Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 281754
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
154 PM EDT Fri Apr 28 2017

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

&&

.SYNOPSIS...
Issued at 154 PM EDT Fri Apr 28 2017

A warm front will lift north across central Indiana on Saturday.
Then, a cold front will sweep through from the west Sunday night.
Record moisture for this time of year combined with the fronts,
upper level support and instability will result in numerous strong
thunderstorms through the weekend with a potential of heavy rain and
flooding. After a brief break, more showery weather is possible for
the middle of next week.

After a muggy weekend, cooler weather will follow next week.

&&

.NEAR TERM /Late this afternoon, tonight and Saturday/...
Issued at 154 PM EDT Fri Apr 28 2017

Latest water vapor imagery was showing an upper circulation over
eastern Utah and western Colorado within a broader Rockies trough.
Meanwhile, a warm front was developing over the Tennessee Valley.

Main focus late this afternoon and tonight will be on heavy rain and
flooding. Very good setup for heavy rain with record precipitable
water values, an advancing warm front and a strong upper jet over
the Great Lakes. Best focus for convergence will be over south
central Indiana near the nose of a 40-50 knot low level jet and in
exit region of 130 knot 250 millibar jet. Frontal forcing combined
with strong isentropic lift on the 305K level, precipitable values
to over 1.75 inches and CAPE to over 1000 j/kg will result in
widespread thunderstorms. The combination of shear and instability
will also support severe storms, especially over our southwestern
counties, where SPC has us in a Day 1 Enhanced Risk. Slight and
Marginal Risks extend further to the north over the county warning
area. Can not rule out isolated tornadoes, mainly south of
Bloomington, otherwise, large hail and damaging winds will be the
main threat. Threat should continue through Midnight or possibly
later.

With such good combination of synoptic forcing, moisture transport,
instability, slow moving front and upper jet dynamics, should see
numerous storms over south central Indiana late this afternoon
through tonight and could see enough for flash flooding. Thus, after
coordinating with adjacent offices, went with a Flash Flood Watch
from Sullivan to Decatur counties and south through noon Saturday.
More flooding concerns are possible if not likely the remainder of
the weekend. So, stay tuned to further updates.

The thunderstorm coverage and intensity may lull a little bit
Saturday morning as the front lifts slowly northward and the low
level jet subsides. However, by afternoon, the best chances for
thunderstorms will shift to our northwestern counties, which will
also be closer to a 45 knot plus jet moving in from the Ozarks. In
addition, model instabilities are much higher Saturday afternoon
then this afternoon and evening. Thus, more severe weather is
possible Saturday afternoon and evening. SPC has a Day 2 Slight Risk
over all of central Indiana for Saturday and Saturday night.
Damaging winds and large hail will be the main threats, but isolated
tornadoes can not be ruled out.

With southerly flow and clouds around, temperatures will not fall
much tonight. All but the far north should likely to be in the warm
sector by Saturday afternoon, so should see a strong southeast to
northwest gradient and highs close to a MOS blend in the mid 60s far
north to lower 80s south.

&&

.SHORT TERM /Saturday night through Monday/...
Issued at 154 PM EDT Fri Apr 28 2017

Main focus will continue to be on heavy rain, including flooding and
severe weather through the weekend.

Low level jet continues to point across areas northwest of
Indianapolis Saturday night. So, will continue with high pops there.
This area may need to be in a Flash Flood Watch soon, but prefer to
see how south central Indiana fares this evening first. Needless to
say, heavy rain and flooding will be hit hard in the Hazardous
Weather Outlook and Weather Story.

Areas southeast of Indianapolis should see a little lull Saturday
night and evening as they will be further away from the front.
Finally, strengthening Rockies low will move into the southern
Plains Sunday morning and then northern Missouri by 12z Monday. As
it does, an associated cold front will sweep through central Indiana
Sunday night and bring with it more widespread thunderstorms with
more heavy rain and possibly severe weather. Things will finally
improve Monday when only scattered lighter showers are possible in
the wake of the cold front.

Accepted the Mild MOS blend temperatures this weekend the southerly
flow and dew point advection.

&&

.LONG TERM /Monday through Thursday/...
Issued at 240 AM EDT Fri Apr 28 2017

Ensembles indicate that system that is expected to affect the area
over the weekend will be in the process of lifting off into the
Great Lakes by Monday. Will keep some PoPs in the forecast for
Monday and Monday night to cover potential shower activity in the
post frontal zone.

There is quite a bit of variance among the ensembles as to the
strength and location of the next trough that is expected to move
into the area around the middle of next week. Some of the members
indicate a strong low pressure system moving into the area, while
others are weaker and/or more suppressed. Differences are
probably due to phasing issues. Will go with PoPs for next
Wednesday and Thursday for now and monitor future trends.

&&

.AVIATION /Discussion for the 281500Z TAF Update/...
Issued at 1026 AM EDT Fri Apr 28 2017

A band of light rain might move through KIND within an hour, but no
impact to visibility or ceiling is expected. Better chances for rain
arrive in afternoon, so only made minor changes to the TAF.

Previous discussion follows...

Layered clouds above 050 expected at the terminals today, along
with the potential for some diurnal cloud development based near
035 by the early afternoon hours.

Although there may be some widely scattered light shower activity
around later this morning, it appears the better threat for
convective activity in the vicinity of the terminals will be
after about 282000Z. Upper disturbance currently located over
eastern Oklahoma/Kansas will begin moving into the area by that
time, accompanied by steepening mid level lapse rates. Brief IFR
visibility restrictions possible in heavier showers. CB bases
035-040.

Surface winds 110-150 degrees at 5-8 kts this morning will
increase to 9-12 kts by this afternoon.

&&

.HYDROLOGY /Discussion for this weekend/...
Issued at 154 PM EDT Fri Apr 28 2017

The heaviest rains tonight will be over our south central counties,
closer to the warm front and low level jet. Then, the action will
shift to areas northwest of Interstate 70 Saturday afternoon and
night as the best moisture transport shifts to that area. Finally,
widespread heavy rain-producing thunderstorms are expected again
Sunday afternoon and night as a cold front moves in from the west.

Extensive flooding will develop along rivers and streams in central
and southern Indiana during the first week of May.  Flooding will
affect numerous local and state roads...considerable agricultural
land and some residential areas.  Flooding will begin possibly as
early as Saturday evening and continue for 10 days or more in
southwest Indiana.

Rainfall of 2 to over 3 inches is expected from Friday evening
through Monday morning.  The most rainfall is expected along the
Indiana-Illinois state line and the least along the Indiana-Ohio
state line.

Significant flooding will develop across much of western Indiana and
bordering eastern Illinois.  Near major flooding is possible in some
areas. Flood crests may be the highest experienced since April 2013
in portions of west central Indiana and the highest since December
2015 and early January 2016 in other areas.

&&

.IND WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 8 PM EDT this evening through Saturday
morning for INZ060>065-067>072.

&&

$$

SYNOPSIS...MK
NEAR TERM...MK
SHORT TERM...MK
LONG TERM....JAS
AVIATION...JAS/50


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