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


Short term...tonight 


Although our low pressure system over the eastern Great Lakes is 
pulling away from the area...snow showers are expected to continue 
into tonight. In fact...expecting an increase in the intensity of 
the snow showers through this evening into the early morning 
hours as the column/airmass cools (especially at 700 mb)...helped 
out by the strong upper wave/cold pool diving southeast from 
Iowa...which is already allowing for some enhancement off Lake 
Michigan. Low level westerly flow will increase out ahead of a 
surface trough...which will be dropping south from northern lower 
Michigan...serving as a focus with the low level convergence 
allowing for at least moderate snow showers. Thinking most of 
Southeast Michigan will receive between 1 and 2 inches. As long as 
the surface trough/Lake Michigan bands remain transient or 
progressively moving south through the night...the higher end 
accumulations should hold or remain below 3 inches. The best 
chance for 3 inches looks to be around the I-94 corridor 
(especially in Washtenaw county). No changes to mins (20 to 25) 
tonight as they are in line with mav guidance. 


&& 


Long term...Thursday through next Wednesday 


The Interstate 94 corridor snow band will be going on Thursday as 
winds pick up moisture off Lake Michigan. While the best moisture 
and snowfall amounts will be across southwestern Michigan...areas 
along Interstate 94 could see an inch of snow on Thursday. 


After a brief break in the action another low pressure system will 
affect the central Great Lakes Friday and Saturday. Despite the 
system being relatively close in days 3 and 4...the models continue 
to struggle to latch onto a unified solution regarding the track of 
the low. The NAM seems to be the odd model out taking the low 
through southern Michigan while the GFS/ECMWF/Canadian take the low 
through the Ohio Valley. This southern track will mean the snowfall 
amounts and snow chances will be adjusted south of the current 
totals...therefore will adjust amounts and probability of precipitation accordingly. The 
models also differ with quantitative precipitation forecast as the European model (ecmwf) dumps almost 0.6 inches of 
quantitative precipitation forecast across southeastern Michigan with closer to 0.4 inches with the 
GFS. Given the model disagreements will lean towards the lower quantitative precipitation forecast 
amounts as a further south track will lower amounts further. With 
this track and amounts around 4 inches of snow is possible across 
the central and southern County Warning Area...with 1 to 3 inches across the 
northern County Warning Area away from the best moisture. 


A very cold airmass comes on the heels of a low pressure system 
moving through the Great Lakes on Monday. 850 mb temperatures fall into the 
middle 20s below zero by Tuesday as 1000 mb to 500 mb thickness 
heights fall below 500 dam. This cold air mass will bring 
temperatures 15 to 20 degrees below normal as highs fail to reach 20 
degrees by Wednesday. Given the agreement of the European model (ecmwf) and GFS 
regarding this cold air will adjust the temperatures down from the 
previous forecast during these periods. 


&& 


Marine... 


Departing low pressure over the eastern Great Lakes this afternoon 
will allow strong northwest winds to kick in tonight...with gusts 
around 25 knots. Gusty winds will continue through the early 
afternoon hours 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. 


&& 


Previous discussion...issued 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. 


&& 


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. 


&& 


$$ 


Short term...sf 
long term....kurimski 
marine.......sf 
aviation.....Sf 




You can obtain your latest National Weather Service forecasts online 
at www.Weather.Gov/Detroit (all lower case).