Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241000

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

330 AM CST

Through tonight...

Low pressure developing over the Southern Plains will deepen and
track overhead or just to our northwest over the next 24 hours,
bringing a prolonged period of precipitation followed by the
passage of a cold front late tonight.

The first part of the forecast period this morning could be
somewhat interesting as light rain moves into an area with
temperatures at or just above freezing. Farther upstream over
portions of Iowa and Missouri there are a handful of stations
reporting mixed precipitation but closer to home the readings are
mostly rain. In general temperatures seem to be nudging above the
freezing rain thresholds as the rain becomes more established in a
location, but as the boundary layer is still in the process of
saturating during the onset of precipitation there could be a
brief period of light freezing rain or even ice pellets. Did not
feel this was going to be widespread or persistent enough to
warrant an inclusion in the forecast, but if trends go a different
direction this would certainly prompt an early morning update.

The first round of precipitation looks to stall somewhere around
the I-80 corridor due to the influence of drier air to the north.
Much of the forecast area except for southern portions could end
up dry this afternoon before the next round approaches this
evening. Along with the rainfall comes a small chance of
thunderstorms. The initial SPC outlook for today included much of
our area in the general thunder category and our southeast third
in a marginal risk for tornadoes and damaging winds. Based on
internal coordination between local forecast offices and SPC this
likely will be pushed back farther south with the next update, so
do not plan to emphasize the severe threat at this time. Local
thinking is that shear is plentiful and instability is present but
limited and likely not surface based this far north.

Beyond precip trends, the other story for tonight will be the
increasing winds after midnight, with gusts by morning likely
exceeding 30 mph especially behind the passage of the cold front
expected during the predawn hours.



330 AM CST

Sunday through Friday...

After an active period today and tonight, ridging becomes more
established for the late weekend into early next week. In
addition to helping us see some sunshine again, the position of
the ridge to our south will allow for 2-3 days of warmer
southwest flow. Even Sunday might see a bit of a warmup after the
cold postfrontal start to the day, but Monday thru Wednesday look
much more promising for widespread 50s and perhaps a few locations
in the 60s farther south.

The next weather system arrives midweek and at least at this
time range appears to be potent. Deterministic models are not in
agreement regarding the strength and track of the low but early
indications are that we stay in the warmer sectors and see
primarily rain, perhaps with some thunder, and another round of
strong winds.

Friday again looks dry with seasonable temperatures, but timing
changes for the midweek system could bring corresponding changes
to the late week trends.



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...Gale Watch...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779...6 PM Saturday to 3 AM Monday.




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