Forum Index > Trail Talk > Learning about the snow forecast - NWS Discussion
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gb
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gb
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PostSun Nov 24, 2019 8:14 am 
This is a very detailed synopsis and forecast analysis from the lead forecaster this morning at the NWS. The detail is amazing and shows the difficulty and expertise required to forecast snow in most Puget Sound events:
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Things begin to noticeably change later tonight into Monday morning. Exposure to the left exit region of the offshore jet streak will continue, but a potent vort max will also dive SE across the eastern Pacific into Oregon overnight tonight. This will likely enhance shower activity across the area and with lowering snow levels down towards all pass levels, Cascade snow accumulation should commence. Although cold air aloft will continue to infiltrate the region during this time, snow levels and near surface temps will remain too warm in the lowlands for snow. This will be a mountain snow event. Should see several inches of snow across the Passes thru Monday morning, especially Stevens Pass. Mid level temps will become even colder thru the day Monday-likely bottoming out below -30C. Not much will change synoptically thru the day, with an unstable, unsettled environment continuing. Hi- res model guidance suggest precip will dwindle into the afternoon hours as the potent vort max slides south. Snow levels will continue to plummet thru the day Monday, likely dropping just under 2000ft by late afternoon. At this time still expecting all mountain snow and lowland rain showers. Things could become interesting on a very local scale Monday night into Tuesday for the lowlands east of Puget Sound. Snow levels will drop to at least 1000ft if not a bit lower overnight, with critical thickness values in the 1000-850mb layer near or below 1300m. This indicates cold air above the sfc, but temps at the sfc will remain above freezing into Tuesday morning, arguing for mostly a rain (maybe brief snow mix) event. Most light shower activity over the area will be rain Monday night, tho wet flakes may mix in at times. The exception to this will be the development of any convergent zones/boundaries; and several hi-res models are hinting at the possibility. Given the environment in place, a convergent zone will have enhanced vertical forcing in place compared to the showers. These zones will have the ability to drive snow levels down significantly in very localized areas.Heavy snow may develop just off the sfc. This can be seen in model sounding data (esp the NAM) where saturation exists thru the dendritic growth zone within these bands. If this scenario were to develop, which it very well could, rain may transition to locally heavy snowfall in a very narrow band. Heavy snow will likely melt upon contact with the above-freezing sfc which will act to lower the sfc air temp. There could end up being very localized snowfall accumulation that may cause minor impacts if a convergence zone develops Mon night into Tues. The problem now is that it is essentially impossible to determine where this would be. Best guess would be the usual; somewhere between central King Co into northern Snohomish Co (maybe even into Island Co and southern Skagit?). This zone could linger into the Tuesday morning commute. Snow levels Tues morning will be even lower than overnight, so if any lingering light showers are around, some may be in the form of light snow showers. No impacts would be expected if this pans out. Conditions will dry out from north to south Tuesday, with cold temps expected. Snowfall totals in the Cascades Sun night thru Tues morning currently in the 8-10 range for Stevens and 4-6 for Snoqualmie. Kovacik

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dave allyn
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PostSun Nov 24, 2019 8:22 am 
Can you translate that into English? Seriously though, there is so much that goes into weather forecasting. Considering all the variables the forecasters do remarkably well.

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gb
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PostSun Nov 24, 2019 8:32 am 
dave allyn wrote:
Can you translate that into English? Seriously though, there is so much that goes into weather forecasting. Considering all the variables the forecasters do remarkably well.
This should help if you have an advanced degree in Meteorology and many years of forecasting experience...... wink.gif
Just kidding, of course, amazing complexity. I've never looked at this from the UW before.

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dave allyn
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PostSun Nov 24, 2019 8:51 am 
Thank you. It's all very clear now. I just needed the graph.

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ale_capone
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PostSun Nov 24, 2019 9:14 am 
I'm reading that it going to snow at my house monday night. Better get the plow ready! It seems we are in a unique spot for weather. Just east of gold bar. All that weather gets bunched up and stuck like traffic on a Steven's pass traffic on a pow day. Can be observed often on highway 2 around pickle farm. I bet if you put a weather station in town and here, you would have widely differing results. I think it's because the valley narrows, and air starts going up due to stickney, index, and the merchants. Bending , cuz it's always breezy here. We are still below 200', but slightly higher then town on a rise. We have yet to have a frost event at our house.

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Nancyann
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PostSun Nov 24, 2019 10:47 am 
Thanks for the heads up gb! Ale, from past experience I agree with your local prediction and will throw in my two cents worth for snow at my house in the foothills above Sultan. agree.gif

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Get Out and Go
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PostTue Nov 26, 2019 9:03 pm 
From Spokane NWS Discussion:
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models bullseye Blewett Pass and the Stuart Range with multiple inches (6 to 12 over a 24 hour period) of snow accumulation tomorrow and Wednesday night..
Between around the clock snow-making + natural, looks promising for Friday on the White Ribbon of Death at Mission. tongue.gif And now a 60-70 car pile-up on I-90 near Spokane. Don't blame the weather; blame the drivers. shakehead.gif https://www.krem.com/article/traffic/multiple-crashes-snarl-traffic-in-both-directions-on-i-90-near-geiger-blvd/293-36b57920-68a1-413a-b600-71afb4ffc0f8

"These are the places you will find me hiding'...These are the places I will always go." (Down in the Valley by The Head and The Heart) "Sometimes you're happy. Sometimes you cry. Half of me is ocean. Half of me is sky." (Thanks, Tom Petty)
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Cyclopath
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PostWed Nov 27, 2019 10:43 am 
Blewett Pass is closed!

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cdestroyer
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PostWed Nov 27, 2019 4:36 pm 
blizzard warning rocky mountain front cold and road closures due to accidents...i90 closed at various points.. check before going

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