Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151137

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
637 AM CDT Tue Aug 15 2017

325 AM CDT

Through tonight...

Light to calm wind regime with low dew point spreads outside of
Chicago will support patchy fog through an hour or two after
sunrise this morning. Variable cloud cover and less rain than
anticipated yesterday evening will preclude more widespread fog.
Can`t rule out very localized dense fog under patches of clearer
sky. A weak cold front will sag south across the area today,
likely stalling in central Illinois and Indiana south of the
Kankakee River, over or south of the southern LOT CWA by early
evening. With height rises aloft and no strong shortwave to aid in
lift, forcing for convection along and south of the front will be
quite weak and rely upon lower level convergence. Temperatures
warming to the lower to mid 80s and dew points around 70 will
support potentially around 1000 j/kg of CAPE. Have held onto
slight chance category PoPs for the potential for a few isolated
thunderstorms in the early to mid afternoon in the southern CWA

Elsewhere, it will be a fairly nice day after any morning clouds
scatter with fair weather cumulus the rest of the day. Expecting
upper 70s to lower 80s north of I-80 and as mentioned lower to mid
80s south based off observed and progged thermal profile, with a
few mid 70s readings along the immediate lake shore given
persistent onshore winds. Tonight will be primarily quiet as
surface ridge holds much of the night. Can`t rule out an isolated
shower/storm into northwest Illinois toward daybreak as warm
advection ensues, though the lionshare of the area should be dry
through the night with near seasonable overnight lows.



235 AM CDT

Wednesday through Monday...

The main forecast challenges/concerns during the period focus
around the chances and timing of showers and thunderstorms late
Wednesday through Thursday.

Low pressure is expected to set up over the Central High Plains
later tonight into early Wednesday in response to mid-level
energy ejecting east-northward from the western CONUS upper
trough. This low is then forecast to shift northeastward across
the Upper Midwest and over the Upper Great Lakes late Wednesday
and Wednesday night. Ultimately, this storm system will lift
today`s cold front back northward over the area as a warm front
sometime late Wednesday into Wednesday evening, then drive another
cold front across the area on Thursday.

With the passage of the warm front Wednesday evening, southerly
winds in the warm sector of this evolving storm system should
bring some healthy moisture (dewpoints nearing 70) back northward
across the area. Lower-level moisture advection will also be
aided by the development of a 30+ KT southerly low-level jet
Wednesday night. With this rapid increase in a high theta-e
airmass, precipitable water (PWATs) values could climb close to 2"
over the area Wednesday night, and this could support some very
efficient rainfall.

The main question that remains is when do the showers and storms
onset over the area? At this time it does appear that much, if
not all of the day on Wednesday will be dry, but there will be an
increasing possibility for scattered showers and storms to
develop as early as Wednesday evening as increasing warm air
advection overruns the surface warm front. The 00 UTC NAM solution
is certainly the most bullish with this potential. However, with
the potential of an overall slower system progression, the better
chance of storms may hold off until after midnight Weds night/Thursday
morning as the parent mid-level disturbance approaches the area.
This timing will certainly need to be refined more as we get a
better handle on the exact evolution of the system. Overall though
the main story will be the potential for a period of heavy
rainfall with these storms Wednesday night, especially given the
high PWATs mentioned above.

A cold front is expected to push across the area sometime around
midday/early afternoon on Thursday. Additional thunderstorm
development will be possible along this front during the day
Thursday, and some of these storms could be strong. However, at
this time there is a large amount of uncertainty on whether the
overnight convection will negatively impact the possibility for
any renewed storm development on Thursday. For this reason, we
have continued with only chance POPs.

Things should quiet down and cool off for a period on Friday.
However, another mid-level disturbance is expected to quickly dig
southeastward towards the area for Saturday. As a result, it
appears that there could be another period of storms around the
area on Saturday. Surface high pressure then looks to produce
another period of quieter weather for Sunday.

For early next week, it appears that the weather could remain
quiet for the eclipse on Monday, with any active weather possibly
remaining to our north until Monday night or Tuesday. Temperatures
do look to warm, possibly well into the 80s on Monday as southerly
flow advects in a warmer airmass. It then appears that another
cold front may try to shift southward towards the area next
Tuesday. This may result in another period of thunderstorms for
the area.



For the 12Z TAFs...

A relatively quiet next 24-30 hours is in store. Main concerns
are end time of FG/BR outside of Chicago this morning and then
the timing of a wind shifts to north and then northeast and
associated speeds this afternoon. Light/calm winds and low dew
point depressions have resulted in FG/BR outside of Chicago this
morning, with sunrise likely to dissipate it fairly quickly. A
bank of IFR stratus was gradually sagging south in WI, but most
recent GOES-16 imagery suggests this is eroding and slowing
southward progress and less likely to make it to RFD.

A cold front will move across the terminals this morning shifting
winds northerly and then northeast, with additional lake
influence expected to enhance speeds a bit at ORD/MDW/GYY. Had
enough confidence in 10+ kt speeds to add them to the ORD/MDW
TAFs. Northeast winds will then gradually diminish this evening,
with light east-northeast winds overnight shifting to southeast
Wednesday morning. CIGs are likely to remain VFR the rest of the
period, but BR/FG is possible overnight into early Wednesday.
Any SHRA/TSRA activity this afternoon will likely be well south
of the terminals closer to the cold front sagging south, with the
next threat of SHRA/TSRA at the terminals Wednesday evening.



305 AM CDT

Northerly winds can be expected across the lake today following
the passage of a weak cold front. It does not appear that the
winds will be strong enough to result in waves over 4 feet over
southern Lake Michigan, so a small craft advisory is not planned.

The main weather story for the lakes region this week will center
around a developing storm system over the Central Plains tonight
into Wednesday. This system is expected to shift northeastward
towards the Upper Great Lakes by Thursday. As it does so, expect
the winds to turn southeasterly and increase to 15 to 25 kt
Wednesday night into Thursday. A cold front is then likely to
shift across the lake sometime on Thursday afternoon. A period of
strong west-northwest winds (25 to perhaps 30 KT) will be possible
following this frontal passage into early Friday. Winds and waves
should then subside late in the week as surface high pressure
shifts back over the lake for a period.






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