Weather

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afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
343 PM MDT Thursday Aug 21 2008 


Short term...tonight through Sunday 
upper low moving into extreme northwest Montana this afternoon with 
996 mb surface low over the NE Big Horn basin. Very dry...warm and 
windy conditions prevailing over the County warning forecast area this afternoon with 
convection staying north in Montana so far. Upper low will move 
east along the Montana/Canadian border tonight with associated 125 
knots jet and 500 mb baroclinic region across southern Montana. 
Atmosphere to become increasingly baroclinic across northern 
Wyoming tonight and forecast has kept isolated to scattered 
showers with isolated thunderstorms overnight. Surface cold front 
will push across roughly the north half of the County warning forecast area this evening 
and hang up across south central Wyoming after midnight. This 
front will bring a cooler and more stable airmass across the area 
Friday with valley and basin highs in the 70s to lower 80s. 


Clear skies and a very dry airmass will result in good radiational 
cooling Friday night with temperatures plunging to near freezing 
in the Upper Green river basin...Jackson Hole and Yellowstone 
National Park. 


Four Corners high strengthens over the weekend with some weak 
disturbances drifting across the northern Great Basin into western 
Wyoming. Middle and high level monsoonal moisture increases across 
the southwest Saturday and across most of west and central Wyoming 
Sunday. Lower airmass remains dry so expecting mainly high based 
virga...some gusty wind and very isolated thunderstorms with this 
late season monsoonal plume. Temperatures to return to above normal 
Sunday with basin highs in the lower to middle 90s. 


Long term...Sunday night through Thursday 
major model differences in regards to potential trough towards the 
end of the medium range period. GFS did a total flip-flop with a 
strong closed low forming over the Great Basin Wednesday night and 
Thursday. The European model (ecmwf) shows a trough swinging across Montana. The 00z 
UKMET is a compromise showing a stronger trough moving into the 
pacnw Tuesday afternoon which definitely supports the potential for a 
stronger trough in the Great Basin around midweek. Hard to see where 
this storm system originates looking at the 500 mb hemispheric charts. 
It appears to come out of the Bering Sea area but the development 
doesn't become trackable until about Sunday. Current forecast is 
following the European model (ecmwf) until we see more consistency with what would be 
a significant change. Models still depicting a minimal amount of 
moisture rotating around monsoonal high leading to potential for 
isolated high based storms late Sunday across the S and central 
portions. Hot...dry SW flow develops Monday with both the GFS and 
European model (ecmwf) showing +19c at 700 mb Monday afternoon around the central basins. 
Have continued trend of bumping up temperatures with widespread 90s east of 
The Divide and 80s out west. European model (ecmwf) shows a series of weak 
disturbances in the flow just brushing the far north early next 
week. Precipitation chances seem minimal with most of the energy in Montana. 
However...any southward shift of this trough or further digging as 
indicated on the UKMET and GFS would change the forecast 
dramatically. Will keep some isolated probability of precipitation across the far north at 
times based on European model (ecmwf) timing. Monday should be the hottest day with 
some moderation toward midweek with several weak frontal boundaries 
moving down from the north at times along with cooler middle level 
temperatures. 


&& 


Aviation... 
breezy to windy conditions to persist most areas this evening as a 
strong trough approaches. The trough will move across southern 
Canada and Montana tonight...dragging a cold front across Wyoming 
shifting the winds to the northwest to north. The winds will 
diminish some in speed...but will still be breezy. Some MVFR ceilings 
may move into the northern Bighorn Basin and Johnson and Natrona 
counties behind the front. 


&& 


Fire weather... 
an upper low moving into extreme northwest Montana this afternoon 
will continue east along the Montana/Canadian border overnight. 
Red flag conditions with gusty southwest wind and low relative 
humidity will continue across much of the area through sunset with 
some decrease in wind speed and relative humidity recovery afterwards. Isolated 
thunderstorms across the north could potentially spark new fires 
through the evening. A cold front along with a sharp wind shift 
from the north will push across northern Wyoming this evening and 
into central Wyoming after midnight. The front will bring a cooler 
and stable airmass over the area Friday along with some relative 
humidity recovery. A warming trend will occur over the weekend 
along with a return of some monsoonal moisture and isolated 
thunderstorms across the south and west. 


&& 


Riw watches/warnings/advisories... 
red flag warning until 9 PM MDT this evening for wyz277-279>281- 
283-289. 


Red flag warning until midnight MDT tonight for wyz140-276- 
285>288. 


&& 


$$ 


Short term...aem 
long term...skrbac 
aviation...twb 
fire weather...aem