Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 150806

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
306 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.



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
todays 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 health 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 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 quite 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 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.



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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