Small Craft Advisory 97, NWS Northern Indiana

Area Forecast Discussion
National Weather Service Northern Indiana
Issued by National Weather Service Indianapolis IN
608 AM EST Thu Dec 24 2020

.SYNOPSIS...
Issued at 230 AM EST Thu Dec 24 2020

A strong cold front will exit the region this morning. Allowing
high pressure and very cold air to arrive tonight and on
Christmas. The cold air and warm Lake Michigan temperatures will
result in the development of accumulating lake effect snow tonight
across southwest Michigan and Northwest Indiana. Thus snow will be
expected downwind from Lake Michigan through Christmas.

High pressure is expected to settle across the Deep south on
Friday Night. This will bring an end to any snow as relatively
warmer air arrives across Michiana...resulting in dry weather for
Friday Night through the rest of the weekend.

Chances for precipitation are expected to return on Sunday night
and again on Wednesday as a pair of weather systems are expected
to push through the Great Lakes.

&&

.SHORT TERM...(Today and Saturday Night)
Issued at 230 AM EST Thu Dec 24 2020

Surface analysis early this morning shows a strong cold front
stretching across Central MI...eastern Indiana to Central KY. The
front extended from deep low pressure over Lake Superior. Radar
shows rain showers exiting the area and broad cyclonic flow was
filling in across the region. GOES16 shows extensive cloud cover
across the region. Aloft a deep trough was in place over the
upper midwest stretching to the southern plains. Very cold air
was found upstream over the upper midwest with surface temps in
the single digits or lower.

Ongoing cold air advection is expected today a showing 850mb temps
falling to near -20C by tonight. This sets up a very favorable
situation fro LE snow with favorable deltaTs. As cold air arrives
this afternoon forecast soundings downwind of Lake michigan
suggest favorable moisture and instability. Best fetch however
does not appear to begin until near 00Z as the mean lower level
wind becomes more westerly. HRRR suggests LE snow development by
late afternoon with the arrival of cold air. Thus will trend
toward a cloudy sky morning with a stray flurry possible within
the cyclonic flow but the main accumulating snow downwind of the
lake starting this afternoon. Given our strong cold air advection
will trend for steady or slowly falling temps through the day.

Tonight forecast soundings suggest West winds initially before
shifting to the northwest by Friday morning. This should result in
widespread cellular lake effect snows downwind of LM...mainly for
Barrien and Cass counties initially. As the wind field shifts
more northwesterly overnight...snows should shift southeast.

On Friday the dome of cold air shifts to east and delta T's rise
to unfavorable levels for LES. This should be a slow progression
of the end of LES during the day on Friday. ECMWF shows strong ridging in place
across the western plains states and NW flow with subsidence will
be in play.

When Complete...Accumulations should be in excess of 6 inches in
Barrien and Cass counties...thus will upgrade to warning there.
With lesser amounts expected south and farther downwind.
Will hold onto advisory for now at those spots.

By Friday Night ECMWF shows strong ridging in place across the
western plains states and NW flow with subsidence will be in play.
Forecast soundings show a very dry column in place on Friday
Night through Saturday night with subsidence and trapped
stratocu. Thus will continue with mostly cloudy skies and given
the stratocu...a stray flurry cannot be ruled out.

&&

.LONG TERM...(Sunday through Wednesday)
Issued at 230 AM EST Thu Dec 24 2020

A northern stream upper trough will move across the area Sunday
night into Monday, with a surface low moving just northwest of the
area. Moisture will be a concern with this system though. Will just
go with some low PoPs mainly Sunday evening for now. At the moment
it appears that rain would be the primary precipitation type, but
some snow may mix in if temperatures cool enough.

High pressure and a brief period of upper ridging will then provide
dry weather for Monday and into much of Tuesday.

An upper level low and surface low pressure system will then
approach the area to end the long term. Differences remain in the
speed of the system, which leads to higher uncertainty on PoPs. Warm
advection ahead of the system could bring some light precipitation
as early as Tuesday afternoon, but confidence is low. By
Wednesday though, the system will be close enough for chance
category PoPs.

How fast the precipitation moves in will determine precipitation
type as cold air will rule still on Tuesday. Will have snow for now
on Tuesday afternoon into Wednesday morning, then when the highest
PoPs arrive for Wednesday, will go with chances for rain and snow as
warmer air moves in.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday Morning)
Issued at 608 AM EST Thu Dec 24 2020

(For the 06Z TAFS through 06Z Thursday Night)
Issued at 1209 AM EST Thu Dec 24 2020

Widespread upstream MVFR Cigs should continue to fill in/persist
across NRN Indiana today.

As the Cold front continues east through Ohio and strongest
pressure rises shift eastward...look for wind speeds to gradually
diminish by midday/afternoon with lake effect snow gradually
ramping as better collocation of lake induced instability/vertical
motion and time overnight.

Anticipate lowering visibilities in show showers at SBN to IFR
late in the forecast period. Limited inland extent of snow bands
to likely maintain little more than flurries at FWA.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory from 4 PM EST /3 PM CST/ this afternoon
     to 4 PM EST /3 PM CST/ Friday for INZ003>005.

MI...Winter Weather Advisory from 4 PM this afternoon to 4 PM EST
     Friday for MIZ079.

     Winter Storm Warning from 4 PM this afternoon to 4 PM EST Friday
     for MIZ077-078.

OH...NONE.
LM...Gale Warning until 7 AM EST this morning for LMZ043-046.

&&

SYNOPSIS...Puma
SHORT TERM...Puma
LONG TERM...Swain
AVIATION...Murphy/JP

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