Showers & Chance of Thunderstorms to Start May

FastCast—Monday, May 1 to Wednesday, May 3:

After a spring heat wave with record high temperatures, a much different pattern is in store for the Pacific Northwest. A slow-moving upper level trough will park off the California coast, bringing SE flow (showers moving from SE to NW over the Cascades) through this week. This pattern will increase instability and lead to a prolonged chance of thunderstorms across the region. Storms are most likely in Eastern Washington, Eastern Oregon, and in the mountains. This a chance that storms will move into Western Washington, and this seems most likely from Tuesday to Wednesday, especially in the afternoons and into the night. However, these systems are difficult to predict, bringing lower confidence in the forecast, especially for Western Washington. Remember to keep your eye to the sky and go indoors immediately if you hear thunder. Regarding temperatures, expect highs in the upper 50s to low 60s on Monday and upper 60s to low 70s on Tuesday and Wednesday, with isolated readings in the mid 70s. Expect lows in the upper 40s to low 50s. Temperatures will cool back to the low 60s by Thursday. Stay tuned to the forecasts and be aware of incoming showers!

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An interesting pattern is ahead for the Pacific Northwest, with a slow-moving trough bringing a chance of thunderstorms and instability in the atmosphere across the region. 

Let’s start with the overall pattern that will bring showers and potential thunderstorms to the region. Below is the UW WRF forecast showing the pattern on Tuesday.

Notice the bright red area with “L” off the Central California coast. That is the trough that will push showers and thunderstorms along the general path of the black arrow I added. This will be the path for thunderstorms in the upcoming week.

One term that will be important to understand in this blog is CAPE (Convective Available Potential Energy), which means the level of instability in the atmosphere, which increases the potential for thunderstorms.

Let’s take a look at CAPE at 6 PM Monday, from the UW WRF forecast.

Notice that most instability on Monday will be confined to far Eastern Washington and parts of SE Oregon and around Idaho. 

Let’s take a look at Tuesday evening, also from the UW WRF model.

Instability will increase through the day Tuesday, and according to the UW forecast, there will be widespread instability across Washington. However, this doesn’t necessarily translate to thunderstorms. 

As of Sunday night, the best chance of thunderstorms will be Monday in Central WA, with showers moving into the Cascades and potentially Western Washington by Monday afternoon. 

More showers will move through the Cascades and lowlands early Tuesday morning.

A good way to see where the strongest showers will be is to look at rain forecasts. Below is the HRRR forecast for rain through 5 PM Tuesday.

Notice the areas of heavier precipitation over Central WA & the Cascades. That is where forecasts show stronger storms, but some storms will be high-based, particularly in the lowlands, meaning that lightning & thunder will occur but little precipitation will reach the ground.

One final aspect of the upcoming forecast is the rebound to warmer temperatures expected for Tuesday and Wednesday. Below is Tuesday’s forecast from the GFS model.

On Tuesday and Wednesday, expect lowland highs in the upper 60s to low 70s, in the low 60s on the coast (low 70s inland), and in the low 70s to low 80s in Eastern WA, warmest north of I-90. 

Stay tuned to the blog & my Twitter for the latest information as the thunderstorm threat develops this week. Keep an eye to the sky and go indoors if you hear thunder!

Last Day of Heat & A Big Cooldown Ahead

FastCast—Saturday, Apr. 29 to Tuesday, May 2:

High temperatures reached record levels across parts of the Northwest on Friday! SeaTac Airport hit a record of 81º, Portland hit 88º, and due to offshore flow, Astoria, Hoquiam, Forks, and Tillamook reached the mid to upper 80s, all records! Another hot day is expected on Saturday, with lowland highs in the mid to upper 70s, with some readings in the low 80s. The coast will be dramatically cooler as marine air moves closer, with highs only in the mid 50s to low 60s. Eastern Washington will be hot, with highs in the upper 70s to upper 80s, with some readings in the low 90s possible. Quite a cooldown is expected for the lowlands on Sunday, with temperatures 15-25º cooler than Saturday, down to the upper 50s to mid 60s, and overcast skies. Similar conditions will continue on Monday, with a chance of showers. By Tuesday, expect temperatures to increase back toward the mid 60s to low 70s. Lows will remain in the upper 40s to low 50s through Tuesday. There is a chance of thunderstorms across the state next week, as an upper level trough will bring showers into the region on multiple days. The extent and placement of thunderstorms is still uncertain, so stay tuned!

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What a beautiful day! Friday brought deep blue sky and record high temperatures across the region (more at the end of the blog). Similar conditions are expected on Saturday, so let’s take a look at some forecasts. 

First, the HRRR forecast for Saturday’s highs.

The HRRR shows the lowlands reaching the mid to upper 70s, except in the low 70s near the water. Areas around Portland will likely reach the upper 70s to low 80s. Eastern Washington will reach the low 80s to low 90s. The coast is significantly cooler, with highs plummeting to the mid 50s to low 60s.

Now, let’s compare this with the NAM forecast, also for Saturday’s highs.

The NAM is warmer for the lowlands and the coast. This forecast shows highs in the lowlands in the mid to upper 70s, all the way south through Portland. Isolated readings in the low 80s are possible. This agrees with the HRRR for Eastern WA, with highs in the low 80s to near 90º. However, this forecast has warmer temperatures on the coast, in the low to mid 60s.

Remember that the water is COLD, with rivers and lakes in the upper 30s to upper 40s, and Puget Sound & the ocean in the mid to upper 40s.

Western Washington is in for a major cooldown on Sunday. Take a look at the temperature difference from 4 PM Saturday to 4 PM Sunday from the NAM model.

Western Washington, mainly the lowlands, will be significantly cooler. Highs on Sunday will likely be 15-25º below Saturday’s highs from Vancouver BC all the way to the Willamette Valley. Notice cooler temperatures are trying to creep into Eastern Washington as well.

What will temperatures drop to on Sunday? Below is the NAM forecast for Sunday’s highs.

Expect a significant drop west of the Cascades, with the lowlands dropping to the mid 50s to mid 60s. Eastern Washington will remain on the hot side, with highs in the upper 70s to upper 80s. 

Forecasts show that due to an upper trough moving down the coast from Sunday to Tuesday (bringing showers to the region) will bring multiple chances of thunderstorms, especially in & east of the Cascades. Stay tuned to future blogs for more on this situation!

Below are some graphics showing highs on Friday. Note the readings in the 80s across the lowlands (hottest near North Bend due to downslope winds) and the incredible highs on the coast due to strong offshore flow, which also brought hot conditions to the Portland metro.

Spring Heat Wave Update

FastCast—Thursday, Apr. 27 to Sunday, Apr. 30:

A spring heat wave is still in the forecast for the Pacific Northwest. On Thursday, expect decreasing clouds, with lowland highs in the low 60s (north) to low 70s (south), with highs in the low 70s from Olympia southward, in the upper 50s to mid 60s at the beaches (low 70s inland), and in the upper 60s to mid 70s with sunny conditions in Eastern Washington. Friday and Saturday will be the hottest days. Expect highs in the lowlands to reach the low 70s to low 80s, with areas near the water north of Everett in the mid to upper 60s, low to mid 60s on the ocean beaches (cooling on Saturday to the 50s), mid to upper 70s inland from the beaches, and in the mid 70s to mid 80s in Eastern Washington. The lowlands will be relatively similar on Saturday, with Eastern Washington being warmer (upper 70s to upper 80s) and the coast being cooler (mid 50s to low 60s). Conditions west of the Cascades will cool to the upper 50s to low 60s on Sunday, with cloudy conditions. Meanwhile, warmth will continue in Eastern Washington, with more highs in the mid 70s to low 80s. Back in the lowlands, expect lows through Sunday to be in the mid 40s to low 50s. Despite warm temperatures, area rivers & lakes are in the upper 30s to mid 40s, with Puget Sound & the ocean in the upper 40s. Though it looks “cooling”, the waters are dangerous at this time of year!

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After a historically cold April, the month will end with a spring heat wave! Let’s take a look at the upcoming forecasts.

Below are Thursday’s high temperatures from the GFS model.

Expect the lowlands to reach the mid 60s to low 70s from Seattle southward and the low to mid 60s from Seattle north. Areas from Olympia south will reach the low 70s, along with inland communities on the coast. The ocean beaches will remain in the upper 50s to low 60s, and Eastern Washington will warm up to the upper 60s to upper 70s. 

Lows will still be crisp in the mornings, especially west of the Cascades. Below is the GFS forecast for Friday morning’s lows.

Expect morning lows in the mid to upper 40s across Western Washington, with lows in the mid 40s to mid 50s in Eastern Washington. This provides for a refreshing cooldown, a luxury of warm spring days that we aren’t afforded in the summertime.

Now for the real warmth…below is the GFS forecast for Friday’s highs.

On Friday, expect lowland highs from Seattle south in the mid 70s to low 80s, warmest near the foothills. Areas from Olympia south through Portland will reach the mid 70s to mid 80s, along with the Coast Range and inland coastal communities in WA & OR. Ocean beaches will reach the mid 60s, and Eastern Washington will reach the low 70s to low 80s.

Record highs may be challenged, mainly west of the Cascades. After months of below average temperatures, Friday will be quite a pendulum swing. Just how far above average will these highs be? Take a look at the GFS forecast for temperature anomalies across the Northwest on Friday.

Wow! Temperatures in the Pacific Northwest will be 5-20º above average for the metro areas and up to 30º above average in the mountains!

Two things to note: Rivers, mainly east of the Cascades, will rise this weekend, with some rivers nearing bankfull. Also, due to rapidly warming temperatures, avalanches are possible in the backcountry, so be prepared. 

Finally, a quick look at Wednesday’s high temperatures, seen below.

Areas from Seattle southward generally reached the mid 60s to low 70s, with the Portland area reaching the mid 70s and Eastern Washington reaching the low 70s to low 80s! Areas from Seattle northward only reached the upper 50s to low 60s, due to clouds from a weather system that moved through British Columbia. With the exception of the aforementioned cooler areas, most temperatures on Wednesday were warmer than forecasts predicted. Keep that in mind as we head into even warmer temperatures through Saturday!

Enjoy the first warmth of the season!

Late April “Heat Wave” Ahead

FastCast—Tuesday, Apr. 25 to Saturday, Apr. 29:

The warmest weather of the year is ahead, as warmer temperatures finally arrive in the Pacific Northwest. Expect partly cloudy conditions on Tuesday, Wednesday, and Thursday, with sunnier conditions in the afternoons. Highs will reach the low 60s on Tuesday and the upper 60s on Wednesday and Thursday. Areas from Everett northward will be cooler, likely in the low to mid 60s. Mostly sunny and warm conditions are expected on Friday and Saturday, with highs skyrocketing to the mid to upper 70s! On Friday, areas from Chehalis southward have a chance of reaching the low to mid 80s. Coastal regions will be slower to warm up, with highs in the low to mid 60s through Thursday, before reaching the low to mid 70s on Friday. Eastern Washington will reach the upper 60s to upper 70s by Wednesday, increasing to the upper 70s to upper 80s by Saturday! Back in Western Washington, expect lows from Tuesday to Wednesday mornings in the low to mid 40s, then in the upper 40s to low 50s from Thursday through Saturday. Remember…although air temperatures will be warm, Puget Sound and lakes/rivers are frigid, with temperatures in the upper 30s in some rivers to upper 40s in Puget Sound and the Pacific Ocean. Enjoy the pleasant weather! 

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Real spring weather…consisting of sun and warmth…is ahead for the Pacific Northwest! Let’s take a look at the upcoming highs!

First, Wednesday’s highs from the NBM model.

Tuesday will be 5-7º cooler than Wednesday. On Wednesday (and Thursday), expect lowland highs in the mid to upper 60s from Everett southward and in the low 60s from Everett northward and on the coast. Areas from Olympia southward have a chance to reach the low to mid 70s. Eastern Washington will reach the upper 60s to upper 70s!

If you want a taste of summer (in April), you will enjoy Friday’s highs, seen in the NBM model forecast below.

Friday will be a summer-like day across the Northwest, with clear skies and warm temperatures. In the lowlands, expect highs in the low to mid 70s, cooler near the water and warmer (upper 70s) near the foothills. Even the coast will reach the low to mid 70s, with some areas a bit inland reaching the upper 70s. Areas from approximately Chehalis southward (including the Portland/Vancouver metro area) will skyrocket into the low to mid 80s! Eastern Washington will reach the mid 70s to mid 80s on Friday.

Saturday is also expected to be warm, but details on temperatures aren’t exactly certain yet, so stay tuned!

What is causing this warmth? Below is the European model forecast for Thursday, showing a large high pressure ridge.

Notice inside the yellow box. That strong ridge of high pressure will deflect weather away from the Northwest, bringing fewer clouds and warmer temperatures (due to sinking air in high pressure ridges). 

How long will it last? We are expecting a cooldown back to the upper 50s to low 60s by Sunday, but what comes next is uncertain. Let’s take a look at the NWS Climate Prediction Center outlooks for temperature and precipitation.

First, the temperature outlook for April 30 to May 4. 

Essentially, areas west of Puget Sound have an equal chance of above or below average temperatures. Areas east of the Sound in WA have a 33-60% probability of above average temperatures, increasing as you go east.

What about precipitation? Let’s take a look at the precipitation outlook, also for April 30 to May 4.

Most of Western Washington is in the “normal” range, with a 33-40% probability of above average precipitation through early May for Eastern Washington.

Stay tuned for more information about the upcoming spring “heat wave”!

Rain Through Monday, Big Warmup Ahead

FastCast—Sunday, Apr. 23 to Thursday, Apr. 27:

After a warm day on Sunday, with highs in the upper 50s to mid 60s, more rain is in store. Rain will fall from late Saturday night through Sunday morning, and again from Sunday night to Monday morning. Across the lowlands, expect 0.25-0.5”, with isolated higher totals where there are heavier showers. Areas on the coast and from Everett northward will receive 0.5-1.25”. Tuesday will bring drier conditions and decreasing clouds by the evening. Through Tuesday, expect lowland highs in the mid to upper 50s, with lows in the low 40s. Wednesday and Thursday will bring more decreasing clouds, with highs jumping into the upper 60s as a high pressure ridge builds. Lows will remain in the mid to upper 40s. There is potential for even higher temperatures (70º+) by Friday. Stay tuned!

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At last…a warmup is ahead for the Pacific Northwest! However, a couple rounds of rain are ahead first. 

Let’s take a look at the European model’s forecast for rain through Monday night.

In the lowlands, expect 0.25-0.5” of rain, with 0.5-1.25” on the coast and from Everett northward. Eastern Washington will get 0.1-0.4”, with 0.4-0.6” from Olympia to Portland. Isolated higher totals are possible across the region, especially in isolated areas of the lowlands under heavier showers.

Now for what everyone is wanting to know about…the potential warmup ahead. Let’s take a look at why significantly warmer temperatures are possible. Below is the European model forecast for upper-air patterns across North America. 

Inside the yellow box is a developing and strengthening high pressure ridge. The position of this ridge will deflect systems away from the Northwest, bringing warmer and drier weather.

An example of what’s in store is seen below, in the NWS NBM model’s forecast for Thursday’s highs.

By Thursday (possibly Wednesday too), temperatures in the lowlands will reach the mid to upper 60s, except in the low 60s near the water. Eastern Washington will warm into the upper 60s to mid 70s, and areas from Olympia southward will warm to the upper 60s to mid 70s. There is a decent potential for even warmer temperatures from Friday to Saturday, so stay tuned!

What about the extended outlooks? Let’s take a look at the NWS Climate Prediction Center outlooks for April 28 to May 2. 

First, temperatures, seen below.

For the first time in months, there is a significant above average signal for the Pacific Northwest. Washington has a 50-70% probability of above average temperatures to end April and start May.

Next, precipitation, seen below.

Also for the first time in awhile, there is a widespread area of below average precipitation across the Northwest. Expect a 40-50% probability of below average precipitation to end April and begin May across the state.

Stay tuned to upcoming blogs, as details about the upcoming “heat” become more certain!

Unsettled Pattern Continues & A Potential Change Upcoming

FastCast—Thursday, Apr. 20 to Monday, Apr. 24:

After a few days of showers and chilly conditions, an unsettled pattern will continue through early next week across the Northwest. Another round of rain will move in from Thursday afternoon through early Friday, with showers at times from Friday through Saturday, followed by another round of rain from midday Saturday through early Monday morning. Across the lowlands, expect 0.25-0.75” of rain through Friday, with more this weekend (another 0.4-0.8”). The coast will get 0.75-1” through Friday, with another 0.5-1.5” through the weekend. Temperatures through Monday will still be below average, but will warm some from the first half of this week. Generally, expect highs in the low to mid 50s, except near the upper 50s to around 60º on Saturday. Lows will drop to the mid 30s on Thursday morning (isolated areas in the low 30s with frost possible) and then will stay in the low to mid 40s through Monday morning, except in the upper 40s on Sunday morning. Forecasts show a potential for a significant warm-up with sunnier weather next week. Details are far from certain and changes are expected, so stay tuned.

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This April has been defined by below average temperatures and many showers, and that will continue. Another round of rain is ahead from midday Thursday through early Friday morning, with showers at times through Friday. Below is the European model forecast for total rain through Friday night.

Across the lowlands, expect 0.25-0.75” of rain, with higher amounts under heavier showers. Areas from Olympia south will get 0.4-0.8”, and the coast will get 0.5-1.25”. Eastern Washington will pick up a trace to 0.4”, most near the Idaho border.

Temperatures will remain on the chilly side, especially Thursday morning. Take a look at the European model’s forecast for Thursday morning lows.

Most of the lowlands will drop into the mid 30s, with areas near the foothills dropping into the low 30s. The coast will reach the mid to upper 30s, and Eastern Washington will be quite chilly, dropping into the mid 20s to mid 30s, warmest in the lower Columbia Basin.

High temperatures on Thursday will be significantly below average across the lowlands, as seen in the European model forecast below.

Expect lowland highs only in the upper 40s to low 50s, far below normal. Even Eastern Washington will only reach the upper 40s to mid 50s.

The NWS Climate Prediction Center (CPC) outlooks for April 25-29 do show a change. 

Let’s start with the temperature outlook below.

This outlook shows a 33-50% probability of above average temperatures for Washington, consistent with extended forecasts that are also showing a warmup.

Next, the precipitation outlook, also for April 25-29.

This outlook shows a 33-50% probability of below average precipitation to end April across the state. Overall, extended forecasts are pointing to a warm-up with less rain by next week. 

However, this same pattern has appeared in extended forecasts over the past month, only to disappear from those same forecasts a few days later, due to the placement of high pressure ridges. A potential warmup still needs monitoring, so stay tuned!

Showers & Thunderstorms Through Wednesday

 FastCast—Monday, Apr. 17 to Wednesday, Apr. 19:

A persistent upper level trough over the Gulf of Alaska will bring persistent showers and a chance of thunderstorms to the region through Wednesday. Across the lowlands, an additional 0.2-0.5” of rain, with isolated areas under heavier showers getting up to 1” through Wednesday. On the coast, expect 1-2” of rain. At the passes, expect snow levels of 1,000 to 2,500 feet through Wednesday. Expect 6-12 inches of snow at the passes through midweek, with any road impacts limited to nighttime, since daytime temperatures will increase to the upper 30s to low 40s. Lowland temperatures will be quite unseasonable, with highs in the upper 40s to low 50s and lows in the mid to upper 30s. Remember…any showers from Monday to Wednesday could contain lightning, gusty winds, hail, and heavy rain. Be aware!

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Consistent showers and a chance of thunderstorms to Western Washington for the first half of the week. Let’s take a look at the total rain forecast from the European model.

This forecast runs from Sunday morning to Tuesday night. Expect 0.7-1.4” in the lowlands, with a rain shadow from Sequim to Everett. Areas from Olympia southward will get 1-1.5”, the coast will get drenched with 2-3.5”, and Eastern WA will pick up 0.1-0.3”. 

Now, let’s take a look at a higher-resolution forecast from the NAM model, for the same timeframe as the previous one.

The NAM generally agrees with the European, but shows more sporadic rain totals of 0.2-0.75” in the lowlands, 1-2” on the coast, 0.5-1.25” from Olympia south, and 0-0.4” in Eastern WA. 

What about the mountains? Due to the chilly nature of the current pattern, snow levels will remain at 1,000 to 2,500 feet. However, since it is springtime, high temperatures at the passes will reach the upper 30s to low 40s, limiting snow impacts to nighttime. Below is the European model forecast for snow through Wednesday night.

Through the first half of the week, expect 6-12 inches of snow at the passes, 1-4” in the Coast Range, and up to 3 feet in the Olympics. Snow-related driving impacts will be limited to nighttime or very heavy snow showers.

Now for the other element of this week’s weather…a chance of thunderstorms. Let’s take a look at the UW WRF forecast for CAPE (measure of instability, which produces thunderstorms). The first forecast is for Monday evening.

This forecast shows CAPE values of 200-400, decent for Western Washington.

Next, let’s take a look at Tuesday’s CAPE forecast, also from the UW WRF model.

This shows more widespread instability and CAPE values of 200-400 across the region.

Overall, be prepared for heavy showers and a chance of thunderstorms through Wednesday. Any showers could contain lightning, gusty winds, hail, and heavy rain.

One final note for this blog…temperatures are unseasonably low. Let’s take a look at Monday morning’s lows, forecasted by the European model.

Expect chilly lows in the mid to upper 30s. Eastern Washington will drop to the upper 20s to mid 30s from I-90 north, and in the low 30s to low 40s from I-90 south.

High temperatures will also be on the chilly side in the lowlands, as seen in the European model forecast for highs on Tuesday.

Expect unseasonably cold lowland highs in the upper 40s to low 50s. Eastern WA will also be on the cold side, with highs in the upper 40s to mid 50s.

Cool & Showery April Continues

 FastCast—Saturday, Apr. 15 to Tuesday, Apr. 18:

Showery and cooler than normal conditions are expected to continue across Western Washington through the start of next week. Expect cloudy conditions on Saturday, with highs in the low 50s. Rain arrives late Saturday night to early Sunday morning, continuing (but becoming more showery) through Monday night. Totals will be 0.5-0.8” across the lowlands (isolated higher or lower totals possible), with a rain shadow over the NE Olympic Peninsula and San Juans. The coast will pick up 1-2”, and areas from Olympia southward will pick up 0.75-1”. In the mountains, snow levels will start around 4,000 feet on Sunday, dropping to 1,500 feet by Monday morning. Most passes will receive 0.5-1” of rain, transitioning to 3-6” of snow. Isolated higher totals are possible at Stevens Pass. Back in the lowlands, expect highs through Tuesday in the upper 40s to low 50s, quite unseasonable for April.

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Our cool and showery April will continue in the days ahead. A system will bring more rain on Sunday, with showers continuing through Monday. Below is the European model forecast for total rain through late Monday night.

Across the lowlands, expect 0.5-0.8”, with 0.75-1” from Olympia south, and 1-2” on the coast. Eastern Washington will get 0.1-0.3”, with even more near the Blue Mountains. 

Snow levels will start out near 4,000 feet on Sunday, dropping to 1,500 feet by Monday. This means that the Passes will likely start as rain (0.5-1”), then transition to snow. The European model forecast for total snow through Monday night is below.

Expect 3-6” at the Passes, with isolated higher totals, possibly around Stevens Pass. The Coast Ranges of WA & OR could get a dusting to 2” of snow, mainly from Monday onward. 

Temperatures continue to be on the cold side. Below is the European model forecast for highs on Sunday.

In the lowlands and on the coast, expect highs in the upper 40s to low 50s. These are significantly below average for this time of year. Eastern Washington will be warmer, but still below average, with highs in the mid 50s to low 60s, except colder (low 50s) near the mountains. 

Below is the European model forecast showing temperature departure from average at 5 PM Sunday.

Notice that most of Washington & Oregon are 5-15º below average for this time of year. 

What about the extended forecast? Let’s take a look at the NWS Climate Prediction Center temperature and precipitation outlooks for April 20-24.

First, the temperature outlook, seen below.

This outlook shows a 50-60% probability of below average temperatures for the entirety of Washington through late April.

Next, the precipitation outlook, also for April 20-24.

This outlook shows a 40-50% probability of above average precipitation through late April for most of Washington. This will continue boosting our reservoir and snowpack ahead of the upcoming El Niño conditions (more on this in a future blog).

Showers on Tuesday, Cooler & Wetter Weather Ahead

FastCast—Tuesday, Apr. 11 to Friday, Apr. 14:

The atmospheric river is moving out of the region as of Monday night. Totals were from 1-1.9” from Seattle southward, 0-0.9” from Seattle northward, and 2-3” near the mountains and on the coast. Another system is moving in and will bring showers to the region on Tuesday and early Wednesday. Expect rain totals of 0.3-0.7” around the lowlands, with areas of higher totals around Everett, in the Convergence Zone. Snow levels, which were over 6,000 feet due to the atmospheric river, will crash to 1,500 feet by Tuesday night. At the Passes, expect 4-12 inches, highest at Stevens Pass. Additionally, there is a slight chance of thunderstorms across the region on Tuesday, so be prepared when showers move in. From Wednesday to Friday, expect partly to mostly cloudy conditions, with only a slight chance of showers on Thursday evening. In the lowlands, expect highs this week in the low to mid 50s, with lows in the mid to upper 30s. 

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After an atmospheric river moved through the Pacific Northwest, another system is ahead on Tuesday and early Wednesday. This system will be mainly showery, but will still bring a decent amount of rain. Let’s take a look at the forecast. 

Below is the NAM high-resolution forecast for total rain through late Wednesday.

Generally, expect 0.25-0.4 inches of rain across the lowlands, with 0.4-0.75” around Everett in a Convergence Zone. On the coast, expect 0.5-1.5”, most in heavier showers. Eastern Washington will get 0.1-0.75”, with isolated higher totals possible. 

Snow levels will crash as this colder system arrives. Below is the NAM forecast for snow through late Wednesday.

At the Passes, expect 6-12”, with the highest totals east of Everett due to a Convergence Zone. The Coast Range will also pick up 1-6” of snow, with highest totals isolated to the highest peaks.

Going into the extended forecast, “spring” isn’t the first word that comes to mind, as temperatures aren’t going to be warming too much. However, more rain is also expected, which will continue to help the water supply and snowpack.

Let’s take a look at the NWS Climate Prediction Center outlooks, starting with the temperature outlook for April 16-20.

Expect a 33-50% probability of below average temperatures, mainly from the Cascades westward. This isn’t too strong of a probability, so more seasonable days are possible.

Now for the precipitation outlook for April 16-20, seen below.

There is a 33-60% probability of above average precipitation for most of the Pacific Northwest, with a 50-60% probability bullseye for Western WA & OR, from about Eugene northward.

Warmer spring weather remains on hold…stay tuned!

Significant Atmospheric River Ahead

FastCast—Sunday, Apr. 9 to Wednesday, Apr. 12:

A significant atmospheric river is ahead for the Pacific Northwest, with steady rain impacting the area from Sunday morning through Monday night, with another system bringing rain on Tuesday. Through Tuesday, expect rain totals of 1-2 inches in the lowlands, with 0.25-1” in the rain shadow (Whidbey Island and the San Juans). Areas in the Cascades (under 6,500 feet), in the foothills, and from Chehalis southward will pick up 2-2.5”. The coast will get 2.5-4”, and isolated areas in the Coast Range and Olympics could get over 4”. In Eastern Washington, expect 0.4-1.25” of rain through Tuesday. Snow levels will rise to 6,500 feet by Sunday night, so all passes will receive rain, though snow levels will crash on Tuesday, dropping to 1,500 feet by Tuesday night. During this time, snow is expected at the passes (stay tuned in future blogs about this). Breezy conditions are expected around the region as well, with gusts of 25-35 mph in the lowlands and 35-40 mph on the coast. Expect highs in the mid 50s on Sunday dropping to the mid to upper 40s by Tuesday, with lows in the low 40s on Sunday, dropping to the mid to upper 30s by Tuesday. Standing water and ponding is expected on area roadways, with rises on area streams and rivers, and moderate flooding on the Skokomish River near Potlatch and Union. 

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The biggest rain event of 2023 is ahead for the Pacific Northwest, with a significant atmospheric river impacting the region from Sunday through late Monday, followed by more rain in a system on Tuesday, with some showers continuing through early Wednesday.

Below is the UW forecast for water vapor transport (how atmospheric rivers are measured) at 1 PM Sunday.

This atmospheric river will be aimed directly up Puget Sound, so there will be no rain shadow for the metro area. Expect a rain shadow from the Olympics to Southern BC, mainly west of I-5. 

Rain totals will be significant. Let’s start with the European model forecast through Tuesday night. This forecast includes the Tuesday system (post-atmospheric river).

In the metro area, expect 1-2” of rain, with 2-2.5” from Chehalis southward and in the Cascades/foothills. Notice the rain shadow north of the Olympics. Additionally, expect 2.5-4” on the coast and 0.4-1.25” in Eastern Washington.

Now for a shorter and higher resolution forecast, let’s take a look at the NAM forecast through early Tuesday morning.

Higher resolution forecasts tend to show higher rain amounts over terrain, since these forecasts can more accurately show terrain features. The NAM agrees with the European in giving 1-2” to the lowlands, though the rain shadow is more pronounced from Everett north and west. This forecast gives the coast and foothills 2-3”, with the Coast Range, Olympics, and Cascades getting 3-5”. Eastern Washington gets 0.3-1.25”, with some areas getting less, especially near the Cascades and in the Palouse.

We will look at one more forecast, the UW WRF ultra high resolution model, through 

The UW forecast shows by far the highest totals for the entire region, with 1.25-2.5” from Everett southward and quite a pronounced rain shadow to the north. The coast and mountains get significant totals of 3-5”, with isolated totals over 5”. This solution is a bit more unlikely than the others, but is still possible.

There is a slight risk that some of this rainfall may bring urban flooding at short notice (flash flooding), as seen in the Weather Prediction Center Excessive Rainfall Outlook for Sunday.

The marginal risk of excessive rainfall exists for the South Washington Cascades, the Olympics, and the coast & Coast Range from Florence, OR northward.

Finally, due to this atmospheric river bringing more mild subtropical air, snow levels will skyrocket to 6,500 feet by Sunday night, remaining from 4,500-6,500 feet through Monday night. Then, as a colder system arrives on Tuesday, snow levels will drop rapidly to 1,500 feet by Tuesday night. Snow isn’t expected at the passes until Tuesday.

Remember that there is a significant risk of urban flooding, standing water, ponding, and rises in small streams & creeks. Rivers will rise, with bankfull stage possible on the Snoqualmie River and moderate flood stage on the Skokomish River near Potlatch. 

Stay safe in all this rain & stay tuned for the next blog by Monday night!