Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250502

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1202 AM CDT Tue Jul 25 2017

940 PM CDT

Northeast winds will ease tonight as high pressure builds, and
this should allow lingering stratocumulus clouds near the lake
front to dissipate. Fog does not appear to a be a big issue
tonight as we will hang onto a small wind component, but patchy
outlying fog is still possible. Otherwise expect a cooler night
with readings on track to head down into the 50s away from the
urban corridor.

Waves at the shore also hold at 3 to 4 ft, so while the beach
hazards statement will expire, waves will remain elevated for a
few more hours.




Through Tuesday...

142 pm...Stratocu is expected to scatter out and dissipate later
this afternoon into this evening but confidence on timing/trends
is low as even the back edge of the clouds over southern Lake
Michigan have shown redevelopment recently. Its possible mostly
cloudy skies continue into the evening with scattering tonight.
There had been some potential for patchy fog tonight and there
still is but confidence is low with the expected slower clouds
scattering out. So no mention of fog with this forecast and trends
will need to be monitored this evening.

High pressure will build across the Great lakes region tonight
and begin shifting east on Tuesday. Light winds overnight will
turn southeast Tuesday morning and a lake breeze is expected by
afternoon keeping the lake shore a few degrees cooler. Low
confidence regarding how far inland this lake breeze may move.
Highs inland likely to reach the lower 80s. cms


300 PM CDT

Tuesday night through Monday...

The main forecast concern and period of interest in the forecast
is Wednesday evening/night into Thursday morning with shower and
thunderstorm potential. This will include a risk for heavy
rainfall rates and at least localized flooding, as well as renewed
river rises. With the saturated grounds and ongoing river flooding
due to the excessive rainfall amounts of the past 2 weeks, the
placement of the axis of heaviest rainfall will determine the
magnitude of additional impacts. Furthermore, there will be some
risk for strong to severe storms with the activity Wednesday

On Tuesday night, quiet and seasonable conditions can be expected
with surface high pressure departing to the east. Warm advection
will increase aloft due to height falls from a strong short wave
trough moving across southern Canada and the far northern tier of
the US. This WAA will bring a brief return of the heat for
Wednesday that will have been shunted south today and Tuesday.
Thunderstorms are expected to be in progress over the northern
Plains and upper Midwest in the cold frontal trough of the deep
surface low associated with the mid- upper level system. This
could carry some cirrus clouds into the area toward daybreak.

The daylight hours on Wednesday should be mainly precipitation
free due to a strong capping inversion centered just under 800 mb,
associated with the southwesterly warm advection. With cloud cover
likely only to be scattered cumulus and broken high clouds, strong
surface heating is probable. Progged 850 mb/925 mb temps of the
upper teens to around 20 Celsius/lower-mid 20s Celsius
respectively should support highs in the upper 80s to around 90. A
warm front will lift north, with southerly low level winds drawing
the moist air mass back northward, featuring upper 60s to low-mid
70s dew points at peak heating. Peak heat index values are
forecast to be in the 90s area wide, with corn belt areas
approaching 100 degrees. Should high cloud debris be more opaque
than currently expected, then temperatures could be a bit cooler,
though it will still be an uncomfortably humid day.

Turning to late Wednesday through Wednesday night into Thursday
morning, the above air mass will yield moderate to strong
instability early Wednesday evening. While there is agreement in
the general idea of cold frontal trough approaching the local
region during this time, the specific details are still rather
uncertain, including onset time of showers/storms, placement of
heaviest rainfall axis and whether coverage of higher totals will
be scattered or widespread in nature. At least some of the
differences center on the possible development of a surface low
within the cold frontal trough, with the 12z runs of the ECMWF and
GEM and NAM favoring this (with varying tracks of this surface
low), while the 12z GFS largely does not. This will affect the
magnitude of low level jet response and possibly the ability of
more widespread robust convection to sustain itself at a less
diurnally unfavorable time by tapping into available MUCAPE.

The models are in general agreement in convection spreading in
from the west/northwest mostly during the evening hours (how
early/late is in question), and then exiting southeast of the CWA
by or during Thursday afternoon with the cold front passage. With
precipitable water values expected to range from 2-2.5",
ingredients for very efficient rainfall rates will certainly be
there. Given uncertainty mentioned above and more progress nature
of system, however, confidence is again low in magnitude of
impacts to the hardest hit river basins (Rock, Fox, Des Plaines)
with respect to renewed rises.

Regarding the risk for strong to severe storms on Wednesday
evening and night, moderate west-northwest mid and upper flow will
support 30-40 of effective deep layer bulk shear, with a higher
chance of strong/severe storms if earlier evening arrival
materializes and/or stronger surface low and low level jet/low
level shear to maintain storms occurs. Primary threats would be
wind and hail.

Gusty northerly winds will develop on Thursday behind the cold
front passage, particularly lakeside, followed by an extended
period of dry surface high pressure setting up through the
weekend. This will result in comfortable temperatures and humidity
through the weekend, daily onshore flow keeping shore areas a bit
cooler, and little to no threat of precipitation to allow us to
dry out and rivers to subside.



For the 06Z TAFs...

Quiet aviation weather through early Wednesday. A high pressure
ridge will move overhead this morning bringing calm, at times
light and variable winds. Cannot rule out shallow patchy fog near
daybreak at outlying sites, but do not foresee this being a big
deal at all. Winds turn southeast after daybreak, with more of an
easterly lake component by mid-afternoon possible at ORD and MDW.
Confidence in afternoon wind speeds being around 9 kt is high.




142 pm...High pressure will build across Lake Michigan tonight
and then shift east to New England Tuesday night. Northerly winds
will slowly diminish this evening then shift southerly Tuesday
morning. An area of low pressure will move across Hudson Bay
Tuesday night with a trailing cold front into the upper midwest.
The gradient will likely tighten ahead of this front with a period
of 15-25kts Tuesday night into Wednesday. A second low will move
across the southern part of the lake Wednesday night into Thursday
morning. Winds will shift northerly behind this low and a period
of 30kts is possible Thursday afternoon into Thursday night...
though there remains some uncertainty on timing and strength of
the low. cms





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