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Area forecast discussion...updated 
National Weather Service Marquette Michigan 
1248 PM EST Monday Nov 23 2009 


Updated for aviation discussion for 18z taf issuance 


Discussion... /issued at 342 am EST/ 


Today through Tuesday...surface ridging extending westward from 
Nova Scotia continues to influence the Great Lakes. Latest WV 
imagery shows moisture streaming northeastward around the 
periphery of this ridge as a cold front tracks east across the 
southern Canadian prairies...remaining northwest of Upper 
Michigan. Broad area of showers have developed within this 
moisture plume aided by weak isentropic lift noted on 305 
Theta-E/pressure analysis. Short term guidance agrees these 
showers will remain west of line from l`anse to Watersmeet...with 
the heavier showers remaining offshore. Expect clouds to remain 
over the far west most of the day with partial clearing and 
periods of sunshine expected over the remainder of Upper Michigan. 
Expect areas of dense fog early this morning under this clearing. 
This should also allow for another mild day with high temperatures 
in the low 50s...some 15 to 20 degrees above normal. A weak short wave 
trough noted in the 500mb flow will pass through the County Warning Area 
tonight...however with limited moisture Don/T expect measurable 
rainfall. In fact expect a dry forecast through Tuesday. 


Tuesday night into Friday...by Tuesday night attention turns to 
short wave trough currently situated over eastern Colorado. This feature 
is prognosticated to shift northeast out the Central Plains on 
Tuesday...crossing Northern Lake Michigan on Wednesday night while 
strengthening slightly. Going forecast appears to handle 
increasing probability of precipitation well...with likely probability of precipitation by Wednesday as deeper 
moisture/best 500mb height falls arrive over Upper Michigan. Main 
changes were related to rain/snow coverage. At this time it appears the 
eastern zones will stay mostly rain per BUFKIT soundings/critical 
thickness values. Cold air is lacking and in fact the necessary 
cold air is situated well to our north on Wednesday over Hudson 
Bay and northern Manitoba. Expect some snow flakes though by 
Wednesday night over some of the higher terrain aided by diabatic 
cooling processes with a rain/snow mix elsewhere except the east 
where all rain is expected through Thursday. Adjusted wording in 
severe weather potential statement to reflect lesser potential for significant accumulating 
snowfall. 


Still disagreement among long term guidance models as to whether 
enough cold air is available for lake effect snow behind this 
system. 23/00z GFS guidance brings 850mb temperatures as cold as 
-9c...which given the Current Lake Superior water temperature of 
5c would be just enough for lake processes to begin. The preferred 
European model (ecmwf) solution offers a much warmer airmass with 850mb 
temperatures never falling below -4c. A quick glance at 850mb 
temperatures across the northern hemisphere shows cold air is 
nowhere to be found over the western hemisphere with virtually all 
the cold air...-20 or colder...locked up over Siberia. 


&& 


Aviation /for the 18z taf issuance/... 


MVFR visible at kcmx should improve to VFR early this afternoon. VFR 
conditions should then prevail through the afternoon. At ksaw...fog/stratus 
has been slow to dissipate this morning. Still expect slow 
improvement to VFR visible/MVFR ceiling this afternoon under weak daytime 
heating. More fog and stratus will redevelop this evening at 
kcmx/ksaw under light winds/nocturnal cooling. Expect lowest LIFR 
conditions at ksaw under light upsloping south-southeast winds. Similar to 
today...cig/vis improvement Tuesday morning will be very slow. 


&& 


Marine /for the 4 am Lake Superior forecast issuance/... 


Expect hi pressure to remain nearly stationary over southeast Canada and the 
NE states into Wednesday while low pressure developing in the plains moves 
slowly NE to Wisconsin. Winds will be under 25 kts through this time. 
As the hi finally retreats...the low center will move gradually east to 
over southeast Ontario on Friday. Plan on a period of stronger north winds to 30 
kts on Wednesday night into Friday on the northwest flank of this low. Since this low 
now appears will develop more slowly than expected before...the north 
gales previously forecast appear less likely. Look for a hi pressure 
ridge to build toward the western Great Lakes on Friday and bring 
diminishing winds. 


&& 


MQT watches/warnings/advisories...none. 


&& 


$$ 


Discussion...Pearson 
aviation...rolfson 
marine...kc