406 
fxus63 kdtx 071708 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
1208 PM EST Wednesday Jan 7 2009 


Aviation... 


The low pressure system over the eastern Great Lakes will continue 
to pull away from the region this afternoon. However...ceilings 
and vsby's will remain predominately in the MVFR category...with 
periods of IFR through the evening hours...as a surface trough 
dropping south from northern lower Michigan enhances the snow 
showers. The snow showers will end and the low levels will dry out 
behind this trough late tonight/early tomorrow morning (as winds 
shift to the northwest between 10-15 knots)...likely leading to 
developing VFR conditions during Thursday morning. 




&& 


Previous discussion...issued 730 am EST Wednesday Jan 7 2009 


Update... 


The Winter Weather Advisory will expire at 8 am as freezing 
drizzle has ended over the region. Periods of snow will continue 
through the morning but with little accumulation until later in 
the morning into this afternoon. This will allow icy conditions to 
improve on treated surfaces as temperatures hover around 30 or 
even rise toward the freezing mark. Highs near 34 still look 
reachable before the trough/front moves through during the 
afternoon and evening, especially during any lulls in the snow 
pattern. 


Previous discussion...issued 315 am EST Wednesday Jan 7 2009 


Short term... 


At press time, freezing drizzle continued in the Winter Weather 
Advisory area, generally along and south of a line from Port Huron 
to Howell. To the north, light snow has been well behaved. We will 
continue the Winter Weather Advisory through 8 am to cover the 
transition from freezing drizzle to snow, which is in progress from 
west to east and from south to north over the area and should be 
complete by 6 am. Light snow will then continue on the steady side 
through the morning and then become more intermittent during the 
afternoon. Accumulation will be an inch or less from the Ohio border 
up to Interstate 69 but could reach 2 inches in The Thumb by the end 
of the day. 


The benefit of satellite and radar observations illustrate the 
complex nature of this system when compared to model data and the 
subtlety involved in the determination of snow vs freezing drizzle. 
Satellite imagery illustrates the dry air in the middle levels 
resulting from the strongly sheared nature of the middle and upper 
level flow on the north flank of the upper jet. This dry air is then 
positioned over the broad, moisture filled surface low and trough 
over the south portions of Southeast Michigan. The middle level dry air 
forcing the lack of ice Crystal formation, combined with plentiful 
low level moisture and forcing, is supporting the freezing drizzle 
pattern. 


Satellite and radar imagery also support model data that show the 
freezing drizzle scenario breaking down before sunrise in favor of 
an all snow precipitation type for the rest of the day. This will be 
driven by the eastward advancement of the long wave trough that is 
carrying yet another small but intense wave that will move from the 
middle Mississippi Valley through the Ohio Valley today. This wave will 
support a new round of dynamic forcing that will intensify Theta-E 
advection in the 850 to 700 mb layer and result in a modest trowal 
structure along and north of Interstate 69 this morning through 
early afternoon. This is where the most persistent snow will be 
today with the tail end of the system pivoting back over the Detroit 
area during the afternoon before diminishing toward evening. 


Long term...tonight through next Tuesday 


Surface low pressure will lift from Lake Ontario into New England 
overnight. Surface troughing will also linger across central 
Michigan overnight...with low level wind convergence and 
baroclinicity increasing during the late afternoon and early 
evening. This area will develop where a thermal trough wrapping 
around the back of the system exists as well. Expect this region of 
forcing will bring an increase in snow activity first over the 
Tri-Cities and thumb this evening...and shift south and east 
overnight to between 1-69 and I-94. Snow amounts are expected to 
remain below 2 inches. Area of snow associated with the low pressure 
system will exit east tomorrow morning...leaving behind a fair 
scenario for lake effect as winds turn to the west northwest off of 
Lake Michigan. Models are keying in on a lake effect band developing 
along and south of I-94...which looks reasonable given flow and 
stability...and have increased probability of precipitation to high chance for this area. 


Surface high pressure will briefly build over the region late 
Thursday and Thursday night before the next low pressure system 
makes its way into the Great Lakes from the northwest. Models 
continue to struggle with how they handle this system...lending to 
much uncertainty with the forecast...especially for that of snow 
accumulations. Various solutions take the system on a west to east 
track Friday night...with the center passing anywhere from northern 
Michigan through the Tennessee Valley. A slightly stronger consensus 
and continuity exists between the GFS and the Gem...and will lean 
most closely to this solution...aside from the earlier arrival on 
the GFS. GFS/Gem take the center of the system along a line from 
Kalamazoo to Detroit. This will spread strong isentropic ascent 
under a coupled jet structure into lower Michigan late Friday 
afternoon...with the surface occlusion and deformation tracking 
across the County Warning Area overnight. Precipitation looks to be all snow...which 
may become heavy at times overnight. Although the swath of heaviest 
snow will be very sensitive to north or southward adjustments in the 
track...it looks as if we could see 2 to 5 inches of snow...with the 
heaviest swath between M-59 and I-69 before wrap-around 
precipitation ends Saturday morning. 


Models continue to indicate another clipper will dive down across 
the Great Lakes for the beginning of next week...but again...vary 
greatly with regard to timing and strength. This could bring another 
round of snow to Southeast Michigan. Will have to watch this system 
carefully...as some runs of the GFS/Gem/Euro have merged it with a 
southern stream system over the central Great Lakes by midweek. 


Marine... 


An area of low pressure centered near Western Lake Erie will 
continue to lift across the eastern Great Lakes today and tonight. 
With the low center so close to the Michigan waters today...winds 
will remain below 20 knots. However...as the low pulls away from the 
area overnight...northwest winds will increase...with gusts between 
20 and 30 knots per latest forecast soundings. Small craft/brisk 
wind advisories are now in effect for all nearshore waters starting 
this evening. Gusty winds will continue through the early afternoon 
on Thursday before high pressure allows them to subside. Winds will 
then remain fairly light through Friday before increasing once again 
on Saturday on the backside of another low pressure system. Small 
craft and brisk wind advisories may again be needed over the weekend 
for all nearshore areas...and winds could approach gale force 
across the open waters of Lake Huron. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 


Lake Huron... 
Small Craft Advisory...nearshore waters from Port Austin to Port 
Huron including outer Saginaw Bay...from 7 PM Wednesday to 1 
PM Thursday. 


Brisk Wind Advisory...inner Saginaw Bay...from 7 PM Wednesday to 1 
PM Thursday. 


Lake St Clair... 
brisk Wind Advisory...from 7 PM Wednesday to 1 PM Thursday. 


Michigan waters of Lake Erie... 
brisk Wind Advisory...from 7 PM Wednesday to 1 PM Thursday. 


&& 


$$ 


Aviation.....Sf 
update.......bt 
short term...bt 
long term....hlo 
marine.......hlo 




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