Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 052318

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

The AVIATION Section has been updated below.


Issued at 142 PM EST Mon Dec 5 2016

A lull in the rather active pattern will prevail today with high
pressure at the surface and zonal flow aloft.  However, the dry
period will be short-lived as the next system approaches from the
southwest tonight.  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 /Tonight and Tomorrow/...

Issued at 142 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, intensifying after Tue 06Z 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
southeast/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 afternoon hours.

Temperatures tonight will range from the low 30s across the
northern counties to upper 30s across the southern counties. Highs
tomorrow will remain above normal with readings in the low to mid


.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 060000z TAF issuance/...

Issued at 610 PM EST Mon Dec 5 2016

VFR to begin at the sites with only some cirrus overhead and light
ESEly winds. During the night high pressure will exit to the east
and isentropic lift will enter 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 shortly thereafter 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. Should note that if precip occurs there, could be a
mix with snow in the early morning hours before column warms
enough in the north for all rain.




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