Monday Night Atmospheric River Update

No FastCast tonight…continue reading the full blog below for an update on the rain and flooding forecast for the atmospheric river.

A significant atmospheric river is impacting the Pacific Northwest, with heavy rain, river flooding, and urban flooding expected. Rain will be steady (heavy at times) on Tuesday, slowly tapering off through the day Wednesday. Let’s take a look at the forecasts.

Additionally, subtropical moisture related to this atmospheric river will bring warm temperatures to the region (the lowlands reached the upper 50s to low 60s on Monday), with the mid to upper 50s continuing on Tuesday.

Let’s start with the European model’s forecast for total rain through Wednesday evening.

This forecast shows 2-3” for the lowlands, with less in the rain shadow NE of the Olympics (1-1.5”). The coast gets 2.5-4”, with the Willamette Valley getting 3-4”, and Eastern WA getting 1-2.5”. The Cascades and Olympics will get 5-8”.

Let’s compare this to the GFS (American) model, also showing total rain through Wednesday evening.

This forecast is much wetter, showing 4-5” for the lowlands, except less in the rain shadow (1-1.5”), which is poorly defined in this forecast. The coast and Willamette Valley get 1.5-3”, along with parts of Eastern WA. The mountains across the region get 5-7” in this forecast.

Now, we will look at high-resolution forecasts, focused specifically on Western Washington. First, let’s take a look at the NAM high-resolution forecast through Wednesday evening.

The NAM forecast shows 3-3.5” for the lowlands, with isolated areas getting up to 4”. A very pronounced rain shadow is seen from Everett northward and westward (0.25-1.5”). The mountains could get up to 10” of rain in this forecast.

Next, the UW WRF forecast for Western Washington, showing rain through Wednesday evening.

This forecast is very similar to the NAM, showing over 3” of rain from Seattle southward, with 1.5-2.5” from Seattle north, excluding the rain shadow area (0.3-1.25”). The mountains will get 3-10” in this forecast.

The last forecast we’ll look at is the HRRR high-resolution forecast, which doesn’t go as far, only to Wednesday afternoon, seen below.

The HRRR forecast shows 2.5-3” for the lowlands, 3-4” for the foothills, and a relatively large rain shadow NE of the Olympics (0.7-1.5”). The mountains get up to 10” in this forecast as well.

Now, let’s take a look at the NWS Weather Prediction Center (WPC) Excessive Rainfall Outlook for Tuesday (Dec. 5).

This shows a Moderate risk (40% chance) of flash flood conditions on the Olympic Peninsula, with a Slight risk (15% chance) for the coast, Cascades, and lowlands from Seattle southward. A Marginal risk (5% chance) exists from Seattle northward in the lowlands.

Finally, let’s take a look at river flooding potential, shown below from the NWS Advanced Hydrologic Prediction Service forecast.

Yellow is bankfull stage, which many rivers will reach, including the Nooksack, White, Cedar, Puyallup, Willapa, and Issaquah Creek. Orange is minor flood stage, which the Carbon, Stillaguamish, and Bogachiel will reach. Red is moderate flood stage, which the Skykomish, Chehalis, Neuwakum, Cowlitz, Nisqually, and Skokomish, will reach. Magenta is major flood stage, which the Snoqualmie/Snohomish and Skagit Rivers are expected to reach.

Remember that in addition to river flooding, these large amounts of rain within a short timeframe will bring urban flooding, water over roadways, and flooding in creeks and small streams. Remember to “turn around, don’t drown” and to not drive through flooded roadways. Stay tuned for further updates!

Major Atmospheric River Ahead: Heavy Rain, River Flooding Expected

FastCast—Monday, Dec. 4 to Wednesday, Dec. 6:

A significant atmospheric river is ahead for the Northwest, with major rain and widespread river flooding expected. Mountain snow levels will rise from 3,500 feet to 10,000 feet, bringing rain to all but the peaks, and melting recent snow. This will bring sharp rises and flooding for most rivers draining the Cascades. Urban flooding is possible as well, so be aware on area roadways. In addition to rain-related hazards, gusty winds and warm temperatures are likely due to the atmospheric river. In the lowlands, expect 2-4” of rain through Wednesday. Temperatures on Monday and Tuesday will rise to the mid to upper 50s in the lowlands, with lows in the low 50s. On Wednesday, highs will drop to the upper 40s, with temps cooling to the low 40s. Winds will gust 30-40 mph in the lowlands as well, from Monday evening to Tuesday morning. Areas north of Everett and near the water could gust higher. Continue reading below for a full update on the atmospheric river.

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Rain will begin on Monday morning, continuing through Wednesday, before tapering off. Let’s take a look at the forecast.

Below is the European model forecast showing precipitable water, aka the amount of water in the atmosphere.

This forecast, for late Monday night, clearly shows the atmospheric river aimed directly at the Pacific Northwest. This plume of moisture stretches all the way to Hawaii (bottom left), making it a true “Pineapple Express.”

Zooming in using the UW WRF forecast, take a look at how the atmospheric river will slam into the Pacific Northwest.

Notice how the atmospheric river is oriented from SW to NE, the perfect position to slam into the Olympics, lowlands, and Cascades all at the same time. This shows why the snow levels in the Cascades will rise to 10,000 feet, since the subtropical moisture of the atmospheric river is oriented directly at the mountains.

Now, let’s take a look at the rain forecasts, starting with the European model. The forecast below is for total rain through Wednesday evening.

The European model shows the lowlands receiving 2-2.5” through Wednesday, with 2.5-3.5” in the foothills, 2.5-4” in the Willamette Valley and on the coast, and 3-7” in the Olympics & Cascades. This shows a slight rain shadow NE of the Olympic Peninsula, with that region getting 1.2-1.5”. Additionally, all of Eastern WA will receive rain, with the most (1-1.5”) from the Tri-Cities northeast to Spokane.

Let’s compare this to the GFS (American) model forecast, also through Wednesday evening, seen below.

The GFS forecast shows substantially more rain for the lowlands, with 3.5-4.5” expected. This gives the foothills up to 5.5”, with the coast getting 2.5-4”, the Willamette Valley getting 2.3-3”, and Eastern WA north of I-90 getting 0.5-2”. This forecast gives the Olympics and Cascades 5-8” of rain. The NE Olympic Peninsula rain shadow is more pronounced in this forecast, showing 0.7-1.5” in that region.

Now, let’s zoom in to Western Washington for a look at the high-resolution forecast, from the UW WRF model.

This forecast shows 1.5-2.5” for the lowlands, 2.5-5” for the foothills, and 2.5-5” for the coast, with the Olympics and Cascades getting 5-10”. The NE Olympic rain shadow is most pronounced in this forecast, with only 0.2-0.75” in the rain shadow.

Now, let’s shift gears to flooding. Below is the flooding forecast from the NWS Advanced Hydrologic Prediction Service, showing river flood levels for this storm.

Most rivers draining the Cascades & Olympics will reach bankfull (yellow). The Chehalis, Bogachiel, and Neuwakum Rivers will reach minor flood stage (orange). The Snoqualmie, Snohomish, Skykomish, Skagit, Tolt, Nisqually, Cowlitz, and Skokomish Rivers will all reach moderate flood stage (red), with the Snoqualmie at Carnation and Skagit at Concrete expected to reach major (magenta).

Next, let’s take a look at the increase in temperatures for the region due to the atmospheric river. Below is the European model forecast for highs on Monday.

This forecast shows lowland highs in the mid to upper 50s, with the Willamette Valley reaching the upper 50s to low 60s, and the coast reaching the mid 50s. Eastern Washington will be colder, with highs in the upper 30s to low 40s, except near Walla Walla and the Tri Cities, where highs will reach the 50s.

Finally, let’s take a look at the wind potential for Monday night to Tuesday morning, starting with the European model, seen below.

This forecast shows lowland winds gusting 30-40 mph, strongest near the water, with winds on Whidbey and Camano Islands reaching 40-45 mph, and the coast reaching 40-50 mph, with gusts over 50 mph possible from Westport southward.

The last forecast we will look at is the NAM high-resolution forecast for wind gusts late Monday evening. 

This forecast is far stronger than the European and is far less likely, but still possible. The NAM shows lowland winds gusting 40-50 mph, with areas from Everett northward gusting 45-55 mph. The coast would gust 45-55 mph, except up to 60-65 mph on the immediate shoreline. Again, this situation is less likely, but still possible.

I will have another update Monday night as the atmospheric river begins to impact the region. Stay tuned and stay safe!

Storm Update: Strong Winds, Heavy Snow, and Heavy Rain Through Saturday

No FastCast tonight…continue reading the full blog below for a storm update.

An impactful storm is ahead from late Friday night through Saturday. The biggest impacts will be strong winds early Saturday morning, heavy mountain snow, and heavy rain, with a potential for thunderstorms as well.

A Wind Advisory is in effect for the lowlands from 11 PM Friday to 6 AM Saturday, for sustained winds of 30-40 mph, gusting up to 50 mph. Tree damage and power outages are possible across the region. Additionally, the strongest winds (likely 12-4 AM) will be accompanied by heavy rain and potential thunderstorms.

Let’s start with the wind. Below is the high-resolution HRRR forecast for peak winds through Saturday morning.

The HRRR forecast shows winds in the lowlands gusting 45-50 mph, with gusts up to 60 mph possible around Bellingham in this forecast. The coast will gust up to 55 mph. 

Let’s compare this to the NAM forecast, showing wind gusts at 3 AM Saturday.

The NAM agrees with the HRRR, showing wind gusts in the lowlands gusting 45-50 mph. Isolated gusts up to 55 mph are possible across the lowlands, especially near the water.

Next, let’s take a look at the HRRR forecast for lightning flash density, which shows lightning is possible. This forecast is valid through Saturday morning.

The highest chance of thunderstorms looks to be from Everett northward to Whidbey Island and from Olympia to Tacoma. Of course, an isolated lightning strike and rumble of thunder is possible anywhere across the lowlands.

Next, the forecast for mountain snow. We’ll start with the European model forecast for snow through Saturday night.

This forecast shows an additional 12-18” at the passes through Saturday night, with up to 4-6” for Spokane, Pullman, Ellensburg, and Wenatchee. 

Let’s compare this to the NAM high-resolution forecast for snow through Saturday night.

The NAM forecast agrees with 12-18” of snow at the passes, but shows less snow for Ellensburg and Wenatchee. This forecast also shows 4-6” of snow for Spokane and Pullman.

Finally, let’s take a look at how much rain is expected through Saturday night, starting with the European model, seen below.

The European model shows a rain shadow over the lowlands (relative to totals for the rest of the region), with lowland totals of 0.5-0.8”. The coast and Willamette Valley will receive 0.9-1.3”. This will all fall as snow in the mountains (3,000+ feet).

Let’s compare this to the HRRR high-resolution rain forecast, seen below, also showing total rain through Saturday night.

This forecast shows less rain overall, with 0.4-0.75” for the lowlands, with 0.5-1” for the coast and Willamette Valley.

Much more active and impactful weather is ahead for the next few days, including a major atmospheric river early next week. Stay tuned & stay safe!