Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 210028

Area Forecast Discussion
National Weather Service Northern Indiana
828 PM EDT Sun Aug 20 2017

Issued at 216 PM EDT Sun Aug 20 2017

A more humid airmass will continue to build into the area tonight
and Monday with a low chance for showers and thunderstorms. Lows
tonight will drop to near 70 degrees, with highs on Monday into
the 80s. A strong cold front will drop through later Monday night
into Tuesday with showers and thunderstorms likely. Drier and
cooler air will then filter in behind this system midweek through
next weekend.


Issued at 804 PM EDT Sun Aug 20 2017

Convectively enhanced vort max across northern Illinois will
continue to push eastward across northern Indiana overnight into
early Tuesday morning. A wing of more pronounced low level
theta-e advection has resulted in a few isolated showers/storms
across far southeast portions of the forecast area over the past
few hours, in more favorable proximity to steeper mid level lapse
rates. Broad area of stronger low level theta-e advection across
central Indiana will lift northward late evening/overnight as low
level flow becomes augmented by approaching MCV. Isolated storms
have been developing along this theta-e gradient/weak warm front
to the south/southwest of the local area this evening. Still
cannot completely rule out a few stronger storms late
evening/early overnight across especially west/southwest portions
of the forecast area where MUCAPEs are maximized. Latest HRRR
trends of more of focus on southwest portions of forecast area may
carry some validity given richer low level low level theta-e
air/maximized elevated CAPE overnight. May tweak PoPs slightly to
reflect this trend, but previous forecast of low-mid chance still
seems reasonable.


.SHORT TERM...(This Evening through Monday)
Issued at 216 PM EDT Sun Aug 20 2017

Zonal flow will continue across the northern tier of the US tonight
into Monday with uptick in low level southwest flow supporting
increasingly humid/warm conditions. This northeastward influx of
moisture/instability in tandem with a series of low
amplitude/convectively aided shortwaves to track east on southern
fringe of westerlies will also bring chances for showers/storms back
into the forecast tonight/Monday, mainly west of I-69. The first
feature to watch is a dying MCV now near the IL/IA border. The
leftover perturbation will translate east through the lower Lakes
tonight. Downstream theta-e advection near a diffuse warm front
lifting north from central IL/IN may be enough to generate a
period of showers/thunder late this evening into early Monday
morning. Opted to introduce PoPs as a result, though only in the
20-40 percent range given subtle timing difference in latest CAMS
solutions...with stronger LLJ focus also displaced well west of
the FA.

Attention tomorrow will turn to the next MCS to likely emerge back
into IA/srn MN later tonight. The expected orientation of the
instability gradient and propagation vectors hint at the possibility
of this system tracking east into the area by sometime in the
afternoon with renewed storm chances. Model guidance is on board
with moderate surface based destablization in advance of this
feature as temps warm into the 80s, with little CIN and sfc
dewpoints near 70F. Deep layer shear not looking all that impressive
but could see some enhanced flow if a more pronounced MCV develops
and holds together east. The chances of this occuring are somewhat
low however as convective complex likely outruns better low level
inflow and weakens with eastward progression. Held onto chance
PoPs regardless as any outflow could generate afternoon convection
given ample instability. If this line/MCS is more organized a
wind threat could materialize. Mainly dry/humid with highs in the
mid/upper 80s otherwise on Monday.


.LONG TERM...(Monday Night through Sunday)
Issued at 216 PM EDT Sun Aug 20 2017

The best opportunity for rain/storms still appears to be later
Monday night into Tuesday morning as an upper trough amplifies
southeast toward the Great Lakes and ingests monsoonal moisture
from the Four Corners. An associated pre-frontal sfc trough and a
series of low amplitude waves will provide the focus as large
scale ascent/height falls overspread during this time.
Precipitable water values pushing 2 inches and a deep warm cloud
layer hints at the potential for some locally heavy rainfall. The
severe threat looks relatively low by this time however given the
poor diurnal timing.

Renewed convection by peak heating Tuesday afternoon will likely
latch onto this pre-frontal trough/composite outflow toward the Ohio
River Valley. This should limit the coverage/intensity of convection
along the main cold front progged to drop through the local area
from nw to se Tuesday afternoon/evening.

Much drier/cooler air will filter in for the middle of the week
behind this system on the leading edge of an expansive high pressure
system. This feature will settle overhead by the end of the week and
next weekend with temps gradually modifying.


.AVIATION...(For the 00Z TAFS through 00Z Monday Evening)
Issued at 804 PM EDT Sun Aug 20 2017

Weak low level warm front and MCV combination should lead to
isolated scattered shower/storm development late this evening into
the overnight hours. Confidence in thunderstorms affecting
terminals is on the low side however given late timing of approach
of MCV and greatest elevated instability west/southwest of
terminals. Low level flow will strengthen late evening/overnight
and given some northward retreat of low level theta-e boundary
with warm front, scattered showers are a possibility in the 06Z-
12Z timeframe. Some lull in precip chances expected Monday
morning as the old MCV tracks east of the area, but will need to
watch potential of upstream Corn Belt convection to possibly begin
to affect northern Indiana Monday afternoon. With a good deal of
uncertainty for Monday afternoon, will keep TAFS dry at this time.
Otherwise, increasing low level moisture and light near sfc flow
may promote some patchy fog formation late tonight/early Monday,
but will keep conditions VFR at this time.




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

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