Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 211044
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
644 AM EDT Mon Aug 21 2017

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

&&

.SYNOPSIS...
Issued at 320 AM EDT Mon Aug 21 2017

An approaching cold front and upper disturbances will interact with
a very warm, moist and unstable atmosphere and result in unsettled
weather over all or parts of central Indiana through Tuesday
evening. The best chance for widespread thunderstorms will be
Tuesday. After that, broad surface high pressure will build in and
bring dry and much cooler weather to the area.

&&

.NEAR TERM /Today/...
Issued at 642 AM EDT Mon Aug 21 2017

Lowered morning pops to 20 percent as convection there has died with
the passing of the upper wave.

Previous discussion follows...
Issued at 320 AM EDT Mon Aug 21 2017

Unsettled weather may have an impact in viewing the eclipse today
over all or parts of central Indiana and will be the main focus to
the near term.

There are currently two main areas of thunderstorms bearing
watching. The first is clusters of strong storms that were dropping
southeast just west of Tipton. These cells were triggered by the
upper wave moving over the lower Great Lakes which was seen nicely
in water vapor imagery. The second area of thunderstorms of concern
were blowing up over western Iowa in a convergent zone along a
stalled front and and being fed by the nose of a 40 knot low level
jet. These storms are almost certain to congeal into an MCS. Could
see fairly extensive cloud cover today due to debris from the MCS
and also from the potential for storm redevelopment over central
Indiana as outflow boundaries from the current convection and weak
upper impulses could be enough to spark more storms today. In fact,
the HRRR, NAM12 and GFS all develop an area of storms near and north
of Interstate 70 in between 2 and 3 pm, which coincidentally is in
the window of time when the sun will be most blocked by the moon.
This time looks reasonable based on convective temperatures in the
mid 80s.

South of I-70, model 700 millibar temperatures of around 10 degrees
Celsius suggest a cap will be in place. With only weak waves and
potential outflow boundaries, the cap will be difficult to break.
Thus, went with only isolated thunderstorms there compared to good
chance pops north. Bottom line, it does not appear conditions will
be optimal for eclipse viewing, especially north of 70, unless the
Iowa MCS debris can mix out early and convective re-development
holds off until after 230 PM.

Cirrus and re-newed convection suggest temperatures may be a degree
or two cooler than MOS and the blend and especially in between 2 and
3 pm, when temperatures could drop a couple of degrees, even if
cloud cover is less than expected. Still, highs in the mid and upper
80s north to the lower 90s south look good.

&&

.SHORT TERM.../Tonight through Wednesday night/
Issued at 320 AM EDT Mon Aug 21 2017

Main focus for the short term will be on thunderstorm coverage,
timing and impact.

Models in good agreement that a cold front will sweep southeast
across central Indiana Tuesday night. The front and an upper wave
will interact with deep moisture and instability and good confidence
that widespread thunderstorms will develop. 30 to 50 knots of deep
layer shear, ML CAPEs to 1000 J/KG and and precipitation loading
suggest damaging winds are possible Tuesday afternoon and evening
over south central sections per the SPC DAY2 Severe Weather Outlook.

Models in good agreement that surface high pressure will build in
behind the front and suggest the storms will be south of the
forecast area by 06z Wednesday with dry and much cooler temperatures
to follow. The blend looked to have a good handle on temperature
trends and thus lends to high confidence in temperatures if not
quite as much in precise numbers.

&&

.LONG TERM (Thursday through Sunday)...

Issued at 150 AM EDT Mon Aug 21 2017

Confidence is high the period will be dry with below seasonal
temperatures. How far below seasonal is somewhat uncertain.

The main models all have high pressure providing dry weather and
relatively cool Canadian air to central Indiana.  There is some
uncertainty about temperatures because the American models are more
aggressive than the European about delivering cool air. Today`s
forecast will be based on consensus, with possible errors of 2-4
degrees.

&&

.AVIATION /Discussion for 211200z TAF Issuance/...
Issued at 644 AM EDT Mon Aug 21 2017

Confidence is good that two or more waves of thunderstorms will
threaten the northern terminals through the TAF period with the most
likely timing 17z-01z and after 09z Tuesday. On the otherhand, more
likely that BMG escapes with VFR conditions and no thunderstorms.
The other sites could briefly see MVFR or worse conditions in
storms. However, timing not precise enough for more than VCTS.

Winds away from storms will become south and southwest less than 10
knots.

&&

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

$$

SYNOPSIS...MK
NEAR TERM...MK
SHORT TERM...MK
LONG TERM....JK
AVIATION...MK



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