Weather

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Area forecast discussion...update 
National Weather Service Marquette Michigan 
640 PM EST Monday Jan 5 2009 


Update... 


Original Winter Storm Warning for the Keweenaw zones was slated to 
expire at 00z...but opted to issue les advisory until 06z Tuesday for areas 
north of cmx to account for persistent low level cnvgc between land breeze SW 
wind S of cmx and more northwest flow over Isle Royale that has developed in 
the wake of shortwave noted on WV imagery/RUC analysis diving southeastward over 
Upper Michigan. This persistent cnvgc area/low pressure trough will likely linger 
until hi pressure builds over northwest Upper Michigan after 06z once shortwave has shifted 
farther southeast. One other factor to consider is persistent west-east oriented 
cloud structure over Western Lake supply that might be feeding shsn over the 
Keweenaw. Spotter in Mohawk (keweenaw county) reports steady 1/2" hour 
snow is continuing. Since this les is light and fluffy and strong winds/ 
blsn have diminished...issuance of advisory vs extension of warning 
seemed more reasonable. 


&& 


Short term (tonight and tuesday)... 
upper air pattern consists of a 500 mb trough across the upper Great 
Lakes stretching down into the southern rockies and desert SW with a 
Flat Ridge over the southeastern U.S. There is also a closed 500 mb 
low across northern Ontario. 


NAM brings another shortwave trough into the area late Tuesday with some 
850-500 mb q-vector convergence moving in as well. GFS shows the 
same thing and will use a blend of the two for this forecast. 


For tonight...lake effect should continue to wind down and will not 
extend the warning for the Keweenaw as there are only two bands 
really hitting it. Winds will continue to decrease as well...so 
blowing snow and heavy snow should not be a problem too much longer. 
Dry air really is having an effect on those bands and this should 
continue overnight as winds become light. Temperatures will quickly 
fall off after sunset tonight and reflected that in hourly 
temperatures. Went below all guidance with light winds and clear 
skies which should allow decent radiational cooling. Cannot see why 
temperatures would not end up to 10 below zero overnight. Late 
tonight...temperatures might actually rise a bit with winds coming 
out of the South. Lake effect should continue over the northern 
Keweenaw tonight as well with a band hanging around and moving all 
night long. 


For Tuesday...clouds should increase across the area and looks to 
remain dry as we wait for the next system and moisture to arrive Tuesday 
Night. Lake effect looks like it will be offshore of the Keweenaw 
with south winds keeping it at Bay. Temperatures should warm up a 
bit as well and did not make too many changes except to lower 
temperatures a couple degrees due to the cold start and not much 
sunshine will be around on Tuesday with high clouds prevailing. 


Long term (tue night through next mon)... 


Tuesday night through Wednesday night...forecast for this period is dominated 
by the interaction of three features...a strong shortwave located over 
far northwest British Columbia...a shortwave over NE Nevada...and an upper 
trough sitting in northwest Mexico. The latter two features are prognosticated to 
come together over the Southern Plains by 00z Wednesday while the shortwave in northwest 
British Columbia dives southeast into southern Saskatchewan. After 00z 
Wednesday...there is a split amongst guidance...mainly with the NAM where 
it keeps the Southern Plains trough moving slower compared to all other 
models. The slower trough results in a longer time for the southern 
Saskatchewan wave to phase with it...which in the end allows Upper 
Michigan to become more affected by the southern Saskatchewan wave. All other 
guidance is faster and allows the southern Saskatchewan wave to come 
close to phasing up with the southern trough...somewhere in the Ohio 
Valley/mid-Atlantic area Wednesday evening. Will be following the faster 
scenario per HPC pmdhmd discussion. For Tuesday night...synoptic snow is 
prognosticated to be working into Upper Michigan from both the west and the southeast. 
The portion from the west is associated with DPVA ahead of the southern 
Saskatchewan wave...while the portion to the southeast is associated with 
isentropic lift seen on the 280-285k surface. The problem the snow will 
have entering the County Warning Area is a dry airmass in place Tuesday evening...noted 
by model precipitable water values around 0.25 inch. Models have 
delayed precipitation moving in...which seems reasonable...so have done the 
same in the forecast. Snow looks more likely on Wednesday...which is when 
the southern Saskatchewan wave dives down into Indiana...allowing to 
catch up with the isentropic lift. However...the moisture advection 
is still not all that impressive into the area...with mixing ratios 
only 1 to at most 2 g/kg. Therefore any synoptic snow is going to be 
on the light side. The snow could be enhanced over the Keweenaw and 
far eastern County Warning Area...due to low level convergence along a surface trough 
over the area. This trough will be associated with a deepening low 
lifting up from the Southern Plains into southeast Ontario. As the southern 
Saskatchewan wave slides southeast...the trough drops south...allowing 
cooler air to filter south. 850mb temperatures drop from -10c on Wednesday to 
-13c by 12z Thursday. The cooling should allow for pure lake effect snow 
to form...impacting all of the northern u.P. Since winds are prognosticated to 
turn due north. Strongest low level convergence is indicated in 
Marquette and Ontonagon counties...so expect the most snow in these 
areas...which could be 2 to 4 inches at least. Regarding temperatures 
during the period...generally followed a blend of met/mav guidance 
given little disagreement between the two. 


Thursday...nearly phased upper trough will be migrating its way through 
the northeast U.S. While another shortwave is indicated to drop southeast in 
northwest flow into Eastern Lake Superior by 00z. The result is a mostly northwest 
low level flow across the area...with gradual lowering in inversion 
heights as drier air associated with a surface ridge to the west advects 
in. This means that lake effect snow showers from Wednesday night should 
shift to mostly affecting the northwest wind snowbelts...along with a slow 
diminishing trend through the day. Amounts will not be that 
significant...given the lowering inversion...probably only an 
additional 1 to 3 inches. Highs will be cooler than on Wednesday... 
partially due to that cold advection on Wednesday night. Lowered highs 
slightly from the previous forecast. 


Thursday night through Monday...models are in pretty good agreement showing 
the mean upper flow through the period consisting of a strong ridge 
along and just off the West Coast...and troughing over eastern North 
America. This upper flow pattern...or a positive pna pattern... 
usually means at or below normal temperatures with potential for 
clippers to move through...along with plenty of lake effect. One 
such clipper models have been showing is indicated to move through 
Friday into Sat. Timing is still somewhat uncertain...which is 
represented at 00z Sat where the 00z European model (ecmwf) has the clipper out in Minnesota 
while the 00z Canadian is in the eastern u.P.. nonetheless...the 
clipper is indicated to come through so high chance probability of precipitation at the 
minimum are warranted. Another clipper is shown to develop over the 
northern plains on sun...but the flow looks too amplified by this 
time to allow the system to come up towards Upper Michigan. Decided to 
follow the 00z GFS ensembles/European model (ecmwf) with the idea of it taking a 
southward track into the Ohio Valley. Outside of the clippers...the 
main concern then is lake effect snow. For Thursday night...the ridge of 
high pressure just off to our west in Minnesota will be progressing into 
Upper Michigan. Northwest winds ahead of the incoming high should favor continued 
lake effect in the northwest snowbelts of the Keweenaw and areas east of 
Marquette. Inland areas might get quite cold as well...prior to high 
clouds moving in ahead of the first clipper. After this clipper 
passes...a fairly strong northwest wind is indicated...which helps bring 
850mb temperatures down to almost -20c by 00z sun. Therefore More Lake 
effect is expected in the same northwest snowbelts. Continued cooling is 
indicated Sat night into sun...and may have to watch for Wind Chill 
Advisory criteria being met at some point for inland areas. The 
cooling air should also make the snow finer...and with a breezy northwest 
wind in place...visibilities could be reduced more for northwest snowbelts. Flow 
looks like it may try to take a turn more SW on Monday...but do not 
want to count on that given the upper flow pattern still suggesting 
a northwest low level flow. So have kept chances for lake effect snow 
showers going in northwest snowbelts into Monday. On that note...Sun night 
also looks like it could be quite cold with a ridge of high pressure 
moving in from the west. 


12z model update...new European model (ecmwf) and GFS look a bit more impressive with 
the clipper Friday night into Saturday. Have raised probability of precipitation to likely in 
many areas for at least Friday night. May need to raise more probability of precipitation as 
well in later forecasts. In addition...areas over western Upper Michigan 
could end up with a period of near blizzard or pure blizzard 
conditions during this Friday night and Sat time period. Will mention 
this in the severe weather potential statement. Continues to look cold behind the clipper for Sat 
night into sun with 850mb temperatures dropping to possibly as low as -25c 
by 12z sun. 


&& 


Aviation (for the 00z taf issuance)... 


Expect les band with lower MVFR to occasional IFR visibilities to persist near 
cmx this evening before drifting to the north and giving way to VFR 
conditions by around midngt with strengthening southerly flow pushing bands 
to the north into lake supply. With south-southwest flow through the forecast period...saw will be 
dominated by Arctic air unmodified by lake moistening and see VFR 
conditions. 


&& 


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


Winds will continue to decrease tonight as high pressure builds into 
the area. Winds stay below gales until Saturday behind a clipper 
type system which will drop south into the Great Lakes and tighten 
the pressure gradient...along with bringing down another surge of 
Arctic air. As winds come down this evening...so should the waves as 
well. Freezing spray will continue to be an issue a while 
longer...but temperatures finally start to moderate on Tuesday. 


&& 


MQT watches/warnings/advisories... 
Upper Michigan... 
lake effect Snow Advisory until 1 am EST Tuesday for miz001-003. 


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


&& 


$$ 


Update...kc 
short term...07 
long term...aj 
aviation...kc 
marine...07 

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