Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270550

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 AM CDT Fri Apr 27 2018

148 PM CDT

Through Friday night...

Features of interest in the short term include a series of cold
fronts tonight and Friday night that will bring some cooler
conditions and also a few opportunities for rain showers.

Our first weaker cold front will move through tonight. This front
will come into a fairly dry airmass, so we just expect some spotty
showers with this one, mainly overnight. Expect breezy north-
northwest winds to accompany the front into early Friday.

The more significant front, evidenced by a more stout shortwave on
GOES water vapor satellite imagery across north-central
Saskatchewan/Manitoba, will dig across the lower Great Lakes later
Friday into Friday night. After some brief clearing Friday
morning, expect another increase in clouds in the afternoon. There
is a decent signal for a period of showers during the mid to late
afternoon. There is even a non-zero signal for some thunderstorms,
though model instability and SREF calibrated thunder
probabilities are fairly low, so these would be isolated, and if
they occur favored north and west. Generally speaking, expect a
bit colder conditions Friday than today.

While we get some mid level drying Friday evening, which should
reduce shower coverage, there could be some lingering rain as the
main upper vort max will still need to shift through during the
evening hours.



148 PM CDT

Saturday through Wednesday...

The weekend will start off on the chillier side, but it will be
heading upward from here. The good news is there will be plentiful
sunshine and no precipitation. Highs will reach then mid to maybe
upper 50s well inland, while lake front areas will top out in the
mid 40s due to a brisk NNE wind through the day. High pressure
overhead Saturday night will make for another seasonably cool
night, with some areas likely getting to or below freezing in
outlying areas, with mid 30s even in the urban areas.

By Sunday, surface high will build southeast towards the Ohio
Valley with winds flipping to southwest across the CWA which will
help moderate temperatures closer to, but still likely below the
seasonal norms. This begins to change as we head into the upcoming
week, though, as strong upper ridge translates east across the
plains into the Great Lakes. Low-mid level flow increases out of
the southwest as this transition occurs which will transport an
increasingly warm and moist air mass into portions of the Midwest.
A more significant warming trend will set up Monday into at least
part of Tuesday when temperatures could top out near the 80F
mark, with breezy southwest winds gusting to 30 mph or so at

A more active pattern will set up midweek. Surface high pressure
will settle over the southeast allowing ample gulf moisture to
stream north into the midwest. Meanwhile, a series of shortwaves
rounding the base of the upper trough over the southern plains and
lifting into the mid/upper Mississippi Valley will result in
periodic chances for showers and thunderstorms. Chances remain low
until the Tuesday night period when a cold front will at least
graze the area and provide a focus for precipitation. The frontal
boundary will linger on Wednesday and then again late Wednesday
into Thursday as the main upper trough will finally dislodge to
the east. In general, the large scale pattern does look stormy
with potential for strong to severe storms in or near the local
area, with the highest probabilities progged just west and south
of our area.



For the 06Z TAFs...

A rather complicated wind forecast for most terminals, but
especially MDW and ORD over the next 18 hours. Light SSW winds will
quickly turn N and gust 20-25kts during the pre-dawn hours as a cold
front crosses the area. A surface boundary will then set-up across
NE Illinois, likely over or very close to ORD and just west of MDW.
Confidence is very low in the wind direction forecast prior to 22Z,
but model guidance is coming into better agreement that the boundary
will shift far enough west to keep an easterly component to the
winds at ORD and MDW through much of the day. A quick moving surface
low will then cross NE Illinois around between 23Z and 01Z, which
will solidify the E to NE winds through the evening. A 2-3hr period
of showers will also accompany this low. There exists a very small
chance of an isolated lightning strike, but confidence remained too
low to include even VCTS at this time.



148 PM CDT

Southwest to south winds will gradually pick up through the
afternoon ahead of an approaching cold front. The front will sweep
down the lake this evening and overnight leaving north winds
gusting to 30 kt in its wake. Conditions appear briefly close to
small craft criteria late tonight into early Friday, mainly for
the Indiana nearshore zones where the fetch is longest, but will
not hoist any headlines at this time. A stronger cold front will
push down the lake Friday evening and night and will provide
another period of north winds gusting to 30 kt that will gradually
ease through the day Saturday. A small craft advisory will likely
be needed for this time frame. Winds should diminish later in the
day Saturday as high pressure builds towards the region and
remain light on Sunday. High pressure will shift to the southeast
early next week, and southwest winds will ramp up over the lake to
around 30 kt Monday and Tuesday as low pressure will shift from
the Rockies to the upper Great Lakes.






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