Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 141644
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1144 AM CDT Sat Oct 14 2017

.UPDATE...
1143 AM CDT

Main concerns through this evening/tonight are the threat continue
to be for additional torrential downpours and flooding along with
some severe potential. Have expanded the Flash Flood Watch to
include Will and Grundy counties, based on 1-3"+ amounts this
morning across northern Will, current radar trends and expectation
of additional rounds of heavy rain through tonight.

Latest surface analysis indicates the warm front roughly along the
Interstate 80 corridor. Regional radar mosaic indicates a large
shield of showers and thunderstorms stretching from southern lower
Michigan to IA/MO border area. A stout short-wave/vort max
embedded in strong west-southwest mid-level flow well ahead of
large scale trough back across the Plains is causing these
reinforcing showers and storms. From this activity, a very
expansive deck of clouds has formed. Latest guidance trends
indicate that there is likely to be any appreciable break in the
convection in this 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 afternoon and then this evening with
cold front. With this all being said, if any SB/ML instability can
be realized, cannot rule out an isolated tornado with any semi-
discrete supercell or QLCS mesovortex.

Near/along and just south of the warm front, surface observations
indicate temperatures into the 70s and dew points into the lower
60s, such as at KPNT. Am most concerned for development of
sufficient surface based instability into this area for a surface
based severe risk, including damaging winds and an isolated
tornado. For additional details on the overall severe risk, see
the SPC Day 1 outlook updates.

There are no significant changes with thinking on the heavy rain
and flooding threat. 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 4 inches in spots already along with
minor road flooding having occurred. Nearly off the charts PWAT
values for mid October of near 1.5 inches on 12z DVN/ILX RAOBS
and possibly as high as 1.8 inches per SPC Mesoanalysis and about
500 j/kg of MUCAPE 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 and as mentioned earlier
have expanded watch to include Will and Grundy.

Castro

&&

.SHORT TERM...
250 AM CDT

Through tonight...

The main forecast concerns for today and tonight are, the
increasing threat of very heavy rainfall from multiple rounds of
showers and thunderstorms, and also the threat of a line of strong
to severe thunderstorms this evening.

Showers and thunderstorms have been developing across portions of
north central Illinois over the past couple hours in response to
an increase in isentropic upglide, and moisture advection atop a
surface boundary located near the I-80 corridor. This activity is
likely to continue to overspread much of northern Illinois over
the next few hours as warm and moist advection continues to occur
atop the surface boundary. The eastward movement to these storms
is likely to set up training of cells over the same areas into
this morning, and unseasonably high precipitable water values (in
excess of 1.5") setting up over the area, expect these storms to
be efficient heavy rainfall producers. Any break in the activity
this morning looks to be short lived, as several additional waves
of showers and thunderstorms are expected to shift across northern
Illinois later this morning into the afternoon.

The surface boundary over the area this morning will try to begin
shifting northward as a warm front later today in response to a
deepening and fast moving area of low pressure shifting over
eastern IA and into WI. The northward movement of this front may
initially be halted due to the extensive shower and storm activity
this morning, but as the low shift towards WI this evening,
expect this warm front to be forced northward.

The wind field through the atmospheric column is expected to
increase dramatically through the day in response to strong
dynamics of this deepening low. As a result, very impressive low
and deep layer shear profiles are expected to set up across much
of the warm sector by this evening. With another line of showers
and thunderstorms expected along and ahead of an eastward
advancing cold front this evening, there is concern that severe
storms could be a real concern over northern Illinois given the
strong wind field. Overall, this still appears to be a high shear-
lower CAPE environment, so we will have to rely on the strong
dynamics of the storm system to help produce the severe storms
this evening. These are always trick types of setups for severe
storms, and as a result, the latest forecast and observational
information will need to be monitored closely today. At this time,
it appears that that damaging winds would be the primary
potential severe concern with a possible squall line shifting
eastward across northern Illinois. There will also be a nonzero
threat of tornados as well given the high low-level shear and
proximately to the area of low pressure entering southern WI this
evening.

Given the likelihood of several rounds of very heavy rainfall
training over the same areas today into tonight, flooding will be
a big concern. For this reason, we have opted to issue a flash
flood watch for areas mainly north of I-80 through tonight.

KJB

&&

.LONG TERM...
319 AM CDT

Sunday through Friday...

The cold front mentioned in the short term discussion should be
shifting over my eastern counties right around daybreak Sunday.
Any lingering showers should quickly come to an end early Sunday
following this cold frontal passage. The main story behind this
front will be the strong west-northwest winds. Winds may briefly
gust to around 40 mph early Sunday morning with the passage of the
front, before gusts slowly ease into the 30-35 mph range by late
morning. Much cooler weather will also setup over the area, with
cloud cover likely lingering over the area. Temperatures on
Sunday are likely to remain nearly steady, or even drop a few
degrees during the day as a cooler airmass works in over the area.

The winds should abate across the area Sunday night as surface
high pressure begins to shift over the region. Skies are also
expected to clear out Sunday evening, and this looks to set the
stage for a very chilly night across the area, with temperatures
likely to drop into the 30s in most areas away from the city. So,
some patchy frost will be possible early Monday morning.

High pressure will remain dominate across much of the eastern
CONUS for much of next week. Overall, this looks to set up a quiet
and increasingly mild weather pattern across the area.
Temperatures are expected to warm back into the 70s by the middle
of next week.

KJB

&&

.AVIATION...
For the 12Z TAFs...

The concerns for the Chicago area airports through Saturday are:

- Winds with mainly an easterly component likely through mid
  Saturday morning, with some variance in speeds

- Another uptick in showers and storms late this morning through
  early afternoon.

- Periods of showers/storms likely again this afternoon into
  tonight, with increasing chances for thunder after dark

- Winds becoming near due south Saturday afternoon and gusting to the
  mid 20s this evening.

- Low Ceilings this morning.

Sort of the day where there will be periods of showers and
thunderstorms off and on through the period. Instead of loading SHRA
VCTS through the period, will attempt to hone in on the more likely
periods of thunder, but note that thunder potential is certainly
there a good portion of the period.

This first wave of showers and storms should depict This activity
will decrease in coverage towards or slightly after daybreak, but
not likely completely dissipate with the warm front lingering and
lifting northward. At this point (mid morning) winds will shift at
ORD/MDW back to more SE and eventually S. Some southward expansion
of IFR or even LIFR ceilings is possible this morning, though
current guidance suggests it will stay north of ORD/MDW. At this
point it has occasionally gotten into ORD which is just north of
the warm front, while MDW is south. Therefore expect occasional
IFR/LIFR at ORD and have TEMPOed it in for a little bit.

Confidence in a minimum in coverage of showers this morning is still
medium high, though there will still be some around, and the gap may
not be long lived as the wave currently across NE/KS lifts northeast
and interacts with the northward moving warm front. The current
activity upstream has plentiful lightning with it and even some
hail, so depicted another increase in TS potential in the 16-18z
time frame, give or take an hour. The warm front will ease northward
during the afternoon and as it passes the Chicago airports winds
will turn to southerly. Speeds will increase, especially after dark
today and some gusts will be occasionally seen.

Periods of showers are favored this afternoon, especially
along/northwest of a PIA to GYY line. It is challenging to say
what thunder coverage would be in these showers, but instability
progs have been trending upward some, so an expansion of the
thunder mention may be warranted in future TAFs. At this point
will just hold the SHRA mention. Ahead of the cold front later
this evening into the early overnight, a band of convection is
likely, some of which will have strong winds, possibly damaging
winds.

Gusty west-northwest winds to 35 kt will be the concern for
Sunday morning along with lingering stratus of the MVFR variety.

KMD

&&

.MARINE...
330 AM CDT

Several waves of showers and thunderstorms will pass over the lake
through tonight, and then the main marine concern is the upcoming
gale, and timing of the onset of the strong winds. Low pressure
across the plains will quickly shift northeast and pass over
central Lake Michigan after midnight. It will strengthen during
this process. Expect a period of gale force winds late this
evening into the overnight hours to the south of the low. After
the low clears the lake late tonight, winds will veer to the
northwest. Expect more widespread gale force winds approaching 40
kt at this time. Have upgraded to warnings, as there is fairly
high confidence in gales for this event, with the morning into the
early afternoon period being the most likely time for gales. A bit
lower confidence on gale force winds for the nearshore waters, but
feel there is a decent chance, possibly briefly this evening, and
more so on Sunday morning. Much quieter conditions are expected
this upcoming week, with occasionally breezy southwest winds.

KMD

&&

.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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