Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210754 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
254 AM CDT Fri Apr 21 2017

248 AM CDT

Through tonight...

Early morning surface analysis depicts low pressure just north and
east of Lake Huron, with high pressure from the Canadian prairies
extending south to the central Plains states. A weak and fairly
subtle cold frontal boundary is found across north central Lake
Michigan and the adjacent coastal sites in Wisconsin, with a more
pronounced cold front across the Ohio Valley.  The main upper
level features are the upper low across the northern Great Lakes,
a blocking ridge across the northern plains, with low pressure
across the central Rockies just south of the blocking low, and a
weaker upper low across the northern Rockies of Idaho/Montana.

Weather today will first be influenced by cool northwest winds
behind the departing low, and then as is normally the case with
these lake enhanced frontal boundaries, a reinforcing shot of
colder air will filter in this afternoon with the northeast winds
from this front. There are still pockets of clouds that will
bring broken cloudiness through the early afternoon. With the
drier air moving in from the northwest, expect some thinning of
the lower clouds later today that came barreling in last night
with the leading cold front. Highs look to reach the mid to upper
50s far southwest, only in the upper 40s downtown.

Several things will work to keep the upper level low across the
central Rockies from impacting the area too much, and the model
trend continues to keep the majority of the area dry to start the
weekend. The main player is the blocking upper ridge, with the
cool and dry air under the surface high enabling the cold front to
remain parked well to our south. Therefore guidance is in fairly
good agreement in dragging the low through central/southern
Missouri into the Ohio Valley. We will start to see some of the
higher clouds with this system already later today, with rain more
likely staying across Central Illinois and points southward,
possibly a few sprinkles across our far southern counties tonight
into Saturday. With high pressure building in and clearing skies,
expect some fairly cool lows, and possibly some patchy frost in
outlying areas under the high across northern Illinois with some
readings heading into the mid 30s or so.



248 AM CDT

Saturday through Thursday...

Elsewhere, a seasonally cool Saturday can be expected, with 50s in
the metro, possibly upper 50s northwest, with cooler readings at
the lake, and possibly south where clouds will linger the longest. The
weaker upper low currently over the northern Rockies will trail
the leading low heading even farther south and will pull the now
west- east oriented ridge axis over head Saturday night into
Sunday and the surface high will shift out southern Lake Michigan.
This pattern will favor more of a seasonal day Sunday, with
moderation as upper level heights slowly build, the sun effect
become more apparent, and the low level airmass responds. It will
remain cooler at the lake.

The now transient ridge will head east overnight into Monday. The
door will then open to milder air as the surface high will move
east, and this but likely at least occasionally wetter conditions
Tuesday and beyond as we get into a milder but more progressive
westerly flow emanating from the Pacific. Several waves will
shift northeast as the upper flow upstream shifts to
southwesterly, the first late Monday night into early Tuesday,
another later Tuesday or Tuesday night, though confidence on
timing these is not super high. Each of these brings precipitation
chances, though certainly less with the first one.

After the second wave a frontal boundary will remain to our south
with a warm and unstable airmass to its south, with our region in
cool north-northeast flow initially. The main upper low will then
shift into the center of the country mid to later in the week,
which will bring better chances of showers and thunderstorms.
While we will be heading in a milder pattern on the large scale
(very warm south of the front) the surface pattern could lock some
areas in the spring cool and clouds, with elevated rain showers,
until the front gets shoved northward and expands the t-storm



For the 06Z TAFs...

1223 am...Only forecast concern this period is a wind shift to
the northeast early this afternoon.

A ridge of high pressure will slowly build across the western
Great Lakes later today and tonight. As it does...northwest winds
will gradually turn more to the north/northwest later this morning
and then shift the northeast early this afternoon. Confidence is
high this shift will occur...but only medium confidence on timing.
Trends will need to be monitored later this morning with some
adjustment to timing expected. Speeds will generally remain in the
10-15kt range through Friday afternoon with some periodically
higher gusts.

Cigs around 3kft are expected to continue through mid morning and
then gradually lift to 4kft this afternoon...scattering out during
the afternoon and by evening. Its possible some breaks may develop
at times but confidence on how long any breaks may last is low.



211 am...Low pressure over Lake Huron early this morning will
move into Quebec today as high pressure builds across the northern
plains and upper midwest. This high will flatten into a weaker
ridge across the lakes region this stronger high
pressure builds across Canada. Low pressure will move across the
southeast U.S. this weekend. Another low will move from the
central plains Monday to the northern lakes it slowly
weakens. cms


LM...Small Craft Advisory...IL nearshore waters until 11 AM Friday.

     Small Craft Advisory...IN nearshore waters until 10 PM Friday.




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