More Snow for Seattle, Heavy Rain For Portland

Remember all that talk about “Snowmageddon 2019” for Portland, with multiple heavy snows for Puget Sound and the Willamette Valley this week? Well, I hate to break it to you folks, but snowmageddon ain’t happenin’ this year, at least for NW Oregon and SW Washington. Parts of Western Washington still look to receive a healthy dumping, but where and how much snow will accumulate remains up in the air.

Forecast Hype – When Will We Ever Learn?

First, a note on forecast hype. Before the weekend snows for Portland had even began, there were articles like this floating around.

Additionally, another forecaster from one of the local news stations posted a graphic with raw model output showing 14 inches of snow over Portland, with the disclaimer that it was only a model chart and not the current forecast. But unfortunately, as the comment thread on the above article shows, that information didn’t necessarily get passed down to the public.

Forecasters learned a great deal about communicating forecast uncertainty back with the Ides of October Storm of 2016. For those who don’t know or need a refresher, some models initially showed the Ides Storm as being one of the strongest storms to hit the Pacific Northwest in the past 100 years. And with social media and the extensive and easily-accessible repository of various weather models online, it didn’t take long before rumors of the threat spread like wildfire. As far as 5 days out, the potential storm drew many comparisons to the great Columbus Day Storm of 1962 due to the similar time of year and origins of the storms (the Columbus Day Storm struck on 10/12 and formed from the remnants of Typhoon Freda, the Ides storm struck on 10/15 and formed from the remnants of Supertyphoon Songda).

But one thing few people emphasized was the amount of uncertainty with this system. Ensembles (many forecasts run from the same model with slightly different initial conditions, giving a model “spread” that increases in time and reflects uncertainty) showed a ton of uncertainty with this system. Moreover, the storm was still a typhoon at the time of these forecasts thousands of miles from the Pacific Northwest. And when future forecasts showed a progressively weaker and weaker storm, this information was difficult to communicate to the public, partially because several outlets didn’t want to “admit defeat” and publish new articles walking back the clickbait, fear-mongering articles they had published just a few days earlier.

I’m gonna make a controversial claim here: there are some journalists and even meteorologists who, in my opinion, cling to the hope that their original (and, in some cases, inflated) forecasts will turn out instead of humbly admitting they were wrong and changing our forecasts to give updated information. Precise, properly-communicated “nowcasts” incorporating the very latest radar imagery, model runs, and observations are crucial during high-impact events, particularly in the Pacific Northwest where our microclimates can result in drastically different outcomes across a very small area. The importance of this was on full-display during the Friday night/Saturday snow event that left NE Portland with several inches but gave most other folks just a slushy inch and folks in SW Portland plain’ ol rain. I’ll talk about microclimates and forecast communication, using Friday night/Saturday’s storm as a case study, in a future blog. In the meantime, let’s talk about the weather forecast for the next few days.

We have two storms – one coming through tonight into Monday morning that will result in a couple inches of snow for Seattle and a dusting for Portland before things switch to rain, and another on its heels Monday that will bring heavy rain to Oregon and SW Washington and spread heavy, wet snow into Washington before precipitation before things switch to rain Tuesday morning for most locations there aside from Hood Canal and the Northwest Interior.

Tonight/Monday morning’s storm:

Much of NW Oregon and Western Washington is experiencing light snow at this time, with heavier rates further north. As the radar and and surface observations below show, precipitation is currently overspreading the area, and though temperatures are above-freezing in most places in the Portland metro and are preventing snow from accumulating, dewpoints are relatively low and are helping ensure that this snow at least falls to low elevations.

The center of the storm is over Western Washington, giving southerly winds to the Willamette Valley. These southerly winds will allow snow levels rise to 1,500 feet and keep precipitation in the Willamette Valley north to Clark County as all rain. Snow could hold on a little longer in the Columbia River Gorge east of Corbett and the higher elevations of Clark County, but amounts here are are only expected to be an inch or two, and the Columbia River Gorge west of the Bonneville Dam should switch to rain later tonight as onshore, westerly winds fill in.

Here’s the 24-hour snow map from the University of Washington’s WRF-GFS model ending 4 am tomorrow. Nothing in the Willamette Valley and relatively paltry amounts over Puget Sound compared to their last few storms this week. However, the radar imagery shows this storm being a little wetter/snowier than modeled, so I wouldn’t be surprised if folks in Puget Sound picked up slightly more than currently forecast – perhaps 2-5 inches through tomorrow morning, with the heaviest amounts in the Cascade foothills, the south Sound, and Snohomish County north.

To summarize: Portland is seeing flurries now being these will switch to rain soon. The morning commute will be fine. Areas from Cowlitz Country north (and perhaps the northern parts of Clark County) will see snow, and based on current radar imagery, I believe it will be slightly more than forecast by your weather app.

Monday/Tuesday Morning’s Storm:

When the Willamette Week was referring to the storm that could be the “Worst in a Century,” this is the one they were referring to. It has much more moisture than any of the storms we’ve seen over the past several days, but it is also much warmer. Initial forecasts had this storm tracking south of Portland and giving the City of Roses a redonkulous amount of snow, but current forecasts show this storm tracking further to the north and giving us even heavier rain while Western Washington gets heavy, wet snow with southerly winds.

The two images below show the 925 mb (approx. 2,500 feet) temperatures/sea level pressure/10-meter winds and the 3-hour precipitation ending at 4 pm tomorrow. With temperatures at 925 mb well-above freezing, Portland will see liquid sunshine, and heavy liquid sunshine at that. Western Washington will see rain transition to snow from south to north, though places north of Snohomish Country should stay all snow.

The changeover from rain to snow is difficult to forecast and communicate, but the Seattle NWS put out a fantastic timeline and graphic doing just that.

Expect heavy rain to begin falling in Portland around the morning commute and stick around through Tuesday morning. A slow-moving band of heavy precipitation will move south throughout Tuesday, bringing heavy rain for Western Oregon and Northern California Tuesday and Wednesday before dissipating Wednesday evening. Forecast 72-hour precipitation totals beginning 4 pm this afternoon are very impressive, particularly when you consider that the UW model is actually drier than the normally superior ECMWF (European) model (to no fault of my alma mater of course… the UW model is initialized from the American GFS, which is inferior to the ECMWF).

Even with no snow here in the lowlands, the 72-hour snow totals in the Cascades are enough to make any skier jump for joy. Our snowpack is well-below-normal, so these storms are huge for helping us get back on track.

To summarize: Monday/Tuesday’s storm will be very moist, with 1-3 inches of rain likely for the Willamette Valley and feet of snow expected for the Cascades. Western Washington could see quite a bit of snow from this system, but areas south of Snohomish County will deal with a switch from snow to rain and this introduces a lot of uncertainty in amounts.

Thanks for reading, have a great week, and stay warm/dry!
Charlie