Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 281750

Area Forecast Discussion
National Weather Service Indianapolis IN
150 PM EDT Sun May 28 2017

The AVIATION Section has been updated below.


Issued at 210 AM EDT Sun May 28 2017

A series of weak fronts will cross Indiana from the west through
Tuesday. High pressure is expected to develop over our state
Wednesday, then another front should arrive Saturday.


.NEAR TERM /Rest of Today/...
Issued at 1017 AM EDT Sun May 28 2017

Mid level vorticity tail currently pushing through northern
Illinois is expected to reach the far northwest zones by early
this afternoon, and into the central zones by early this evening
based on short term model guidance. Instability axis currently
runs across central Indiana, so expecting scattered convection to
develop near this feature by the early to mid afternoon hours.
This morning`s upper air indicates adequate mid level lapse rates
remain over the area for severe convection, however deep layer
shear is quite weak. Short term models suggest deep layer shear
may increase later this afternoon with the approach of the
vorticity tail.

Overall, current forecast looks good. Only minor changes made.
Previous discussion follows.

The main issue is the threat of thunderstorms today.

This looks high. The GFS and NAM have similar handling of a
disturbance arriving aloft before 24Z. They also agree about large
amounts of CAPE surging over Indiana in advance of the disturbance.

It`s possible clouds will reduce destabilization.  However satellite
shows we`re already on the northern side of the cirrus shield, which
is eroding. Subsidence ahead of the short wave this afternoon will
also work against clouds. We should at least be partly sunny. With
the high late May sun, that supports considerable CAPE.

Likely to categorical POPs for thunderstorms seem best for the
afternoon. As the HRRR indicates, the morning should be dry as
instability builds.

Uncertainty about cloudiness means some uncertainty about
temperatures, but most methods indicate a consensus should be
accurate to within 2 degrees.


.SHORT TERM (Tonight through Tuesday night)...

Issued at 210 AM EDT Sun May 28 2017

The forecast problem is POPs.

For tonight, the eastern CWA needs a chance in case anything that
fires during the afternoon continues past 00Z. In the west,
subsidence behind the previously mentioned disturbance should keep
things dry. The east should be dry as well after this evening.

The models have disagreements Monday through Tuesday night, but
concur the flow will stay basically cyclonic. With the high sun
angle, that means there be at least some chance of storms every
afternoon and evening. Beyond that, consensus should identify
periods when rain chances are highest.

Skies should be partly cloudy as weak perturbations pass through
the cyclonic flow.

Given the models differ about rain chances, they have differences
with temperatures as well.  In absence of a strong reason to go one
way or the other, consensus will be used to minimize errors.  These
should be 3 degrees or less.


.LONG TERM /Wednesday through Saturday/...

Issued at 310 AM EDT Sun May 28 2017

Models start the period with a middle/upper level low over southeast
Ontario with a short wave trough that will swing from the Upper
Great Lakes southeast through the Ohio Valley early in the period.
Mid/upper level flow becomes somewhat zonal as another low
develops over Lake Winnipeg and drops southeast and deepens into
southeast Ontario by Friday evening. Weak embedded short waves
will move through the region with this trough starting on Friday.

After the trough passes on Wednesday morning, looks like any rain will
taper in the morning as high pressure moves into the region for
late Wednesday through Thursday. Then, as that next trough begins
to move southward into the Upper Great Lakes. Looks that GFS
overdoes moisture return through the Mississippi Valley, but the
Blend does begin to bring a chance for showers and thunderstorms
on Friday through Sunday. Given that the ensembles are struggling
with this solution, have a low confidence in this scenario at this
time. But, it looks like that rain and the occasional
thunderstorm will be possible end of the week into the weekend.

Temperatures look that they will be around the average during this
period with highs in the mid-upper 70s and lows in the upper 50s
to around 60.


.AVIATION /Discussion for the 281800Z TAFs/...

Issued at 150 PM EDT Sun May 28 2017

Mainly VFR Conditions are expected this TAf period.

Showers and TSRA are expected at the TAF sites through the first
3-5 hours of the TAF period as a cold front over NW Indiana will
push southeast across Indiana. Favorable conditions exist ahead
of the front with forecast soundings suggesting steep lapse rates
and CAPE over 1500 j/kg. Brief MVFR to IFR conditions will be
possible with any thunderstorm that strikes a TAF Site.

Subsidence and drying is expected in the wake of the front as
time height sections show a dry column into Monday.




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