Area Forecast Discussion
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FXUS63 KLOT 142016

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

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

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


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.



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.



For the 18Z TAFs...

Low pressure is centered over the mid Missouri Valley this
afternoon with a warm front draped east across northern
Illinois. The warm front has been the focus for repeated rounds of
showers and thunderstorms so far today and expect this to
continue through the evening. Thunderstorms have been increasing
and then decreasing slightly in coverage in response to low
amplitude waves that have thus far proven hard to time. Have taken
a stab at timing these peak and lull trends including tempo
groups for lower conditions, but confidence in these specific
times is low and bottom line is that thunderstorms will continue
to be an issue across much of the region through the rest of the
day. As the surface low lifts into southwestern Wisconsin late
this evening lifting the warm front north of the terminals, this
should result in somewhat lower coverage and less thunderstorm
activity for the terminals late this afternoon into early evening.
A trailing cold front will then sweep across the region late this
evening and overnight with a line or broken line of thunderstorms
along and ahead of the front. This will likely be the last chance
for thunder at the terminals. Winds swing westerly behind the
front with very strong gusts in excess of 30 kt developing during
the predawn and early morning hours Sunday. Strong gusty west to
northwest winds will persist through the day along with gradually
improving MVFR level cold-air stratocumulus.



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.



IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     until 4 AM Sunday.

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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