Area Forecast Discussion
Issued by NWS Indianapolis, IN

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858
FXUS63 KIND 281921
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
321 PM EDT THU JUL 28 2016

.UPDATE...
The SYNOPSIS...NEAR TERM and SHORT TERM Sections have been updated
below.

&&

.SYNOPSIS...
Issued at 321 PM EDT Thu Jul 28 2016

A remnant frontal boundary across the Ohio Valley will interact with
mid level disturbances to produce periodic threats for thunderstorms
through much of the weekend. The front will briefly drop south of
the region late weekend into Monday before returning back north.
This will set the stage for additional daily chances for storms
through the middle of next week.

&&

.NEAR TERM /Rest of This Afternoon and Tonight/...
Issued at 321 PM EDT Thu Jul 28 2016

Have seen convection continually fire along a leftover boundary
extending from central Illinois toward the lower Wabash Valley with
additional isolated cells now forming on what appears to be another
remnant boundary south of Interstate 70 between KHUF and KBMG. Still
plenty of clouds over the forecast area this afternoon but temps
have been able to warm into the 80s most everywhere as of 19Z.

The remnant front...with several outflows littered across the region
and an approaching wave aloft should contribute an uptick in
convective coverage for the rest of the afternoon. The greatest
focus for scattered convection should be over the southwest half of
the forecast area in closer proximity to the primary surface
boundary...and in an area of greater instability and deeper
moisture. Overall boundary layer shear and flow through the mid
levels is downright paltry so storms are pulsing in intensity and
disorganized in general. One potential drawback to better convective
coverage is the lack of more substantial forcing aloft as the main
energy with the aforementioned wave over eastern Missouri tracks
south of the forecast area closer to the Ohio River while a
secondary wave currently generating convection over northern
Illinois slides east. For that reason...cannot justify going any
higher than 35-40 pops for the rest of the afternoon and evening.

Primary concern with convection will be the heavy rainfall and for
waterlogged southern counties...localized flash flooding. Precip
water values have dropped back from values this morning as axis of
deepest moisture has shifted further south and east of the region.
Still though with PWATs generally above 1.5 inches and the overall
slow and erratic movement of cells...locations that get underneath
storms will get soaked. Will have to monitor any convection that
moves across areas south of a Sullivan-Bloomington-Columbus-
Greensburg line as many locations south of this line have seen
several inches of rain over the last 72 hours.

Most convection will diminish after 06Z as instability diminishes
and the forcing aloft north and south of the region move away. Model
soundings do favor potential for patchy fog formation in the soupy
airmass during the predawn hours as well as potential for low
stratus as well.

Temps...hard to argue with a model blend which aligns with MOS
nicely. Should see lows fall into the upper 60s and lower 70s. Still
thinking Indy may stay at or above 70 for one more night. Indy was
last under 70 degrees on the morning of the 18th.

&&

.SHORT TERM /Friday through Sunday/...
Issued at 321 PM EDT Thu Jul 28 2016

Forecast challenges focus on precip chances through much of the
short term.

Dirty upper flow pattern will persist through much of the period as
weak waves continue to track through the Ohio Valley in a quasi-
zonal pattern aloft. The surface boundary will remain in the area as
well...with weak troughing noted at 850mb and 700mb. Combined with a
moist and unstable airmass over the region...scattered periodic
convective threats will continue for central Indiana through
Saturday with a greater potential for drier weather Sunday as drier
air briefly tries to advect into the area.

This kind of a sloppy setup aloft combined with mesoscale features
such as remnant outflow boundaries playing a more critical role in
where scattered storms form creates a scenario that makes it
difficult to generate any specific convective detail or timing. Have
therefore largely placed blanket chance pops into the region through
Saturday...focusing higher pops Friday night into Saturday as a
slightly stronger wave aloft moves across northern Illinois and
Indiana. Continued paltry flow through 500mb suggests little to no
BL shear...poor lapse rates...and overall sluggish and disorganized
convective mode and movement. PWATS will remain at or above 1.5
inches through Saturday with heavy rain and localized flooding
continuing as the primary storm threats.

Forcing aloft will shift east of the region Saturday night into
Sunday with weak northerly flow trying to advect slightly drier air
into the forecast area. Despite the presence of the drier
airmass...still expect some diurnal instability and cannot entirely
rule out a few storms on Sunday...especially over the southern
forecast area closer to the remnant front dropping into the
Tennessee and lower Ohio Valleys.

Temps...The low confidence convective forecast translates into a
lower than desired confidence in high temps through the period as
well. Felt comfortable leaning towards a model consensus with highs
generally in the low to mid 80s through the weekend. Slightly cooler
nights than that experienced as of late will arrive over the weekend
as well.

&&

.LONG TERM /Sunday Night through next Thursday/...
Issued at 223 PM EDT Thu Jul 28 2016

Ensembles in good agreement in expanding heat ridge, initially
over the southern Rockies, northeast into the Midwest and Ohio
Valley by the middle of next week. This may serve to cap off
convective potential by that time. For now, will keep some
chance PoPs going throughout the extended. If a trend towards
drier conditions continue, PoPs may be trimmed back next week.

&&

.AVIATION /Discussion for the 281800Z TAFS/...
Issued at 1256 PM EDT Thu Jul 28 2016

Weak vorticity lobe currently over Illinois is progged to drift
eastward through the local area later this afternoon and evening.
Scattered convection possible in association with this feature,
but confidence is low in direct impacts at this time. Short term
models suggest this vorticity lobe will pass east of the terminals
around 290400Z, so convective threat should diminish by then.
Brief IFR visibility restricitions in heavier convective cells. CB
bases near 020.

Otherwise, should see some visibility restricitions develop in
light fog after dark due to the high moisture content of the air
mass and little wind. Pattern favorable for widespread IFR/LIFR
conditions after 290600Z.

Surface winds generally below 7 kts through this evening.

&&

.IND Watches/Warnings/Advisories...NONE.

&&

$$

SYNOPSIS...RYAN
NEAR TERM...RYAN
SHORT TERM...RYAN
LONG TERM....JAS
AVIATION...JAS



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