Weather

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fxus63 kdtx 060510 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
1210 am EST Tuesday Jan 6 2009 


Aviation... 


VFR conditions will prevail with clear skies and light winds during 
the night...replaced by a gradual increase in middle/upper level clouds 
at or above 12kft later this morning. Clouds will further thicken/lower 
late in the forecast as a disturbance approaches from the 
southwest. Even then...ceilings will remain lower VFR (aoa 5000 
feet) until 22z-00z before light snow begins to work into the area 
during the evening and ceilings/visibilities trend towards 
MVFR...and IFR by 06z into the overnight. 


&& 


Previous discussion...issued 328 PM EST Monday Jan 5 2009 


Short term...tonight 


Clear skies along with light/calm winds this evening/tonight will 
result in a cold night as the center of the weakening surface high 
slides through the Northern Ohio valley. Current surface dew points 
around 10 degrees looks to be a nice Center Point for tonight's 
mins. Went with the colder mav guidance and still undercut temperatures by 
a degree or two (based on 18z comparison vs actual obs)...resulting 
in middle single numbers over The Thumb region to middle teens south...as 
high clouds are expected to arrive by morning south of I-69...along 
with steady low level warm advection overnight (925 mb temperatures warming 
to around -4 c). 


Long term...Tuesday through next Monday 


The current water vapor imagery clearly shows the ingredients for 
the development of an eastern US storm system by middle week. A 
positively tilted long wave trough extends from the Central Plains 
through the Baja California peninsula...while an upper low is in place across 
the Gulf of Alaska. The southern stream trough is forecast to track 
into the Appalachians by Wednesday morning...while a middle level wave 
quickly moves from the Gulf of Alaska to the northern plains/upper 
Midwest. Overall...the 12z model suite is in rather good agreement 
with respect to surface and upper level features with this storm 
system. The exception is the NAM which is weaker and farther south 
with an associated surface low...which looks very suspect given the 
nature of the upper air pattern. Overall...model initializations 
were good. There was however a lack of good sampling of the upper 
low over the Gulf of Alaska which will lead to some degree of 
uncertainty in the strength and amplitude of the northern stream 
wave. 


Overall the consensus among the 12z GFS/Canadian/UKMET/ECMWF show 
the southern stream trough becoming sheared as it lifts across the 
Tennessee Valley Tuesday night...with the bulk of the moisture tracking 
across the southeastern US. Surface low pressure is still forecast 
to develop over Ohio Tuesday night...then track across Lake Erie Wednesday 
morning as the northern stream wave amplifies across the upper 
Midwest. The upper wave will then develop into a closed circulation 
by Wednesday afternoon...allowing the surface low to track across the 
eastern Great Lakes...with a lingering trough extending back 
across lower Michigan. While moisture will increase Tuesday 
night.../mixing ratios at 850mb forecast to rise to +3 g/kg/ large 
scale lift does not look to be overly impressive. Lower to middle 
level isentropic ascent will overspread southern Michigan 
Tuesday...persisting trough Tuesday night. Lift will then 
transition to some weak middle level deformation by Wednesday. 


It will take some time to overcome the dry air on Tuesday. 
Thus...have backed off on the chances of snow considerably Tuesday 
afternoon. The best isentropic lift along with some elevated 
instability should lead to some minor accumulating snowfall Tuesday 
night...with general weak deformation leading to occasional light 
snow into Thursday morning. Even though this is expected to be a 
long duration event...the majority of model ensemble members agree 
in total quantitative precipitation forecast remaining on the light site /generally anywhere from 
a tenth to quarter inch over the entire 36 hour period/. A general 
10-15 to one inch ratio supports total snowfall in the 2 to 5 inch 
range...a bit lower than earlier models and forecasts suggested. 
These lower snowfall totals over such a long duration do not 
warrant the need for headlines at this time. 


After brief surface ridging Thursday night...a fast moving northern 
stream wave is forecast to impact the region Friday afternoon 
through Friday night. This system will have good low to middle level 
isentropic lift within some upper level entrance region dynamics. 
This system warrants increasing probability of precipitation Friday afternoon into Friday night. 
While the GFS is rather bullish with quantitative precipitation forecast over Southeast Michigan...trends with 
this system have been farther north...which may actually end up 
taking the heaviest snow north of the area. An amplifying long wave 
trough over the eastern US in the wake of the Friday system will 
bring a blast of Arctic air into Southeast Michigan...with 850mb temperatures forecast 
to plunge to around -20c by Saturday afternoon. This will spell 
temperatures a good 15 degrees below normal toward the end of the 
forecast period. 


Marine... 


Have dropped all headlines...as winds along the nearshore waters are 
generally gusting to 20 knots or less. Weakening high pressure will 
continue to slide across the Northern Ohio valley tonight...allowing 
the pressure gradient over the central Great Lakes to decrease 
further. 


Easterly winds will then increase tomorrow...ahead of a deepening 
low pressure system...which will track northeast into the eastern 
Great Lakes on Wednesday. Cold northwest flow behind this system 
will result in gusts of 25 knots or greater across all marine areas 
Wednesday night into Thursday. Winds will slowly decrease on Friday 
as a ridge of high pressure builds into the area by Friday night. 


&& 


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


&& 


$$ 


Aviation.....Dg 
short term...sf 
long term....SC 
marine.......sf 




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at www.Weather.Gov/Detroit (all lower case).