Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152324

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
624 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 00Z TAFs...

Other than the potential for some patchy ground fog during the
predawn hours Wednesday, anticipating generally VFR conditions
through the TAF cycle. Best chances of fog is DPA, GYY, and
perhaps RFD, though if high cloudiness remains fairly extensive it
could cut back on fog some, conversely less high clouds would mean
a more significant fog problem that could even result in vsby
reductions at the urban terminals (ORD & MDW). Any fog will
rapidly dissipate after sunrise with lake breeze likely to
develop and freshen up the easterly winds again Wednesday
afternoon. Some potential for showers and t-storms during the last
few hours of the TAF cycle, but confidence in timing is low, so
will defer the introduction of precip to later TAF cycles.

- Izzi


325 PM CDT

High pressure centered over the northern part of the lake this
evening will move east overnight and give way to influence of low
pressure over the Plains. East to southeast winds will increase
quickly on Wednesday evening. By overnight Wednesday night, winds
will have increased to 20 to 25 kt with even a few higher gusts
possible, especially north and central. The low will strengthen
on Thursday over Wisconsin and then as it passes over the far
northern Thursday night, a cold front will advance across the
lake. Westerly winds will increase back to at least near 25 kt.
Behind this, the wind field will be more relaxed for the weekend,
especially Sunday.






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