Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 292032
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
432 PM EDT Sat Apr 29 2017

.UPDATE...The AVIATION section has been updated below.

&&

.SYNOPSIS...
Issued at 320 PM EDT Sat Apr 29 2017

A stationary front will lift north across central Indiana tonight.
Then, a strong cold front will sweep east across the area Sunday
night. Copious amounts of moisture, along with strong lift and
instability, will result in numerous thunderstorms over central
Indiana through Sunday night. Heavy rain and flooding will continue
to be a threat. Severe weather is also possible.

A few showers will linger into Monday in the wake of the cold front.
Then, high pressure will bring dry and cool weather to the area
Tuesday. Showers will return midweek courtesy of a southern system.
Finally, next weekend looks dry and continued cool under high
pressure.

&&

.NEAR TERM /Late this Afternoon and Tonight/...
Issued at 320 PM EDT Sat Apr 29 2017

The stationary front should start lifting slowly northward late this
afternoon, in part due to the decrease in coverage of convection, as
the low level jet has weakened and moved off to the east. The front
was extending northeastward along the Ohio River valley from a low
pressure center over east central Oklahoma. Meanwhile, an upper low
was pivoting southeast over central New Mexico.

Models are pretty consistent in moving the upper low to southwestern
Kansas and the surface low to east central Kansas by 12z Sunday. As
the surface low lifts northward, it should drag the warm front to
near Lafayette and Kokomo by Sunday daybreak.

All but areas northeast of Lafayette and New Castle are in a Slight
Risk for severe weather late this afternoon and tonight. This seems
reasonable as CAPES have been creeping up south of Indianapolis.
Also, 40 plus knot low level jet will migrate into our west central
counties this evening and overnight which will provide more fuel for
thunderstorm development. In addition, the High Resolution Rapid
Refresh suggests there will be a few storms around this evening with
increased coverage overnight, mainly west central within low level
jet and deep moisture transport. The HRRR has handled the shower
evolution very well this morning. So, will follow it closely and
stick with high pops there, especially overnight and taper to chance
pops southeast. With record precipitable water amounts for this time
of year, above 1.75 inches, and good dynamics and instability, heavy
rain will be a possibility once again tonight. The main severe
threats will be damaging winds and large hail. All of central
Indiana is in a Slight Risk for excessive rainfall tonight.

Blend temperatures look ok overnight with little drop expected.

&&

.SHORT TERM /Sunday through Tuesday/...
Issued at 320 PM EDT Sat Apr 29 2017

Main focus for the short term will be on timing and coverage of more
heavy rain producing thunderstorms.

Models in good agreement that a low pressure system will lift from
Kansas to northern Iowa by 12z Monday and to just north of Sault
Ste. Marie by 12z Tuesday. As the system moves northeast, it will
drag a strong cold front across central Indiana Sunday night.  Ahead
of this system, record deep moisture will advect northward over the
area via a 50 plus knot low level jet. Precipitable water values
will be above 1.75 inches, which continues to exceed the max moving
line at ILX and ILN for late April. There was also be sufficient
instability with CAPES 400-800 j/kg and deep uni-directional shear.
So, there could be some damaging winds and possibly large hail per
the SPC Day2 Marginal Risk of severe weather. There will be
thunderstorm chances all day Sunday with upper impulses moving
through the fast southwest flow, but the best chance will be Sunday
late morning through Sunday night with cat pops needed. Potential
for another inch or more on top of saturated soil warrants a Flash
Flood Watch for all of central Indiana through 8 am Monday. 1 hour
Flash Flood Guidance of slightly more than half an inch supports
starting the Watch off at 00z tonight rather than later.

Could see a few instability showers Monday and Monday evening in
cyclonic flow behind the cold front, otherwise it will be much
cooler behind the cold front with sunshine returning by Tuesday. MOS
blend looks to capture this with pre-frontal highs Sunday in the
upper 70s to mid 80s and below normal post-frontal highs only in the
upper 50s to mid 60s Monday and Tuesday.

&&

.LONG TERM /Tuesday Night Through Saturday/...
Issued at 214 PM EDT Sat Apr 29 2017

Highly variable model solutions lead to low confidence, so the model
blend initialization was accepted.

Models continue to have different solutions on how an upper level
system will develop during the long term, including individual model
variability from run to run.

Thus as noted above, confidence is low. Left the initialization
alone which leads to some PoPs for several periods during the long
term.

Temperatures will remain below average. (Some models have highs
nearly 20 degrees below average at times.)

&&

.AVIATION /Discussion for 292100Z TAF Update/...
Issued at 432 PM EDT Sat Apr 29 2017

Winds have veered at all terminals except KLAF as the warm front
continues to lift north through the area. Much of central Indiana
enjoying a lull in the precip and convection at this time with
ceilings lifting from the southeast as briefly drier air works in.
Will continue to see impacts from IFR ceilings at KLAF into the
evening but elsewhere...expect VFR conditions outside of convection.
Any storms should remain isolated to scattered into the early
evening with an uptick in coverage during the second half of the
veering into the overnight as the next wave aloft moves into the
area.

18Z discussion follows.

Low confidence forecast regarding timing of flying conditions.
However, current thinking is IFR and worse conditions will improve
to MVFR or better 18z-20z. In addition, with instability and
moisture increasing, pop up thunderstorms are possible ahead of the
slow moving warm front over southern Indiana. Kept LAF and HUF with
at least VCTS through the night with a low level jet pumping in deep
moisture there, and the HRRR hinting at such.

Southeast winds to 11 knots will gradually become southerly 02z-11z
as a warm front moves in from the south.

&&

.HYDROLOGY /Discussion for this weekend/...
Issued at 320 PM EDT Sat Apr 29 2017

Widespread thunderstorms with heavy rain are expected late tonight
near and northwest of a Terre Haute to Kokomo line. Also, widespread
heavy rain producing thunderstorms are expected again Sunday
afternoon and night from west to east as a cold front moves in from
the west.

With more heavy rain to come, extended the Flash Flood watch for all
of central Indiana through Monday morning. WPC and the OHRFC QPF
looks good. Rainfall of 2 inches or more is expected over all or
parts of central Indiana. The most rainfall is expected near and
northwest of a Terre Haute to Kokomo line.

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.

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 through Monday morning for INZ021-028>031-035>049-
051>057-060>065-067>072.

&&

$$

SYNOPSIS...MK
NEAR TERM...MK
SHORT TERM...MK
LONG TERM....50
AVIATION...MK/Ryan
HYDROLOGY...MK



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