Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180119

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
819 PM CDT Tue Apr 17 2018

816 PM CDT

Have growing concerns about expected p-type for tomorrow`s
forecast. Forecast soundings from NAM/RAP/GFS all strongly suggest
that as the initial wave of WAA driven precip moves through the
area from mid-morning through early afternoon that min temps in
the cloud bearing layer look to be -5 to -10C range, which
suggests very little threat of ice nuclei. In the absence of ice
nuclei, sleet and snow would be out as possible p-types. By onset
time of precip, sfc temps will likely be above freezing, meaning
primary p-type (assuming forecast soundings are correct) would be

One caveat, plume of very steep lapse rates and a modest amount
of MUCAPE suggests a threat of widely scattered thunderstorms or
at least some higher topped heavier showers. Any higher topped
convection could get into temp range with high probs of ice
nuclei, meaning we should see mainly rain, except in the higher
topped convective elements probably able to produce sleet, snow,
and/or graupel. Growing increasingly concerned that any
accumulating winter precip with the initial band will be very
spotty and much more the exception than the rule. Given the
convective chances, am reluctant to make any changes to winter
weather advisory, though think the chances of any impactful winter
wx through mid afternoon is quite low.

In the wake of the initial WAA driven band of precip, expect
drizzle/very light rain through the afternoon, again with cloud
temps far too warm to support any ice nuclei through mid
afternoon. By late afternoon (probably after 21z/22z), cooling
cloud tops should allow drizzle/very light rain to change over to
snow over north central IL. This transition should continue
southeast across the CWA during the evening hours. It is during
this time period that there is the greatest threat of some
accumulating snow, helped in part by sunset as the high April sun
angle will make daytime accumulation very difficult with marginal
temps. It looks to be a 3-6 hour period of snow, probably not
until after 00z in the Chicago metro area. It`s possible during
this time window that we could see an inch or two of snow, mostly
on grassy/elevated surfaces, though temps after dark look to cool
sufficiently to allow for some slushy accums on roadways as well.

Will likely do some tweaking in the grids for Wednesday, but not
planning any big overhauls to the forecast on this shift. Rather,
will brief the oncoming overnight shift about the concerns and let
them get a look at the full suite of 00z guidance before making
any changes.

- Izzi


304 PM CDT

Through Wednesday...

A surface ridge just west of the CWA will cross the area tonight.
Cloud cover should gradually scatter out this evening after the loss
of daytime heating. Would not be surprised to see a few patches of
fog develop primarily across the south half of the CWA where
dewpoints are mixing out to only the upper 20s this afternoon.

Focus then turns to more winter weather expected Wednesday. A mid-
level low and associated surface low currently over the lee of the
central Rockies will track eastward, with the surface low passing
just south of the CWA and the mid-level low passing just north of
the CWA during the afternoon. An area of 285-295K isentropic ascent
and low-level moisture transport ahead of the system will lift a
shield of moderate precip NE across the CWA late Wednesday morning.
Thermo profiles support precip to begin as sleet and possibly some
freezing rain north of a rough line from Dixon to Elgin to Evanston.
Elsewhere, precip should remain rain through the early afternoon.
Also, marginal elevated instability along this initial precip surge
supports a slight chance of thunder. Either way, the precip will
have somewhat of a convective look to it, which should lead to high
embedded higher precip rates.

The mid-level low will begin to shear out while begin absorbed into
deep troughing across eastern Canada by the afternoon. As this
occurs, an elongated area of low-level frontogenesis will settle just
across the far northern CWA in conjunction with increasing low-level
CAA. Rain will make a quick transition to snow from NW to SE through
the afternoon, beginning at Rockford just after noon and reaching
Chicago around 6pm. Accumulating snow is expected, with 1-3 inches
north of the aforementioned line.

Given the expected high precip rates of sleet in the morning, and
the accumulating snow beginning in the afternoon, a Winter Weather
Advisory has been issued for the northern tier of counties plus Ogle
County. If early Monday`s snow is an precursor to what could happen
with the snowfall, some slushy conditions can be expected. Did not
expand the advisory farther south and east as confidence on the
initial precip being all sleet is low at this time.



304 PM CDT

Wednesday night through Tuesday...

Main concern will be the ongoing wintry system. As the surface low
continues to shift toward the eastern Great Lakes Wednesday,
evening, enough cold air will funnel in from the northwest to
transition most areas to snow/rain and eventually all snow. While
the higher QPF is expected during the midday hours Wednesday,
cyclonic flow associated with the upper low and a healthy
deformation field should allow precipitation to continue or even
pick back up a bit for most areas through the evening. Northern
areas near the Wisconsin border, and areas closer to the lake may
still pick up another inch or so of snow in the evening, with
some wet accumulations mainly on grass farther south. Higher
pressure will funnel in after midnight which will quickly cutoff
the deeper moisture, with some snow possible overnight downwind of
Lake Michigan.

We will remain in cool northwest flow on Thursday, and while not
the bitter cold airmass we have had earlier this week, we will
still have highs only in the 40s for most locations, coolest

Upper ridging will develop downstream of a cutoff upper low across
the southwest for the weekend, and this will allow expansive
surface high pressure to take up residence across the Great Lakes
Friday and through the weekend. The good news is dry weather will
be place into early next week. With the high centered over or
east of the lake, northeast winds will maintain our seasonally
cool airmass, with highs only slowly creeping up, not nearing
seasonal norms until Monday. It will be much cooler at the lake
in this pattern. Saturday will also feature more clouds than other
days as the western upper low will shift through the lower
Mississippi valley, being held south by a mini rex block (ridge
due north of the low), so this will keep most of the precip out of
our area.Warmer southwest flow will bring another seasonal day
Tuesday ahead of cold front which would signal our next chance
precip mid week.



For the 00Z TAFs...

Lake breeze is moving across ORD at this time and could see an
hour or so of NE winds around 10 kt before winds subside shortly
after sunset. Easterly winds will increase and become gusty early
Wed morning as next system approaches the area. An arc of showers
and scattered thunderstorms will move across northern IL late
morning through early afternoon. Forecast soundings suggest no ice
in the clouds, so precip type should be primarily rain. Isolated
thunderstorms or higher topped showers could have cloud tops cold
enough to support some sleet/graupel/snow mix with this band. In
the wake of the band, look for low CIGS (IFR to perhaps LIFR) with
drizzle or light rain and light fog through the afternoon. Late in
the day at RFD and during the evening in Chicago area, the cloud
tops should cool enough for a switch over to snow, with perhaps a
brief period of heavier snow possible. Winds should gradually veer
from easterly early Wed morning to northerly Wed evening.

- Izzi


304 PM CDT

High pressure will briefly transit the lake tonight which will
allow gusty northwest winds to relax and ease the hazardous
conditions for small craft.

Winds will quickly shift northeast to east by Wednesday morning
ahead of the next low. The low will pass just south of the lake
Wednesday afternoon and reach New England Thursday afternoon.
Northeast winds will ramp up with gusts in the 20-25 kt range as
early as Wednesday afternoon. Model guidance continues to suggest
northerly gales may be possible for the open waters following the
low passage Wednesday night into early Thursday, but they may be
relatively short lived. Therefore will mention gale force gusts in
the forecast. Gusty northwest winds will linger Thursday behind
the low which will keep waves elevated through Thursday evening
along southern Lake Michigan. High pressure will build over the
Plains behind the low and shift to the western Great Lakes Friday.
The high will linger across the Great Lakes into early next week
keeping relatively quiet conditions in place.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ005-ILZ006-
     ILZ008...10 AM Wednesday to 1 AM Thursday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...1 PM
     Wednesday to 7 PM Thursday.

     Small Craft Advisory...LMZ744-LMZ745...4 PM Wednesday to 1 AM




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