Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 212257

Area Forecast Discussion
National Weather Service Northern Indiana
657 PM EDT Fri Jul 21 2017

Issued at 315 PM EDT Fri Jul 21 2017

Showers and thunderstorms will develop across the region
tonight. Severe weather is expected, mainly from 11 pm EDT
tonight through 8 am EDT Saturday. The primary threat is damaging
winds, along with heavy rain and localized flooding. The best
chance for flooding will be in areas already impacted by recent
heavy rains. An isolated, weak tornado is possible in any stronger
lines of storms that develop. Severe weather potential is most
likely in the northwest near Lake Michigan, then spreading
southeastward through the night. Monitor our website and weather
radio for the latest watches, warnings, and statements.


.SHORT TERM...(This Evening through Saturday)
Issued at 315 PM EDT Fri Jul 21 2017

The short story is: SPC has us in a slight-enhanced risk for severe
weather tonight, and we agree with this assessment based on our
analysis (details below). We have high confidence in thunderstorms
occurring. The primary threat will be damaging winds and heavy
rain/flooding-with best chances between 11 pm tonight to 8 am EDT
Saturday morning. The tornado threat tonight on the lower end, but
given the sufficient low level shear, the warm front, and moist
boundary layer-cannot rule out an isolated spin up with any
stronger convective lines that develop. Best chances for this
would be in the SPC enhanced risk area (as of 18z).

As far as location, everyone has a risk of damaging winds
tonight, but the better chances will be in the enhanced risk area
across the NW. As the system progresses southeastward we`re
thinking it will transition to more of a heavy rain/flooding
threat-especially in areas that have seen flooding issues already.
This could continue into Saturday, depending on how any lingering
outflow boundaries/etc. set up after overnight convection.

The Long Story (details):

Many of the models have come into better agreement as far as the
timing and general location of precipitation for tonight into
Saturday, with most of the recent runs favoring the MCS diving
southeastward along the current stationary boundary/moisture &
instability axis. Given that this has been the trend with the
previous MCS`s, will follow that thinking as far as the highest QPF.

Unlike the other MCS`s that crossed into our CWA this week, this one
will have decent upper level support. At the 250mb level, we have
divergence associated with the right entrance region of a 115 kt jet
streak as it rounds the base of the low to the northeast of Hudson
Bay. This divergence is focused across the north/central portions of
the CWA, and is most intense initially in the far NW, then
diminishing in intensity Saturday as it moves E/SE.

At the 500mb level, we have a weaker shortwave arriving around the
00-3Z timeframe in the northwest near Lake MI, and then the main
and much stronger wave arriving towards the 3-5Z time. The
stronger shortwave and associated wind maximum blasts through
towards the southeast by early Saturday morning, and drifts
eastward out of our area by late Saturday afternoon/evening. This
jet max is what increases the risk of damaging winds during the
11pm-8 am EDT timeframe. Thinking the best risk will be in the
northwest (enhanced area as of 19Z from SPC outlook).

Dropping down to the lower levels-at 850mb we see a strengthening of
the LLJ and thus moisture transport into our area, with PWATS
projected to climb to just over 2.3 inches (nearly 200% of normal).
Warm cloud depth will reach as high as 13-15Kft, so with that being
said heavy rain is a real threat tonight. Flooding concerns are a
bit trickier, but the best chances will be where flooding/heavy rain
has been the past month. Luckily, the system seems pretty
progressive-but the concern is that any outflow boundaries lingering
could cause backbuilding of storms/training, and this could be
problematic. Will hold off on any headlines for flooding at this
point, but monitor things closely tonight as we see how things
evolve. Again- Damaging winds will be the biggest threat earlier on
and further west, but then as we move overnight we may transition to
more of a heavy rain/flooding event (greatest potential in areas
that have had a ton of rain/flooding issues already).

At the surface, we`ll see the once stationary front lift
northeastward as a warm front by Saturday morning as advection
increases with the deepening sfc low and strengthening LLJ. ML CAPE
will be near or upwards of 2500 J/kg, and as far as effective bulk
shear we could see values of greater than 40 knots north of US 30.
What is more concerning is that 0-3 km bulk shear is around 20-30
knots at times this evening-and depending on how any stronger lines
of storms develop tonight and their orientation to the shear vector
(westerly-southwesterly)-we could see some brief spin up tornadoes.
Think the 2% tornado by SPC is reasonable for our area, with the
best chances in the enhanced risk and areas north of US 30. We`re in
the vicinity of the warm front, and LCL`s are pretty low given the
abundant moisture. Low confidence in this of course, as it depends
on how storms evolve, but it`s something to keep in mind.

Look for additional AFD updates based on current observations as
things evolve this evening.

Saturday, we`ll see the low move through and bring additional
chances for precipitation/storms. Still a severe threat with any
storms in the morning with the exiting system, but chances for
storms/severe weather threat later in the day will be heavily
influenced by the previous nights MCS. If there is additional
development along any lingering outflow in the south/southeast, we
could see additional rainfall and the potential for flooding. At
this point-would rather hold off on a detailed discussion about this
given the dependence on the overnight MCS. Pops for tomorrow may be
slightly overdone, but decided to leave them in for now to coord
with neighbors and to account for any additional development.


.LONG TERM...(Saturday Night through Friday)
Issued at 315 PM EDT Fri Jul 21 2017

Uncertainty for the mid-range and long term forecast continues, as
mentioned in former discussions. Saturday night/Sunday depend
largely on the evolution of the MCS that plows through during the
day on Saturday. The good news is that models appear to be coming
into agreement as far as the synoptic scale goes, with generally
slight differences in the major features at play.

Thankfully, the zonal flow aloft and associated parade of MCS`s will
move out. 500mb flow becomes meridional, with a cut off low/trough
(depending on the model of choice) swinging from MN southeastward
through Upper Michigan Sunday afternoon and into southern Ontario
late Sunday night. Models agree on the track of the surface low from
IA Saturday to the lower Great Lakes Sunday-with differences in the

What that means for us on Saturday night is we`ll be dealing with
the cold front moving through, and any lingering MCS remnants
associated with the warm front. Kept the going pops, with highest
chances in the evening, then tapering off from west to east
overnight into Sunday morning. As the upper level low/trough move
through the Great Lakes during the day, we could see a few
showers/storms initiate with the daytime instability and then
dissipate by the evening, but the main forcing is further north so
have lower confidence at this time. Models disagree significantly as
far as where/if we get any afternoon/evening development Sunday.

Behind the upper level trough/cut off low Sunday night, the flow
becomes more amplified as another stronger low drops into the
Canadian Prairie, serving to amplify the upper level ridge over the
Great Lakes through Wednesday morning. Some of the guidance
generates thunderstorms on Monday with another weak shortwave
rotating around the low, but left any chances out of Monday/Mon
Eve forecast for now. The subsidence from the building ridge may
end up suppressing things.

This ridge will bring us a drier, cooler airmass for the middle of
next week-much to the relief of many. Highs Monday into Wednesday
will be in the 70`s and low 80`s, with less humidity than we`ve seen

With northerly flow down the long axis of Lake Michigan on Monday,
expect the potential for a moderate-high swim risk for LaPorte Co
IN/Berrien Co Michigan in the afternoon. Wave directions will be
from the north-northwest-at about a 40-80 degree angle relative to
shore, with periods of around 5-6 seconds. Though wave heights will
only be in the 2-3 ft range, we could see the development of strong
rip/structural currents at area beaches. This will be something to
keep aware of should weather be favorable for higher beach
attendance. All will depend on how strong northerly winds become.

Wednesday will be the next chance for precipitation as a
vertically stacked low pressure system drops into northern
Ontario, just south of James Bay. This will bring a cold front
through the forecast area during the Wednesday afternoon to Thursday
afternoon timeframe. Another developing low to our southwest will
also lift northeastward around that time, stalling the front out
before building high pressure in the wake of the low forces it
southward again. This will warrant slight chance-chance pops for
now, and have not made any major changes from previous forecast.
Highs Thursday into Friday will be in the upper 70`s and low 80`s.


.AVIATION...(For the 00Z TAFS through 00Z Saturday Evening)
Issued at 657 PM EDT Fri Jul 21 2017

Meso high in wake of early morning MCS continues to hold moist and
very unstable theta-e ridge off to the sw this evening. However
approach of upstream MCV over srn MN will yield ramping
destabilization across the terminals overnight as swrly low level
jet spreads east and upticks in excess of 40kts. Would suspect some
storms will develop with increasing warm advection wing regime ewd
of substantial line segment that evolves through nrn IL this
evening...03-06Z time frame across nw zones. Otherwise primary
feature of note is incoming organizing squall line. Based on near
term CAMS solutions/trends have added svr mention invof KSBN yet
held off invof KFWA given late night timing. Will reaccess as the
evening progresses. Nonetheless some mvfr-ifr restrictions likely





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