Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181209

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
609 AM CST Sun Feb 18 2018

330 AM CST

Through tonight...

Fog is developing across portions of central/north central
Illinois early this morning. A dense fog advisory will be hoisted
for these areas with patchy fog possible farther east. Otherwise,
breezy but mild conditions will develop across the area this

High pressure is centered over the Ohio Valley this morning with
ridging extending into northern Illinois. This ridge will quickly
move east through the morning and a moderately strong southerly
gradient will develop ahead of a clipper system impacting portions
of the Upper Midwest today. Breezy southerly winds will result
with gusts increasing into the 30 to possibly 35 mph range by mid
to late this afternoon and continuing into the evening. Southerly
flow will help advect warmer temperatures into the region with
highs today expected to top out in the low 40s north to mid 40s
south. Dew points creep up as well rising above freezing late this
evening and overnight and helping to melt remaining snow pack.

Meanwhile, along with West Coast, a strong upper wave will dig
across the Great Basin carving out a deep trough over the
Intermountain West tonight. Downstream, broad height falls will
overspread the Great Plains with a surface low deepening over
Colorado and pushing east towards Kansas City late tonight.
Initial wave of showers and thunderstorms are expected to form
as a strong low level jet sets up across the mid-Mississippi
Valley overnight. Through daybreak Monday, thinking that
precipitation should stay south of I-80 and may not arrive until
late tonight into the predawn hours Monday.



330 AM CST

Monday through Saturday...

An active and spring-like pattern is expected to set up across the
Midwest early though the middle of next week, characterized by
deep troughing over the Intermountain West and high pressure just
off the East Coast. This allows for a broad corridor of southerly
flow from the Gulf of Mexico into the Midwest and strong moisture
transport will result in unusually high PWats over the region
rising to over an inch Monday and topping out around 1.4-1.5
inches Tuesday. These numbers will be 350-400 percent above normal
for this time of year and will set the stage for heavy rainfall
along with the potential for flooding.

Aforementioned surface low is progged to lift from near Kansas
City early Monday morning to near Dubuque Monday evening with
accompanying surface warm front lifting north across the CWA by
late afternoon or early evening. Showers and thunderstorms are
likely with frontal passage Monday and temperatures are likely to
surge well into the 50s with a few spots tagging 60 Monday (and
continuing on Tuesday). Frontal trough looks to stall across far
northern Illinois Monday evening and will serve as the focus for
continued convection as LLJ restrengthens over the region with
moisture convergence focused near the boundary. GFS/ECMWF still
favoring far northern IL/southern WI for the axis of heaviest
rainfall Monday night though the ECMWF has shifted slightly
farther north whereas the GFS has been more consistent. Meanwhile,
the NAM has placed the axis of heaviest precipitation much
farther southeast. Needless to say, there remains some uncertainty
in placement, but favoring the GFS/ECMWF for now.

Surface trough makes little progress into the the day Tuesday as
positively tilted upper wave rounds the base of the longwave
trough and lifts into the Upper Midwest. Another surface low
develops in response lifting from near Kansas City Tuesday morning
taking a similar track across northwest Illinois Tuesday evening.
This stronger wave should finally help to move things along with
a cold front advancing across northern Illinois and northwest
Indiana Tuesday evening and night. Showers and thunderstorms
should increase in coverage once more as the cold front moves
across the region. Copious moisture and deep layer flow largely
parallel to the alignment of the front will result in a gradual
fropa with training showers and thunderstorms.

Through daybreak Wednesday, guidance is still in reasonable
agreement with area-wide one inch precip totals (Monday though
Wednesday morning) with localized amounts of 2 to 4 inches,
though placement of these highest totals varies between the
models. Will continue to highlight the flood hazard with a
Hydrologic Statement this morning given the potential for
unseasonably heavy rainfall, some additional snowmelt yet to come,
and frozen grounds which will promote rapid runoff. Flooding
seems increasingly likely on many area streams and basins and
cannot rule out some areal flooding as well for flood-prone low-
lying areas.

Finally, while flooding is the primary concern, will have to
continue to maintain vigilance for any potential severe threat on
Monday and Tuesday. Surface low tracks remains favorable for
severe  weather in our area and strong winds/shear are not in
doubt. As is typical with cold season severe events, though,
instability remains a big question mark, and forecast soundings
generally favor weak elevated CAPE with poor lapse rates up to
about 850mb. If surface temperatures end up outperforming current
expectations, there would be more cause for concern.

Temperatures turn cooler (seasonable) behind the front later
Wednesday into at least early Thursday. Should get a brief respite
from precipitation during this time frame. Does appear that the
baroclinic zone will lift back into the region late in the
week/weekend though there remains some notable timing differences
in the models, but does appear the region will remain under an
active weather pattern.



For the 12Z TAFs...

Updated 1209Z...fog continues to spread east of DPA toward ORD
and MDW early this morning. Went ahead and amended ORD and MDW
down to 1/2SM based on satellite trends (GOES-16 10.3-3.9 micron
Night Fog product).

Previous AFD text

Dense fog has spread across much of northern IL overnight, and is
the primary near-term concern for the terminals this morning.
Areas west/south of Chicago proper will experience a few hour
period of VLIFR conditions this morning. Low confidence in how
much this will affect ORD/MDW, with high-res model guidance
depicting worst of the fog outside of the city terminals. Will
definitely need to monitor extent of fog over the next few hours
as related to ORD/MDW vicinity. Increasing south winds later this
morning, and little higher cloud cover should aid in mixing out
fog a few hours after sunrise.

Main concern later today is with gusty south winds. Gusts around
25 kts appear likely by midday as the pressure gradient tightens
in response to low pressure moving into the upper Midwest. Gusty
winds should persist into this evening before winds gradually
try to decouple tonight. Strong low level jet above the boundary
layer later today and this evening will also support LLWS, with
winds at the 2000 ft agl level from the southwest at 55-60 kts.

Moisture surges into the area later tonight with the strong
south-southwesterly low level flow, with showers developing
around/after midnight. Gradual increase in low level dew points
over cold ground and snow cover, further moistened by rain, is
expected to lead to deteriorating cig/vis conditions early Monday.
Model guidance hitting this pretty hard with LIFR/VLIFR conditions
after sunrise. Confidence is a bit low on this more than 24 hours
out, so have not gone quite as low as some of the guidance would
indicate. Isolated thunder is not out of the question early
Monday, though instability looks somewhat marginal this far north
Monday morning. Thunderstorm potential is higher beyond the end of
this TAF period.



301 AM CST

Primary focus in the near term is with the quick ramp-up of south
winds today, as low pressure develops out of the High Plains and
moves to the northern Great Lakes by this evening. Model guidance
continues to indicate a period of gale force winds this afternoon
and evening across much Lake Michigan, in response to the
strengthening gradient and surface pressure falls across the
region. Winds should diminish overnight, as the low fills and
moves into southwestern Ontario. A gale warning remains in effect
for the open waters of Lake Michigan from noon-midnight CST. On
the south end of the Lake, small craft advisories are in effect,
and will persist into pre-dawn hours of early Monday morning as
winds diminish.

A cold front trailing from the low will settle half way down the
lake by early Monday, before another low pressure wave translates
along the boundary from the Central Plains to the south half of
the Lake by Monday evening. This will result in northerly winds
across northern Lake Michigan north of the front, and south winds
to it`s south. Another surface low will ripple along the front
along roughly the same track Tuesday, before the front eventually
drifts south of the Lake Tuesday night. Northerly winds will
affect the entire lake by that time, as strong high pressure
builds in from the west.

High pressure spreads across the Great Lakes region Wednesday
afternoon and night, then moves east Thursday. Weaker ridging
lingers across the western Lakes however, within the western
periphery of the departing surface high.



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023-ILZ032-ILZ033-ILZ039 until 10 AM Sunday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 10
     AM Sunday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...noon Sunday to 3 AM Monday.

     Gale Warning...LMZ777-LMZ779...noon Sunday to midnight Monday.




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