Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 142334
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 PM CDT Sat Oct 14 2017

.UPDATE...
633 PM CDT

Mesoscale Update...

A quasi-linear convective system is moving into the I-39 corridor
and poses a wind and isolated tornado threat south of I-80 and an
isolated wind threat north of there.

The effective warm front per observations and ILX radar is lifting
north very near Interstate 74 as of 630 pm. This is timed to
enter Livingston and possibly the panhandle of LaSalle County 7-8
pm. South of this feature and in particular right along and
immediately north of it, there will be a tornado threat with the
intersection of the MCS, and have already seen some rotation
behavior in the southern DVN CWA. If this continues as the MCS
enters our CWA, we may need to locally expand the tornado watch
eastward for at least Ford, Iroquois, and Kankakee Counties.

Further to the north...along and north of I-80...the QLCS has had
some noteworthy storm structure including mid-level convergence
across Bureau County and into parts of Lee County. While some
sites have gusted from the west, they have not had overly
pronounced gusts thus far. Some if not all of that is likely due
to the stable layer near the surface from the rain-cooled air. As
the low-level jet increases and pressure falls strengthen, this
pre-storm stable layer will likely become more shallow. So some
wind threat may increase in tandem with improving convective
structure. Have touched base with SPC and it is possible a small
Severe Thunderstorm Watch will be needed for this area along/north
of I-80 including into the Chicago metro area. While the tornado
threat is non-zero there, the wind hazard would probably be the
greater one.

Rainfall rates are heavier with this MCS than the previous round
or two of storms. Will need to in particular watch the Mendota to
Chicago area which was hardest hit with 2-5 inches of earlier
rain. Progressive storm motions may limit true flash flooding
threat, however at least localized areal flooding is a good
bet...and possibly more widespread depending on convective
morphology and if another round moves through later.

MTF

&&

.SHORT TERM...
315 PM CDT

Tonight Through Sunday...

Through Early Overnight:

Main concerns through this evening/tonight are the continued
threat for additional torrential downpours and flooding along
with some severe potential ahead of a powerful cold front.

Latest surface and radar analysis as of 3pm CDT indicates the
effective warm front has sagged south to roughly just south of
BMI to RZL corridor, with summer like temps in 80s and dew points
in 60s south of boundary. Convection south of I-80 is aiding the
southeastward propagation of an outflow boundary noted on regional
radar mosaic. Where this boundary ends up over the next few hours
will likely serve as effective warm front position heading into
this evening. Additional short-waves/vorticity maxima embedded in
strong west-southwest mid-level flow well ahead of large scale
trough back across the Plains is causing boundary reinforcing
showers and storms to continue to develop back to our west and
southwest. From this activity, a very expansive deck of clouds has
remained in place. Latest trends indicate that there is not
likely to be any appreciable break in the convection over the
area.

Due to continued rounds of convection and cloud cover, it is quite
uncertain how much if any surface based instability can be
realized along and north of the I-80 corridor through this evening.
Even in a cool season set-up with upper echelon deep layer and
low level shear such as we have today, the possible lack of any
surface based instability casts doubt on a higher coverage surface
based severe risk in aforementioned area (along/north of I-80).
It appears the main threats will perhaps be isolated-widely
scattered instances of hail and wind. The potential wind threat
even with no SB/ML CAPE would be owing to extremely strong wind
fields punching through low level stable layer in any bowing
segment that develops this evening and tonight until powerful
cold front sweeps across the area after midnight.

Am most concerned for development of sufficient surface based
instability into areas near wherever the effective warm front
lifts to this evening for a surface based severe risk. There is
uncertainty regarding where the effective boundary sets up as the
surface low lifts northeast this evening, so observational trends
will need to monitored closely over the next several hours.
However, at this time it appears along/south of I-80 will be more
probable location. Severe threats would include damaging winds and
isolated tornadoes from semi-discrete supercells or QLCS
mesovortices, with perhaps isolated hail. With this all being
said, if any SB/ML instability can be dynamically driven and
realized farther north, cannot completely rule out an isolated
tornado risk given the very impressive low level shear. For
additional details on the overall severe risk, see the SPC Day 1
outlook updates.

Regarding the heavy rain and flooding threat, have made no changes
to ongoing Flash Flood Watch. Radar estimates and reports from
hardest hit northern La Salle to central Cook corridor indicate
storm total amounts of 2.5 to as much as 5 inches in spots already
along with minor road flooding having occurred. Nearly off the
charts PWAT values for mid October of 1.6 inches on 19z DVN RAOB
and possibly as high as 1.8 inches per SPC Mesoanalysis, about
500 j/kg of MUCAPE and impressive 850 mb LLJ/moisture transport
will continue to yield extremely efficient rainfall rates in
thunderstorms through tonight. With the rain that has already
fallen serving as a priming mechanism, remain concerned for flash
flooding potential. The primary uncertainty with the heavy
rain/flooding risk tonight is whether potentially farther south
effective front augments axis of heaviest additional rain a bit
farther south, as well residence time of most intense rainfall
rates.

Castro

Late tonight through Sunday...

Cold front sweeps across the CWA late tonight with winds turning
west to northwest in its wake. Steep low level lapse rates develop
during the predawn hours Sunday coincident with an axis of 5-
6mb/3hr pressure rises overspreading the region. These factors will
result in a sharp increase in wind speeds behind the front early on
Sunday with winds gusting in excess of 30 mph through the morning
with a few 40 mph gusts possible. Gusty conditions will persist
through the afternoon as well, but should very gradually taper in
magnitude and then more quickly drop off around or shortly after
sunset Sunday evening. Calendar day highs will occur at midnight
with temperatures falling sharply behind the front into the low 50s
and upper 40s Sunday morning. Temperatures will then hold steady
near 50 through the afternoon with only a degree or two diurnal bump
if any at all expected.

Deubelbeiss

&&

.LONG TERM...
301 PM CDT

Sunday night through Saturday...

Overall quiet weather expected during the upcoming week as high
pressure builds across the midwest behind the departing cold front
early in the week then settles along the east coast through the
end of the week. Southerly flow develops later Monday into Tuesday
behind the surface ridge axis allowing the area to undergo a
warming trend early through the middle of the week. Temperatures
return to unseasonably warm levels Tuesday on as upper ridge
gradually amplifies through the period.

Deubelbeiss

&&

.AVIATION...
For the 00Z TAFs...

Main aviation concerns are timing periods of thunderstorms this
evening/overnight, along with periods of MVFR/patchy IFR cigs, and
an eventual wind shift from southeast to southwest overnight and
to west-northwest by Sunday morning. West-northwest winds are
expected to be gusty 30+ kts early in the day Sunday, gradually
easing through the afternoon.

Rounds of rain/thunderstorms continue to spread eastward across
the forecast area early this evening, in a region of strong moist
ascent north of a warm frontal boundary stretched out across
central IL. Strongest storms are currently from near DKB to west
of VYS and MQB, and are expected to pass largely south of the RFD
terminal, and affect the Chicago metro terminals early this
evening. While there could be a bit of a lull behind this line of
storms, additional showers and thunderstorms will likely continue
to develop into the overnight hours as the warm front eventually
lifts north as low pressure passes northwest of the terminals.
Thunderstorm threat should end before sunrise as the trailing
surface cold front pushes east across the area.

Wind directions have been disturbed by rain/thunderstorm outflow
which has pushed the effective warm frontal boundary well south of
the terminals this afternoon. Winds have turned southeast and were
increasing due to pressure falls ahead of the low across the upper
Midwest, though an expected shift to the southwest with the
northward moving warm front will likely be delayed until later
tonight. With the cold front moving through early Sunday, winds
will shift to the west and eventually northwest, with gusts over
30 kts expected during the morning. Winds will gradually ease
later in the day Sunday. MVFR cigs expected with/behind the front,
which will also slowly rise into higher MVFR/minimum VFR in the
afternoon.

Ratzer

&&

.MARINE...
254 PM CDT

Low pressure over the mid Missouri Valley this afternoon will
continue to gradually deepen as it lifts across central Lake
Michigan late this evening and overnight. Strong southerly winds
develop south of the low track while moderately strong northeast
flow will be in place north of the low track through this evening.
Gales will develop across the lake this evening and overnight
ahead of the low, then as the low shifts east of Lake Michigan
Sunday morning, winds will further strengthen to 40-45 kt as
winds turn north to northwesterly and colder air overspreads the
region behind a strong cold front. Winds should very gradually
weaken through the day Sunday and drop back below gale force by
early to mid evening. South to southwest flow returns Monday and
is expected to stay in place through most of the upcoming week as
high pressure becomes anchored along the east coast. Flow may
strengthen to gale force or near gale force at times during the
upcoming week.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022
     until 4 AM Sunday.

IN...None.
LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...7 AM
     Sunday to 4 PM Sunday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...10 PM Saturday to 7 AM Sunday.

     Gale Warning...LMZ777-LMZ779...10 PM Saturday to 7 PM Sunday.

&&

$$

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