Mostly Dry to Begin May, Rainy Weekend Likely

 FastCast--Wednesday, May 1 to Sunday, May 5:

After a few days of showers, a relatively dry remainder of the work week is expected. Partly sunny conditions are expected on Wednesday, with highs reaching the upper 50s to low 60s. Thursday will be the best day of the week, with mostly sunny skies and highs in the mid 60s across the region. Clouds will return on Friday, but highs will remain in the mid to upper 60s. However, rain will move in by late Friday evening, continuing at times through late Sunday. Precipitation totals look to be on the heavier side for this round of rain, with amounts of 0.5-1.5" possible, depending on which forecast we look at. Temperatures on the weekend will decrease to the mid 50s, with lows from Friday to Sunday in the mid 40s. Notably, lows on Wednesday and Thursday will be in the mid to upper 30s, with a slight chance for frost in some outlying areas on Wednesday morning. Stay tuned over the next few days for more information about the weekend forecast.

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A couple nicer days are ahead across Washington, following a few days of showers. Let's take a look at the forecast, and a look ahead to a potentially wetter weekend.

Below is the forecast for morning lows on Wednesday from the NWS NBM model.

Expect morning lows in the mid to upper 30s outside the metro area, with the cities remaining in the upper 30s to low 40s. Eastern Washington will drop to the low to mid 30s. Any areas below approximately 35-36° have a potential for frost, so be aware of this possibility.

Highs on Wednesday will be an increase from Tuesday, as seen below in the NWS NBM forecast.

On Wednesday, expect lowland highs in the upper 50s to low 60s, coastal highs in the mid 50s, Willamette Valley highs in the upper 50s, and Eastern Washington highs in the upper 50s near Spokane and Ellensburg to the mid to upper 60s in the Columbia Basin.

Next, let's take a look at highs on Thursday, which will be the sunniest day in Western Washington, albeit not the warmest. The NWS NBM forecast is below.

On Thursday, the lowlands will reach the low to mid 60s, with the Willamette Valley and coast reaching the upper 50s to low 60s. Eastern Washington will reach the low 60s near Spokane to upper 60s in the Columbia Basin.

Friday will be the warmest day for awhile, but Western Washington's skies will be mostly cloudy as our wet weekend system moves in. Below is the NWS NBM forecast.

Expect highs on Friday to reach the mid to upper 60s in the lowlands, with the coast reaching the upper 50s, the Willamette Valley reaching the upper 50s to mid 60s, and Eastern Washington reaching the mid 60s to mid 70s.

Quite a wet weekend is possible, especially on the European model forecast. We'll look into the weekend forecast more over the next few days, but below is the forecast for total rain in the 48-hour period from Friday evening to Sunday evening.

This forecast shows heavy rain totals of 1-1.5" from Seattle southward through the Willamette Valley, with 0.3-0.9” from Seattle northward. This forecast doesn’t necessarily encompass rain totals for the whole region over the entire weekend, so stay tuned for more information over the next few days!

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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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!

Wet Weather Returns to Western Washington

 FastCast--Thursday, April 25 to Monday, April 29:

After a relatively dry month, by Seattle standards, a rainy pattern will return to Western Washington to round out April. Thursday will be the rainiest day of the pattern, with 0.4-0.8" of rain across the lowlands. A chance of showers will persist every day through at least Monday, and likely beyond. Conditions will be overcast, except for some potential sunbreaks on Monday. High temperatures will be below average, with highs on Thursday only reaching the low to mid 50s, and remaining in the mid to upper 50s through Monday. Wetter and cloudier conditions will keep lows in the upper 40s through Sunday, decreasing to the upper 30s to low 40s by Monday. Despite a wetter pattern, snow will remain mostly above 5,000 feet, with a trace to 1" of snow possible at the higher passes through early next week.

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A wet pattern is returning to Western Washington after a relatively dry past few weeks. Let's take a look at the forecast!

We'll start with the European model forecast for rain through midday Friday.

This forecast shows areas from Seattle south and Mount Vernon north getting 0.5-0.8" of rain by midday Friday, with a rain shadow from Seattle to Mount Vernon, and west along the Strait, where 0.3-0.5" of rain is possible. The coast and mountains will receive 1-2" of rain, most in the Olympic Mountains. Parts of Eastern Washington, mainly mountainous areas, will get 0.1-0.3" of rain.

Let's compare this forecast to the high-resolution NWS NBM forecast, also showing total rain through midday Friday.

This forecast largely agrees with the European model, except it shows a lesser rain shadow, which is mainly confined to areas west of Whidbey Island. This forecast shows 0.5-0.9" of rain in the lowlands, with 1-2" on the coast, and up to 3.75" in the mountains, most in the Olympics and Southern Cascades.

What about mountain snow? Below is the European model forecast for total snow through midday Friday.

As we get later in spring, it gets harder to get significant snowfall. The winners for snow will be the volcanic peaks, with Paradise and Mount Baker being the only recreation areas to get some snow, as the snow level remains around 5,000 feet.

You'll definitely notice that temperatures will be on the cooler side, as seen below in the NWS NBM forecast for Thursday's highs.

This forecast shows highs on Thursday only reaching the low to mid 50s west of the Cascades. Eastern Washington will reach the low to mid 60s. Expect temperatures across the state to warm a few degrees on Friday.

We'll end this blog by taking a look at the extended outlooks for the end of April and first few days of May. These outlooks are from the NWS Climate Prediction Center (CPC). 

First, the CPC temperature outlook.

For April 30-May 4, this outlook shows a 50-70% probability of below-average temperatures in Washington, highest for Western Washington.

Next, let's see how the temperature and precipitation outlooks correlate.

This outlook shows a 40-50% probability of above-average precipitation for Washington through the first week of May. 

Stay tuned for more information, and enjoy the rainy weather while we have it! All the rain counts toward more water for the summer!