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Area forecast discussion...updated 
National Weather Service Glasgow Montana 
927 am MST Sat Nov 21 2009 


Short term...for northeast Montana...today through Monday... 
a vigorous Pacific trough and cold front are in the process of 
crossing northeast Montana this morning. A strong west wind is 
developing in the wake of the front with the western zones 
beginning to experience wind speeds in excess of 20 miles per hour...with 
gusts above 30 miles per hour. The wind should increase across the entire 
area today with cold air advection and strong subsidence behind 
the trough. The Current Lake Wind Advisory and its time duration 
remain reasonable. Overnight relative humidity recovery ahead of 
the front was very poor in the Little Rockies and Missouri breaks 
region. However...dewpoints are increasing...and temperatures are 
falling slightly hence relative humidity is recovering this 
morning. A brief period of low relative humidity is still expected 
this afternoon...but the morning recovery should preclude any fire 
weather highlights. The overall forecast is in good shape and the 
primary update involved blending overnight relative humidity 
percentages to forecast values in order to diagnose whether a 
fire weather highlight was necessary or not. Ajz 


Previous short term discussion... 
an energetic short wave trough and cold front sweep over the 
zones today. The main effect will be strong winds westerly winds 
behind the front. A strong surface trough anchored by a low 
centered in southern Saskatchewan will maintain a strong pressure 
gradient across the surface. These winds will be reinforced by 
850mb winds of 35-40kts resulting in surface wind gusts of equal 
strength. The lwa seems like a slam-dunk so will renew the npw 
during this package. Moisture is rather anemic with this system so 
expect mostly cloud cover with little precipitation. A vorticity maximum 
around the base of the upper level trough could produce some 
moisture in southern Alberta/Saskatchewan that might spread a bit 
across the international border. So will maintain probability of precipitation just below 
text mention for now. The other previously mentioned concern was 
the expected low relative humidity values for today which could produce near 
critical fire weather conditions. See fire weather separate discussion 
below. 


Tonight/Sunday...upper flow veers to the northwest late tonight as 
the upper trough base moves east. A short wave upper ridge follows 
but is quickly followed by the next short wave trough. This next 
wave sends a weak cold front into the County Warning Area by afternoon which could 
produce light precipitation over the western zones. 850mb 
temperatures remain near 0*c or just below so mixed precipitation 
is a possibility. With the atmosphere decoupling at sunset...the 
relaxing gradient will allow winds to drop below lwa criteria. 


Sunday night/Monday...the base of the trough drops south into 
northern Colorado...taking most of the energy to our south. 
However the inverted surface extends north into northeastern 
Montana which will keep conditions a bit unsettled. While 
NAM...UKMET and ec stay dry...based on GFS...Gem and sref...slight 
chance probability of precipitation seem appropriate for snow overnight. Conditions do 
not change much for Monday so will keep slight chance probability of precipitation going. 
Surface highs will drop down much closer to normal while 850mb 
temperatures hover around -4*c. So expect any precipitation to 
fall as snow and possibly rain showers. Quantitative precipitation forecast amounts will not 
produce accumulations. Scattered 


Long term...Monday night through Saturday... 
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. A mix of middle and high level clouds will move 
through the area today with ceilings above 10 000 feet and no 
obstructions to visibility. Skies clearing tonight. West winds 
this morning will increase to 20 to 25 knots by 18z with higher 
gusts. The winds will diminish to around 10 kts after sunset. 


&& 


Fire weather... 
a strong west wind will develop today behind a cold front. Wind 
speeds are expected to reach 20 to 30 miles per hour...with gusts to 40 miles per hour 
during the late morning and afternoon hours. Afternoon minimum 
relative humidity values are expected to briefly drop into the 20 
to 30 percent range before beginning a steady recovery near 
sunset. Therefore...a brief period of critical fire weather 
conditions are possible this afternoon...but highlights are not 
expected at this time due to the short duration of low relative 
humidity due to shortening sunlight hours. The most susceptible 
areas will be the Little Rocky Mountains and the Missouri breaks 
region of zones 119 and 121...which experienced a rather poor 
relative humidity recovery last night. Ajz/scattered 


&& 


Glasgow watches/warnings/advisories... 
lake Wind Advisory until 5 PM MST this afternoon for Fort Peck 
Lake. 


&& 


$$ 


Weather.Gov/Glasgow