Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 202232
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
632 PM EDT Fri Oct 20 2017

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

&&

.SYNOPSIS...
Issued at 227 PM EDT Fri Oct 20 2017

High pressure will provide warm and dry weather through Saturday.
Then, a pair of frontal systems will bring showers to all or parts
of the area Saturday night through next Wednesday. The latter front
will also result in much cooler weather starting next Tuesday. Highs
will start off in the mid 70s Saturday but will only be in the 50s
by Tuesday.

The threat for showers will return next Friday as yet another
frontal system moves through the area.

&&

.NEAR TERM /Tonight/...
Issued at 227 PM EDT Fri Oct 20 2017

Models in good agreement that surface high pressure will drift east
over the Appalachians tonight. Meanwhile, an upper ridge over the
Ohio Valley will shift to the Appalachians. This should allow some
upstream high clouds to stream northeast over the area in the fast
southwest flow aloft. Satellite trends support this.

Light southerly flow supports overnight temperatures several degrees
warmer than last night per the blend. Good confidence in lows in the
lower to middle 50s for most or all locales.

&&

.SHORT TERM /Saturday through Monday/...
Issued at 227 PM EDT Fri Oct 20 2017

The main focus for the short term will be on timing and coverage of
showers late this weekend and early next week.

The models were having some issues with their handling of an
upstream upper trough and frontal system. That said, the 12z GFS and
ECMWF were well within ensemble clustering. Both of these models
show an upper low breaking off the southern portion of the upper
trough and spinning up a surface wave along the front and over the
lower Mississippi Valley Sunday night. This would tend to slow the
front up as it moves over central Indiana Sunday night into Monday.
However, this would also tend keep the heavier rains to the south.
The 12z Canadian did not develop a southern upper low and was thus
quickest with the trough and front.

For Saturday, some of the models generate a shower or two over west
central Indiana as upper impulses lift northeast across the Ohio
Valley in southwest upper flow. However, model rh time sections were
showing very dry lower and mid levels. So, should see another dry
and warm day with just passing cirrus. The southerly low level flow
around the high combined with filtered sunshine should support highs
in the middle 70s, per the blend, with good confidence.

Model rh time sections and synoptic forcing suggest the western
counties could see a few overnight showers Saturday night. Look for
the showers to gradually spread east Sunday into Monday with the
upper trough and cold front getting closer and slowly moving
through. The blend is trending slower with their QPF fields. So,
will accept it and go with with likely pops west tapering all the
way down to dry far east Sunday. Then, will go likely to categorical
northwest to chance pops southeast and finally likely east and good
chance west Monday. Confidence in the slower shower trend is
moderate to good.

Kept thunder out for now with only sporadic very weak elevated model
instability. Moderate QPF should not cause any widespread water
issues, although would not rule out brief heavy rain at times Sunday
and beyond.

Will stick with a blend regarding temperatures as the it appears to
handle increase in clouds and showers and frontal passage well.

&&

.LONG TERM /Monday Night Through Friday/...
Issued at 222 PM EDT Fri Oct 20 2017

Models are close enough that the model blend initialization
was accepted for most items.

Confidence is high that an upper low will move into the area Tuesday
and linger on Wednesday. The low will keep chances for showers
around as well as bring much colder air into the area. Temperatures
may struggle to reach 50 across parts of central Indiana on
Wednesday.

Will have to watch for some frost potential Wednesday night with
clearing skies and light winds.

The low will move out allowing ridging to move in for Thursday,
allowing drier and warmer conditions to briefly return.

On Friday another upper trough will approach the area, bringing more
rain chances with it.

&&

.AVIATION /Discussion for 210000Z TAF Issuance/...
Issued at 632 PM EDT Fri Oct 20 2017

VFR conditions are expected throughout the forecast period.

The center of high pressure will shift east into the mid Atlantic
region through Saturday...but remain in close enough proximity to
central Indiana to maintain dry conditions through Saturday
evening. Return flow on the back side of the high will bring
increasing high level moisture steadily into the Ohio Valley with
cirrus expanding in coverage slowly from the west by Saturday
afternoon.

Southerly winds will peak near 10kts on Saturday. Low level wind
shear may become an issue Saturday evening as winds increase
through the boundary layer. Will likely address this in greater
detail for upcoming forecasts later tonight and early Saturday.

&&

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

&&

$$

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



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