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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 190250

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

850 PM CST

GOES-16 differential water vapor RGB this evening shows shortwave
trough over the central plains embedded within the broad mid-upper
level moisture plume associated with the subtropical jet. The 00z
evening upper air analysis showed a significant moisture gradient
at 850mb between bone dry air mass sampled at KILX/KDVN and
seasonably moist air mass at KSGF/KTOP. Evening RAOBs already were
sampling 50 to nearly 60kt southwesterly flow around 850mb with
KDVN WSR-88d showing a 10-15kt increase in low level flow in the
past 3 hours. This very strong low level jet should transport the
more moist air mass upstream rapidly northward tonight above sharp
frontal inversion. This supports the high-res model solutions
which show convective precip blossoming over eastern IA/western IL
later this evening. Anticipating showers and isolated t-storms to
quickly develop toward or just after midnight to our west before
spreading east across mainly the southern half of our CWA
overnight into early Monday morning.

At the surface, moderately strong and occasionally gusty southerly
winds should result in near steady temps through the night. The
higher dewpoints are still well south toward the Missouri Ozarks
region, but should begin to surge northward tonight with warm
frontogenesis over central IL later tonight. As the low level
moisture begins to impinge on the snow covered areas of northern
IL and northwest IN believe that dense fog will become an 
increasing threat, particularly after sunrise Monday morning and
lingering into the afternoon north of the northward advancing warm
front. As warm front lifts north, look for a quick improvement in
visibilities, but there is still some uncertainty how much/if at
all, the snow cover will impede the warm front's northward advance
Monday. The NAM gives snow pack apparent super powers and it is
likely too cool with temps and slow with the FROPA, with a
compromise solution between the NAM and RAP probably most
reasonable at this point. 

Strongly considered issuing a dense fog advisory for tomorrow, but
fog can be fickle and have been burnt before with apparent slam
dunk fog events, so opted to "message" the dense fog in a
graphical nowcast and will let later shift issue the NPW. 

Flood watch looks on target and no changes made this evening. 

Updated grids have been saved and will be firing out a new ZFP and
LFP shortly. 

- Izzi


202 PM CST

Through Monday...

Temperatures this afternoon will depend on cirrus overhead. Locations
that see breaks in the cirrus should see temperatures rise into 
the 40s. Locations that don't see breaks will stay in the 30s. 
Winds will also be strongest where sunshine can get mixing going.

The main focus for this period is when rain will start Monday, 
how far north the warm front will get, and thunder chances. Low 
pressure will take shape over the High Plains tonight and reach 
the mid Mississippi Valley mid Monday afternoon. Rainfall will 
spread south to north. Areas along and south of I-80 should see 
rain before 6AM. Forecast soundings feature a bit of elevated CAPE
and a very stout cap. I think the cap will limit showers from 
tapping into the CAPE, so I do not have any thunder mentioned 
prior to 6AM.

The GFS and ECMWF lift the warm front north into southern WI, 
while the NAM stalls the front near I-88. The NAM has a slower 
progression of rain to the north, but all of the forecast area 
should see moderate rain showers by Monday afternoon. Embedded 
thunderstorms are possible, especially in the afternoon as the 
warm front lifts and the low's center reaches the region. Severe 
storms are not expected, but increased rainfall rates could lead 
to localized flooding especially with the snowpack. 

Dewpoints will soar into the 50s, and areas of fog are likely 
late tonight through Monday afternoon, especially if the warm 
front stalls over northern IL.



202 PM CST

Monday night through Sunday...

Flooding is the main long term concern. As mentioned
in previous discussions, precipitable water (the amount of
available moisture in the atmosphere) values, and moisture
transport (the movement of moisture into our region) are forecast
to reach the maximum values ever reached in the month of February
per ILX/DVN sounding climatology. Rivers and small streams will
have sharp responses and flood prone areas should be alert in the
coming days. A flood watch will be issued for the entire area 
given some uncertainty in the location of highest impacts, but at 
this time we are focusing on the Tuesday into early Wednesday time

Current forecasts have the Kankakee River at Shelby getting close
to major flood stage from the combination of snowmelt (we have 
0.85" of liquid here in Romeoville in the snowpack), seasonally 
frozen ground limiting absorption of moisture, and the heavy rain.
While the axis of heavy rain is not always nailed down at this 
distance and does shift some, the signal is high for a few 
periods/areas of concern at different times. 

The issue out of the gate Monday night is the position of the warm
front that the global models surge through the area during the
day, while the NAM (possibly catching onto the snowpack in place)
keeps the front locked across I-80/I-88 and does not lift it
through until late Monday night. These uncertainties will affect 
the position of highest QPF Monday night. 

In spite of these uncertainties, meteorologically the setup is
very favorable for flooding concerns in the region, especially 
on/near rivers and streams. High pressure will be positioned 
across the southeast United States, with a digging neutrally 
tilted upper trough over the western U.S.A. Deep southwest flow 
ahead of the trough will allow gulf moisture to be advected due 
north into our region. Sometimes we get robbed of some of this 
moisture due to convection to our south, but it appears we will 
not have a problem getting it up here. With a cold front slowing 
shifting southeastward into the region, the upper level jet will 
strengthen and enable not only a prolonged period of upper level 
lift, but will also allow several low pressure waves to ride along
the front and keep it stationary. While the initial concern will 
be over the Rock River Basin area (north central Illinois - I.E. 
Rockford area, I-88 Northward west of Chicago), the GEFS ensembles
gradually shift this axis to Chicago Tuesday and along the 
Kankakee/Iroquois basins (east central Illinois/Northwest Indiana)
later Tuesday and Tuesday night. 

These subtle shifts of the heaviest axis are helpful to
reduce flooding, but am increasingly concerned with the response
on Tuesday after snow has melted and then throw a bunch of rain on
top that, plus the fact the GEFS ensembles are indicating several
6 hr periods of probabilities of greater than 0.6" of QPF in 6
hours (which is a high number for an ensemble, deterministic
models and the real atmosphere can produce much more). Areas 
across Chicago southeastward appear like they have the highest 
potential for more prolonged rainfall as the upper jet 
strengthens, the front stalls some, storm motion values decrease 
significantly, and precipitable water values increase further 
Tuesday/Tuesday evening. 

While the main heavy rain axis will shift southeast of the area
Wednesday as high pressure is forecast on all guidance to shift
from the upper Midwest into Wisconsin and push the main cold front
into Indiana, we remain in strong southwest flow aloft such that
there could be some additional rain along and southeast of I-57
into Wednesday, and if the precip gets hung up farther northwest
into the colder air, there is lower threat of some wintry precip
on the back side both Tuesday night and Weds.

Regarding thunderstorms and severe weather potential...Forecast
soundings feature very little instability across northeast Illinois
and northwest Indiana. The profiles do show deep enough moisture
for charge separation, thus chance of thunder (and enhancing
rainfall rates) are warranted. Severe weather chances while not
zero given the shear profiles, are not very high in this setup.
There is a low probability if some surface based cape can be
realized ahead of the cold front Tuesday afternoon. 

Beyond this time, we do still stay in active southwest flow aloft,
which will likely bring waves of precip later in the week, but the
main frontal boundary appears to remain south of the area which
will keep the heavier rain focused across the Ohio and Tennessee
River valleys. 



202 PM CST

The combination of snowmelt (we have 0.85" of liquid here in 
Romeoville in the snowpack), seasonally frozen ground limiting 
absorption of moisture, and heavy rain, expect 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 axis of heavy rain is 
not always nailed down at this distance and does shift some, the 
signal is high for flooding concerns. Current forecasts have the 
Kankakee River at Shelby getting close to major flood stage. 



For the 00Z TAFs...

553 pm...Several forecast concerns this period including cigs/vis
Monday...periods of showers Monday...chance of thunderstorms 
Monday afternoon/evening...low level wind shear this evening and 
again Monday evening.

South/southeast winds will remain gusty this evening into the
20-25kt range. Speeds will slowly diminish overnight as winds turn
more southerly. Speeds will diminish further Monday morning as
wind directions turn back southeasterly. A warm front will move
across the area Monday afternoon...turning winds back to the
south/southwest with speeds/gusts increasing. Low level wind shear
in the 45-50kt range is expected this evening and then again
Monday afternoon into Monday evening and may need to be included 
for this time period with later forecasts.

Much warmer and moist air will spread over the region Monday
morning with cigs/vis steadily lowering. While there still remains
uncertainty regarding how low visibilities will drop...trends are
suggesting vis will lower under 1sm everywhere Monday morning with
dense fog possible. Have continued the trend down with tempo 1/2sm
with tempo 1/4sm at rfd. Its possible these may become prevailing
vis for a time midday. As the warm front lifts north in the
afternoon...vis will improve and possibly more so than currently
advertised. While some improvement in cigs are possible with the
warm front passage...confidence is low regarding cig improvement.

Showers are expected to develop south of the terminals early
Monday morning and continue through late morning and there could
also be some thunderstorms. An isolated shower is possible over
the terminals...but confidence is low enough to keep them dry
through about sunrise and there could still be some periods
without showers Monday morning...though the low cigs/vis may allow
drizzle to develop. A steadier rain is expected Monday afternoon
into Monday evening which may also aid with improved visibilities.
This is also the time period for the best chance of any isolated
thunderstorms for the terminals...but confidence is still too low
for even prob mention with this forecast. cms


202 PM CST

South winds will still ramp up a bit late this afternoon
and evening in the stronger pressure falls ahead of low pressure
across the upper Midwest. Some of this may be mitigated by an area
of low clouds and fog over the water. The stronger winds are
favored over the northern half of the lake, though the nearshore
waters will still see some gusts to 25 to 30 kt too. Winds will
weaken overnight as the low weakens and moves into Ontario.

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. Despite the varying winds, the low
pressure systems are not that strong and therefore winds will
remain below gale concerns. High pressure will keep concerns low
over the lake beyond this time. 



IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     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...Small Craft Advisory...nearshore waters 
     until 3 AM Monday.

     Gale Warning...LMZ777-LMZ779 until midnight Monday.