December Storm shift: ‘atmospheric river’ now aimed at King County, mountain snow changes to heavy rain, Snoqualmie Valley flood risk increases

UPDATE | DECEMBER 20, 2019, 6:30AM

Like Ryan Porter of Snoqualmie Weather warned yesterday, if the ‘atmospheric river’ of moisture with this December storm shifted a bit north, it could change the flooding risk in the Valley – and that’s what happened overnight.

Ryan said with that northerly shift, the heaviest moisture is going right over King County and pulled in more warm air from the south – that changed heavy snow at Snoqualmie Pass to heavy rain last night. It is also expected to stay rain in the higher elevations for much of Friday.

This means the Snoqualmie River is now forecasted to minor to moderate reach flood stages at both Falls and particularly in Carnation. By early Friday morning, the Snoqualmie River at the Falls had reached 13,200 cfs, considered level 2 flood stage

Snoqualmie has received about 3.5″ of rain already with this storm. According to Ryan, the Snoqualmie River near Carnation may reach 35 kcfs (major flood level is ~41 kcfs) and near theFalls 27 kcfs (minor going on moderate).

The National Weather Service says 4-5 inches of rain is possible in Snoqualmie through Saturday. There is also an avalanche warming posted for the mountains due to heavy snow followed by heavy rain. Urban flooding has been reported in areas of the Puget Sound.

ORIGINAL STORY

[Ryan Porter of Snoqualmie Weather contributed to this article]

After a relatively mild December, some stormy weather is upon us just as we officially flip seasons from fall to winter. If you’re taking off for the holiday this weekend and plan to drive over our mountain passes, you’ll want to prepare for winter driving conditions.

Winter storm warnings are up for the mountain passes and an Area Flood Watch is in place for the Snoqualmie Valley – Thursday, December 19th through Saturday, December 21st.

Conditions at Snoqualmie Pass on Thursday afternoon, 12/19/19

Snoqualmie Weather’s Ryan Porter says forecasters are expecting tons of snow in the mountains, but it’s unlikely that Snoqualmie will reach more than minor flood stage near the Falls. Carnation, though, might have somewhat higher levels. 

Ryan said even with Snoqualmie Pass on the ‘snow-rain’ bubble on Friday, there shouldn’t be much snowmelt at higher elevations – which is a positive when it comes to potential flooding around here.  That doesn’t mean there won’t be localized flooding around susceptible streets in Snoqualmie Valley, but Ryan says we most likely won’t have the high river flows we saw in October when the mountain passes got 9 inches of solid rain. 

Note:  if the storm track makes a late shift further north, it will draw in more warm air and NOAA could upgrade the threat of potential flooding.

It’s going to be a soaker in the lowlands! Ryan is forecasting about 3″ of rain in the Snoqualmie /North Bend area for the 24 hour period ending Friday and possibly 5″ through Saturday. 

You can expect periods of gusty winds, too, adding to the the overall “storminess” category for the next few days.  Winds started picking up on Wednesday through the Cascade gaps and will eventually turn more southerly as the brunt of the storm moves through on Friday.  Ryan said even Seattle could see gusts of 40mph+ according to the Euro and NAM weather models as the pressure gradient turns more N-S on Friday.

See storm rain, snow and river level estimates below:

Rain projections from Euro model for City of Snoqualmie (solid red blue lines should be the focus)
Accumulated snow projections Snoqualmie Pass
Local river flow forecasts from NOAA displayed courtesy of Floodzilla.com.  Note 10/22 realized flows were more than double the current forecasts

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