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Area forecast discussion...updated
National Weather Service Chicago/Romeoville, Illinois
954 PM CDT Wed Jun 28 2017

Mesoscale discussion...
952 PM CDT

West-to-east oriented convection across the northern tier of
northern Illinois counties is bringing an enhanced flash flooding
threat that will last for a few more hours. This includes for
Rockford, Belvidere, and the northern Chicago suburbs. There have
been reports of numerous stalled vehicles on flooded roads in
Winnebago County. The severe weather and tornado threat has waned
due to the cold pool influence, with still a threat of isolated
cores of hail, and some threat on the western end (now Stephenson
county) of wind and a small tornado threat.

The deep convection is part of a now elongated mesoscale convective system across Central
Lake Michigan back into far northern Illinois. A 45-50 kt low-
level jet (llj) as sampled by vwp continues to replenish this area
with moisture, with this low level jet canceling out cold pool propagation
speed. So while there will still be a southward sag of this area
of storms, the main focus remains over and just south of the ffw
counties. The focus for heavy rain will continue to be across the
four counties presently in a Flash Flood Warning, and even though
there is a present western edge to these storms, redeveloping
storms are possible into the overnight given the moist advection
regime. One thing that may help some is the active area of
convection in northern Missouri that may rob some moisture
replenishment and focus deep into the overnight.

Radar has estimated rainfall of 4+" across Winnebago and parts of
Boone County, with some mesonets having indicated 1.50"-2" in the
past hour. Significant flash flooding, at least for part of the
ffw areas in Winnebago and Boone counties, will likely continue
into the overnight hours. The message we are stressing is that
this is particularly dangerous for those near creeks and motorists
the remainder of the night, including in and near several urban
areas such as Rockford and Belvidere.

Mtf

&&

Short term...
221 PM CDT

Through Thursday...

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

This afternoon, decaying line of showers from early morning mesoscale convective system
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 County Warning Area 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 west-southwest to east-northeast with
east-northeast 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 low level jet 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
area.

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
County Warning Area can once again become destabilized allowing thunderstorms to
redevelop during the afternoon, potentially severe assuming we
realize the strong instability.

Deubelbeiss

&&

Long term...
239 PM CDT

Thursday night through Wednesday...

A period of unsettle weather is setting up for the long term
forecast period. Conditions are settling back to more typically
seasonable weather, with periodic chances for showers and
thunderstorms, high temperatures in the low to middle 80s and
dewpoints in the 60s to low 70s.

The greatest chances for organized showers and thunderstorms will
come early in the period, from Thursday night through Friday as a
quasi-stationary frontal boundary expected to lay out across
southern Iowa, through northern Illinois to Southern Lake Michigan
and Southern Lower Michigan. This boundary will be the focus for
convective activity ongoing into Thursday evening. The boundary
will slowly push through the cwa, finally pushing east of the County Warning Area as
the associated upper long wave trough pushes ewd across the Midwest.
However, while the longwave trough aloft crosses the Midwest, weak
instability enhances by cyclonic curvature aloft will keep a chance
for some scattered showers or thunderstorms for the afternoon hours
on Saturday, with the best chances for any shra/tsra likely to be
over ncntrl/nwrn Illinois and should be largely diurnally driven.

The period most likely to be dry through the longer term forecast
period should be from Saturday night through Sunday as weak
upper ridging and sfc high pressure build across the region.

For Sunday night through much of next week, the period will trend
more towards a low amplitude long wave pattern with relatively light
flow aloft as the stronger westerlies shift well north into Canada.
This will lead to decreasing confidence in sensible weather features
as weather will largely be driven by weak shortwaves tracking
through the relatively weak mid level flow. While the various
longer range model guidance are in relatively decent agreement on
the longer term long wave trends, there are considerable differences
in the timing and strength of individual shortwaves which would
drive the location, timing and intensity of pcpn. So, while the
extended forecast may sound or look like a broken record of chances
for showers and thunderstorms, but it is more likely that there will
be more dry weather than wet weather. With the high level of
uncertainty in the smaller scale details pops will be limited to the
slight chance to low chance ranges. Given the low amplitude,
progressive pattern, day to day temperature changes will be minimal
with highs generally in the low to middle 80s and overnight lows in
generally in the 60s.

&&

Aviation...
for the 00z tafs...

Concerns with the Chicago area tafs are:

- strong to potentially severe storms mid-evening with some
showers and scattered storms continuing into overnight.

- Gusty southerly winds this evening with 50 kt southwest winds in
the 1000-2000 ft layer late this evening and overnight bringing
near low level wind shear criteria.

- Potential for more storms late in the day Thursday into Thursday
evening.

Organizing storms across southwest/south central Wisconsin will
progress southeast into northern Illinois through the evening.
Storms are continuing to show a strong wind threat (40+ kt) that
will enter northern Illinois. Whether or not this extends as far
south as the taf sites will be close, but more developing
thunderstorms further to the west will at least move over the area
too. So felt it prudent to note higher convective gusts in the taf
tempos. Confidence in timing is medium-high especially for rfd,
dpa, and Ord. Confidence drops more on specific timing a bit
further south, including mdw and gyy.

While an organized cluster of storms is favored to move in/through
the area during mid-evening, regenerating storms back to the west
or over the area will potentially be a problem through the
overnight. This is a challenge to say how late this will continue.
Do expect that threat to ease southward, but it is certainly
possible storms will be seen a couple hours longer into the
overnight than noted in current tafs.

There looks to be a gap in upper level disturbances for Thursday
morning and a good part of the afternoon, but potential for storms
does return later Thursday into the evening.

Southerly winds will be gusty through this evening. Convectively
enhanced gusts are likely at least near the airports and that will
disrupt the wind field some for late this evening into early
overnight. Off the deck, a strong low-level jet late this evening
will support near low level wind shear cartier.

Mtf

&&

Marine...
245 PM CDT

Main marine forecast concerns are with a period of gusty south winds
tonight, as an area of unseasonably deep low pressure tracks just
north of the lake.

Deep low pressure tracking across the upper Mississippi Valley and
upper Great Lakes, combined with high pressure parked over the
sern Continental U.S. And eastern Gulf of Mexico will keep strong sly-swly
winds over Lake Michigan through the night and into Thursday
morning. A weak cold front should cross the lake tomorrow
allowing winds to relax and veer to more westerly. With the
gradient strongest this afternoon and into the evening hours
tonight, winds to 30 kt are likely with an occasional higher gust
to 35 kt or so. High pressure will then build across the northern
plains through the Great Lakes region through the weekend and into
early next week, bringing an extended period of relatively light
and variable winds. By midweek, the high pressure should finally
build east of the lake, with a series of weak low pressure systems
lifting out of the south High Plains and tracking toward the
western Great Lakes. In advance of these systems, flow will trend
to generally southerly. Flow should be relatively weak as the lows
should be weak systems and the pressure gradient over the lake
should be weak as well. These systems should bring periodic
chances for thunderstorms to Lake Michigan, but confidence in
timing is low given the weak flow pattern.

&&

Lot watches/warnings/advisories...
Illinois...Flash Flood Watch...ilz003-ilz004-ilz005-ilz006-ilz008-ilz010-
ilz011-ilz012-ilz013-ilz014 until 6 am Thursday.

In...none.
Lm...Small Craft Advisory...nearshore waters
until 1 PM Thursday.

&&

$$

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