Weather

396 
fxus63 kmpx 282001 
afdmpx 


Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota 
301 PM CDT Thursday Aug 28 2008 


Discussion... 


Afternoon GOES moisture channel imagery shows a less amplified 
pattern across North America than we have seen lately. Sharp upper 
trough axis which contributed to the abundant and much needed 
rainfall of yesterday and last night continues to pivot across 
Minnesota with its center along the western shore of Hudson Bay. 
Fairly flat west-northwesterly flow is trailing the trough along 
the U.S./Canadian border stretching westward into the eastern 
Pacific. Another sharp trough was swinging eastward south of the 
Alaska coast. This trough will continue eastward and force weak 
ridging into the flat west-northwest flow ahead of it by Saturday. From that 
point the trough will dig into central rockies before ejecting 
northeastward Monday night. This send a surface low across the area 
later Tuesday...but there are some questions with regard to the time 
of its passage. The models further diverge for the rest of next week 
with low confidence in the forecast during that time. 


Some details... 


Upper trough axis is swinging across the County Warning Area this afternoon with 
associated vorticity axis and advection triggering scattered showers 
and thunderstorms across northern Minnesota. Surface pattern is a bit complex 
with several subtle trough axes across the area while main cool 
frontal boundary has pushed to the east of the County Warning Area. Will continue a 
slight chance for spotty rain showers/thunderstorms and rain into the early evening from about 
St. Cloud to New Richmond to west of Eau Claire as the upper vorticity 
axis shifts across the area. An isolated shower or thunderstorm is 
possible across central and south central Minnesota as well given the low 
upper heights and a weak surface trough/dew point boundary advancing 
across the western County Warning Area at 1930z. Not enough of a chance to have a 
mentionable pop though. Strong drying as entered behind this boundary 
with dew points falling into the low to middle 40s. The upper trough 
axis will be east of the area later this evening. A ridge of high 
pressure will nose northeastward into southern/eastern Minnesota and 
western WI overnight. Skies will be clearing so expect temperatures to take 
a tumble into the upper 40s to around 50 most areas with l/M 50s in 
the Twin Cities metropolitan area with light winds. A weak upper trough 
axis will move eastward across southern Canada Friday with a surface 
reflection extending well into the northern plains. The surface trough 
will enter western Minnesota by early afternoon but begin to wash out 
as it moves eastward. Very dry atmosphere and weakening convergence 
and minimal upper level support favor a dry forecast Friday ahead of 
the trough. Have temperatures in the lower 80s in the southwest and near 80 
in the east. Southwest winds and associated downslope component may 
yield a few warmer readings in the far southwest. 


Warmer conditions will arrive for Saturday. Surface cyclogenesis 
commences in the Lee of The Rockies Saturday in response to the 
approaching West Coast trough. High pressure will still be in place 
locally keeping quiet weather in the area but southerly flow will be 
increasing by late Saturday and be strong for Sunday. The upper flow 
continues to amplify ahead of the trough as well with sharp ridging 
quickly developing overhead Saturday before shifting eastward 
Sunday. High Saturday are expected to be in the low to middle 80s with 
the warmest readings west. A warm front becomes established to the 
north and west of the County Warning Area by Sunday morning as 850 mb temperatures surpass 20 
c...supporting even warmer highs Sunday and Monday with values in the 
upper 80s. 


The next decent chance for rainfall will occur with the passage of 
the upper trough and associated cold front in the Tuesday/Wednesday 
time frame. The European model (ecmwf) has been slowing the arrival of the front with 
the 00z/28 and 12z/28 versions having the front arrive in the west by 
Tuesday evening. The 06z GFS was much faster than the European model (ecmwf) as well 
as its earlier runs while the 12z has come in slower but still 
faster than the European model (ecmwf). Given the amplification of the pattern prefer 
a slower solution at this time which would favor later portions of 
Tuesday and early Wednesday for showers and thunderstorms. Plenty of 
moisture looks to be in place ahead of the front so rainfall does 
look promising if the pattern verifies. High pressure is expected to 
at least briefly build in behind the front Thursday. After that 
medium range guidance is at odds with the GFS bringing broad 
troughing across the western U.S. While the European model (ecmwf) holds southwest 
upper flow overhead while another very strong longwave trough digs 
into the western U.S. Will also need to watch progression of 
tropical system Gustav towards the end of next week. ..mdb.. 


&& 


Aviation.../18z taf issuance/ 


Main surface front just about to push through Eau Claire...with 
secondary weak surface trough nearing axn. Behind this second one will 
come some wind gusts to axn by mid-afternoon. Ahead of the 
trough...cumulus development has begun and some more is 
expected...especially in ctrl Minnesota into western WI. An isolated 
thundershower is possible in this area due to middle-level 
temperature cooling and associated positive instability...but not 
enough chance to warrant mention in tafs. Winds to gradually veer 
to the west-northwest this afternoon. These will then again back later tonight to 
the SW as the flow becomes weak. Some near daybreak fog possible 
at eau...given the wet ground and higher surface dew points. Winds expected 
to increase in strength from the SW on Friday ahead of a weak surface 
front pushing east across ND and northern Minnesota. 






&& 


Mpx watches/warnings/advisories... 
Minnesota...none. 
WI...none. 
&& 


$$ 


Mdb/mtf 

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