Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 202237

Area Forecast Discussion
National Weather Service Indianapolis IN
637 PM EDT Sat May 20 2017

The AVIATION Section has been updated below.



Issued at 302 PM EDT Sat May 20 2017

A cold front will slide east across central Indiana overnight and
Sunday. The front will interact with a warm, moist and unstable
airmass and result in widespread thunderstorms. The storms will be
ending from west to east overnight and Sunday morning. High pressure
will usher in seasonably dry weather through Monday. Then, an upper
low pressure area will bring more showers and thunderstorms and cool
temperatures to the area through the middle of next week. Warmer
temperatures and more showers are possible next weekend as another
frontal approaches from the west.


.NEAR TERM /Tonight/...

Issued at 302 PM EDT Sat May 20 2017

Main focus tonight will be on timing and coverage of thunderstorms.

Models in good agreement that an upper low will lift from Nebraska
to the northern Plains by 12z Sunday. In response, a surface low
will lift from southwestern Iowa northwestern Wisconsin. The
thunderstorm complex, that went through central Indiana earlier, has
now moved into northern Indiana. Satellite was showing breaks in the
cloud cover which should allow the southern Indiana front to move
northward and the atmosphere to de-stabilize once again ahead of a
cold front, that was sagging south from the surface low, across
western Missouri and the southern Plains. The front should be just
to the west of a Kokomo to Indianapolis to Bedford line by daybreak.
The front will interact with the very moist and unstable atmosphere
and should result in numerous thunderstorms. The thunderstorms will
be ending overnight west of the front and continue east of the
front. Pre-frontal precipitable water values will be near-record
around 1.70 inches at ILX and ILN. In addition, central Indiana will
be within the right rear entrance of a 105 plus knot jet streak over
western Wisconsin. All this suggests heavy rain is possible. In
addition, 30-40 knot 1000-500 bulk shear suggests a few strong or
severe storms are possible, mainly during the late afternoon and
early evening, when surface base CAPES should exceed 1000 j/kg.

Blend lows in the lower and mid 60s look good tonight with the
cooler temperatures in the wake of the cold front.


.SHORT TERM /Sunday through Tuesday/...

Issued at 302 PM EDT Sat May 20 2017

Main challenge for the short term will be timing the showers and
thunderstorms out of the area in the wake of the cold front and also
will be on timing and coverage of the return of unsettled weather
associated with an approaching upper low.

Models are in good agreement that the showers will be ending near
and west of a Kokomo to Indianapolis to Bedford line by 12z Sunday
and over with across all of central Indiana by 18z, as the front
moves into Ohio. Skies will be decreasing Sunday afternoon with
seasonable blended highs in the lower and mid 70s looking
reasonable. It will be cool Sunday night under high pressure. Some
areas may dip to the 40s. Look for slightly below normal
temperatures Monday despite a good deal of sunshine expected.

Models then in good agreement that an upper low will rotate
southeast across the western Great Lakes by late Tuesday. This will
promote more showers by Tuesday. Although areas near and west of
Tipton and Terre haute could see an overnight storm Monday night.
For now chance pops look good for most of the forecast area, but
chances look a little better northwest. With extensive cloud cover,
below normal temperatures are a good bet.


.LONG TERM (Tuesday night through Saturday)...

Issued at 150 PM EDT Sat May 20 2017

There is good confidence it will rain either Tuesday night or
Wednesday, and that Friday should be dry. Otherwise there is much
forecast uncertainty.

The American and European models disagree on the timing of
systems that would cause precipitation. The exception is they both
have Indiana covered by dry high pressure Friday. Depending on
which model ends up best, POP errors may be as high as 20 percent.

The American and European models also disagree about cool Canadian
air arriving in the Hoosier state. This makes temperature forecasts
progressively more uncertain. Errors through Wednesday night should
be 2 degrees or less. Errors late in the period could be 5 degrees.


.AVIATION /Discussion for the 210000Z TAFS/...
Issued at 636 PM EDT Sat May 20 2017

Convection is starting to develop over eastern Illinois and
western Indiana in advance of an upper level vorticity lobe
pushing east across Illinois. Convective threat at the terminals
looks be rather high throughout the evening and into the late
night hours as this feature interacts with an unstable air mass
locally and a diffuse frontal zone. Brief IFR visibility
restrictions and gusty shifting winds in and near heavier cells.

Short term model guidance suggests this feature will pass off to
the east of the terminals around 210900Z, so should see convective
threat diminish by that time.

Otherwise, expecting lower ceilings 010-015 to develop towards
sunrise Sunday in the wake of upper level feature. Surface winds
150-180 degrees at 8-11 kts this evening will gradually veer
around to 200-230 degrees by sunrise Sunday.




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