Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240832

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
232 AM CST Sat Feb 24 2018

243 PM CST

Through Saturday Night...

For the remainder of this afternoon and into the evening, breezy and
cloudy conditions will persist as the the system that brought some
additional rain overnight moves off to the east.  A zone of height
rises over the region in between an exiting low and high pressure
building across the northern plains to the middle and upper
Mississippi Valley.  As the high builds across the Great Lakes
overnight tonight, a warm front associated with a southern stream
system will set up along the Ohio Valley.  A shallow layer of cold
air will remain in place at the surface across the region, but
aloft, warm, moist advection will set up in advance of the southern
stream system, setting up the potential for some freezing/frozen
precipitation in the morning.  There should be a relatively sharp
cutoff to the precipitation on the northern edge, where the cold air
will be in place.  There is still some uncertainty as to the
progression of p-type as the precipitation shield spreads northward,
but the general trend should be for an area of all snow on the far
northern edge, with a narrow zone of a wintry mix transitioning over
to all rain to the south.  Through the morning hours, both the nrn
edge of the pcpn shield will spread north while the zone of frozen
and freezing pcpn also shifts north, to be quickly followed by the
transition over to all liquid as sfc temperatures rise into the
middle 30s by late morning.

However, of greater concern for Saturday into Saturday night, will
be the additional rainfall moving across areas already impacted by
major flooding.  Latest guidance suggests an area of 0.25 to 0.50
inches of additional rainfall for the period from early Saturday
morning to early Sunday morning.  As of this afternoon, most rivers
across the region, including the Illinois, Des Plaines, Du Page and
Fox Rivers have either crested or will crest soon.  Portions or the
Rock River are not expected to crest until Sunday or Monday as the
crest wave moves downstream.  The Iroquois and Kankakee Rivers and
locations near these rivers will remain a concern through the
weekend and into next week.  The Iroquois River at Chebanse is still
rising as water from upstream on the Iroquois river as well as flow
from Sugar Creek and smaller tributaries.  The Kankakee River in
northwestern Indiana will remain above major flood stage well into
next week as the river is particularly slow to subside there.  The
Kankakee River in Kankakee County will also be slow to subside.
While any additional rainfall will not be welcome at areas already
impacted by significant flooding, since the rainfall will occur over
a 24-30 hour period, it should not contribute to significant rises,
and perhaps delaying the river levels falling.



Sunday through Friday...

153 pm...Primary forecast concerns are the potential for more
rain Wednesday/Thursday and the chance of mixed precip/snow

As low pressure lifts north across the upper lakes region
Sunday...strong/gusty westerly winds will diminish on Sunday but
there could be a period Sunday morning when wind gusts are still
in the 35-40 mph range.

High pressure will move from the central Plains Sunday to the
Ohio Valley Monday with dry conditions expected through Tuesday
evening. With some sunshine expected...temps likely to moderate
back near 50 on Monday and into the 50s on Tuesday. With no snow
pack...its possible these temps could be a bit low but can trend
higher as this time period approaches.

Low pressure is expected to develop over the Plains Tuesday night
into Wednesday and move near northern IL by Thursday morning.
Still considerable differences with the gfs showing a rather
strong low at that time vs. a weaker ecmwf low. Ahead of this
low...showers will be possible Tuesday night into Wednesday with
a period of heavier precip possible Wednesday night into Thursday
morning. While the timing looks similar...the large differences in
the strength of the low make for an uncertain forecast. In
addition...small track changes could make the difference between
all rain or a stripe of accumulating snow...which currently would
be most favored across far northern IL into southern WI. Current
qpf forecasts would suggest the potential for a half inch to an
inch of rain with this system. High pressure will then build
across the region Friday. cms


For the 06Z TAFs...

The main forecast concern continues to center around CIG trends
though the period, as well as the potential for more IFR to LIFR
conditions this evening with the next batch of rain and fog.
Strong westerly winds are then looking likely late tonight into
Sunday morning.

Surface high pressure over southern Minnesota will shift eastward
towards the Western Great Lakes around daybreak. As this occurs,
expect light northerly winds overnight becoming easterly during
the day and gradually increasing into this evening. The main
concern for the winds is overnight tonight into Sunday morning.
Following the passage a surface low and associated cold frontal
passage late tonight, it is looking like a period of strong
westerly winds (gusts of 30-35 kt) will set up over the area into
early Sunday morning.

The main concern in regards to CIGS and visibilities will be this
evening. During this period the chances for rain and fog will
increase across the area in advance of the next area of low
pressure approaching the area from the southwest. It appears
likely that the area will get a period of very low CIGs (LIFR) and
lower visibilities in fog and rain, especially during the evening.
Conditions should improve overnight with the passage of the cold
front and the increase in westerly winds.



232 AM CST

Low pressure over the TX panhandle will rapidly deepen as it
quickly tracks northeast across the western Great Lakes tonight
and continue to rapidly deepen as it moves into Canada Sunday.
Expecting a period of easterly gales to develop ahead of the low
this evening, except over far southern portions of the lake where
milder air over the colder lake will result in somewhat more
stable conditions. For that reason, opted to run with a small
craft advisory this evening for the easterly winds for the
nearshore water. Behind the front, look for a period of high end
southwesterly gales behind the fast moving and quickly deepening
low. Quite concerned that there will be a window, perhaps up to 6
hours long or so, of southwest storm force winds over the far
northern portions of the lake, especially toward the Straits.
Strongly considered a Storm Watch for that area, but opted to just
introduce storm force gusts in the north for now and will let the
day shift get a look at latest guidance later this morning and
consider upgrading to a storm warning at that time. Gales will
quickly subside later Sunday afternoon and evening from south to
north with fairly quiet conditions on the lake the first part of
next week before the next storm approached later in the week.

- Izzi


LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...6 PM Saturday to 3 AM Sunday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...3 AM
     Sunday to 3 PM Sunday.

     Gale Warning...LMZ777-LMZ779...7 PM Saturday to 3 PM Sunday.




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