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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
927 PM EDT Sat Mar 20 2010 


Update... 


Observations support earlier expectations of a diminishing trend 
in the pattern of snow tonight. As mentioned in the day shift 
discussion, this weakening trend will be the result of weakening 
middle level frontal forcing above 850 mb through the rest of the 
night. Cross sections from the latest model data show the front 
becoming weaker judging from the gradient of Theta-E and this 
weakening is strongly associated with the entrance region of the 
upper jet departing into eastern Canada and the resulting loss of 
sustaining ageostrophic flow. The cross sections also indicate very 
low stability, and even some instability, in the 700 to 500 mb 
layer which will continue to result in periodic narrow flare ups 
in the otherwise broad area of weakening lift. This stability 
regime is rooted closer to the surface front over Ohio and Indiana 
and is supporting pockets of convection which could move/develop 
into our area during the night, even if the pattern of snow over 
our north is reduced to flurries by then. Therefore, still expect 
at least a chance of rain/snow showers in the Detroit area and 
points south. The pattern of low stability/instability is shown to 
linger through Sunday, subjecting our area to additional rain/snow 
shower development but on a more disorganized basis due to the 
inconsistent forcing for lift. Our going forecast has these 
overall trends covered well with only cosmetic adjustments to the 
database anticipated for the rest of the night. 


&& 


Previous discussion...issued 705 PM EDT Sat Mar 20 2010 


Aviation... 


A broad area of light snow, with pockets of heavier snow showers, 
will persist through this evening as middle level frontal forcing 
continues over Southeast Michigan. This pattern will have a slow southward 
drift with varying snowfall intensity, as exhibited by the flare up 
of a band over SW lower at press time, before a slow weakening trend 
gets underway. Pockets of IFR visibility, mainly in the fnt area, 
will give way to VFR before reaching the Detroit area toward 
midnight. Just flurries are then expected south of Flint through the 
early morning hours under MVFR ceiling. From Flint north, there will 
be enough dry air in the low levels to improve or maintain VFR 
ceiling through the night. There is some concern for another flare 
up of snow south Flint around sunrise Sunday morning. This is model 
indicated and confidence is not high enough to include a new 
restriction in this forecast package but this trend will be 
monitored for possible later inclusion. After that, the very slow 
southward progression of the moisture pattern will allow improving 
conditions at all terminals during Sunday afternoon. 


Previous discussion...issued 354 PM EDT Sat Mar 20 2010 


Short term...through tonight 


Quasi-stationary h850 frontal boundary remains in place over 
Southeast Michigan this afternoon. Moisture streaming up along this 
feature has allowed steady snowfall to occur today north of a line 
from Lansing to Flint to Port Sanilac. Area of snowfall is just 
starting to expand southward...as the frontal boundary drops very 
slowly to the South. Strong frontogenetical forcing has been present 
along the front today...and has been evident by the banded look to 
snow on area radars. GFS centers the fgen right at 850mb...and shows 
a marked decrease in this forcing after 00z tonight. This looks very 
reasonable as a building ridge of surface high pressure over 
northern and central Michigan will allow for a decrease in winds at 
850 mb. 


Expect the h850 frontal boundary to remain active through the 
evening as it sags very slowly south...with a gradual decline in 
activity overnight as convergence along this boundary wanes. Chances 
for snow remain very high north of a line from Pinckney to Pontiac 
to St Clair...with a marked decrease in probability of precipitation as you go towards the 
Ohio border. Most of the area will see all snow...however a 
combination of rain and snow is possible from the northern suburbs 
of Detroit to the Ohio border. Additional snow amounts will be 
fairly light...less than an inch. Reports and web cams across 
Southeast Michigan also indicate that snow accumulations have been 
limited to elevated and grassy surfaces. 


Long term...Sunday through Saturday 


The upper level pattern will remain split much of the week with the 
Central Lakes largely falling in between the two main storm tracks. 
The southern stream will provide the most significant system for the 
Continental U.S. During the period as the large upper low over the Southern 
Plains tracks through the deep south and on up the eastern Seaboard. 
For Southeast Michigan...little has changed with respect to this low 
as there remains just a small chance of rain on the southeast fringe 
of the area as this system begins its turn to the northeast early in 
the period. 


During this time frame...extending from Sunday into at least parts 
of Wednesday...the northern stream jet will remain north of the 
Great Lakes and allow for moderating temperatures...from the middle to 
upper 40s Sunday to the middle/upper 50s Tuesday and perhaps again on 
Wednesday. 


Once the southern stream storm system lifts into far eastern Canada 
and eventually phases with the northern stream to produce a rather 
powerful storm east of Hudson Bay...the northern stream begins to 
assert itself more over the region as several shortwaves passing 
just north of the region drive cold fronts through the region and 
bring cooler readings...back into the 40s for highs by Friday and 
perhaps only upper 30s in some locations by the weekend. With the 
predominantly northwest upper flow during the period...any chances 
for precipitation appear to be minor as a general lack of notable 
moisture prevails. 


Marine... 


A strong ridge of high pressure will build over the northern Great 
Lakes tonight and hold through Monday. This will bring a return to 
quiet conditions over Lake Huron. A cold front will remain stalled 
near the Ohio border during this time frame...as a low pressure 
system lifts into the southern Ohio Valley. This will keep chances 
for rain and snow over the southern Great Lakes...and allow 
northeast winds to continue at 10 to 15 knots. Gusts are expected to 
remain below Small Craft Advisory criteria across Western Lake Erie 
and Lake St Clair. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 
Lake Huron...none. 
Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Update.......bt 
aviation.....Bt 
short term...hlo 
long term....dg 
marine.......hlo 




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