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Area forecast discussion
National Weather Service Indianapolis in
632 PM EDT Mon Oct 23 2017

the aviation section has been updated below.


issued at 236 PM EDT Mon Oct 23 2017

Low pressure over Indiana will continue to bring clouds and rain
showers to central Indiana tonight. The low is expected to drift
northeast through Tuesday night...keeping clouds and rain chances
in the forecast.

Dry air is expected to arrive in the area on Wednesday morning
through Thursday as Canadian high pressure settle across the
area. This will result in more below normal temperature and
perhaps some frost on Wednesday morning.

Another frontal system and reinforcing shot of cooler air will
come late in the week, bringing additional chances for
precipitation and continued cool conditions.


Near term /tonight/...

Issued at 236 PM EDT Mon Oct 23 2017

Surface analysis early this afternoon shows low pressure over
Indiana. Radar shows areas of light rain showers circling across
the state. Aloft water vapor showed a deep negatively tilted
trough stretching from the upper Midwest...across Indiana to the
south eastern states. Water vapor shows the trough axis across SW
Indiana...pushing northeast and poised to bring some drier air
into the state from the southwest. This dry air may bring a short
break in the precipitation this evening...however broad cyclonic
lower level flow and wrap around moisture is expected to return as
the surface low moves northeast overnight.

Forecast soundings continue to shows a saturated column
tonight...particularly overnight as wrap around moisture arrives.
Time heights also show a deeply saturated column with best lift
arriving after 06z. Thus will trend pops toward highest values
overnight. Strong cold air advection continues overnight as clouds
and rain are expected to linger. Thus will again trend toward
expected wet bulb temperatures.


Short term /Tuesday through Thursday/...

Issued at 236 PM EDT Mon Oct 23 2017

Water vapor images this afternoon show a secondary trough and
short wave over the Dakotas...pushing southeast within the flow
aloft. This reinforcing wave will keep the large upper trough in
place across Indiana on Tuesday and allow for more showers
chances. Models suggest the surface low moves to the
northeast...however broad cyclonic flow looks to remain across
Indiana...keeping plentiful clouds around. Forecast soundings and
time heights keep good lower and mid level moisture across the
area albeit with weaker lift. Forecast soundings show favorable
lapse rates on Thursday...with convective temps only in the upper
40s. Thus given the arrival of the next wave...wrap around
moisture...and cyclonic flow will trend toward higher pops on
Tuesday...particularly across the northeast where the set-up will
be best. Again given clouds and rain will trend highs at or below
forecast builder highs.

By Tuesday night...forecast soundings start showing dry air
intrude into the column. Aloft...the broad trough axis begins to
drift east...exiting the region as highly amplified northwest flow
begins to stream into Indiana in its wave. Models continue to
suggest another short wave arriving on Wednesday...but best
forcing appears to be across the Great Lakes to the
northeast...where the trough axis has moved. Thus will keep some
chc pops on Tuesday evening for the lingering departing
showers...but will trend toward a dry forecast overnight and on
Wednesday as ridging aloft and drier northwest flow begins to take shape
over Indiana. Temps on Wednesday morning should be cold enough
for frost...however the expected pressure gradient may result in
continued mixing preventing frost formation. Will need to wait
and see on this for now. Will stick close to the forecast builder
blend on temps.

The GFS shows the northwest flow transitioning to westerly by Thursday
with little in the way of dynamics across the area. Forecast
soundings again show a dry column and ridging is apparent within
the lower levels. Thus will trend toward a dry forecast on
Wednesday night and Thursday...with temperatures near the forecast
builder blend.


Long term /Thursday night through Monday/...

Issued at 243 PM EDT Mon Oct 23 2017

Weak upper level ridging will begin to break down Thursday as
a cold front moves into central Indiana late Thursday into Friday.
The aftermath of the front will bring the coldest temperatures of
the fall so far with the first chance for a freeze across much of
the state. Lows are expected to fall to at or below freezing on
both Friday night and Saturday night. The axis of the trough
associated with the passing cold front has shifted further east
compared to earlier model runs which now indicates that the best
chance for precipitation will be late Friday into Saturday morning
and across eastern Indiana. The season's first snow shower may be
possible early Saturday morning, but with warm surface
temperatures, no accumulation is expected. Dry weather is expected
through the rest of the period with only a Few Lake effect
showers across northern Indiana. High temperatures will only reach
into the upper 40s and lower 50s through the period, and lows
will generally be in the low to mid 30s.


Aviation /discussion for the 240000z tafs/...
issued at 631 PM EDT Mon Oct 23 2017

Short term model guidance suggest deformation zone currently
pushing into western Indiana will be east of the terminals by
240100z. As a result, rainfall should diminish by that time, as
well as rapid improvement in ceilings.

Secondary upper trough near the Mississippi River is progged to
move over the terminals roughly 240200z-240800z, so there may be
another period of enhanced shower potential at that time. It
appears ceilings probably won't be as low with this secondary wave as
they are now, given current surface observations in that area.

In the wake of the secondary wave, ceilings should settle in the
015-030 range.

Surface winds 270-300 degrees at 8-13 kts early this evening will
gradually back around to 220-250 degrees after 240600z.


Ind watches/warnings/advisories...


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