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Area forecast discussion...update 
National Weather Service Marquette Michigan 
1233 PM EST Wednesday Jan 7 2009 


Updated for the 18z taf issuance 


Update (issued at 1100 am est)... 


A grid/zone forecast product update has been sent. This incorporates a slight warmup 
to our high temperatures today...and increased probability of precipitation with light 
accumulation snow occurring across much of the County Warning Area today. Another 
change includes adding more definition to the probability of precipitation tonight...with 
100 percent over Ontonagon and Gogebic counties given ll convergence 
and colder air moving in aloft. 


&& 


Discussion (issued at 445 am est)... 
WV imagery and RUC analysis indicated a middle level low over northwest 
Ontario with a trough axis southward through MN/IA/Mon. Several weak 
shortwaves were embedded in the cyclonic flow associated with this 
feature. Light snow over lower Michigan and eastern WI was expanding to the 
northwest...supported by weak to moderate 700-500 mb qvector conv and 
280k-285k isentropic lift. Visibility in the upstream snow area was 
generally in the 2-5sm range. With the relatively weak 
dynamics...the precipitation was slow to move to the northwest given the 
relatively dry low levels...per 00z kgrb sndg. 


Today and tonight...expect the areas of precipitation to expand over the County Warning Area 
with occasional light snow. However...given the relatively weak 
dynamics...accumulations should generally remain around an inch or 
less. So...going probability of precipitation only in the 60-70 range still looks on track. 
By later today...as the surface trough axis over Western Lake Superior 
slides to the south enhanced low level conv and sufficient 
instability and deep moisture for lake enhanced snow (850 mb temperatures 
around -11c) may bring a period of moderate snow vicinity Ironwood and 
Ontonagon. Low level conv with developing cyclonic nearly flow into north 
central Upper Michigan could also bring a period of moderate snow vicinity 
Marquette. With only weak instability and gradually backing winds to 
northwest...snowfall amounts only in the 2-4 inch range west and 1-3 inch 
range north central are expected. 


Thursday...as the middle level trough shifts to the east and a surface 
ridge builds into the upper MS valley...northwest winds will continue to 
back. Cold air advection pushing 850 temperatures into the -14c to -17c and weak lift/ 
lingering 850-700 moisture ahead of a clipper shortwave sliding through 
northwest Ontario will help sustain les for northwest flow favored locations. 
The shifting/backing winds through the period should help limit snow 
accumulations to the 1-3 range with some local 4 inch 
amounts...mainly for locations east of Marquette. An advisory may be needed 
if models suggest potential for more persistent stronger bands. 


Friday and Friday night...models were in agreement that another 
shortwave trough and associated surface low will move quickly from the 
northern plains to the Great Lakes. However...the 00z GFS and 00z NAM 
continue to place this feature and precipitation swath farther north than 
the UKMET/ECMWF. The forecast leaned toward CMC glbl Gem which 
represented a reasonable compromise. So...the precipitation area (greater 
pops) were shifted only slightly northward. Snowfall potential with 
this expected track would only be in the 1-3 inch range...greatest 
south. Cold air advection with 850 mb temperatures dropping to near -18c will bring les 
again to locations favored by northwest flow by late Friday night. 


&& 


Aviation (for the 18z taf issuance)... 


Widespread snow this morning is beginning to wane as the inverted 
trough slowly slides south. Now the main form of moisture will be 
lake effect snow showers...as winds become more northerly late this 
afternoon/evening...and northwesterly towards the end of the 
forecast period. Drier...more stable air will advect in on Thursday 
as the low over the lower Great Lakes races on to the East 
Coast...but with a favorable wind direction off Lake 
Superior...little to no improvement will be realized at cmx. 


&& 


Marine (for the 4 am Lake Superior forecast issuance)... 


Winds will remain below gales through the forecast. However...there 
are two times in the forecast that are of some concern. The first is 
tonight...with north winds of 20 to 30 knots on Eastern Lake Superior. 
These blustery winds are behind a weak trough swinging south across 
Lake Superior this evening. The next period of concern is on 
Saturday...where north to northwest winds will increase to 30 knots 
behind a clipper system moving through upper Mississippi River 
valley into the lower Great Lakes. 


&& 


MQT watches/warnings/advisories... 


Upper Michigan...none. 
Lake Superior...none. 
Lake Michigan...none. 


&& 


$$ 


Update...kf 
discussion...jlb 
aviation...kf 
marine...dlg