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Area forecast discussion...updated 
National Weather Service Glasgow Montana 
502 PM MST Sat Nov 21 2009 


Updated to expire lake Wind Advisory 


Short term...for northeast Montana...tonight through Monday... 
current GOES water vapor imagery reveals a longwave trough now in 
place over the western United States. The leading shortwave trough 
has now lifted over Saskatchewan and has drug a cold front through 
northeast Montana into the Dakotas. A tight surface pressure 
gradient has developed over northeast Montana in the wake of these 
features and combined with cold air advection to produce a strong 
west wind. A secondary frontal system and associated vorticity 
maxima is currently dropping southeast over southeast Alberta and 
southwest Saskatchewan. The vertical ascent associated with this 
system is combining with diurnal heating and creating a few snow 
showers in Canada. As these showers move south they encounter a 
very dry airmass and hence at this point it appear to be primarily 
virga along the western portion of the hi-line. 


This secondary frontal system will cross the the local area this 
evening. The showers should weaken and begin to dissipate after 
sunset...but there may be enough upper forcing to maintain a few 
weak showers into this evening. Therefore...isolated flurries were 
added to the northwest zones this evening although its main 
affect will be increased cloud cover across the north and it will 
also maintain the wind through the evening. Albeit much weaker as 
low-level lapse rates lessen after sunset...so the current timing 
of the lake Wind Advisory should suffice. Shortwave ridging begins 
after midnight...so cloud cover should quickly decrease along with 
the wind. Minimum temperatures will be seasonally cool across 
most of the region. 


Cloud cover will begin to increase Sunday afternoon as another 
shortwave trough dives into the Pacific northwest. This trough 
will traverse across the local area Sunday night and early Monday. 
The upper dynamics with this system are decent...but the lower 
portion of the atmosphere will remain dry...so it will be 
difficult to get much moisture to the surface. Therefore...only 
slight chance probability of precipitation are forecast with precipitation type as light 
snow. Temperatures will be cooler Sunday and Monday...but still 
above seasonal averages. Ajz 


Long term...Monday night through Saturday... 
GFS and ec models generally agree through Friday. Forecast is 
fairly dry for the long term with a series of passing shortwave 
troughs and ridges. Outside chance for a rain or snow shower 
Tuesday or Tuesday night as a somewhat dry clipper-like system 
drops into the central US. Most significant feature in the long 
term is ridging Wednesday into Thursday...which should give clear 
weather and above normal temperatures for Thanksgiving Day. 


Ridge breaks down with a sharper trough in the ec model...which 
suggests a higher chance for some snow on Saturday than seen in 
the GFS. Martin 


Previous long term discussion...shortwave trough drops southeast 
out of Saskatchewan during the day Tuesday and may bring a slight 
chance of rain or snow. This fast moving system will have limited 
moisture with it. 


An upper ridge builds into the area on Wednesday and will bring 
warm and dry weather through Thanksgiving day for sure. 


Models have some differences on track of a shortwave that moves 
inland from the Pacific on Friday. European model (ecmwf) takes the system south of 
the area while the GFS brings it across Montana. Either way...a 
cold front will affect the forecast area on Friday with cooler 
temperatures expected next weekend. Will keep the forecast dry for 
the most part for now. Forrester 


&& 


Aviation... 
VFR conditions. Strong west winds this afternoon should abate 
after sunset...decreasing to around 10 kts. Winds turn 
southeasterly on Sunday up to around 15 kts. Mostly clear to 
scattered skies on Sunday with ceilings lowering out west Sunday 
evening. Martin 


&& 


Glasgow watches/warnings/advisories...none. 


&& 


$$ 


Weather.Gov/Glasgow