Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281925

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
225 PM CDT Wed Jun 28 2017

221 PM CDT

Through Thursday...

Multiple rounds of showers and thunderstorms, potentially severe,
possible from the remainder of this afternoon through the day

This afternoon, decaying line of showers from early morning MCS
has mostly dissipated with moderate instability building in its
wake across portions of Iowa and northwestern Illinois. A couple
isolated thunderstorms will be possible in the very near term on
the leading edge of the instability gradient as it spreads east
across the CWA, but main concern for more widespread convection
is starting to initiate over north central Iowa early this
afternoon where temperatures have rebounded into the low 80s with
dewpoints around 70. An upper level disturbance is noted moving
slowly east across the Dakotas with a modest surface low
developing in response over west central Minnesota. Convection
should begin to grow upscale late afternoon into the evening as
low/mid level flow strengthens out ahead of these features
allowing convection to congeal and become more linear with time.
Exactly where this occurs remains a bit uncertain with some of the
more recent HRRR runs trending farther west this evening with the
more intense convection. Not quite ready to bite off on this
scenario as the local CWA is expected to quickly destabilize
through the afternoon with partial clearing already developing in
the western counties and moderately strong southerly winds at the
surface transporting high theta-e air into northern Illinois.
Guidance continues to show large cyclonically curved hodographs
this evening which points towards a supercell threat with any
discrete or leading storms, but as the system evolves more
linearly expect the primary severe threat to become damaging
winds. Training and back building also appear possible later this
evening if portions of the line become oriented WSW to ENE with
ENE storm motions expected while Corfidi vectors drop to 5kt or
less out of north or northeast late this evening. Depending on
where this sets up (as mentioned HRRR has shifted this area
primarily west of the CWA), a strong SW LLJ should be able provide
a good feed of unstable high PWat air (in the neighborhood of
1.7-1.8 inches) which will support convection deep into the
overnight hours with an enhanced concern for flooding over this

Upper wave will continue slowly east across Lake Superior and the
Upper Peninsula of Michigan Thursday with deep southwest flow
continuing across much of the midwest out ahead of the wave.
Guidance indicates steepening mid level lapse rate will advect
over the area tomorrow superimposed over dewpoints pushing to
around 70F resulting in much stronger instability as compared to
today. Mid and upper level winds are not forecast to be as high
resulting in more modest deep layer shear, but still sufficient
for a severe threat. Convective debris/cloud cover from overnight
storms always a concern in this type of setup, but given the deep
southwest flow over the region, seems reasonable that much of the
CWA can once again become destabilized allowing thunderstorms to
redevelop during the afternoon, potentially severe assuming we
realize the strong instability.



330 AM CDT

Thursday through Tuesday...

Period of unsettled and potentially active weather appears in
store for the area into the holiday weekend, as a quasi-stationary
frontal boundary lays out west-east beneath fast zonal mid-upper
level flow. Periods of thunderstorms are expected from Thursday
through Friday night, modulated by a series of mid-level
disturbances, diurnally maximized instability and surface

One larger amplitude mid-level wave and associated surface low
pressure center is progged to be passing north of the area early
Thursday. West-southwesterly low level jet of 40-50 kts is
expected to aid in forcing nocturnal convection during the pre-
dawn hours, which will likely be weakening across the area after
sunrise with the jet veering more west and stronger forcing
transition off to the northeast. This convective activity is
likely to result in an outflow boundary across the southern parts
of the cwa or even south of it through early afternoon. Weak
height rises behind the morning wave, combined with some
stabilizing effect of the cold pool/outflow from morning storms
may allow for a lull in activity during the late morning and
midday hours...though persistent southwest 20-30 kt flow may
support some low-coverage festering cells at times. Thunderstorm
potential should increase during the afternoon again as diurnal
heating leads to renewed destabilization. Any outflow from earlier
storms may be the focus for these new storms...with a general
increase in convection also expected to the west of the cwa as
another short wave approaches and backs/increases low level flow.
Thus expect a gradual increase in pops from the afternoon into the
overnight hours associated with this scenario. Storms that
develop will likely present a locally heavy rainfall threat, as
well as the potential for severe weather. Forecast soundings from
both the NAM and GFS depict MLCAPE values 2000-2500 J/kg Thursday
afternoon, with 0-6 km bulk shear gradually increasing from 20-30
kts during the day to 30-40 kts toward evening as wind fields
strengthen ahead of the approaching wave. SPC day 2 outlook covers
the cwa, with slightly higher risk over the southern 2/3 of the
area where low level convergence and instability may be better
focused along outflow from morning storms.

Upstream short wave continues to translate eastward into the
northern and western Great Lakes region into early Friday, as the
upper flow pattern amplifies into a broad trough across the
region. Surface low develops along the front and lifts north of
the area through Friday, with the trailing cold front slowly
pushing southeast across the forecast area. This will maintain the
probability of thunderstorms from Thursday night into Friday,
along with some severe and heavy rainfall threat persisting. With
the front and the axis of moist and more unstable low-level air
pushing southeast of the area Friday night, the precip threat
should diminish to start the weekend. With our location beneath
the southern periphery of the upper trough however, the potential
for some isolated to scattered mainly afternoon/early evening
showers or thunderstorms can`t be ruled out on Saturday.

Moisture begins to spread back north/northeast across the Plains
Sunday however, as a pacific short wave ripples east. Warm/moist
advection redevelops into the region as early as Sunday afternoon
(though there is inconsistency in timing between the global models
by this time). A warm front, and a weak surface wave then are
progged to slowly traverse the area from Sunday night through
Tuesday, producing the potential for additional periods of showers
and storms. Would expect many dry hours in between convective
clusters over the holiday, despite multiple periods of
thunderstorm potential.

Temperatures will surge into the 80`s Thursday, though lingering
cloud cover and outflow from morning storms may prevent readings
from reaching their peak potential, with mid 80`s a good bet in
many areas. With the front pushing across the area Friday, weekend
temps look to range from the upper 70`s far north to the low 80`s
across the rest of the area. Muggy conditions expected Thursday
and along/south of the front on Friday.



For the 18Z TAFs...

There is considerable uncertainty particularly regarding the
convective timing and coverage this evening and how additional
thunderstorms will play out on Thursday.

Early this afternoon, decaying line of showers continues to slowly
push east across northeastern IL and northwest IN with stable
conditions immediately in the wake of this precipitation.  Winds are
expected to jump up over the next hour or two though as we start to
lose the lower cloud cover and mix more deeply. In the meantime,
there may be some variability in the winds between 160 and 200 but
expect winds to trend to a more solid 190-200 by mid afternoon as
wind gusts increase to around 30 kt.

Focus will shift back west into Iowa and northwest Illinois by
late afternoon and evening where destabilization is occurring and
additional strong to severe thunderstorms are expected to develop.
Hi-res convective allowing models are not in the greatest
agreement with respect to timing and placement of convection, but
anticipate by 00Z there should be thunderstorms ongoing over
portions of northwest and north central Illinois. Initially
scattered convection should congeal into a line and spread across
the Chicago metro at some point this evening or possibly
overnight. While individual storm motions should be off to the
east, anticipate new development southwest of ongoing storms which
could result in multiple rounds of showers and storms over any
given location.

Expect a lull in activity late tonight through Thursday morning, but
additional showers and thunderstorms are expected to redevelop
Thursday afternoon. For now, chances appear greatest mid/late
afternoon into the evening for the terminals, but confidence remains
low in the details at this distance.



330 AM CDT

Main marine forecast concerns are with a period of gusty south winds
of 30-35 kt today and tonight, as an area of unseasonably deep low
pressure tracks north of the lake.

Low pressure of 995 mb was analyzed over the Dakotas early this
morning. This low will track northeast across the Upper Mississippi
Valley by this evening, and across the northern Lakes while starting
to fill a bit Thursday morning. Tightening of the surface pressure
gradient is already underway...with south winds expected to increase
to 30 kts across Lake Michigan today. Warm air over cool water will
make for a stable marine boundary layer, though even with a
relatively shallow mixed layer there may be occasional gusts to gale
force across the lake today and mainly over the northern part of the
lake into this evening. The low will then move east of Lake Superior
and north of Lake Huron Thursday afternoon, with winds diminishing
across Lake Michigan. Another weaker low will follow across the
region Friday and Friday night, with a trailing cold front
eventually settling south of the lake Friday night. Periods of
thunderstorms, some strong, will be possible from later this morning
through Friday night.

Weak high pressure will move across the lake Saturday night into
Sunday, before the front lifts back toward the southern end of the
lake early next week. Modest northeast winds should keep most of the
thunderstorm potential over the far south end at that time.



LM...Small Craft Advisory...nearshore waters
     until 1 PM Thursday.




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