Area Forecast Discussion
Issued by NWS Indianapolis, IN

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494
FXUS63 KIND 261925
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
325 PM EDT Wed Jul 26 2017

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

&&

.SYNOPSIS...
Issued at 325 PM EDT Wed Jul 26 2017

A cold front will drop southeast across central Indiana Thursday and
Thursday evening. The front will interact with a warm, very moist
and unstable atmosphere and trigger widespread thunderstorms
overnight and Thursday. A few storms will linger into Friday as an
upper waves dives southeast over the Great Lakes. Then, high
pressure will bring dry weather and cooler temperatures to the area
for the weekend.

Temperatures will warm back up to near normal next week, and a few
storms are possible south by Wednesday as an upper wave moves across.

&&

.NEAR TERM (Tonight and Thursday)...
Issued at 325 PM EDT Wed Jul 26 2017

Timing and coverage of thunderstorms as well as location of heavy
rain axis will be the main concerns for the short term. Severe
weather can also not be ruled out.

Models are having some timing difficulties regarding an approaching
weak cold front, that currently is residing over Lake Superior into
Nebraska. The 12z NAM and GFS both have it over our northern
forecast area 00z Friday. However, the 00z ECMWF has it near the
Ohio River at that time. The 00z Canadian was something of a
compromise with the front over south central Indiana. This goes
along better with the WPC frontal position and is thus preferred but
with low confidence. Models also have timing differences with a pair
of weak impulses set to move through the area tonight and Thursday.
At any rate, the synoptic forcing will interact with precipitable
water values to 2.4 inches, courtesy of a 30 to 40 knot low level
jet, which is above the daily max line of 2.1 inches for ILN from
the SPC Sounding Climatology Page. In addition, despite extensive
cloud cover, the instability will be plenty enough with surface
based CAPES to around 200 J/KG overnight and over 1000 J/KG
Thursday.

Current forecast has small chance pops this evening, except far east
and likely pops all but far south and far east by 11z Thursday.
Despite some timing differences in model QPF, this looks to be a
good compromise and better to leave things as is with the
uncertainty that goes with those timing concerns. For Thursday, all
models and would suggest widespread thunderstorms. Confidence is
high with the front moving in.

With the moisture so impressive and freezing levels above 15,600
feet, there is certainly a threat for heavy rain overnight and
Thursday. Models are not surprisingly having issues with placement
of the heaviest rain axis. However, model trends are now a little
further south with the heaviest rainfall axis across our far
southern counties and into Kentucky. So, will go along with adjacent
offices and mostly use the 18z WPC QPF which reflects this thinking.

The Day 1 Excessive Rainfall Potential Outlook has parts of our
northwestern counties in a marginal or slight risk. However, Flash
Flood guidance has recovered with 1 hour numbers mostly 2 to 2.3
inches and 3 hour numbers mostly 2.5 to 3.2 inches. Taking that into
account along with the model consensus issues but further south
trends, and after coordinating with adjacent offices, will hold off
on a Flash Flood Watch, but include heavy rain in the grids
overnight and Thursday south and hit that and localized flooding
hard in the Hazardous Weather Outlook and Weather Story.

Due to extensive cloud cover, confidence in severe weather is not
great for Thursday. However, with nice veering with height, the low
level jet and a 70 plus knot upper jet over Michigan, can not rule
it out. The SPC Day2 Outlook has a Marginal Risk over our southern
half.

Blended temperatures look ok. Prefer temperatures at or below a
blend Thursday with the widespread convection opted to keep it from
warming up as much as otherwise.

&&

.SHORT TERM (Thursday night through Saturday)...
Issued at 325 PM EDT Wed Jul 26 2017

The main concerns with the short term will be timing the convection
of the area.

Models and ensembles all suggest the front will move south of the
area Thursday evening. This should result in the bulk of the
thunderstorms ending from northwest to southeast before Midnight.
For the evening, good chance pops look good close to the front
tapering off to slight chance northwest. After that, a distinct
upper wave will drop southeast across the Great Lakes on Friday.
This supports model small pops, despite limited moisture. There
should be just enough instability to justify thunder.

By Friday night, should see little to no clouds and cooler
temperatures, per model blend as high pressure builds in from the
upper Midwest.

&&

.LONG TERM /Saturday night through Wednesday/...
Issued at 228 PM EDT Wed Jul 26 2017

A strong upper level low pressure system will exit the Midwest and
move into the Appalachians Saturday night leaving behind northerly
flow that will persist through much of the long term period.  At
the surface, north to northeasterly winds will keep temperatures in
the low to mid 80s and keep dewpoints in the low to mid 60s.

There is high confidence for dry weather through Tuesday. A weak
upper level disturbance will move across the Midwest on Monday and
Tuesday, but with very dry air aloft, chances for precipitation are
low. Better chances for rain for Wednesday with several models
showing an upper level shortwave strengthening over the High
Plains and moving east into the Midwest.


&&

.AVIATION /Discussion for 26/18Z TAF Issuance/...
Issued at 112 PM EDT Wed Jul 26 2017

VFR conditions are expected through 6Z tonight before scattered
showers and thunderstorms move into central Indiana and bring
possible MVFR ceilings and visibility.

High pressure will continue to weaken this afternoon as an upper
level disturbance approaches the state bringing good chances for
showers and thunderstorms tonight. Exact timing is still uncertain
at this time, but the best chances look to be between 6Z and 14Z
with a line of thunderstorms moving across the state and trailing
stratiform behind. Another round of showers and thunderstorms are
possible Thursday afternoon, but with low certainty it will be
left out at this time.

Winds will generally be light and variable through the period
except for instances of thunderstorms. Ceilings will fall into
MVFR conditions shortly after sunrise and will slowly rise
through the afternoon.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SYNOPSIS...MK
NEAR TERM...MK
SHORT TERM...MK
LONG TERM....White/Tucek
AVIATION...White/Koch



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