The storm is almost here, and its going to be a tricky one! Its going to be a battle between those dry east winds (Cascadias), and this large plume of moisture coming in off of the Pacific. Heres a look at the relative humidity values and the water vapor image at noon.

Today’s model runs have been coming in with a bit more snowfall for Western Washington compared to yesterday’s runs. Starting with the global runs, both the 12z Euro and 12z GFS bring about 6-7″ of snow to Seattle.

The two areas that the deviate is with the amount of snowfall south and west of Seattle, absolutely burying areas south of Tacoma. They also disagree about how much the dry air limits totals in King county. The GFS depicts a sharp gradient between totals in Seattle and the eastside with totals on the eastside between 2-5″ while the Euro shows pretty consistent totals throughout King county. The UW WRF-GFS, which is higher resolution and does a better job of taking our local topography into effect, thinks those east winds will be a bit more powerful than the deterministic GFS run. The WRF only brings 1-4″ for the eastside and 3-6″ in Seattle.

Taking a look at another high res model, the NAM also shows a region of lower totals for portions of the eastside with a noticeable gradient between areas west and east of Lake Washington. Unlike the WRF, the NAM does show that heavy snow making it further north and includes parts of Snohomish county and the north sound in the significant snow party.

Last night’s extended HRRR model (an hourly rapid refresh model) was showing those east winds absolutely cranking, only giving Seattle 1-2″ of snow and a dusting on the eastside. It has since reversed its course and aligns closely to the NAM with a noticeable gradient, but still 5-8″ in Seattle and 4-6″ out on the eastside.

The National Weather Service in Seattle has issued a winter storm warnings the central sound with anywhere from 3-7″ of snow along with 40mph wind gusts. In the south sound, their winter storm warning is for anywhere from 5-10″ of snow.

The additional precipitation show in the models along with a slightly weaker east wind is encouraging. We do have a ton of moisture with this storm system, and I am optimistic that cascadia winds wont eat up all of the snow. With that being said, the further east you go in King county, the lower the totals will be, same goes for the further north you go out of Everett. Along with that, slight shifts in the strength of those winds could have big implications for the central sound snow totals.

Another caveat that is worth mentioning, the snow ratio. Snow ratio is the ratio of liquid to snow accumulation. The maps above use a 10:1 ratio, so for every 10 inches of snowfall accumulation is equal to 1″ of liquid water. The colder the air is, the higher the snow ratio due to changing processes in ice crystals. In this setup, some meteorologists are using a model output called the Kuchera Ratio, which takes into account the temperatures in the lower half of the atmosphere to calculate a snow ratio. Depending on how cold it is tonight, we could see snow ratios higher than 10:1 which would bump up these totals. However, i’m a bit skeptical of that because the models have generally been overdoing the amount of cold air with this setup. So for now im sticking with 10:1 ratios.

Now after throwing all of these model images at you, I should probably give you an actual prediction. Right now in Seattle proper, I think 5-7″ is a pretty good middle range estimate with a moderate chance of being closer to 4″ or 8″. Out on the eastside, depending on your exposure to east wind, you’ll probably end up with anywhere from 2-6″. The south sound and Kitsap peninsula will be dumped on where 7-12″ seems like a good bet. This storm could be a boom or bust situation though, so just about anything is possible.

We’ve been talking a lot about this storm, but what I havent talked about is that the models have trended cooler on Sunday into Monday as another system moves in. We could end up with another couple inches of snow Sunday night into Monday before more of that cold air is scoured out and we transition back to rain. This storm would likely favor areas north and east of Seattle where it will be cooler, but ill have an update on that after this storm.