Area Forecast Discussion
Issued by NWS Northern Indiana

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171
FXUS63 KIWX 220035
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
835 PM EDT Mon Aug 21 2017

.SYNOPSIS...
Issued at 820 PM EDT Mon Aug 21 2017

Rain chances will ramp up overnight ahead of a strong cold front
progressing out of the Plains. The best chances of showers and
thunderstorms will exist late tonight across the northwest and
early Tuesday morning southeast. Otherwise much drier and cooler
air will overspread the are in behind this system midweek through
next weekend.

&&

.UPDATE...
Issued at 820 PM EDT Mon Aug 21 2017

Low level moisture transport is beginning to ramp up across the
Mid MS Valley to the far western Great Lakes early this evening as
southwest flow gradually strengthens in advance of stronger upper
level short wave ejecting out of the Central Plains. The
combination of the leading edge of this stronger moisture
transport and a weaker smaller scale vort working across the
southern Great Lakes this evening has been enough to allow for an
increase in isolated-scattered thunderstorms across northeast
Illinois past few hours. Some question as to the longevity of
these showers and thunderstorms eastward into north central
Indiana over the next few hours. Waning of sfc based instability
and eastward advection of mixed layer aloft may tend to temper
instability magnitudes initially this evening. However, during the
overnight hours, more substantial low level moisture surge will
allow MUCAPE axis across Mid MS Valley to rapidly advect northeast
into the area.

Will maintain trend of previous forecast with ramp
up of likely PoPs across the northwest half of area coincident
with greatest MUCAPE axis as stronger upper support moves in
overnight. Severe threat does appear to be on the minimal side,
with a low end potential of an isolated instance of hail/wind
given fairly steep lapse rates aloft and moderate amount of
elevated instability. May need to watch potential of some pockets
of heavier rainfall also overnight, given PWATS increasing up
around 2 inches and strengthening low level jet allowing for some
upwind development. The window of greatest potential of isolated
strong storms/localized heavy rain potential appears to be in the
06Z-10Z timeframe. Just some minor adjustments made to PoPs to
account for current trends but overall no major changes
anticipated at this time.

&&

.SHORT TERM...(Tonight and Tuesday)
Issued at 200 PM EDT Mon Aug 21 2017

Challenging fcst this period owing to messy mesoscale. Decaying mcs
over cntrl IL attm with remnant MCV over nw IL. Expect this
feature will continue downstream and initiate new convection this
evening invof well established lake breezy boundary over sw MI.
Secondary additional development likely within elevated low level
warm advection zone through cntrl IL which would then carry newd
through wrn zones into late evening.

Otherwise large scale cold front in association with potent sw
trough digging out of cntrl MB will continue sewd overnight. Ewd
passage of prefrontal trough expected late tonight with cold front
to follow Tue morning for most. Far se zones lag with perhaps some
isolated development possible Tue aftn pending degree of convective
overturning that occurs in the morning. Regardless 12Z guidance
across the board quite tepid with pops. Best coverage/most
appreciable qpf expected through nw zones within zone of maximized
low level mass flux along nose of low level jet late. But given
timing and relative stable near sfc layer severe weather threat
looks minimal if at all.

&&

.LONG TERM...(Wednesday through Monday)
Issued at 200 PM EDT Mon Aug 21 2017

Little of note this period as broad sfc ridging becomes established
through the wrn lakes with an extended period of delightful late
summer weather. Some semblance of return flow seen toward Mon where
some token low chc pops were retained yet sys aloft/return flow
both look pretty weak.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday Evening)
Issued at 820 PM EDT Mon Aug 21 2017

The leading edge of stronger low level moisture transport and
small scale vort max tracking across the southern Great Lakes have
allowed isolated/scattered showers and thunderstorms to develop
from southwest Lower Michigan to far west central Indiana.
Instability magnitudes at least initially this evening, may be a
bit more unfavorable across northeast Indiana, and some question
as to how these showers will hold together over the next hour or
two. Better chance of showers and thunderstorms should still hold
off until the overnight hours when intensifying low level jet
allows for continued uptick in moisture transport, and as stronger
upper level short wave approaches from the west. Did maintain
mention of MVFR vsbys with precip...and then in association with
the front on Tuesday. LLWS criteria may be met for a short time
late tonight into early Tuesday morning in advance of the front.
Otherwise, transition back to VFR conditions expected Tuesday
afternoon with some northwest post-frontal gusts into the 15 to 20
knot range expected.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Beach Hazards Statement from Tuesday morning through Wednesday
     afternoon for INZ003.

MI...Beach Hazards Statement from Tuesday morning through Wednesday
     afternoon for MIZ077.

OH...NONE.
LM...Small Craft Advisory from 5 AM Tuesday to 5 PM EDT Wednesday for
     LMZ043-046.

&&

$$

SYNOPSIS...Marsili/T
UPDATE...Marsili
SHORT TERM...T
LONG TERM...T
AVIATION...Marsili


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