Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240200

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
900 PM CDT TUE AUG 23 2016

856 PM CDT

Made modest changes to the forecast this evening, mainly to refine
PoP timing and introduce a lull in precip centered around midday
tomorrow. Latest runs of the RAP and HRRR continue to show
convection sprouting across portions of eastern IA into northwest
IL after midnight tonight as the low level jet ramps up to our
west in advance of the upper wave currently lifting across
portions of the mid-Missouri Valley. These thunderstorms are still
expected to move across the local CWA overnight into tomorrow
morning, with activity exiting east by mid to late tomorrow
morning. While cannot rule out some isolated to widely scattered
activity during the day tomorrow, expect northern Illinois to be
somewhat stabilized from the first round of convection with
nebulous forcing mainly from mid level WAA. Better forcing
arrives later in the day, by late afternoon or early evening, as
the cold front approaches from the northwest and another mid level
disturbance lifts across the region.



203 PM CDT

Through Wednesday...

Quiet conditions are expected through this evening with increasing
chances for shra/tsra late tonight and into tomorrow.

For the remainder of the afternoon and this evening, expect
generally quiet conditions across the region.  Temperatures this
afternoon should top out in the low 80s under partly cloudy skies.
Developing sswly flow has lead to increasing low level moisture
with sfc dewpoints returning into the lower 60s across the area.
Gulf moisture should begin to return in earnest late tonight as
the moist air can finally make it`s way around the sfc ridge
extending into the lower Mississippi Valley and expect sfc
dewpoints in the middle to upper 60s by daybreak. Warm/moist
advection and modest isentropic lift should bring increasing cloud
cover with deeper layer moisture. Timing of pcpn onset is still
a bit uncertain as the models have difficulty latching onto any
particular detailed solution, but expect the general trend should
be for increasing pcpn chances durg the early morning hours with,
at least, sct shra/tsra through the morning hours. There is some
guidance suggesting that there may be a lull in the pcpn chances
durg the early afternoon hours following the passage of a warm
advection wave. The second round of convection would be more
associated with a cold front pushing across the upper Mississippi
valley and Great Lakes. The srn portion of the front should stall
out as high pressure remains parked over the sern CONUS and lower
Mississippi Valley. The progression of the front will define where
the second round of pcpn lays out, with the general consensus
being that the front should push at least into cntrl or srn IL/IN.
Regardless of whether the front lays out across cntrl IL/IN or srn
IL/IN, confidence is high enough to keep the likely PoPs across
portions of the area along and south of a Chicago to Peru line and
then tapering off to the north and perhaps some drying over nwrn
IL by late Wednesday afternoon. Warm/moist advection should
continue through the day tomorrow, with temps reaching the lower
to middle 80s and sfc dewpoints in the lower to middle 70s. Max
heat index values should reach the lower 90s across the swrn
portions of the CWA.


325 PM CDT

Wednesday night through Tuesday...

Primary concern is with the ongoing convection Wednesday night and
placement and intensity of it. In the most favored corridor, the
potential exists for at least isolated strong to severe
thunderstorms, as well as torrential downpours and flooding
concerns. Would expect wind field to be a bit disturbed from what
occurs during the daytime, with any lingering outflow boundaries
possibly playing a role in focusing low level convergence. Frontal
trough will gradually slide southeast, focusing convergence on the
larger scale, and multiple short waves in disturbed west-southwest
30-50 kt mid-level flow will move across the area. SB or MLCAPE
will diminish diurnally in the evening. Thus concern is there for
a window in the early evening for the best chance for any severe
storms with about 2000-2500 j/kg of CAPE and about 30 kt of 0-6km

Main threat will be gusty to damaging winds and large hail.
Confidence is not the highest, but it appears the best focus will
be into the southwest 1/3 of the CWA on the nose of the LLJ and
overall better threat for any severe storms mainly south of I-88.
There will likely be additional waves of thunderstorms overnight,
again highest threat along/south of I-88, but with a lesser severe
threat as instability wanes. Regarding flooding concerns, 12z
upper air analysis depicted well the high moisture content that
will surge back into the area with PWAT values near or above 2".
Most intense convection will be very efficient rainfall producers
given the above and warm cloud depths near 13kft, so at least
localized flooding is certainly a threat to monitor.

On Thursday, cold front will slide southeast across the area, with
a chance for scattered showers and a few storms from lingering
overnight convection or some regeneration in the morning as
instability rebuilds. 70+ dew point axis will linger ahead of the
front into the afternoon along and east of I-55. Temperatures in
the lower to mid 80s and high dew points will again yield around
2000 j/kg of MLCAPE in the presence of 30-35 kt of 0-6km shear.
Focus for scattered convection will shift southeast with the
frontal boundary, and assuming instability can build as progged,
the aforementioned parameters add concern for at least an isolated
severe wind and hail threat, as well as heavy rain/local flooding.

The air mass will dry out considerably Thursday night and Friday,
with a very pleasant end to the work week and start of the last
weekend of August expected. Highs Friday will reach the mid 70s
to lower 80s along with dew points in the 50s to lower 60s.

Surface high will gradually give way over the weekend, with moist
air mass returning and southwest flow short waves moving over the
area due to trough passing to our northwest and Bermuda high along
the east coast. Low to mid POP shower and thunderstorm chances
will return on Saturday and continue into Sunday, with generally
seasonably warm and humid air mass lingering through early next
week. A weak surface cold front may press south Monday into
Tuesday. However, maintained low POPs from initialization as
instability reservoir will remain nearby on northern periphery of
strong mid-level ridging over the southeast.



For the 00Z TAFs...

Main aviation concerns are with identifying periods with greater
potential for showers/thunderstorms, and also with the potential
for mvfr ceilings to develop tonight and linger through Wednesday

Weak surface low pressure was over the mid/upper-Missouri Valley
early this evening, with a somewhat diffuse surface warm front
extending southeast across eastern Missouri. Broad southwesterly
flow was in place aloft, which will provide persistent warm/moist
advection across the region through the TAF period. Satellite
imagery depicts a couple of mid-level disturbances lifting
northeast across the region, which are expected to provide forcing
for the development of showers/thunderstorms late tonight into
early Wednesday, and TAF`s reflect timing of greatest threat
generally from the pre-dawn hours into mid-morning Wednesday.
Isolated or scattered showers cannot be ruled out at other times,
given the weak but persistent moist ascent, but have tried to
limit mention to the times of highest probability. With low level
moisture increasing from the southwest, lower clouds are on the
increase, and even without precip it appears that a period of mvfr
cigs is likely overnight/Wednesday morning. Winds tonight into
Wednesday are expected to back slightly southeast this evening in
advance of the warm front, eventually veering back to south-
southwest 200-210 degrees as the warm front lifts northeast of the
area Wednesday.

Mid-level disturbances move out of the area by mid-late Wednesday
morning, suggesting a lull in organized shower/thunderstorm
potential during the day. While destabilization is a concern
during the afternoon, the lack of large scale forcing and
extensive cloud cover lowers confidence in tsra. Another
disturbance lifts toward the area Wednesday evening, with
agreement in much of the available model guidance in developing
greater coverage of shra/tsra at that time.



356 PM CDT

Low pressure over the Plains and strong high pressure from the
eastern Lakes to the East Coast is resulting in stout southerly
winds over the lake. The strong winds have resulted in waves up to
6 ft at the northern buoy Tuesday afternoon. The strong south
winds will continue tonight and into Wednesday until veering
southwest and easing Wednesday afternoon as a cold frontal trough
approaches from the west. The cold front will sweep across the
lake Wednesday night through Thursday morning, with winds shifting
to lighter westerly. As high pressure spreads in from the west
Thursday night, a secondary cold front will push down the lake,
causing a period of NW wind speeds/gusts to 20-25 kt. Generally
lighter winds will return once the high spreads over the lake
Friday, followed by southerly winds returning over the weekend as
the high departs to the east.






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