Area Forecast Discussion
Issued by NWS Indianapolis, IN

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422
FXUS63 KIND 191652
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
1252 PM EDT Sat Aug 19 2017

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

&&

.SYNOPSIS...
Issued at 302 AM EDT Sat Aug 19 2017

A weak frontal system will drop southeast of central Indiana today
as an upper wave pivots through from the west. These features will
trigger a few morning thunderstorms over central Indiana. Then, a
broad area of surface high pressure will bring dry weather to the
area through Monday. Strong storms are possible next Tuesday and
Tuesday night as another system swings through. The rest of next
week will be cooler and dry as high pressure builds in in the wake
of the cold front.

&&

.NEAR TERM /This Afternoon/...
Issued at 1225 PM EDT Sat Aug 19 2017

UPDATE...Weak convergence boundary noted in the observational data
aligned just south of Interstate 70 at 16Z.  This may be the
remnants of the frontal boundary remnants which was becoming
increasingly diffuse since Friday night. CAMs have been hinting for
the last few hours at this boundary becoming quasi-stationary in the
area from the I-70 corridor south through Bloomington and Columbus
with MLCAPEs at 1000-1500 j/kg pooling in its vicinity.  While the
better forcing aloft is already in the process of departing off to
the east with the upper wave...the presence of instability and low
level convergence near the convergence zone may be enough to
generate isolated convection within a short window late this
afternoon and early this evening before the capping inversion
reestablishes.

Not sure expected coverage warrants reintroducing a low pop at this
point...but this will be something to monitor through the afternoon
and ahead of the release of the main forecast package between 19-
20Z.

The presence of a lingering low level thermal trough in wake of the
upper wave supports development of some cu this afternoon...
especially over the southeast half of the forecast area. Bumped high
temps up a couple degrees as most area should warm into the mid 80s.

Zone and grid updates forthcoming.

&&

.SHORT TERM /Tonight through Monday night/...
Issued at 302 AM EDT Sat Aug 19 2017

Main focus for the short term will be cloud trends, especially
Monday for the eclipse, as well as timing and coverage of convection
Monday night.

Models in good agreement that zonal flow will be over central
Indiana through Monday night overtop a broad dome of high pressure
over the southern states. At the surface, broad high pressure
will shift east of the area on Sunday allowing light return flow
and gradually warming. Upper impulses will interact with residual
low level moisture and could produce scattered diurnal cu and a
few mid and high clouds Sunday per cu development progs and rh
time sections. Although, 850 millibar moisture thins on Monday, rh
time sections hint at increasing mid and high clouds. Still,
think partly cloudy will do both days and the eclipse will still
be viewable. Although, the eclipse could briefly retard
temperatures, 21 degree 850 millibar temperatures suggest 90
degrees is possible Monday, mainly south. It will feel very warm
and humid regardless for those outdoors. Blend temperatures look
reasonable and will be accepted.

By Monday night, models hinting at more numerous waves in the zonal
flow along with an approaching frontal system and increased moisture
and instability. Thus, should see a few evening storms northwest
with more coverage overnight and beyond per the blend. Confidence in
exact timing is not great but in the trend is good.

&&

.LONG TERM /Tuesday Through Friday/...
Issued at 232 AM EDT Sat Aug 19 2017

Ensembles are in good agreement with the upcoming weather pattern
during this period. Short wave energy is expected to drop southeast
through the Great Lakes on Tuesday, eventually carving out a long
wave trough over the eastern parts of the country for the rest of
the extended.

Will keep PoPs in the forecast for Tuesday and Tuesday night as the
upper trough and associated cold front move through. In the wake of
this front, it appears a drier and more stable air mass will move
into the area. Will go dry after Tuesday.

&&

.AVIATION /Discussion for 191800Z TAF Issuance/...
Issued at 1252 PM EDT Sat Aug 19 2017

Other than brief visibility restrictions from fog early Sunday
morning...VFR conditions are expected.

Weak convergence zone lingering just south of I-70 early this
afternoon...likely the remnants of the frontal boundary that moved
into the region earlier this morning. Hi-res guidance starting to
highlight potential for very isolated convection to fire in the
vicinity of the boundary in the 21Z to 02Z window. While forcing
aloft is negligible...instability pooling in the convergence zone
may be just enough to kick off a stray shower or storm. Confidence
nowhere near where it needs to be to consider any sort of a VCSH
mention but should any convection develop...all terminals except
KLAF could be impacted briefly. Will continue to monitor.

Capping inversion reestablishes this evening with weak ridging
aloft developing by Sunday morning. Could see fog develop at
outlying terminals near sunrise Sunday. Cu will once again develop
by early afternoon with light southeast flow.

&&

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

&&

$$

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



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