National Weather Service Raw Text Product

Displaying AFOS PIL: AFDIWX Received: 2017-12-07 11:58 UTC

FXUS63 KIWX 071158

Area Forecast Discussion
National Weather Service Northern Indiana
658 AM EST Thu Dec 7 2017

Issued at 435 AM EST Thu Dec 7 2017

Lake effect snow is expected today, mainly near and north of US 20.
Accumulations of 1 to 2 inches are possible. Dry conditions are 
expected tonight and Friday but another system will bring 
widespread snow to the entire area on Saturday. Locations outside
of the lake effect snow belt region will likely see 1 to 3 inches
of snow while the snow belt could see 6 or more inches of
accumulation before the snow ends Saturday night. 


.SHORT TERM...(Today and Tonight)
Issued at 435 AM EST Thu Dec 7 2017

Slower arrival of short wave and surface trough has delayed veering 
winds and onset of lake effect snow. Regional radar mosaic showing 
leading edge of pcpn now entering far northern forecast area as 
winds gradually shift northwest. However...upstream obs indicate 
winds already beginning to back over eastern WI with dew points 
dropping into single digits. WNW trajectories and drier air 
entrainment should help offset larger delta T values and modest 
instability this morning. Shrinking window for optimum lake effect
snow should also help keep amounts in check with 1 to 2 inches 
still possible today and maybe a localized 3 inch in extreme
northern areas of our Michigan counties. Flow slowly backs 
westerly this afternoon with inversion heights dropping so snow 
showers should become confined to Michigan with a weakening trend.
Clearing works in tonight away from flow off the lake. Gradient 
winds to remain in the 5 to 10 mph range which will help keep 
boundary layer mixed. Despite this low temperatures expected to 
still drop into middle and upper teens with cold airmass in place.


.LONG TERM...(Friday through Wednesday)
Issued at 435 AM EST Thu Dec 7 2017

Active and cold pattern continues through the long term. Main 
focus on clipper system Friday night into Saturday with lake 
enhancement and transition to lake effect likely yielding 
measurable snow for entire area and some heavier amounts closer to
Lake Michigan. 

Blocked pattern continues with upper low remaining over Hudson Bay 
through the weekend with staunch ridge over west coast and active 
jet squeezed between the two. Pacific energy will top ridge Friday 
and dive south towards the Midwest Friday night. Left front region 
of 500mb 100 kt jet streak will provide good upper level divergence 
while sharpening mid level trough pivots across the area Saturday. 

Brief period of isentropic lift develops in warm air advection 
regime late Friday night and this should help spread light snow 
across western two thirds of the area before daybreak Saturday. As 
weak surface low drops into Michigan Saturday morning, low level 
flow will become northwest and period of moderate lake enhanced snow 
expected in the northwest while light snow spreads across remainder 
of forecast area. General accumulations of 1 to 3 inches look likely 
away from the lake from synoptic system.

The snow belt region should see a good snowfall from this event 
while long fetch and strong flow will likely yield several inches 
well inland as well. Low level winds veer northerly Saturday 
morning with convergent signature down axis of Lake Michigan with 
upstream pre-conditioning off of lake Superior. Delta Ts not 
extreme but do climb into upper teens and eventually low 20s by 
Saturday afternoon. 0-2km theta e lapse rates are impressive 
between -2 and -3 K/km Saturday over southern Lake Michigan while
inversion heights remain above 10kft most of day. Strong lift is 
centered within saturated DGZ through the day as well and snow 
ratios about 20:1 expected. These ideal factors should lead to a 
rather intense dominant single band for much of the day with 
snowfall rates of 1 to 2 inches easily attainable. Blowing snow 
will also be a concern as momentum off the lake will bring 15-30 
mph winds well inland. About the only questions...assuming models 
continue with these solutions...are whether band will be 
transitory and spread the snow around or remain more stationary 
with concentrated amounts. Also some concern for the daytime 
timing as we often see partial disruption to organization with 
low daytime heating and mixing. For now have painted a rather 
broad swath of heavier amounts on Saturday and carried these well 
inland given strength of flow and length of fetch which 
climatologically favor areas as far inland as Marshall and western
Kosciusko counties. These amounts will likely be narrowed down to
a smaller swath once higher resolution models come into play. 

Expecting headlines will eventually be needed for this event, at 
least closer to the lake where heavier amounts expected. Since this 
will be the first widespread synoptic snow of season, advisory may 
be needed away from lake as well. For now will play up in HWO and 
weather story. Thankfully this snow will be falling on a Saturday 
with fewer vehicles on the road. Travel conditions along interstates 
over NW Indiana to see some significant impacts with combination of 
heavy snow, low visibilities and strong winds.   


.AVIATION...(For the 12Z TAFS through 12Z Friday Morning)
Issued at 651 AM EST Thu Dec 7 2017

Lake delta T values were near 17C early this morning and were 
favorable for lake effect snow, but with a marginal fetch, 
conditions at SBN will not be optimum for lake effect snow during
this TAF period. An initial round of snow dropped the visibility 
at SBN below 2 miles. Conditions should improve later today past 
the best window for lake effect snow as winds back a little more
to the west.


LM...Small Craft Advisory until 10 PM EST Friday for LMZ043-046.



SHORT TERM...Lashley 
LONG TERM...Lashley 

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