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fxus63 kmpx 110055 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
655 PM CST Wednesday Mar 10 2010 


Update... 


Added the Twin Cities and west central WI to the dense fog 
advisory for tonight and Thursday morning. Widespread dense fog to 
the west and south of the cities will slowly spread north and east 
this evening as dryer middle level air pushes in...with high boundary 
layer saturation. 


&& 


Previous discussion... /issued 140 PM CST Wednesday Mar 10 2010/ 


Latest water vapor imagery and RUC upper air analysis continue to 
show a somewhat monotonous pattern across the Continental U.S.... with a 
broad upper trough over the central US bracketed by ridging off 
the West Coast and along the East Coast. Individual shortwaves 
continue to rotate around the upper low... and look to bring 
successive waves of precipitation to the area over the next several days as 
low clouds and areas of fog persist due to the abundant boundary 
layer moisture which is continually being replenished by the 
dwindling snowpack. The various model guidance is in decent 
agreement on things over the next few days... and in line with 
continuity from previous forecasts... so made no significant 
changes to things other than to add a bit more specificity in the 
shorter range. 


A break in the widespread heavier precipitation looks to occur tonight as 
the current shortwave shears out and departs to the northwest. 
However... this will be short-lived since the next wave is already 
rounding the base of the trough and lifting northeast through the 
Southern Plains. Good 850-500mb q-vector convergence in 
association with this shortwave looks to move in from south to 
north late tonight and during the morning on Thursday... bringing 
another round of precipitation. There is even some elevated instability 
prognosticated per the 850-500mb differential Theta-E values of below 
zero which work toward the southeast part of the area on Thursday. 
Left thunder out of the forecast for now... but it may be a 
possibility south and east of the metropolitan Thursday morning into 
early afternoon. The upper trough will cutoff entirely as we head 
into Friday then slowly work its way eastward toward the Ohio 
Valley. However... shortwaves will continue to rotate around it 
and bring period better chances for precipitation to the area into the 
weekend. It is a bit tough to time each window of higher 
probabilities beyond Thursday night at this point... so trended 
toward more generic Middle-Range probability of precipitation from Friday through Saturday. 


Dense fog should continue to be an issue through tonight over much 
of the area... with the best potential in the areas which are 
currently seeing the lowest visibilities given the light winds and 
high boundary layer moisture along with the lack of falling precipitation. 
Will likely extend the dense fog advisory through the night for 
those areas... while continuing to mention areas of fog across the 
remainder of the area. 


For the extended period... Sunday through Wednesday... the pattern 
looks to turn drier but remain mild as the cutoff upper low 
continues to move east and upper ridging establishes itself across 
the area. The GFS... European model (ecmwf)... and their respective ensembles are 
reasonably similar with respect to the large scale features through 
the early part of the week then start to diverge toward midweek on 
the degree to which energy from the Pacific will crash east through 
the upper ridge. These differences Don/T appear as though they will 
have a major impact on the sensible weather in our area during the 
current 7 day forecast period... but certainly would beyond that 
point. Used a blend of the GFS and European model (ecmwf)... which matches the HPC 
guidance quite well. This results in a dry forecast for most of the 
period with a continuation of above normal temperatures. Included 
some low probability of precipitation on Sunday which may linger while the upper low 
slowly retreats east. 


&& 






Aviation.../00z taf issuance/ 


Main issue for tafs tonight will be dense fog. Surface low right now 
is centered near rwf...with dense fog settling in across most of 
southern Minnesota at this time thanks to the slackened pressure gradient and copious 
amounts of low level moisture present from the combination of 
rather rapid snow melt and rainfall the last couple of days. 
Seeing rather extensive 1/2sm or less visibility across the entire 
region...so thought going into taf was to bring visibilities down quickly 
and keep them there through the morning. However...have started to 
see sporadic improvements in visibilities around the Twin Cities since 
issuing the tafs...so may not be as continuous on the low visibilities as 
painted in tafs...will try to be patient when it comes to 
amendments though. This surface low is expected to fill and slide 
further west during the night...which should lead to a tighter 
pressure gradient setting in after 09z with stronger winds 
possible...helping to improve conditions earlier than forecast. 
After that...will be watching the next strong short wave to impact 
the area. This is currently over Kansas and looks to spread rains 
back into the terminals after 15z...with the main push waiting 
until after 18z Thursday. Models are hinting at this wave and 
associated rain stalling out over the area...keeping rain going at 
terminals through most of Friday as well...so no appreciable 
improvements in conditions lie ahead. 


Kmsp...strange things happening here this evening...as 1/4sm and 
1/2sm visibilities at taf issuance have rapidly improved to 6sm within 
30 mins. Culprit here is likely the winds becoming more 
srly...reaching as far south as 160 degree...but as of 0030z...winds 
have backed to 140 degrees. Gfslamp keeps winds in this direction 
until 09z...when they head back toward 090 degree. Have brought visibility 
back down then. Hard to say how low it will go...but will at least 
trend lower visibilities returning with the more Erly winds. Next batch 
of rain looks to begin spreading in around 15z...with the heaviest 
rain holding off until after 21z...when it looks to remain over 
the field until the end of the taf period. 


&& 


Mpx watches/warnings/advisories... 
Minnesota...dense fog advisory until 9 am CST Thursday for Anoka-Benton-Blue 
Earth-Brown-Carver-Chippewa-Chisago-Dakota-Faribault- 
Freeborn-Goodhue-Hennepin-Isanti-Kandiyohi-Lac Qui Parle-Le 
Sueur-Martin-McLeod-Meeker-Nicollet-Pope-Ramsey-Redwood- 
Renville-Rice-Scott-Sherburne-Sibley-Stearns-Steele-Stevens- 
swift-Waseca-Washington-Watonwan-Wright-Yellow Medicine. 


WI...dense fog advisory until 9 am CST Thursday for Barron-Chippewa- 
Dunn-Eau Claire-Pepin-Pierce-Polk-Rusk-St. Croix. 


&& 


$$ 


Trh/mpg 

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