Weather

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afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
304 am MDT Monday Oct 13 2008 


Short term...today through Wednesday night 
wrap around moisture associated with the departing upper low 
pressure system continues to bring light snow showers over northern 
Wyoming. This wrap around moisture has also brought some middle to 
high level cloud cover over the northwestern half of the County Warning Area this 
morning...helping to alleviate fog over these areas so far. Water 
vapor imagery demonstrates a weak area of drying slipping 
southeast towards northwestern Wyoming this morning...this slot 
should allow clearing skies to develop over the forecast area this 
morning. With recent snow cover...inversions will become more 
prevalent this morning...with associated fog over low lying areas. 


High pressure will be found over the forecast area for the next 
couple of days. Continued wrap around moisture will leave very 
isolated shower activity over the western mountains this 
morning...with these storms expected to dissipate by the late 
morning hours. Snow cover will help to keep temperatures on the cool 
side today...with many areas not getting out of the middle 30s today. 
Skies will continue to clear during the evening hours tonight...with 
another round of early morning fog anticipated over low lying areas 
of the County Warning Area. Generally dry conditions will continue through Tuesday 
morning. Temperatures will warm a bit on Tuesday...with the low to 
middle 40s expected to be present over the majority of the forecast 
area. 


The GFS demonstrates a shortwave moving over northern Wyoming 
Tuesday afternoon and evening. This shortwave will bring enough 
instability and moisture to bring isolated light snow showers over 
northwestern Wyoming. This shortwave will continue to move through 
the area Wednesday afternoon...with isolated light snow anticipated 
to linger over northwestern Wyoming. Temperatures will continue to 
warm on Wednesday afternoon as 700 mb temperatures continue to warm. 
Another slightly stronger shortwave will move towards northwestern 
Wyoming Wednesday afternoon and evening. A pending jet streak will 
decide the strength of this shortwave. The GFS is the more 
aggressive solution with the nose of a 120 knots jet pushing into 
western Wyoming Wednesday evening. The NAM meanwhile...leave the jet 
streak further west with little to no impact over the forecast area. 
For now...have followed previous shifts in siding with the GFS 
solution of the jet dynamics and light snow showers over the 
northern and western mountains. 


Long term...Thursday through Monday 
models in reasonable agreement to start the medium range period 
but then differences develop towards the end of the weekend and 
into early next week. The GFS shows a weak trough developing off 
the West Coast Saturday and then spreading inland Sunday with an 
increasingly moist SW ahead of it. Most of the quantitative precipitation forecast that the GFS 
is showing stays across Utah and Colorado but it is right up to the southwestern 
Wyo border. The European model (ecmwf) on the other hand just shows a shortwave 
approaching the far west late Saturday night in the overall 
westerly flow...moving across the state during the day Sunday. 
This European model (ecmwf) scenario would probably warrant a chance of showers to 
much of the central and northern zones. The GFS has a much more 
northern shortwave passing across southern Canada and Montana with only the tail 
end brushing our far north. And then on Monday...the GFS has a 
negative tilt trough approaching the west along with that moist SW 
flow interacting to spread widespread quantitative precipitation forecast across most of the area. 
The European model (ecmwf) just has another shortwave passing through Montana 
affecting only our far northern zones. Not sure how this will play out 
yet since the 12z run of the European model (ecmwf) had a very different solution 
for early next week with a major trough developing again over the 
Great Basin just past this period and now it has a ridge. Will not 
buy into the moist SW flow the GFS shows but otherwise will follow 
generally a compromise as much as possible toward the end of the 
period. With that said...Thursday will see a chance of showers 
across the north and these probability of precipitation may have to be beefed up over the 
next couple shifts if trends continue. Brief break then until late 
Saturday or Saturday night when the next shortwave brings another 
chance of showers to mainly the northwest and northern mountains into Sunday. 
Monday's forecast sides more with the European model (ecmwf) than the negative tilt 
trough GFS for now. Temperatures will continue tricky for at least 
the start of the period with warm 700 mb temperatures returning but with snow 
cover and moist ground continuing. Will be interesting to see how 
much our early season snow can hold US down as 700 mb temperatures rising to 
at least 5c Friday and Saturday in the central basins. Without 
snow cover that would be 70 degrees or more. We will hold those 
temperatures down in the upper 50s to lower 60s for now and see how the 
next couple days end up. 


&& 


Aviation... 
VFR conditions will prevail today into this evening. Areas of fog 
may occur in the basins until 16z with IFR to LIFR conditions in 
places. 


&& 


Fire weather... 
wrap around moisture will continue to bring isolated light snow 
showers over northern and western Wyoming this morning...though 
showers should continue to taper off through the morning hours. 
Areas of fog will be found across the forecast area this morning 
and will dissipate by the middle to late morning hours today. An area 
of high pressure will bring generally drying conditions over the 
forecast area by this afternoon. This morning some continued 
moisture associated with the departed upper low pressure system. 
Will be enough to keep isolated showers over the western 
mountains. Dry conditions anticipated to develop and prevail this 
afternoon as high pressure continues to build into the forecast 
area. Temperatures will remain on the cool side today with 
elevated humidities. Temperatures will warm over the next couple 
of days. Fog will again be possible Tuesday morning as inversions 
continue to develop over the snowfall from the last couple of 
days. 




&& 


Riw watches/warnings/advisories... 
none. 
&& 


$$ 


Short term...branham 
long term...skrbac 
aviation...Arkansas 
fire weather...branham 

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