Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151924

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

221 PM CDT

Through Wednesday afternoon...

A weak cold front has settled across portions of central Illinois
this afternoon while high pressure is building across the Upper
Midwest and western Great Lakes region. This is resulting in light
north to northeast flow across much of the CWA and modest relief
in the humidity with dew points in the mid to upper 60s north of
the front. Near the front, dew points in the low 70s is
contributing to moderately strong instability of 1000-1500 J/kg
MLCAPE and weak low level convergence may be sufficient to kick
off a few isolated to widely scattered showers and thunderstorms
south of I-80. Weak low level forcing and subsidence aloft from
building upper ridge will result in low coverage this afternoon
and evening, so expect most areas including areas south of I-80 to
remain dry.

Humidity returns to the entire CWA through the day Wednesday as
low pressure lifts across the plains turning winds southeasterly
in response allowing 70 degree dew points to spread back north
across the outlook area. Through at least the afternoon hours,
appears most areas should remain dry outside of a stray storm or
two, but precip chances do increase slightly through the day from
the southwest. Models do differ in how much and how widespread
precipitation will be during the day with the ECMWF and NAM on the
drier side while the GFS remains quite bullish on precipitation.
Considering we will still be under the influence of the upper
ridge with main shortwave energy off to our west, will keep PoPs
capped at slight chance/low chance through 00Z with better chances
arriving later in the evening and overnight as discussed in the
long term portion of the AFD.



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 18Z TAFs...

Fairly quiet weather is expected over the latest TAF period for
the northern Illinois and northwest Indiana terminals. A cold
front has settled across central Illinois this afternoon and will
likely kick off a few isolated to widely scattered showers and
thunderstorms this afternoon, but activity will remain south of
the terminals. For the terminals through the rest of today, expect
variable scattered-broken cumulus clouds, but should maintain VFR
conditions throughout the period. There is some potential again
for patchy fog at RFD/DPA late tonight into early Wednesday, but
confidence is not high enough to include in the TAF at this time.

North to northeast winds have overspread the terminals behind the
front this afternoon, and expect winds to increase to around 10
kt through the afternoon for the Chicago metro terminals with some
enhancement from Lake Michigan and as high pressure strengthens
north of the terminals. Winds will transition to southeasterly
Wednesday morning as the ridge axis shifts east and low pressure
lifts across the plains.



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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