Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 170536

Area Forecast Discussion
National Weather Service Northern Indiana
136 AM EDT Thu Aug 17 2017

Issued at 725 PM EDT Wed Aug 16 2017

Chances for showers and thunderstorms will arrive tonight into
Thursday as a strong frontal system moves through. There is a low
chance for isolated strong to severe thunderstorms Thursday
afternoon into Thursday evening. Lows tonight will only drop to
near 70 degrees...with highs on Thursday in the upper 70s to mid
80s. Cooler and drier air will filter in behind this system
Thursday night into Friday. This will followed by low rain chances
on Saturday as an upper level disturbance tracks through.


.SHORT TERM...(This Evening through Thursday)
Issued at 308 PM EDT Wed Aug 16 2017

A well defined shortwave and attendant sfc reflection will track ene
into the Upper Midwest tonight and the Northern/Western Great Lakes
by tomorrow. Associated height falls will elicit a decent LLJ
response downstream (30-40 kt), which will force a tight theta-e
gradient/warm front northeast through the Lower Great Lakes this
evening into early Thursday morning. Scattered showers/storms are
expected with this elevated WAA, best chances north of US 24 in an
area of more pronounced moisture convergence and upper
divergence. Brief heavy downpours will be possible as PWATS
approach 2 inches.

A somewhat narrow corridor of forcing/deep moisture then follows
along a pre-frontal trough later Thursday morning through nw iN/sw
Lower MI and points east late morning through mid afternoon...with
the actual/main cold front to follow late Thursday into Thursday
evening. The result will be additional chances for a couple of
rounds of showers/storms...though timing/coverage remains in
question as better upper level forcing bypasses north.

SPC upgraded areas east of US 31 into a slight risk for severe
storms Thursday afternoon into Thursday evening as breaks in cloud
cover could push MLCAPE values to 1500-2000 j/kg in pockets. 40-50
kt mid level westerly flow over top this moist (sfc dewpoints low-
mid 70s) could allow for some multicell organization...with 20-30
kts of 850 mb flow and low LCL`s also hinting at an isolated
tornado threat. However, extensive cloud cover and poor lapse
rates...with several CAMS firing renewed afternoon/sfc based
convection east of the area (near pre-frontal trough) with little
activity along the actual cold front...suggest chance PoPs and
low/conditional severe threat the way to go at this fcst range.


.LONG TERM...(Thursday Night through Wednesday)
Issued at 308 PM EDT Wed Aug 16 2017

Drier/cooler air will filter in post-frontal Thursday night into
Friday as the above mentioned upper trough weakens northeast into
eastern/southeast Ontario. The next shortwave will dig southeast
under this feature bringing renewed chances for showers/iso storms
later Friday night into Saturday. Forcing per 5-3H Q-Vector looks
good later Friday night into Saturday morning with this wave,
though moisture quality will really be lacking this time around
resulting in nothing more than 20-30% type PoPs.

Flow pattern flattens out leaving dry/warming conditions Sunday into
Monday...followed by another chance for showers/storms later Monday
night through Tuesday night as another larger scale upper trough
amplifies into the Great Lakes forcing a cold front through.


.AVIATION...(For the 06Z TAFS through 06Z Thursday Night)
Issued at 130 AM EDT Thu Aug 17 2017

Increased lift and moisture was allowing isolated showers and
possible thunderstorms to develop. While both sites may see a
brief shower or 2, coverage and impacts warrants no more than the
inherited shower mention with vcts. Hi res models show more
expanded development with next wave and precipitation back across
SW Illinois. Will keep TAFs rather simple for now with amendments
possible as evolution becomes clearer. Precipitation on the actual
front still looking questionable later this afternoon before
drier air arrives Thursday night.




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

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