Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 192350

Area Forecast Discussion
National Weather Service Northern Indiana
750 PM EDT Wed Jul 19 2017

Issued at 111 PM EDT Wed Jul 19 2017

A complex of showers and thunderstorms across the Mid Mississippi
River Valley will track across northeast Illinois and northwest
Indiana during the early portions of the overnight hours. Some
gusty winds may accompany these storms before they weaken during
the overnight. Thereafter, a weak front along with weak upper
level disturbances will bring a chances for storms from Thursday
through Saturday. The first instance of this will come early
Thursday morning. Highs will be near 90 the rest of the week with
lows around 70 to 75. Afternoon heat indices will mainly be in the
90s the next 4 days.


Issued at 717 PM EDT Wed Jul 19 2017

Ongoing MCS across northeast Iowa/southeast Minnesota/southwest
Wisconsin has become the dominant MCS, which hires guidance has
been somewhat slow to latch onto. Severe wind gusts have been
reported with this MCS and orientation of instability gradient and
both upwind/downwind propagation vectors would support east-
southeast propagation into northeast Illinois/northwest Indiana
overnight. Still some uncertainty regarding severity of this
complex for the long haul across northwest Indiana, given perhaps
some tendency for weak low level CIN to develop downstream and
somewhat more marginal low level and deep layer shear profiles
with eastward extent. Have had a few instances of mesovortex
generation at leading edge of bow accelerations upstream across
northeast Iowa, but with more marginal low level shear across
southern Great Lakes, currently not expecting this to be a first
order concern for the overnight hours. Main concern will be
lingering strong wind gusts across western portions of the
forecast area in the 04Z-08Z timeframe as this MCS makes a run at
western portions of the area. Propagation vectors also suggest
perhaps a bit more aggressive southeast propagation across far NE
Illinois/NW Indiana late tonight. 12Z CAMs largely missed
evolution of this earlier complex, and impacts from this initial
MCS could translate to a more delayed initiation of next upstream
convective complex for tomorrow. Will adjust PoPs shortly to
account for slightly higher PoPs across western portions of the
forecast area for the 04Z-09Z period tonight.


.SHORT TERM...(This Evening through Thursday)
Issued at 302 PM EDT Wed Jul 19 2017

A few spotty showers/storms cannot be ruled out late this afternoon
into early this evening across nrn IN/far nw OH/sc Lower MI as low
level moisture and MLCAPE continues to increase near a diffuse sfc
trough. Lack of any forcing/flow aloft and weak convergence should
limit coverage/intensity with most locations remaining
dry/warm/humid into tonight.

MCS on target to develop east into the Upper Midwest tonight will
likely hold together east-southeast into the Great Lakes region
later tonight into Thursday morning given ample moisture
return/convergence with a 40-50 kt low level jet...and decent 60-70
kt mid lvl jet associated with seasonably strong shortwave embedded
in west-northwest flow aloft. Latest CAMS solutions favor areas
mainly north of US 30 to be on the southern fringe of this feature,
though a farther south track is possible per propagation vectors
and orientation of main instability axis. The progged wind field
and high moisture content hints at a damaging wind threat if any
bowing segments emerge.

The placement of the trailing composite outflow boundary in wake of
the MCS will be the primary driver for temps and renewed convection
Thursday afternoon into Thursday evening. Latest guidance favors
areas along/south of US 30 later in the afternoon/early evening for
scattered convection. Coverage/intensity of any convection may be
limited by subsidence/warming aloft in wake of the above
mentioned shortwave. However, any storms that do break through
could become severe given expectations for moderate boundary layer
destablization and 0-6km bulk shear near 30-35 knots near
outflow. Otherwise, afternoon heat indices may near advisory
criteria (100F) southwest of US 30.


.LONG TERM...(Thursday Night through Wednesday)
Issued at 302 PM EDT Wed Jul 19 2017

A weak low level anticyclone into the Great Lakes will help orient
the frontal boundary more nw to se Thursday night into Friday
morning. The result will be a brief push of drier/less humid air
into into ne IN/nw OH/Lower MI, with areas along/southwest of US 30
in nrn IN not out of the woods for isolated to scattered convection
during this time near boundary/instability axis. Models have come
into a little better agreement in timing of the next convectively
aided shortwave/possible MCS in perturbed westerly flow to move
through the lower Great Lakes Friday night into Saturday. Again,
moisture/instability/flow progs suggest a heavy rain/severe
threat during this time, though confidence in timing/impacts at
this fcst range remains low given low predictability of mesoscale
features in this pattern. Cooler/drier weather is then in store by
early next week as a more pronounced mid level trough through the
Great Lakes/Northeast forces active frontal boundary off to the


.AVIATION...(For the 00Z TAFS through 00Z Thursday Evening)
Issued at 717 PM EDT Wed Jul 19 2017

Challenging aviation forecast this cycle with upstream MCS across
southwest Wisconsin/eastern Iowa being the obvious near term
concern. Strong to severe wind gusts have been noted past few
hours at leading edge of this complex, and propagation should
tend to favor east-southeast along pronounced instability gradient
that would support movement across northern Indiana in the 05Z-
08Z timeframe. Will likely include tempo SHRA mention at KSBN
during this period, and will monitor trends over past few hours
for possible TSRA inclusion. Complex should weaken across northern
Indiana late tonight with a possibility of additional upstream
convection moving in again after daybreak Thursday. Given a good
deal of uncertainty with mesoscale details after 12Z, will limit
any precip mention to VCSH. While upstream convective complex may
tend to weaken after 04Z, some gusty winds still possible in the
05Z-08Z timeframe and will continue to monitor upstream trends for
possible amendment.




SHORT TERM...Steinwedel
LONG TERM...Steinwedel

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