Atmospheric River Impacts Southeast Alaska

A large plume of concentrated moisture labeled an  Atmospheric River (AR)  moved through Southeast Alaska (SEAK) starting late November 30th, 2020 lasting through December 2nd, 2020.

This event brought widespread record-setting rainfall to locations across Southeast Alaska. For some, precipitation started as snow before transitioning into rain as temperatures rose with the warm front. With warming temperatures and falling rain, snow began to melt that had fallen prior. These conditions lead to more water runoff impacting towns such as Haines, Skagway Juneau, Gustavus, Hoonah, and Tenakee Springs. In addition to heavy precipitation, strong winds were also reported across the panhandle along with heavy snow in the town of Hyder near the British Columbia border as the storm moved through.

These combined weather conditions on top of already saturated soils created mass wasting events (MWE) such as landslides along with inundation flooding, storm surge, and powerline damages across SEAK.

As the front approached from the west, high wind gusts began in Yakutat as early as November 30th, with 63 mph wind gusts reported at PAYA at 11:38 pm AKST. The front pushed eastward and then stalled over the southeast panhandle on December 1st keeping the high wind threat around for much of the day. Juneau Airport (PAJN) reported a 60 mph wind gust as early as 4:53 am AKST that morning, then frequent strong wind gusts through the day in the Juneau area. The strongest winds during this event occurred along the outer coast and in the southern panhandle as gusts reached over 70 mph. Salmon Landing (SLXA2) reported a wind gust of 67 mph at 6:45 am while Zarembo Island (ZMBA2) reported a 64 mph gust at 10:55 am. The Sitka area saw the highest wind gusts for sea-level sites during this event with a 72 mph gust at the Sitka ASOS (PASI) at 10:42 pm. Surface winds began to diminish into the early morning hours of December 2nd with high winds only reported at upper elevation sites. Eaglecrest (JECA2) reported a 99 mph gust at 12:45 am on Dec 1st and a 79 mph wind gust at 6:45 am on December 2nd, however, at this time sea level sites were not reporting high winds. 

The event began with an air mass supporting snow at sea level with cooperative observer (COOP) reports of 2-5 inches at low elevations and 8-12 inches at higher elevations on December 1st. Some standouts include the Haines 40NW site which received 6.5 inches and Eaglecrest top's 2 day total of 15.7 inches. On December 2nd, warm air advection caused a change over to rain. Higher elevation sites remained as snow longer increasing snow depths at Flower Mountain (2510 ft) by 34 inches and Ripinski Ridge (800 ft) by 59 inches through December 2nd. The image to the right shows Rapinski Ridge while the image below shows Flower Mountain. Notice the sharp increase in snowfall associated with the onset of the atmospheric river.

Another area that received significant snowfall was Hyder. Snow fell on the days leading up to Dec 1st and 2nd with about 35 inches at sea level and much more at higher elevations. As temperatures warmed through Dec 2nd, heavy rain fell on the snowpack to increase runoff and slope instability. Multiple debris flows and avalanches isolated Hyder, Alaska from Stewart, British Columbia, Canada for 2 day by blocking the only road connecting the communities. Hyder was also without power.

A preliminary damage assessment reported over $29,000,000 worth of damage to public infrastructure, not including private property losses due to the event.

Both the Governor of Alaska and the President of the United States declared this event a major disaster, allowing a greater level of response from the Federal Emergency Management Agency (FEMA) and other assets. Local governments also declared disaster emergencies, including the municipalities of Haines, Gustavus, Hoonah, and Juneau. 

As a result of the weather events on November 30th through December 2nd, federal funding became available to state, tribal, eligible local governments, and certain private nonprofit organizations on a cost-sharing basis. The funding was used for emergency work and to repair or replace facilities damaged by the strong winds, flooding, and landslides. Federal funding also became available on a cost-sharing basis for hazard mitigation measures statewide.

While this AR set multiple precipitation records across Southeast Alaska, it was a fairly well-forecasted event with multiple numerical model solutions suggesting extreme values days in advance.

Flood watches were issued ahead of the bulk of precipitation, with a flash flood watch and then a flash flood warning issued for additional landslides in and around the city of Haines. Decision support services associated with this storm system culminated in dispatching the senior service hydrologist and a lead meteorologist to Haines. They provided in-person support for the Haines Emergency Operations Center for weather information relating to search and rescue and recovery operations. 

Today, lessons learned from this event are being used to better serve communities in Southeast Alaska in storm preparedness and response messaging by NWS Juneau. In addition to enhanced messaging, new tools have been developed at WFO Juneau to better assist meteorologist/hydrologist in identifying potentially impactful rain events for flooding and landslides.

A term you will often hear for large flood events is an Annual Return Interval (ARI). An example would be a 50-year rainfall event. Another way to think of the ARI is the Annual Exceedance Probability (AEP). For a 50-year ARI event the AEP would be a 2% chance of occurring in a given year. So the higher the ARI, the lower the annual probability, and the more rare or extreme the event is compared to historical data.

One of these tools is an interactive dashboard that displays live rainfall measurements and compares them to ARI. This display below shows the tool now developed for meteorologists/hydrologists to detect what locations are receiving significant rainfall amounts at certain time frames. It allows NWS Forecasters to better assess the potential threats that could occur and inform emergency management of potential hydrologic threats from flooding to landslides.

NWS Juneau continues to work with partners to establish key weather/hydrologic hazard thresholds. The office has hosted tabletop exercises (discussion-based sessions where team members meet to discuss their roles during an emergency and their responses to a particular emergency situation) with core partners, including emergency managers, city/borough officials, and fire/police. From these exercises, key thresholds have been established as well as improvements to how NWS Juneau messages information to the public during high-impact weather events. From all the lessons learned from this event and continued conversations with NWS Juneau's partners, the office continues to enhance its Impact-Based Decision Support Service (IDSS) capabilities and services provided to the public.

Total and Layered Precipitable Water: By December 1st at 00z, a long plume of high Total Precipitable Water (TPW) (1.25-1.75”) extended northeast from south of 30°N into the south-central Gulf of Alaska. This plume was along the warm side of the strong frontal zone and indicative of a developing Atmospheric River (AR). The plume then pivoted north with the warm sector of the front and moved over Southeast Alaska at 12z December 2nd.

By 00z on December 3rd, the main plume of moisture had weakened and was confined along the cold front over the eastern Gulf of Alaska. The cold front moved through the panhandle of Alaska by 12z on December 3rd with no significant amount of total precipitable water across SEAK.

250 millibar (mb): On December 1st at 12z, a 250mb trough of low pressure was in the process of digging into the North Pacific. The strong ridge over the west CONUS directed a north/south-oriented jet streak over the Gulf of Alaska and produced a long southwest fetch of onshore flow over Southeast Alaska. By 12z on December 2nd, the trough had become much sharper with a very deep fetch of moisture into the North Pacific. There was also a large increase in the wind speeds with a jet streak over 200 knots across the Gulf of Alaska. The entire pattern shifted to the east by 00z on December 3rd with the trough becoming negatively tilted and weakening of the jet max as winds became more southerly.

700-500mb: A strong 500mb ridge extended north from the west coast of the CONUS and into northern Canada with a digging trough over the west coast of Alaska and into the North Pacific. There were also strong steering winds and high vorticity associated with the digging trough. This increased the vertical ascent and enhanced precipitation as it propagated eastward. By 12z on December 2nd, the upper ridge had sharpened and drifted to the east with the axis over the western CONUS and the base of the upper trough moving northeastward. Due to the strength of the onshore flow, the thermal gradient, and the mountainous terrain over Southeast Alaska, there was a tremendous amount of vertical motion (Omega). This very strong dynamical forcing for ascent along with lower-level orographic effects from the strong low-level jet (LLJ) enhanced precipitation development especially with the amount of TPW over the area available to condense.

850-700mb: On December 1st at 12z, the amount of moisture transport within the AR was focused on the warm side of a strong warm front pointed right at Southeast Alaska. Precipitation production increased from the amount of moisture transport, the strength of the upper-level dynamics, and orographic effects. By 12z on December 2nd, the flow had backed to the south with a clear indication of the warm front position and continued high moisture transport in the 850-700mb layer, particularly over the northern third of Southeast Alaska. One thing to note is that the southerly flow at this layer is a preferred direction for the northern third of the panhandle as it limits downsloping.

There was a southwest 850mb jet over Southeast Alaska which extended into the North Pacific. This low level jet was greater than 85 knots and was directed right into the terrain enhancing geography at 12z on December 1st. As the Atmospheric River (AR) drifted east and the warm front lifted north, the 850mb winds backed to the south and remained very strong over the entire panhandle. Notice the maxima over the north central inner channels in the animation below. The strong southerly 50 knots winds over the far northern inner channels aided in moisture advection.

A very warm air mass is associated with the warm sector of the Atmospheric River (AR) with temperatures near +10℃ over the southeast Gulf of Alaska. This warm air surged north through Dec 1st as the winds backed to the south and as the digging upper trof became negatively tilted. By 06z on December 2nd, the very warm and moisture-laden air had eroded all the cold air away with all areas over Southeast Alaska significantly above 0°C at 850mb. Consequently, snow levels rose above 4,000 feet which produced an environment for significant warm rain processes. 

Boundary-Layer (Surface-850mb): The magnitude of moisture transport and winds in the 925-850mb layer was very similar to the 850-700mb layer. This amount of lower-level moisture transport and warm rain processes enhanced orographic effects across the complex terrain of Southeast Alaska. The very tight surface pressure gradient over the inner channels and Southeast Gulf of Alaska along with the latent heat release from the precipitation development; produced a very strong low-level jet of 70 to 80 knots at 925mb.

The strong south-southwest winds extending into the North Pacific enhanced the orographic effects from the complex terrain of Southeast Alaska. These south-southwest winds also mixed down to the surface. The strong low-level jet drifted northward through the inner channels and by 12z on December 2nd, the 925mb winds had backed around to the south-southeast ranging from 50 to 70 knots. This continued to aid in sustained moisture transport from the abundant total precipitable water (TPW) that was available throughout the entire column. The animation below shows the CIRA advected layered TPW which splits the atmosphere at different levels to see how deep the moisture is on December 2nd. This type of information is very important to forecasters as when all levels through the atmosphere line up with moisture as indicated below it is indicative of heavy precipitation. It also shows that the region will continue to see heavy precipitation due the length of the moisture plume.

Satellite imagery can be very useful to forecasters when observations are limited throughout the region and the lack of surface base radar coverage over the inner channels of SEAK. For example, satellite-derived rain rate products can give a good estimate of how much rainfall is falling over an area. The images below show 3 different satellite-derived rain rate products each with own strengths and weaknesses along with an snowfall rate product that shows the snowfall rate within the cloud. Notice the highest rain rates and snowfall rates occur through the northern panhandle as indicated by the dark blue to orange colors.