Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192340

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

317 PM CST

Tonight through Wednesday...

Primary forecast concerns today are the risk for heavy rainfall
and flooding, potentially quite significant, followed by a more
uncertain risk of freezing rain Tuesday night.

Overall synoptic pattern is characterized by a strong mid-upper
level ridge over the southeast CONUS and a deep trough over the
west, with a strong jet stream between them. Strong and deep
southerly flow on the west side of the ridge is draw absolutely
mind-boggling moisture values north into our area with the plume
of deep moisture originating from the Gulf of Mexico and western
Caribbean. Guidance is in excellent agreement on a large swath of
precipitable water values around 1.5" from Texas north into
northern IL and northwest IN. Hard to quantify just how anomalous
PWAT values of this level are this time of year, but to say that
these are record value would be an understatement. Using the SPC
sounding climo page, the forecast PWAT values would not only be
the highest PWATs ever for the month of February, but would also
likely break the record for the highest March PWAT ever for the
combined (ILX/PIA/RAN) climo period dating back to 1949.

The >5 standard deviation above average PWATs in and of
themselves would be concerning enough, but compound that with this
axis of moisture remaining largely stationary for a 24-36 hour
period with multiple weak impulses resulting in potentially more
than one round of heavy and potentially flood inducing rains.
Freezing levels tonight through Tuesday are forecast to be at or
above 10kft, resulting in remarkably efficient heavy rain
production due to the deep warm cloud layer, the likes of which
are rarely seen this time of year.

Given the the amped up nature of the moisture in the atmosphere it
will not take much to produce heavy rainfall and the forcing
portion of the heavy rain equation looks to fall into place
tonight as well. Very strong low level jet (>70kt at 850mb) will
result in extremely strong low level moisture convergence coupled
with strong upper level divergence in the broad right entrance
region of a 140kt+ anticyclonically curved 250mb jet stream over
the upper Great Lakes region. While nothing off the chart,
sufficient CAPE should be present to support embedded
thunderstorms further raising the heavy rainfall threat. Finally,
the low level jet will be aligned with the mean mid-upper level
flow setting the stage for potential training of heavy rain
producing convection tonight. In addition to the general areal
flood threat, am concerned there is a very real threat of flash
flooding, potentially significant in our CWA, likely north of
I-80, but pinning down exactly where is difficult. Some of the
higher resolution explicit convective allowing model show the
potential for max rainfall totals of 3-4" tonight, which given the
frozen ground and recent snow melt could result in significant
flash flooding and eventual river flooding.

An additional round of two of showers and heavy rain producing
thunderstorms look to be in the offing Tuesday, potentially over
or near the same areas that get hit tonight, then one last wave
Tuesday night over mainly our southeastern CWA. Conditions really
look no less favorable for heavy/extreme (for February) rainfall
amounts Tuesday into Tuesday night.

In addition, by Tuesday night, a strong cold front is forecast to
move southeast across the CWA sending temperatures rapidly
plummeting. Ahead of the front and in the warm sector we should
see temperatures (and dewpoints) mostly in the upper 50s and 60s
tonight through Tuesday and until the cold front moves through.
The NAM somehow thinks that some snow will linger on the ground
and is exhibiting its typical cold bias in temps both in the warm
sector and behind the front Tuesday night, so its 2m temp
forecast has been largely disregarded. Even the more reasonable
GFS and ECMWF both drop temp behind the front Tuesday night to
solidly below freezing. The million dollar question at this point
is how quickly the precipitation will end behind the front. The
GFS shuts off precip much more quickly and would be the best case
scenario, however the NAM and ECMWF have a slightly more amplified
wave lifting north and keep the precip going in the cold air much
longer, posing a risk for a several hour period of freezing rain
Tuesday night.

The very warm/humid antecedent conditions will mitigate the icing
threat on roadways, however, if the more worrisome NAM/ECMWF
solutions were to pan out then an icing event is possible with ice
accumulations likely, mainly on elevated objects such as trees,
powerlines, and bridges. The most likely area of ice potential
looks to be from Chicago to LaSalle-Peru and points south in our
CWA. Wintry precip should end over far southeast CWA early
Wednesday morning with seasonable temps expected behind the front
Wednesday. Given the uncertainty and wide spectrum in
possibilities for ice accums, no headlines were issued for Tuesday
night at this time, but eventually a winter wx advisory could be




Wednesday night through Monday...

317 pm...Forecast concern is for a fairly active extended period
with the potential for 3 separate systems to affect the region
Friday through Sunday.

High pressure will be moving across the upper midwest and northern
lakes region Wednesday into Thursday and this will likely keep the
next system for Thursday morning generally south of the cwa across
the Ohio Valley with low pops in the far south but just about all
of the models keep the bulk of the precip south of the cwa.

The first of three potential systems to affect the area arrives
late Thursday night into Friday morning. Gfs/ecmwf are in decent
agreement and show a band of heavier qpf across the southern cwa
in portions of central IN/IL. While precip is lighter further
north...temps will be cooler and the gfs is much cooler then the
ecmwf...suggesting the potential for a wintry mix or mainly light
freezing rain across the northern cwa. Confidence from this
distance is fairly low for both the heavier qpf axis and any
potential wintry mix but this time period will need to be watched
as it approaches. Right on the heels of this a second
wave for Friday night into Saturday morning with another axis of
heavier qpf likely across central IL/IN...perhaps mainly south of
the cwa. Surface pattern would suggest all northerly winds across
the area with this second wave though currently models suggest
temps would be warm enough for all rain. The third potential wave
would follow quickly Saturday night into Sunday but this is
dependent on where and if a surface low develops over the southern
or eastern lakes. Specifics are still several days away but axis
of any additional heavier qpf will need to be monitored with later
forecasts. cms


352 PM CST

In addition to the threat of areal and flash flooding, significant
river flooding is likely, especially over the Iroquois and
Kankakee River basins where record or near record flooding is
possible should QPF pan out as forecast. Farther north, current
river forecasts are less dramatic, but it is important to remember
that is based on a lower QPF. Some potential does exist for
heavier rainfall totals and in shorter duration tonight, which if
that were to occur river crests could end up higher and sooner
than current forecasts suggest. One possible factor that could
work in our favor is the rapid drop in temperatures that is
forecast Tuesday night as temp drop below freezing the run off
should shut down as water freezes. Water that has already made it
into tributaries would obviously still result in rises on main
stem rivers, but freezing temps could lock up some of the water
before it makes it into the streams and rivers.



For the 00Z TAFs...

540 pm...Primary forecast concerns include...

Two rounds of thunderstorms overnight and midday Tuesday.
Ifr/Lifr cigs/vis.
Low level wind shear.

Low pressure over eastern Iowa will slowly move to central Lake
Michigan overnight. While visibilities have improved over much of
the area...dense fog remains at rfd and far northern IL. While
some improvement is possible overnight...some fog may redevelop
Tuesday morning but confidence is low regarding how low vis drops
with any additional fog.

While light rain/showers will be possible through the period...
there are two periods of focus for heavier rains and at least
isolated thunderstorms...perhaps scattered thunderstorms. Those
two periods are overnight...mainly prior to sunrise and then
during the late morning through mid afternoon Tuesday. First
period already had tempo thunder so no changes there but added
tempo thunder for the second time period. While some tweaks to
timing may be needed...confidence is medium/high for these two
time periods. In addition to the thunder...rain may be heavy at
times. Confidence is low on how low vis may drop with any heavier
rain but ifr vis is possible. After the second wave...showers will
still be possible later Tuesday afternoon into Tuesday evening but
overall coverage/intensity should be diminishing.

Cigs have begun to slowly drop late this afternoon and expect
mainly ifr cigs this evening. Confidence for cigs decreases
overnight into Tuesday morning. There may be quite a bit of
variability during periods of showers/storms and with a possible
lull in the precip after sunrise...cigs may drop into lifr/vlifr.

Gusty southerly winds are expected to diminish some at dpa/ord
this evening...perhaps at mdw then increases again Tuesday morning
with some gusts into the mid 20kt range at mdw/gyy. Winds will
shift more westerly later Tuesday afternoon and then northwest
Tuesday evening as a cold front moves across the area. Low level
winds look to remain very strong through much of the period so
included low level wind shear through Tuesday afternoon for the
Chicago area terminals. cms



352 pm...Low pressure over eastern IA will slowly move across
central portions of Lake Michigan tonight into early Tuesday
morning and then accelerate into Quebec Tuesday night. Northeast
winds to 30kts on the north end of the lake this evening will
diminish overnight and turn more easterly. Winds are expected to
shift southerly for a short time Tuesday and then a cold front
will move across the lake shifting winds northwest Tuesday night
across all of the lake. Areas of fog will also be possible across
the lake through Tuesday evening...especially in central portions
of the lake where the surface low tracks. High pressure will move
from the northern plains Wednesday to the northern lakes region by
Thursday morning and then to New England by Friday morning. Low
pressure may move across the southern lakes region Saturday night
but confidence is low. cms


IL...Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012 until 6 PM Tuesday.

     Flood Watch...ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
     ILZ032-ILZ033-ILZ039 until 6 AM Wednesday.

IN...Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019 until 6 AM

LM...Small Craft Advisory...IL nearshore waters until 4 AM Tuesday.

     Small Craft Advisory...IN nearshore waters until 6 PM Tuesday.




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