Lowland Showers & Mountain Snow to End April

 FastCast--Monday, April 29 to Friday, May 3:

Showers will continue on Monday and the first half of Tuesday across Western Washington. Generally, expect 0.3-0.6" of additional rain, most under Convergence Zone bands and from Olympia south. Some areas may get over 0.6" of rain through Tuesday. There is also a region-wide chance of thunderstorms on Monday and Tuesday, specifically in the afternoons. Remember that any showers have the potential to produce thunder, lightning, and gusty winds. Be prepared if you are outside. Snow levels in the mountains will decrease to 2,000-2,500 feet on Monday and Tuesday. Expect higher passes like Stevens and White to receive up to 8" of snow, with Snoqualmie Pass picking up 2-4". Some winter driving conditions are possible, so don't be caught off-guard by late April snow! Regarding lowland temperatures, Monday and Tuesday will be on the chilly side for April, with highs in the low to mid 50s both days. Wednesday will be mostly sunny, with highs increasing to the upper 50s to low 60s. Clouds and showers will return on Thursday, but temperatures will remain in the upper 50s to low 60s. Friday will be the nicest day of the week, with mostly sunny skies and highs likely in the mid to upper 60s! Expect overnight lows to generally be in the upper 30s to mid 40s, coldest on Monday and Wednesday mornings.

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Continue reading the full blog below!

More showers are expected on Monday and Tuesday, along with a region-wide chance of thunderstorms. Additionally, snow levels will be down to 2,000-2,500 feet on Monday and Tuesday, bringing a chance of late April snow at the passes.

Let's start with the snow forecast. Below is the NWS NBM high-resolution snow forecast through Tuesday evening.

This forecast shows the higher passes (Stevens & White) getting 6-10" of snow, with Snoqualmie Pass getting up to 3-5", Paradise and Crystal Mountain getting 6-10", and Mount Baker and the North Cascades Highway getting 4-5". This event shows that mountain snow is still possible at the passes into April. Be prepared for potential winter driving conditions over the passes, especially on Monday.

Now...back to the lowlands. Let's take a look at expected rain through Tuesday night. Most showers will be on Monday, with showers decreasing in coverage on Tuesday. Below is the European model forecast for rain through Tuesday night.

Generally, expect an additional 0.3-0.5" of rain across the lowlands. However, with the showery nature of the incoming precipitation, totals will likely be a bit more scattered. This forecast shows areas of higher rain totals between Tacoma and Mount Vernon, due to Convergence Zone bands (0.5-0.9"), and areas south of Olympia getting 0.6-0.8". The coast gets 0.5-1.5" through Tuesday, with parts of Eastern WA near the Cascades and near the Idaho border getting 0.2-0.4". 

Let's compare this with a higher-resolution forecast, which will show the potential areas of heavier precipitation due to showers and thunderstorms. Below is the NAM high-resolution forecast for total rain through Tuesday night.

This forecast shows areas of showers producing paths of increased precipitation, on both sides of the Cascades. Overall, the lowlands receive at least 0.25" of rain, with areas of heavier showers getting up to 0.8", with isolated higher totals, especially in Convergence Zone bands and on the coast.

The cause of these strong showers that could bring high rainfall totals is a region-wide chance of thunderstorms, on Monday and Tuesday, but with the higher chance being on Monday. Below is the NAM high-resolution forecast for CAPE (instability in the atmosphere) on Monday afternoon.

This forecast shows CAPE values of 200-500 across Washington on Monday afternoon. While these values would be considered quite low in the Midwest, they're at least respectable for Washington.

Another way of looking at thunderstorm potential is lightning flash density, essentially the chance of lightning at a given spot over a certain time period. In this case, we're looking at 3-hour lightning flash density on Monday afternoon, forecasted from the European model.

This forecast shows a lightning potential for most of the lowlands, with the highest potential in Eastern Washington, especially around the Tri-Cities, Walla Walla, and the Eastern Columbia River Gorge. The thunderstorm chance in Western Washington is going to be highest around the Convergence Zone, so likely somewhere between Seattle and Skagit County.

Any strong showers or thunderstorms that do develop will bring heavy rain, gusty winds, a potential for small hail, and of course, a chance of thunder and lightning. Visibility will likely decrease while driving, and areas of standing water are possible too.

There's a good amount of weather going on through Tuesday, with mountain snow, lowland showers, and a statewide chance of thunderstorms! Be prepared out there!