Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 191931

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

The SYNOPSIS...NEAR TERM and SHORT TERM Sections have been updated


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 of This Afternoon and Tonight/...
Issued at 331 PM EDT Sat Aug 19 2017

Cu field has become more firmly established from a Rockville to
Muncie line and points south this afternoon with less cu further
north where drier air has advected in evidenced by dewpoints in the
upper 50s and lower 60s. Weak convergence boundary remains present
just south of Interstate 70. 19Z temperatures were largely in the
mid 80s.

First issue focuses on any isolated convective potential that would
exist through 01-02Z in the vicinity of the boundary. As mentioned
at the midday update...suspect this convergence boundary is actually
the remnants of the weak front that tracked into the region earlier
this morning. This boundary will remain quasi-stationary through the
rest of the afternoon and evening and serve as a focal point for low
level convergence and pooling instability with MLCAPEs maxing at
1000-1500 j/kg.  While forcing aloft remains negligible over central
Indiana...the combination of subtle enhancement of instability and
convergence may be enough to generate isolated convection in the
vicinity of the boundary through early evening.

CAMs continue to pick up on this thinking and with that...confidence
is a bit higher to warrant the inclusion of a 20 pop through the
middle part of central Indiana through about 02Z with the I-70
corridor on the north and a Sullivan-Bloomington-Columbus line on
the south. It should be stressed that most locations within this
zone will likely not see a drop of rain...but could certainly see a
few short-lived showers and perhaps a storm.

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

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 /Tuesday Night Through Saturday/...
Issued at 205 PM EDT Sat Aug 19 2017

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

An upper trough will be across the area through much of the long
term, keeping temperatures cooler than average for late August.

Kept some lower PoPs Tuesday evening to account for any lingering
precipitation from an exiting cold front. Afterward, high pressure
will build in and provide dry conditions.


.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.





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