Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 060451

Area Forecast Discussion
National Weather Service Indianapolis IN
1150 PM EST Mon Dec 5 2016

The Aviation Section has been updated below.


Issued at 720 PM EST Mon Dec 5 2016

A system will approaches from the southwest late tonight bringing
chances for precipitation to the area. Precipitation will generally
be in the form of rain with this system except for some snow
initially mixing in at times across the northern counties late
tonight.  The rain will move out rather quickly tomorrow afternoon
though and another lull will commence through Wednesday. At that
point, yet another system will bring much colder air and a snow
threat Wednesday night/Thursday morning. Light snow accumulations
will be possible with this system.
Further out, high pressure will result in dry conditions into the
weekend before the next system brings additional chances for rain
and snow late in the weekend.


.NEAR TERM /Rest of Tonight and Tomorrow/...

Issued at 720 PM EST Mon Dec 5 2016

Clouds will increase from the south tonight as a low pressure system
traverses the southeast U.S. Precipitation will also accompany this
system, moving in after Tue 06Z from the southwest as isentropic
lift strengthens. Precipitation will generally be in the form of
rain, but some snow could initially mix in across the northern
counties during the night. All precipitation should be rain by
daybreak tomorrow though and remain that type through the duration
of the event. Highest pops will be across the southern/eastern
portions of the forecast area.

Time cross sections suggest a rapid drop in moisture in the mid
levels starting at Tue 18Z, so will continue to trend toward dry
conditions across western and central portions of the forecast
area after that. Meanwhile, slight chance pops across the eastern
counties remain reasonable through the early afternoon hours.

Temperatures tonight will range from the low 30s across the northern
counties to upper 30s across the southern counties and will occur
fairly early as temperatures will increase during the overnight due
to warm advection with the isentropic lift. Highs tomorrow will
remain above normal with readings in the low to mid 40s.


.SHORT TERM /Tomorrow Night through Thursday/...

Issued at 142 PM EST Mon Dec 5 2016

A bit of a lull in the active pattern will resume tomorrow night,
resulting in dry conditions through Wednesday. The pattern will
change drastically by Wednesday night though as much colder air
filters into Central Indiana with the next system. Daytime highs
starting Thursday will only be in the 20s with overnight lows on
Thursday night in the upper teens.

As far as precipitation chances, it looks like models are now
trending toward the drier GFS. Confidence is high that any
precipitation that falls with this system will be in the form of
snow, but accumulations will be low with amounts around one-half
inch to 1 inch from Wednesday night into Thursday.


.LONG TERM /Thursday night through Monday/...

Issued at 125 PM EST Mon Dec 5 2016

ECMWF depicts a bit of more settled pattern during this run than
its previous run. Models vary a bit on timing of late weekend
weather system and GFS has come in colder and wetter than previous
run. Confidence is low on precipitation outcome given this
variability but upcoming runs hopefully will better align with
each other.

Flow aloft depicts short wave trough embedded in eastern Canadian
low rotating across the Great Lakes Friday with snow showers in
vicinity of the Great Lakes. More zonal flow with embedded weak
wave passes across the Great Lakes on Saturday Night, moistening
the atmosphere, but for moment, generating no precipitation.

Onward in time, Canadian is a bit more progressive with arrival
of more substantial wave Saturday night while GFS exhibits fairly
strong over running Saturday Night. ECMWF, later still, shows
over running by Sunday. Upshot of all this will be precipitation
can be expected Sunday, but timing, the amount of warm-up before
precip onset, and precipitation type are still all in question.
For now, expectation is for a start-up as snow by or before
daybreak Sunday and transition to snow/rain mix Sunday afternoon
as warm air continues to infiltrate. Latest GFS,being colder,and
now with a bit more moisture, develops several inches of snow
before transitioning to mix though ensembles range from slight
snows in the north only, to widespread 2"-4" across most of the
forecast area. Again, this far out in time, several adjustments
can be expected.

As for temperatures...coldest of the season to start Friday
morning with gradual moderation through the forecast period,
but overall, temperatures will remain below normal this period.


.AVIATION /Discussion for 060600z TAF issuance/...

Issued at 1150 PM EST Mon Dec 5 2016

VFR to begin at the sites with only some cirrus overhead and light
ESEly winds. High pressure is exiting to the east and isentropic
lift is entering the area from the southwest. Cloud decks will drop
during the overnight and by around 11z MVFR ceilings should arrive
at KBMG and KHUF and the other sites thereafter. Rain should start
around the same time as well and continue through mid morning to
early afternoon when it swings out of the area and dry air behind
the system starts to mix ceilings back up a little. During the best
lift (in the morning hours) should see ceilings drop to IFR and
could see vsbys there as well, but more uncertainty on the vsbys. Up
at KLAF, rain probability is low enough to just go with VCSH and no
explicit rain mention. If precip does occurs at KLAF, could be a mix
with snow in the early morning hours before column warms enough in
the north for all rain. VFR conditions should return by around 0z if
not before.




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