Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152015

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
315 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.



310 PM CDT

Wednesday night through Tuesday...

Emphasis in this portion of the forecast was mainly placed on
thunderstorm with heavy rain potential on Wednesday night.
Additional but low chances of storms are present at the start of
the weekend.

Two well-defined short wave troughs will be moving east across
the upper and central Plains during Wednesday and are agreed upon
in guidance to phase on Wednesday night into Thursday over the
Upper Midwest. In response, strong upper forcing for ascent and a
deep moisture plume will overlap in the region on Wednesday
evening into overnight. Would expect to see showers and storms
increase in coverage fairly quickly during the evening. While
storms should be moving within an increasing wind field, there is
potential for fairly widespread coverage and redevelopment which
could pose excessive rainfall concerns. Moisture magnitudes of
around 2" are observed upstream (1.98" at FWD this morning) and
forecast by model guidance for Wednesday night. Will continue to
note heavy rain potential in the HWO and Weather Story.

In terms of severe threat, that remains fairly low but non-zero.
The highest amount of instability in the warm sector, driving by
low-level dew points in the 70s, will arrive Thursday. So on
Wednesday night instability would likely be a limiting factor,
though a widely scattered strong to severe storm or two is
possible. On Thursday, as the system advection patterns
strengthen, immediately ahead of the cold front in a muggy air
mass should be a decent focus for at least scattered storms. This
includes northern Illinois in the morning hours, and then in the
afternoon for mainly northwest Indiana. The latter would have some
chance for scattered severe storms with mainly a wind threat.

Beyond, few changes made to what blended guidance provided. The
cold front will not provide much for a temperature drop, with
highs on Friday still around or just a bit above 80. Another
strong short wave is forecast to move through Friday night into
early Saturday. This will not have anywhere near the moisture
return than its predecessor, and its currently forecast poor time
of day passing would limit any high impact weather threat. Have a
chance of storms noted in the forecast at this time. There should
then be a period of quiet weather lasting the rest of the weekend
as a short wave upper ridge moves over.



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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