additional convective chances
Wednesday afternoon with the cold front, but the EC unlike the GEM
has that axis over the entire area, whereas the GEM/GFS are more
focused I-55 southeastward. Either way the later Tuesday night
and Wednesday time frame is the first one to watch. Precipitable
water values are not overly high, but some moderate rains could
occur. Thunder chances do exist, but held at chance for now.

Wednesday night into Thursday we take a quick breather behind the
cold front with surface high pressure expected to briefly return.
A deeper upper low emanating from the Pacific will strengthen
across the mountain west and lead to an active thunderstorm day
across the western high plains and central plains. Locally, the
cold front that had made its way to the Ohio valley will head back
northward as a warm front Thursday. Elevated showers and
thunderstorms appear plausible later Thursday into Friday. Cool
northeast flow will be the rule for Thursday, and the warm front
will still remain south across central Illinois, but strong
moisture transport and warm advection ahead of the low would
suggest a favorable pattern for showers with increasing chances
for thunderstorms through the night into Friday as the sloped
front shifts north. Very heavy rain is a concern in this pattern
with the gulf of Mexico wide open, and depending on instability
and a potentially capped warm sector a severe threat would be on
the table as well, but way too early for details in
timing/location yet and at this point the threat would be
southwest of the local area. With much cooler temperatures
initially, expect a significant surge in temperatures Friday as
the front moves south to north. Still large uncertainty at this
week out distance how quick the front will lift through.

Then we will have to await the main upper low which will bring
those anxious for a return to dry weather a bit more uneasy as
another round of heavy rainfall producing showers and
thunderstorms to begin the weekend as early as late Friday night
into Saturday, though model timing on the evolution of deep upper
lows is not always stellar at this distance.



For the 12Z TAFs...

625 am...Dry/vfr through the period. Northeast winds will
increase to 10-15kt this morning with gusts 18-20kts possible.
Speeds/gusts will diminish quickly with sunset and as a ridge of
high pressure settles across the area tonight...winds will become
light/variable or calm at many locations. High clouds will
scatter/thin through the day with skc expected tonight. cms



218 am...A ridge of high pressure extends from the plains to the
northern lakes this morning and will slowly sag south and build
east into New England Sunday. A weak area of low pressure will
move from Ontario today to northern Quebec Sunday morning with a
weak trailing cold front moving across northern Lake Michigan
Sunday into Sunday evening. This front will likely stall across
central portions of the lake as the ridge slowly weakens Monday.
An area of low pressure will move from the western plains Monday
morning to near Lake Superior it weakens. Southerly
winds will increase ahead of this low but currently only expect
up to 30 kts since the low is expected to weaken. Another low is
forecast to develop over the central plains Tuesday and move
northeast to the western lakes region Wednesday. cms


LM...Small Craft Advisory...nearshore waters until 10 AM Saturday.




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