Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 150602

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


936 PM CDT

For Evening Update...

Have continued to lower pops/thunder potential this evening and
overnight, with weak mid-level lapse rates and weak low level
convergence apparently working against favorable large scale
forcing and resulting in little shower/thunderstorm development.

Weak surface low pressure was analyzed over far northeast IA late
this afternoon, with a subtle quasi-warm frontal feature extending
southeast into northern/central IL. Despite fairly strong mid-
level short wave digging southeast across the Upper Mississippi
Valley during peak diurnal heating, and MLCAPE values 500-1000
J/kg (around 700 J/kg per DVN 00Z sounding), little organized
convection developed across the region. While broad areas of
modestly enhanced cu were noted in GOES-16 vis imagery, low level
convergence remained fairly weak, and mid-level lapse rates of
5.5-6 C/km resulted in little development other than a few
isolated cells over northeast IA. A few isolated showers were
noted over parts of northern IL as boundary layer cooled and
instability become a bit elevated above with sunset, but these too
have fizzled for the most part over the past hour or so. High-res
convection allowing guidance has been significantly overdone in
developing convection across northern IL since this afternoon, and
successive runs continue to back off on coverage overnight. While
current radar depiction would suggest little threat of precip
overnight, weak but persistent theta-E advection above subtle warm
frontal feature may continue to produce some festering weak
convection at times overnight beneath the southern periphery of
the aforementioned mid-level wave just to our north. Thus not
comfortable going dry, though have limited pops to 30 percent or
less and many areas will likely see no rain.

Trailing weak surface cold front/trough is progged to sag across
central IL Tuesday. Guidance continues to indicate some light qpf
in the vicinity of this boundary into the afternoon hours, though
mid-level ridging develops and suggests little support beyond a
little convergence right near the boundary. Based on this have
also lowered or removed pops except for south of a VYS-VPZ line,
and lowered pops slightly there.

Updated digital forecast/zfp available.



234 PM CDT

Through Tuesday...

Debris clouds from early morning thunderstorms kept temperatures a
bit cooler than expected this morning. However, those clouds are
dissipating as they push east, and temperatures are rebounding

Weak low pressure over Iowa will pass over southern Lake Michigan
tonight.  Scattered storms are expected to form across north-central
IL this evening.  Guidance has been struggling with convection most
of today and many models are already overdoing thunderstorm
coverage. Therefore, while I expect scattered storms, I am uncertain
about coverage and exactly when thunderstorms will form. The latest
RAP guidance suggests the atmosphere is in the process of
destabilizing, but surface CIN remains.

Therefore, expecting scattered storms to form near the Rockford area
between 00-02Z/ 7-9PM CDT.  Storms will shift east through the
evening. A couple of storms may become strong, but I am not
expecting widespread severe weather. CAPE values should be 1000 J/kg
or less and 0-6km shear will be around 40 kt. The strongest
storms may produce gusty winds.

Weak winds and recent rainfall will probably lead to patchy fog
in outlying areas tonight. The low`s cold front pushes south early
Tuesday morning with winds becoming north to northeast behind it.
12Z guidance has the cold front stalling across central IL, a bit
further south than previous runs. As such, thinking the focus for
thunderstorms Tuesday will be further south. Kept up to a chance
of showers and storms south of I-80, but the main focus may even
be further south across WFO ILX`s forecast area. Temperatures
Tuesday will be similar to today except for cooler along the lake.



255 PM CDT

Tuesday night through Monday...

With Tuesday looking drier than previously anticipated, weak ridging
should be in place and a diffuse stationary front parked just south
of the area by late Tuesday night and early Wednesday. Northeasterly
low level flow Tuesday night will transition to south to
southeasterly by midday on Wednesday as the stationary front lifts
through as a warm front and starts feeding higher dewpoints into the

The GFS and ECMWF disagree on how quickly to develop showers during
the day Wednesday, apparently in part due to differences in the
degree of low level convergence in the warm sector, afternoon
destabilization, and supporting shortwaves aloft. Using the NAM as a
tiebreaker would support the much slower ECMWF solution and keep
much of the area dry until evening.  For now will plan to carry
slight chance to chance POPs for most of Wednesday and go with
likelies for the evening and overnight.

Somewhat better model agreement supports hanging onto at least
chance POPs for most of Thursday as well.  A cold frontal passage
was previously expected a bit earlier in the period but now looks
to be on track for the middle of the day Thursday.

Friday looks dry and cooler with northwest winds but residual low
level moisture and steep midlevel lapse rates likely will support
increasing cloudiness in the afternoon along with the possibility of
a few sprinkles.

The picture for the weekend is of course far less certain but
current trends suggest Saturday could see scattered showers with a
passing small amplitude trough aloft, while Sunday presently looks
dry under the influence of increasing heights aloft and ridging at
the surface.

And then there is the question for Monday and the eclipse.  A very
early look gives hope that the ridge anticipated to be overhead
Sunday may still be blocking moisture return and providing large
scale subsidence through the first part of Monday.  Or perhaps the
pattern will slow down a bit and not move the ridge into the area
until Sunday night or Monday. It remains too early to have much
confidence in any specific solution, but for now we will start
with a dry forecast and partly cloudy skies.



For the 06Z TAFs...

A relatively quiet period is in store. Main concerns are BR
potential outside of Chicago this morning and then the timing of a
wind shift to northeast and associated speeds this afternoon.
Light/calm winds and low dew point depressions should support MVFR
BR outside of Chicago this morning through 12 or 13z, with IFR
possible but lower confidence. Have removed IFR CIGs from RFD
based off regional observations lending low confidence in them
occurring, though will monitor trends. 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. 10+ kt speeds are possible and may
be added to next TAF. Northeast winds will then gradually diminish
this evening, with light east-northeast winds overnight. Outside
of lower CIG potential at RFD, CIGs should be VFR throughout.

Have removed VCSH from TAFs except from GYY based off radar
trends. Any additional SHRA/TSRA activity this afternoon will
likely be well south of the terminals closer to the cold front
sagging south.



255 PM CDT

A weak cold front dropping across the lake tonight will turn winds
northeast and then north. The front will stall well south of the
lake Tuesday before lifting back north as a warm front on
Wednesday with strengthening south and southeast winds behind
it Wednesday night and Thursday. A stronger cold front then
crosses the lake Thursday and returns winds to a more
northwesterly direction.






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