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Area forecast discussion 
National Weather Service northern Indiana 
656 PM EST Tuesday Dec 2 2008 


Aviation.../00z tafs/ 
strong low pressure over the northern plains and upper Midwest 
will continue to push east toward the region during the period. 
VFR conditions are expected through the majority of the 
period...with deteriorating flight conditions from west to east 
near the end of the period...as frontal zone pushes into the 
region. Perhaps the most important flight concern will be the 
development of strong southerly winds just off the surface in response 
to strong warm air advection associated with the approaching low pressure system. 
Sustained winds in the 1.5kft to 2kft will vary from 30 to 50 
knots through 09z at 220 degrees. A wind shear hazard may develop 
if a strong enough surface inversion develops allowing surface winds to 
back more easterly. Tightening pressure gradient is anticipated to 
curtail this scenario...but will monitor and amend if necessary. 
Ll winds will diminish some by middle morning. Otherwise...nsw 
expected until Wednesday afternoon/evening. 




&& 


Previous discussion... /issued 345 PM EST Tuesday Dec 2 2008/ 


Short term... 
amplifying upper air pattern next 36hrs as shrtwv's over Pacific northwest and 
Ontario dig/merge forming longwave trough over central U.S. By 12z 
Thursday. Surface cold front over northern plains this afternoon will strengthen through Wednesday 
night and Gulf moisture will be tapped as a result of the upper level 
trough phasing. Deep fgen forcing should cause a band of precipitation 
along and behind the cold front as it moves through our area. Precipitation 
will likely start as rain or a mix Wednesday afternoon... and then change 
over to snow from northwest-southeast as colder air moves rapidly into the area 
behind the front Wednesday evening. Expect snowfall with the front will generally 
be in the 1-3" range with highest amounts northwest due to an earlier 
changeover to snow. Cold air advection will allow lake effect snow showers to 
develop late Wednesday night over SW Michigan and North Central Indiana. Little 
additional accumulate expected from the lake effect late Wednesday night 
since Delta t's will only be in the teens with inversion rather 
low and drier air in low levels advecting into the area from the 
northwest. Even so... some slick roads are likely across the County Warning Area Wednesday 
night due to the freezing of wet roads as temperatures fall well below 0c 
overnight. 


As for tonight... low clouds being scoured out across western portion 
of County Warning Area by intially dry SW flow. Tight gradient overnight along 
with incrsg high clouds should keep temperatures from falling much and 
probably will rise a few degrees by morning. 


Long term... 
long term... 
Thursday through Tuesday. 


Active pattern to remain in place with several systems to move 
through the general northwest flow bringing chances for synoptic snowfall 
ahead and along each one and then lake effect potential in the wake 
as continued reinforcing shots of cold air arrive. 


First system will be departing the area with lake effect likely 
already underway at the start of the period as 850 mb temperatures drop 
into the -15 to -17 c range yielding Delta t's in the lower 20s. 
Saving Grace to a significant les event at this point is inversion 
heights remaining generally at or below 8500 feet...wind fields a bit on the 
strong side and trajectory more northwest then north-northwest/north has has been the case 
in the last few events. Trends suggest more of a multiple band setup 
with potential for one or 2 more dominant bands. Current thinking is 
amounts of 1 to 2 inches looks to be a good bet where bands persist 
the most. Have continued with trend of overnight shift and increased 
to 60 probability of precipitation and went with numerous wording. Main reason for holding 
back on any higher probability of precipitation is due to a bit of question as to exactly 
how trajectories will set up. Have left grids alone for Thursday night 
into early Friday as trajectory becomes even more problematic with 
bulk of snow showers possibly shifting more north of the area. 


Next system will quickly be moving towards the area possibly as 
early as Friday night. Models hinting at a slower arrival of 
things...but with such a fast moving flow...am inclined to hold 
things as it. Did increase probability of precipitation in the north areas closer to what 
appears to be the better area for precipitation chances as main low tracks 
north of the area and most precipitation associated with the cold front. Another 
round of lake effect will setup behind the system with a similar 
scenario to the one at the start of this period. Inversion heights are 
somewhat higher so bit more potential for concern of moderate 
accumulations. Too early to do much with this and severe weather potential statement has it in 
hand at this point so will leave things as is. 


Yet another clipper looks to move in towards the end of the period. 
Normally would be inclined to hold off on adding anything for probability of precipitation 
that far out but given pattern and collaboration...will go ahead and 
introduce low chance probability of precipitation Monday night into Tuesday. Precipitation type could come 
into question with European model (ecmwf) warmer in lower levels indicating potential 
for mixed precipitation. Depending on amount cover in some areas...may have 
to watch for risk for sleet or freezing rain situation but too early 
to do much more than monitor things. 


Temperatures will remain a good 10 to 15 degrees below normal much 
of the period with only modest warm ups ahead of each system. Again 
the degree of warming with these waves will be somewhat dependant on 
amount of snow cover so only minor changes. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...Small Craft Advisory until 10 am EST Wednesday for lmz043-046. 


&& 


$$ 


Short term...Taylor 
long term...Fisher 
aviation...Chamberlain