Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 210034

Area Forecast Discussion
National Weather Service Northern Indiana
834 PM EDT Thu Jul 20 2017

Issued at 826 PM EDT Thu Jul 20 2017

Hot and humid conditions will remain the story into Friday with
low chances for showers and thunderstorms tonight into Friday. Highs
through Saturday will range from the mid 80s to near 90 degrees,
with lows tonight near 70 degrees. Better chances for showers and
thunderstorms arrive later Friday into Saturday as a disturbance
moves through the region. Locally heavy rainfall and strong to
severe thunderstorms will be possible during this time. Cooler and
less humid conditions then return Sunday into early next week.


Issued at 826 PM EDT Thu Jul 20 2017

Scattered showers have been trying to develop across primarily
southern half of the forecast area this evening. Precip has
struggled to develop to this point likely as a result of weak
lingering capping issues and lack of coherent forcing signal in
wake of earlier day MCS. Primary area of convection this evening
has been well to the west across portions of southern western
Iowa along old outflow boundary and southward sagging synoptic
boundary. Water vapor imagery does suggest very subtle short wave
across the Corn Belt likely aiding in convective initiation
across these locations. Still some uncertainty as to how upstream
convection will evolve but if storms congeal, propagation vectors
would once again suggest an E-SE tendency, which could result in
storms affecting southwest/western portions of the area late
tonight/early Friday morning. An axis of impressive instability
noted from southern Iowa into central Illinois, with main
uncertainty for the overnight regarding possibility of some weak
mixed layer CIN persisting north of this instability gradient
across western Indiana as strongest west-east oriented theta-e
ridge advects across extreme southern portions or just south of
forecast area. May make just a slight upward adjustment to PoPs
across the western portions of the area given upstream convective
trends, but overall not anticipating significant changes to going
forecast at this time.


.SHORT TERM...(This Evening through Friday)
Issued at 215 PM EDT Thu Jul 20 2017

A complicated short term forecast overall-with much of the
forecast area beneath zonal flow on the periphery of an upper
level ridge through the period. Thus, we will see precipitation
chances with any shortwaves that ripple through the flow,
especially given the moist airmass forecast to be in place. At the
surface, a stationary front currently north of the area will take
a dive southward early this evening...where it will again stall
out over our CWA-and lead to precipitation/storm chances through
Sunday as the various waves aloft move through.

For this evening, though there might be an isolated shower/storm
along the boundary as it sinks southward, thinking that subsidence
behind the passing wave from this mornings precipitation will limit
any real development through the evening. As a result, have the
best chances for showers/storms south of US 30 after 3Z, which is
when we have increased confluence near the frontal boundary. Kept
chances limited to 20-30 percent given model disagreement, and
weaker lift. While there is a threat of heavy rain within any
storms that develop, the risk for severe weather is extremely low
overnight. If any strong updrafts manage to develop within the
most unstable areas, think it`s possible to see isolated damaging
winds- but again- confidence in this occurring is low.

Friday is another complicated story. There may be a few
scattered/isolated showers along the boundary (south of US 30) early
in the AM, but otherwise expect most of the forecast area to be dry
before the next shortwave approaches. Highs will make another run up
to the 80`s and low 90`s, particularly if cloud cover is limited.
Humidity will continue yet again, so those participating in outdoor
activities will need to plan accordingly for sweltering, muggy
conditions. Chances for showers/storms approach from the west and
southwest towards late afternoon. Severe weather is not expected
during the day. SPC has our area in a slight risk for day 2, but
this is mainly due to the potential incoming MCS Friday evening,
which will bring the potential for damaging winds/hail.


.LONG TERM...(Friday Night through Thursday)
Issued at 215 PM EDT Thu Jul 20 2017

Several concerns this weekend-including heavy rain/flooding
potential, heat/humidity, and severe wx potential. However, threats
are very conditional. As mentioned in the short term discussion,
precipitation chances are focused around the boundary that settles
across our CWA, and the various shortwaves that move through in the
relatively zonal flow aloft. We have lower confidence with
regards to precipitation/storm chances in these situations given
that each MCS that develops impacts the precipitation/storm
chances for the next system-so minor changes can lead to bigger
errors further out.

That being said, mid-range model guidance suggests that a more
potent shortwave and associated surface trough will shift through
Friday night into Saturday, and then linger through Sunday. From
the 12Z model runs, it almost looks like we could see a repeat of
last night/today on Friday night/Saturday, with convection along
the stationary front in our CWA and another MCS developing over
WI in the evening. This will sneak into our area and bring
widespread rain with embedded storms through Saturday
morning/afternoon. Most of the models keep the precipitation
focused north of US 30-which is what our previous forecaster had
highlighted with likely POPS. I kept this trend in the latest
forecast package.

Severe weather-wise, SPC has our area in a slight risk through
Sunday morning given the moist airmass, sufficient instability (up
to 2K Sfc Based CAPE), and wind shear ranging from 20-40 knots.
Assuming those values are accurate we could see some strong to
severe storms Fri night-Saturday-but given the uncertainty of the
overall forecast pattern have low confidence. Despite this, it
would be wise to maintain situational awareness through Friday
night into Sunday as we`ll be watching the evolution of this
system closely.

Confidence in any one model solution goes down Sunday, as they are
in general disagreement as to how things play out. Think there will
be a break in precipitation from the overnight system before we see
some redevelopment when the better forcing from the cold front
arrives Sunday night.

High temperatures through the weekend will be in the 80`s and even
possibly low 90`s, as 850 temps linger around 20-22C. We`ll see
continued threats from heat and humidity during this time-but how
hot we get will depend on the cloud cover/precipitation evolution-
which is uncertain.

A final concern will be heavy rain/flooding potential Friday night
into Sunday, with PWATS of greater than 2" (175-200% above normal)
and warm cloud depths forecast to be upwards of 12K ft. Moisture
transport is maximized into our area at this time, and given the
antecedent conditions-thinking this could exacerbate problems in
areas that have already experienced flooding. Again-certainty is low
given we are unsure about the convective evolution but it is
something to be mindful of this weekend.

Monday afternoon into Thursday we see some relief in regards to
precipitation chances, which will be ever decreasing as a region of
high pressure sinks over our area. Cooler temperatures will come
with this, with highs expected to be in the 70`s and low 80`s. Some
isolated chances Monday as the weekend system exits out-but
otherwise fairly quiet.


.AVIATION...(For the 00Z TAFS through 00Z Friday Evening)
Issued at 826 PM EDT Thu Jul 20 2017

Showers have struggled to develop in wake of earlier day MCS,
with some weak convective inhibition persisting early this
evening. Modest westerly low level flow will allow a west-east
oriented theta-e ridge across central Illinois to advect eastward
across northern/central Indiana later this evening into the
overnight. This may allow at least an uptick in elevated
instability that would be more conducive for scattered showers and
thunderstorms developing. However at this time, it appears better
potential should remain south of the terminals. Otherwise weak
sfc flow to persist tonight with synoptic boundary draped across
the area, eventually favoring southeast Friday afternoon as sfc
pressure falls strengthen to the west-northwest.





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