Area Forecast Discussion
Issued by NWS Indianapolis, IN

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000
FXUS63 KIND 200646
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
246 AM EDT Sun Aug 20 2017

.UPDATE...
The LONG TERM Section has been updated below.

&&

.SYNOPSIS...
Issued at 331 PM EDT Sat Aug 19 2017

Warm temperatures and mainly dry weather is expected through the
rest of the weekend.  The approach and passage of a strong cold
front will bring the best chance for widespread rain and
thunderstorms experienced in a few weeks beginning Monday night and
Tuesday.  Drier and much cooler weather will arrive for the second
half of the work week.

&&

.NEAR TERM /Rest Tonight/...
Issued at 927 PM EDT Sat Aug 19 2017

Radar indicates showers beginning to die out.  Will keep low chances
over portions of our southwest and southeast for another hour or two.
Satellite shows mostly scattered clouds south of I70 outside of
a few showers and mostly clear north.   All in all current forecast
is on track and no other changes are needed at this time.

Portion of previous near term discussion follows...

Remainder of the overnight will be generally quiet as the region
comes under the influence of weak high pressure. Loss of heating and
the reestablishment of a capping inversion should shut down any
convective threat with skies mainly clear overnight. With light
flow...expect some patchy fog to form early Sunday morning. High
confidence in the forecast from mid evening through the overnight
once any isolated convection diminishes.

Temps...low level thermals were more supportive of the cooler MAVMOS
guidance as lows should drop into the 60 to 65 range across much of
the forecast area.

&&

.SHORT TERM /Sunday through Tuesday/...
Issued at 331 PM EDT Sat Aug 19 2017

Forecast challenges focus primarily on potential cloud cover impacts
for Eclipse viewing Monday afternoon...and the potential for strong
to severe storms Tuesday ahead of a strong cold front.

Weak ridging aloft will expand across the Ohio Valley Sunday which
should support an overall dry pattern across the forecast area. One
potential caveat which at the very least needs to be mentioned. Flow
through the column remains light and with surface flow reverting
back to a more southeasterly direction...any erosion of the low
level moisture field that occurs tonight will be lost on Sunday.
Models...most notably the NAM12...NAM3 and the HRRRX are all hinting
at isolated diurnally-driven convection developing over the southern
half of the forecast area during the afternoon.

With little forcing aloft and model soundings indicating the capping
inversion still in play...only factors that seem plausible in
generating convection Sunday afternoon would be lingering low level
moisture and instability. Now with that being said...plan to
maintain a dry forecast over the entire forecast area for two main
reasons. One...models mentioned above appear to be grossly overdoing
forecast dewpoints in the low to mid 70s on Sunday afternoon...
especially the NAM12 and NAM3. And two...the presence of the cap
albeit in a weakened state should serve to mitigate any convective
development.

After a quiet night Sunday night.once again..Monday continues to be
a primary focus for the short term with respect to viewing the
eclipse. While model soundings and RH progs continue to highlight
expectation of diurnally-driven cu formation Monday afternoon which
should do little to hamper viewing...the greater concern continues
to be any high level cloud debris from convection that develops over
Iowa and Missouri Sunday night and Monday morning. Trends continue
to point to potential for at least some of this cloud debris to
filter into the forecast area during the eclipse timeframe. Have
nudged up cloud coverage a bit as a result...with highest impacts
likely over the northwest half of central Indiana. Cannot entirely
rule out isolated convection developing over northwest counties
Monday afternoon as well...although appears most development will
hold off until after the end of the eclipse...20Z and beyond.

Remaining focus within the short term is on potential for rain and
severe storms late Monday night and Tuesday as a strengthening low
pressure tracks across the Great Lakes and pulls a cold front into
the area by the end of the forecast period. Nice dynamical setup
with this system with best potential for severe weather focused over
the eastern half of the forecast area east into Ohio in an unstable
airmass just ahead of where a strong upper jet and low level jet
interact. High pops remain warranted across central Indiana on
Tuesday and it does appear that this will likely be the best chance
for much of the area to see rain...not only in recent weeks but in
the coming 7 days as well.

Temps...Low level thermals are supportive of warm temperatures
Sunday and Monday with the possibility at many locations making a
run at 90 degrees Monday despite the potential for higher clouds
coming in. Heat indices will peak in the 90s on Monday as dewpoints
rise into the 70s. Lows will rise back into the lower 70s by Monday
night as the cold front approaches. Overall a model blend worked
well for temperatures through the period.

&&

.LONG TERM /Wednesday through Saturday/...
Issued at 244 AM EDT Sun Aug 20 2017

Ensembles are in good agreement during this period, depicting a long
wave trough over the eastern parts of the country, resulting in
northwesterly upper flow locally.

Ensembles suggest a dry and stable air mass will reside over the
local area, with little threat for precipitation in the extended.
Will go with a dry forecast.

&&

.AVIATION /Discussion for 200600Z TAF Issuance/...
Issued at 1155 PM EDT Sat Aug 19 2017

Mainly VFR...but a brief period of MVFR fog possible at KHUF
and KBMG around sunrise.

Area of high pressure across northern Indiana will move on to the
east coast.  This will result in a very light east flow overnight
and south 5 to 7 knots late Sunday morning on.  As mentioned
earlier...could be a brief period of MVFR fog at the outlying
TAF`s around sunrise.  Otherwise...VFR with Few to scattered
CU from late Sunday morning on.

&&

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

$$

SYNOPSIS...Ryan
NEAR TERM...Ryan/JH
SHORT TERM...Ryan
LONG TERM....JAS
AVIATION...JH



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