Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221229
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
629 AM CST Mon Jan 22 2018

.UPDATE...

533 AM CST

Regional radar composite shows a remarkably well developed MCV
moving northeast across central IL early this morning. Radar
estimates have shown hourly rainfall rates of 0.3-0.5" per hour
with the heavier returns with sfc obs confirming these rates.
Given elements with these rates, still looks reasonable that a
swath of around an inch to perhaps an inch and half of rain will
fall near the track of the MCV, slicing right across the Chicago
metro area. Given the frozen ground and moderate to heavy rates,
issued an areal flood advisory earlier to cover the threat of any
minor flooding/ponding with this activity early this morning.

The rain has done a nice job of washing out the dense fog with
visibilities improving substantially in the rain. Once the rain
ends, could see fog re-develop north of the warm front, but
confidence isn`t high enough to extend what`s left of the dense
fog advisory beyond its scheduled 12z expiration. If dense fog
does redevelop this morning, it should be temporary until the warm
front lifts north to the WI border by around midday.

- Izzi

&&

.SHORT TERM...
252 AM CST

Through tonight...

A little something from the weather department for almost
everyone the next 24-36 hours. On the docket weather-wise, we have
ongoing dense fog, showers & t-storms, locally heavy rain/minor
flooding threat, unseasonable warmth, small threat of isolated
severe weather, possible change over to wet snow, and windy/colder
conditions.

This morning:
Slug of showers and thunderstorms over MO/western IL will continue
to move and develop northeast into northern IL and northwest IN
prior to sunrise and continue for a bit after sunrise. Still looks
like there will be a swath of heavy rainfall totals in excess of
an inch in less than 6 hours, probably extending northeast into
the Chicago metro area. Given the frozen ground, very little of
the rain will get absorbed into the ground and rather will go
straight to run off. Result could be some minor flooding/ponding
of water issues this morning. Widespread showers and thunderstorms
will end during the mid-late morning hours as a pronounced dry
intrusion pivots into the area. Warm front is slowly lifting north
now, but is expected to more rapidly surge north into southern WI
by mid-late morning. Look for dense fog to improve both as showers
and storms begin and also as the warm front lifts north. May shave
a few more counties off the dense fog advisory in addition to
those set to expire at 09z.

This afternoon:
The pronounced dry intrusion will provide a break in precip
chance late this morning into the start of the afternoon hours.
Based on satellite trends upstream and conceptual model as we
break into the warm sector, would anticipate some partial clearing
to take place with at least some breaks for sunshine. Have
trended temps upward toward and a bit above the warmest guidance
today as stout southerly winds and perhaps some breaks for
sunshine allow temps to climb well into the 50s area-wide with
perhaps a 60F reading here or there this afternoon.

Ingredient this afternoon look likely to come together to support
a broken arcing band of convection this afternoon as steep mid-
level lapse rates/cold mid level temps pivot eastward and become
juxtaposed over top the narrow warm sector characterized by
dewpoints in the low/mid 50s. The result should be a ribbon SBCAPE
around 500 j/Kg developing just ahead of surface trough. Strong
DCVA and low level convergence along the boundary should support
rapid convective development during the early-mid afternoon hours
over western IL, with storms moving northeast across the area
late in the afternoon into the early evening hours.

Model forecast wind profiles suggest effective shear values
should be sufficient to support some threat for low topped
supercells, though veer-back-veer profile over northern IL isn`t
usually particularly favorable for tornadoes. However, strong
ambient vorticity in the presence of the approaching upper low
could offset the potentially questionable wind profiles. Certainly
doesn`t look like a tornado outbreak set-up, but does look to be
a window of opportunity for storms to produce a couple tornadoes
this afternoon. Low freezing levels could allow for some hail,
mostly sub-severe.

Tonight:
Broken band of convection should be in a weaken phase as it
rotated northeast and east out of our CWA early this evening.
Widely scattered showers will likely linger across the area
tonight with precipitation gradually mixing with then changing to
some wet snow overnight from northwest to southeast as colder air
filters into the region.

There is a strong signal in various model guidance that
strengthening f-gen on the northwest flank of the system will
promote an intensifying band of snow overnight into early Tuesday
morning. While there is strong agreement in that scenario, the
placement of this feature varies widely in the model guidance. The
strong signal is over southern WI, but a few models do suggest
that the intense trowal could affect portions of northern IL
overnight. Given the presence of very steep lapse rates in the
presence of strengthening f-gen circulation, potential exists for
heavy snow and couldn`t even rule out a couple lightning strikes
in this band given the very steep lapse rates. Unfortunately, due
to the mesoscale nature of this potential band of heavier snow, it
really is going to be more of nowcast type issue. Plan to
continue with accums of an inch or two near the WI line late
tonight into early Tues, but in all likelihood accumulations will
be more confined to the f-gen driven band where heavier precip
rates will be able to augment the somewhat marginal temp profiles
and allow for accumulations. Outside of that heavier snow band
(where ever it may set up) not expecting much accumulation given
the mostly above freezing temps and warm/wet ground. While the
most favored area for the heavier band of snow is southern WI and
far northern IL, the Hires WRF-NMM is right over the Chicago area,
so will need to closely watch the evolution of this system.

- Izzi

&&

.LONG TERM...
252 AM CST

Tuesday through Sunday...

Any linger snow should end by mid morning Tuesday leaving just
windy and cold (compared to today) temps in its wake. In reality,
temps Tuesday should hold steady in the low/mid 30s which is close
to the avg high for late January. Wednesday look dry with
seasonable temperatures.

Another long wave trough is forecast to dig into the western CONUS
mid-late week with downstream amplification of upper ridge over
our area. Surface low is forecast to track east across southern
Canada late in the week with broad and rather stout southerly flow
developing across the central and eastern U.S. and setting the
stage for another bout of unseasonable warmth locally. Thursday
should see the moderating trend begin, but by Friday it looks like
high temps should make it into the 50s most areas. The southerly
flow looks to be relatively void of moisture through Friday
locally, so stratus isn`t much of a concern and cirrus should be
more prominent closer to the jet to our north, so Friday looks
like it could be a really nice day with sunshine and temps in the
50s.

Cold front looks to move across the area Saturday afternoon, so
after a mild start look for a chance of showers and a return to
more seasonable temps late in the weekend into early next week.
Operational runs of the medium range models tonight are all in
good agreement on the late weekend cool down (return to avg temps)
being short-live with another bout of unseasonably warm
temperatures possible heading into next week.

- Izzi

&&

.AVIATION...
For the 12Z TAFs...

SHRA and embedded TSRA are lifting northward across the area, with
the current TSRA threat ending shortly for the eastern
terminals/prior to 13z. Low CIGs are already showing signs of
improving at the eastern terminals and this process will quicken
as a warm front lifts north this morning. Southeast winds could
briefly gust in the 15-20 kt range for 2-3 hours as the front
lifts north. This will be followed by south or south-southwest
(170-200 deg direction) winds gusting to 15-20 kt for a few hours
until they diminish. North of the warm front at RFD, after the
current round of SHRA ends, can`t rule out FG redeveloping for a
few hours until the front clears north of there by mid day.
Expecting conditions to improve to VFR across the area by the
early afternoon with a lull in precipitation. The next round of
scattered SHRA and possibly isolated TSRA looks to develop in
western and northwestern IL during the mid afternoon and then lift
northeast into the evening. Covered this activity with VCSH and
temporary SHRA for on station impacts. Did not have enough
confidence for TSRA inclusion, which will need to be monitored.

With the surface trough overhead later this evening and overnight
and lighter southwest winds, should see lowering CIGs once again
and likely VSBY outside of any SHRA. Scattered SHRA should be
around much of the night. Later tonight into Tuesday morning,
there is concern for the rain changing to snow and possibly a
narrow band of moderate to heavy snow somewhere over northern IL
and southern WI. Have covered this lower confidence potential with
a PROB30 mention in the ORD/MDW/RFD/DPA TAFs. If steady snow
occurs at any of the terminals, then a slushy accumulation of wet
snow is possible, with currently RFD most favored. Conditions will
improve by mid Tuesday morning with brisk northwest winds gusting
in the 20-25 kt range.

Castro

&&

.MARINE...
141 AM CST

There are multiple concerns through Tuesday as a strong storm
system impacts the region. Northeasterly 35-40 kt gales will
develop on the north half of the lake this morning as the low
pressure moves into southern IA, tightening pressure gradient
between it and high pressure north of the lakes. Meanwhile, a warm
front lifting north spreading unseasonably mild and moist air
over the south half will keep favorable conditions in place for
dense fog through mid evening. Northeasterly gales will continue
north of the surface low path, which will be over northern
Illinois this evening to Lake Huron by Tuesday morning. Once the
low shifts east, gales will turn northerly on the north half of
the lake, with north-northwesterly gales spreading down the south
half. A Gale Warning is in effect from 9am CST this morning
through 6pm Tuesday for the north half and 3am CST to 6pm CST
Tuesday for the south half.

For the nearshore waters, expecting the dense fog to dissipate
this morning with steady rain moving in and warm front lifting
north. A brief period of near hazardous southerly winds will occur
this morning through early afternoon for mostly the Indiana
nearshore. East-southeast winds north of the warm front could
also produce waves up to 3-5 feet along the Illinois shore from
mid morning through early afternoon. Will hold off on Small Craft
Advisory issuance with some uncertainty on the speed/gust
magnitude and very short duration of the higher winds for the
Indiana shore and waves just shy of criteria for the Illinois
shore. A Small Craft Advisory will then be needed Tuesday into
Tuesday evening for the Illinois shore and through Tuesday night
for the Indiana shore during the strong north-northwest winds. A
brief period of gale gusts appears possible for the Indiana shore
during the mid morning through early afternoon timeframe, but do
not have enough confidence for an additional gale headline.

The next period of strong wind concern is south-southwesterlies
Friday through Friday night, with a period of gale force winds
currently appearing probable over the open waters. Winds will
shift westerly and diminish but remain elevated behind cold
frontal passage on Saturday.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory...LMZ777-LMZ779 until 9 PM Monday.

     Gale Warning...LMZ777-LMZ779...3 AM Tuesday to 6 PM Tuesday.

&&

$$

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