Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 141944
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
244 PM CDT Sat Apr 14 2018

.SHORT TERM...
244 PM CDT

Through Sunday...

No big changes planned in the near term. Strong mid-latitude
cyclone is slowly moving east across the central CONUS. Another
arc of rain is blossoming near the MS River in advance of the
closed mid-upper level circulation. Morning soundings did show
pocket of some mid-level steep lapse rates, which may be enough to
support some weak elevated CAPE and allow a thunderstorm or two to
develop later this afternoon into early this evening, mainly south
of I-80. As the arc of rain pushes east, guidance suggests
increasing coverage/intensity, so anticipating a fairly
widespread, soaking rain across the area through tonight.

A number of models have 2m temps over our northern tier of
counties dropping to near or just below freezing tonight. Plenty
of warm air aloft would suggest a freezing rain threat developing
tonight for our WI border counties. Climatology would certainly
not favor freezing rain in April, but given the fairly good model
agreement in sub-freezing temps see no reason not to maintain
freezing rain chances in the forecast for tonight. Sfc freezing
line is currently well north into central WI and sfc winds are
largely parallel to the thermal gradient, so advection is not that
strong despite the strong winds. Given the uncertainties and
expectation that ice accums tonight (if any) would be mainly
confined to elevated surfaces, opting to not issue any headlines.
Will brief the oncoming evening shift and allow them to monitor
trends this evening to determine if a winter wx advisory may be
needed.

Wind over and near the lake have largely been under performing
today. Earlier ship obs indicate that stability issues are likely
playing a role and preventing the stronger winds aloft from mixing
down. Have seen some occasional obs with wind advisory criteria
winds well inland across north central IL, while closer to the
lake and over the metro area winds have generally been solidly
below criteria. It is still windy and given the sporadic advisory
level gusts over western CWA, plan to just let the wind advisory
continue through its scheduled expiration time at 00z.

Given winds have been under performing over the lake, waves have
also not been nearly as high as previously forecast. Checking with
local official suggests that the lake shore flood impacts have
been minimal thus far and of the nuisance variety. The event is
underway, so don`t see much upside to changing the lakeshore
flood warning back to an advisory. Rather, at this point, plan to
just let the warning continue for now, as winds begin to come down
this evening, the warning can likely be cancelled early.

Finally, Sunday looks like we should get dry slotted with steadier
rain likely tapering off to drizzle. Sunday looks drizzly and
cold, though by late in the afternoon into the evening deeper
moisture wrapping around the cyclone should provide for ice nuclei
needed for drizzle to change over to some snow as the column
cools below freezing. This change over looks to take place over
northern central IL late in the afternoon or early in the evening
slowly spreading east across the remainder of the CWA Sunday
night. Some minor accums (generally less than an inch) could occur
Sunday night on elevated and grassy surfaces. Northwest winds will
begin to pick up again, only adding to the winter-like feel of the
weather Sunday night.

- Izzi

&&

.LONG TERM...
231 PM CDT

Monday through Saturday...

By Monday morning, conditions will be settling down as the deep low
pressure system continues to slowly lift through the eastern Great
Lakes and into sern Canada.  The sfc and upper lows will become more
vertically stacked, with strong deep layer cold advection in nwly
flow and slightly cyclonic curvature aloft.  Some light snow should
linger into the morning hours and then quickly change over to a
rain/snow mix and then all rain through the day.  Pcpn chances
should shut down through the afternoon as sfc high pressure begins
to build across the Middle and Upper Mississippi Valley.
Temperatures should start out the day in the upper 20s to lower 30s
and then rise into the upper 30s to lower 40s.  The upper ridge and
sfc high pressure should build across the region overnight Monday
night and Tuesday as the old sfc low sits nearly stationary over
sern Canada.  A relative warming trend will begin Tuesday as a new
low develops over the Central Plains and upper ridging builds over
the Midwest.  Temps through the rest of the period should be closer
to seasonal normal levels with highs in mainly in the 50s and lows
in the 30s.  The next significant chance for pcpn will be late
Tuesday night and Wednesday and the next low approaching the region.
 This system is expected to be more progressive and take a more
northerly track than the system currently impacting the region, so
while some light rain-rain/snow mix-snow will be possible, winds
will likely not be nearly as strong, the pcpn will move through the
area much quicker and temperatures will not be as low.  More mild
and dry conditions are expected for the end of the period as the
system moves off to the east.  However, with the general pattern
expected to remain relatively high amplitude and progressive, the
longer range guidance is suggesting that another system may bring
another round of pcpn for next weekend.

&&

.AVIATION...
For the 18Z TAFs...

No significant change in conditions expected through Sunday
morning. IFR CIGS may have a tendency to build downward to LIFR in
times, particularly overnight into Sunday morning. Drizzle and
sporadic showers today will build to a steadier/heavier rain early
this evening that should largely prevail through the night before
tapering to drizzle Sunday morning. Strong/gusty northeast winds
expected through tonight, tapering off late tonight and more so
Sunday morning.

- Izzi

&&

.MARINE...
231 PM CDT

Gales are expected to continue over the lake as a low pressure
system passes through the Mid Mississippi Valley tonight and strong
high pressure remains parked over ern Ontario and wrn Quebec.  The
low will slowly lift across the Eastern Great Lakes Monday, reaching
swrn Quebec Tuesday morning.  The low will then weaken and dissipate
by Wednesday. Winds should gradually diminish below Gale Force from
south to north through the day on Sunday as weak high pressure
approaches the Mississippi Valley.  Another low is forecast to move
over the lake Wednesday afternoon or evening, but the models are
indicating some timing and intensity differences, so the general
confidence in exact wind trends associated with the passage of this
system is relatively low at this time.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Lakeshore Flood Warning...ILZ006-ILZ014 until 10 AM Sunday.

     Wind Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ014-ILZ020 until 7 PM Saturday.

IN...None.
LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ777-LMZ779 until 10 AM
     Sunday.

     Gale Warning...IN nearshore waters until 1 AM Sunday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...1 AM Sunday to 5
     PM Sunday.

&&

$$

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