Area Forecast Discussion
Issued by NWS Chicago, IL

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105
FXUS63 KLOT 190926
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
326 AM CST Mon Feb 19 2018

.SHORT TERM...
326 AM CST

Today through Tuesday night...

Several forecast concerns through the near term of the forecast
this morning, including dense fog potential today and heavy
rainfall tonight through Tuesday night. A flood watch remains in
effect for the entire forecast area tonight through Tuesday night.
Also, could be a period of wintry mix with some glazing potential
Tuesday night into early Wednesday morning.

Deep south-southwest lower and middle tropospheric flow was in
place across the Midwest early this morning, ahead of an area of
surface low pressure over Kansas. Unseasonably warm and moist air
was spreading north across Missouri and downstate IL in
association with a developing warm front, and this air mass will
continue to spread north into the forecast area today. Surface dew
point temps in the low-mid 40`s were as close as central IL at
this hour, and as this moist low level air spreads north over our
colder ground and lingering snow pack, fog is expected to develop
and may become dense in spots later this morning.

Increasing deep moisture, persistent southwesterly flow aloft,
and a series of low-amplitude mid-level short waves will bring
several rounds of precipitation to the area today, with the first
round of showers and a few thunderstorms having traversed the
central and southern parts of the cwa overnight. High-res guidance
depicts the next surge arriving this morning, from northeastern
Missouri where showers thunderstorms were already developing per
regional radars. Another mid-level speed max emerges from the
western CONUS trough tonight, with another surface wave lifting
along the front across the region into Tuesday. Guidance remains
in good agreement in developing relatively heavy rainfall amounts
of between 1.50" and 3.00"-4.00" tonight through Tuesday evening,
before the front to our west eventually moves across the area.
Heaviest amounts are progged across the southeast third of the
forecast area, though frozen ground and runoff from melting of the
lingering snow pack across the north/northwest will all aid in
presenting the potential for flooding and rises on area rivers and
creeks. Thus have maintained the flood watch, with no changes to
timing or aerial extent.

With the warm front lifting across the area today, temperatures
will warm substantially from the south. Forecast highs range from
the upper 40`s along the IL/WI border, to the lower 60`s across
the south. Not much fall-off expected tonight as south winds
persist in this warm and moist air mass, and temps on Tuesday are
again expected to be in the 50`s and 60`s. The cold front begins
to slowly push across the area Tuesday afternoon, and gets more of
a push Tuesday night as high pressure builds across the Northern
Plains. Temps will fall below freezing Tuesday night from
northwest to southeast behind the front, and will likely support
precip becoming a mix of rain/sleet and snow, and possibly a
little freezing rain before ending. The bulk of the QPF looks to
occur prior to temperatures becoming cold enough for mixed precip,
though there could be some issues with light icing or sleet/snow
accumulation Tuesday night with this wintry mix. Will have to
continue to monitor this situation with future model runs, but
could foresee needing some sort of winter weather advisory Tuesday
night into early Wednesday morning if freezing rain were to
producing some glazing.

Ratzer

&&

.LONG TERM...
326 AM CST

Wednesday through Sunday...

Pattern looks to remain somewhat active through the latter part
of the week and the weekend. Temperatures are expected to run
above average, except for Wednesday behind the cold front, though
the frontal zone lingers off to our south through the period
beneath persistent southwest flow aloft. Periodic low-amplitude
short waves translating along the low level baroclinic zone will
have the potential to produce a few periods of light precip for
the area, with guidance focusing the best chances Friday into the
weekend.

The cold front clears the forecast area Wednesday morning, with
any lingering precipitation limited to the far southeastern part
of the cwa early in the morning. The front then settles south of
the Ohio River, but becomes nearly stationary as it is parallel to
the southwesterly flow aloft. Weak surface high pressure spreads
across the upper Midwest to our north through Wednesday night,
with seasonably colder, average temperatures. While most of the
guidance is dry, the 00Z ECMWF does indicate some very light QPF
along the ILX/LOT border, and have held on to some low chance pops
over the far south tier of counties late Wednesday night. Thermal
profiles are marginal for snow, with soundings supporting the
potential for a light wintry mix if it does indeed precipitate
there.

Guidance depicts another short wave lifting northeast from the
southwestern U.S. Thursday, renewing warm advection across the
frontal zone to our south. GFS/ECMWF both develop light precip in
a region of isentropic ascent across IA/western WI and northwest
and north central IL by afternoon, with precipitation eventually
becoming more widespread across the area later Thursday night.
Thermal profiles across our northwest counties are most conducive
for snow during the night, though thicknesses increase during the
day Friday. This would support snow or a mix which would change to
all rain during the day Friday as temps warm back into the 40`s
in most areas.

Similar conditions persist into the weekend, as another slightly
more amplified upper wave is depicted in guidance. Daytime temps
remain in the 40`s both Saturday and Sunday supporting primarily
rain with this system, though a brief period of rain/snow mix is
possible later Friday night and early Saturday if the slightly
cooler ECMWF thermal field were to verify.

Ratzer

&&

.HYDROLOGY...
326 AM CST

The combination of snowmelt (0.85" of liquid in a core sample from
here in Romeoville), seasonally frozen ground limiting absorption
of moisture, and heavy rain, will likely lead to sharp rises
over area rivers and streams in the next few days. Most areas will
see at least an inch of rain, with 2 to 4 inches possible in some
locations. Expect several rivers to get into flood, and numerous
rivers to get above bankfull. Ice jam breakup may also contribute
to flooding in the coming days. While the location of the axis of
heavy rain is not a certainty, there is a strong signal for
potential flooding concerns. Current forecasts have the Kankakee
River at Shelby getting close to major flood stage.

KMD/Ratzer

&&

.AVIATION...
For the 06Z TAFs...

Several forecast concerns continue for this period including,

- Lifr/vlifr cigs/vis with dense fog.
- Low level wind shear overnight and Monday afternoon/evening.
- Chance of showers overnight...showers/rain Monday/Monday
  evening.
- Chance of thunderstorms Monday afternoon/evening.

Overall...no significant changes for vis/cigs. Dense fog is still
expected to develop over much of the area Monday morning...mainly
after sunrise but there remains uncertainty regarding how low vis
drops at ord/mdw as well as is duration. Its possible that a
period of 1/4sm vis may develop and trends will need to be
monitored. There is still expected to be some improvement with vis
as a warm front moves north across the area Monday afternoon. Cigs
however may remain low well into Monday evening.

Low level wind shear currently should begin to diminish during the
early morning hours but then will increase again south of the warm
front Monday afternoon and continue into Monday evening...and
included with this forecast. Southerly surface winds may be gusty
for a few more hours but then will diminish during the early
morning...turn more southeasterly after sunrise then turn back to
the southwest with the passage of the warm front...with speeds and
gusts increasing.

Scattered showers have developed across the area late this evening
and these should quickly move across the terminals in the next few
hours. A rumble of thunder is possible but not expecting too much
lightning with this activity. Additional activity is expected to
focus mainly south of the terminals during the early morning hours
so only have vicinity mention with this forecast. Additional
rounds of showers and rain are expected from later Monday morning
into Monday night. The best chance of thunderstorms appears to be
from mid afternoon through early evening...but confidence remains
low so no mention with this forecast. cms

&&

.MARINE...
303 AM CST

A series of lows will move across the region through midweek.
Early this morning, one area of low pressure is located over the
Central Plains with an elongated trough extending northeast
across far northern Lake Michigan towards Quebec. This first low
will move across central Lake Michigan this evening and fill, and
the elongated trough will stall over central portions of Lake
Michigan. Expect north to northeast flow gusting to 30 kt across
the north half of the lake while south winds of 25 kt or so will
be in place across the south. Meanwhile, an unseasonably warm and
moist airmass will be in place south of the trough. Areas of fog
appear possible within the moist air mass through Tuesday along
with the potential for a few thunderstorms.

Another low will form over the mid Mississippi Valley Tuesday and
lift across central Lake Michigan Tuesday evening. This low will
consolidate and deepen with a trailing cold front eventually
pushing south across the remainder of Lake Michigan by late
Tuesday evening with moderate north winds gusting 25 to possibly
30 kt behind. Strong high pressure will build across the Northern
Plains in the wake of the front and spread across the western and
central Great Lakes by Thursday allowing winds to diminish late in
the week.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
     ILZ032-ILZ033-ILZ039...6 PM Monday to 6 AM Wednesday.

IN...Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019...6 PM Monday
     to 6 AM Wednesday.

LM...None.
&&

$$

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