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CW3E Event Summary: 13-23 January 2024

29 January 2024

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Four Atmospheric Rivers Highlight Active Weather Period Across US West Coast

A series of four distinct atmospheric rivers made landfall along the US West Coast between Jan 13 and Jan 23 2024
This prolonged period of active weather for the region resulted in a variety of liquid and frozen precipitation impacts
The first AR made landfall along the OR/CA border early on Sat 13 Jan alongside a low pressure system, bringing heavy precipitation to the OR/CA border and the Southern Cascades.
A cut-off low pressure system brought the second AR to the USWC. The burst of IVT with the AR alongside the persistence of the low pressure system resulted in heavy precipitation in the PNW, including significant freezing rain in the Portland Metro, and heavy snowfall in the Cascades.
The third AR in the sequence developed as the persistent low pressure system shifted into the Gulf of Alaska, resulting in counterclockwise moisture transport around the cyclone, leading to southerly IVT transport along the US West Coast
A fourth AR developed over the eastern North Pacific, with a robust corridor of elevated moisture transport extending greater than 2,500 miles from north of Hawaii to the US West Coast
The highest 10-day precipitation totals (> 10 in.)were observed along CA/OR border and over Northern Sierra Nevada
Snowfall accumulations during this period ranged from 2-6 feet in the Cascades, Sierra Nevada, and the higher terrain in the Upper Colorado River Basin
Addition impacts during this active period included river level rises due to heavy precipitation and a multi-day freezing rain event in the Pacific Northwest

Click images to see loops of GFS IVT/IWV analyses Valid: 0000 UTC 13 January – 0000 UTC 23 January 2024

Summary provided by M. Steen, S. Bartlett, J. Kalansky, and P. Iniguez; 29 Jan 2024

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CW3E AR Update: 29 January 2024 Outlook

January 29, 2024

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Pair of Strong ARs Forecast to Bring Heavy Precipitation to USWC

An active weather pattern for the US West Coast is forecast to continue through Fri 2 Feb as two more strong ARs make landfall along the USWC.
The first AR made landfall late 28 Sun Jan into British Columbia and the PNW. A second pulse of IVT associated with this system continues AR conditions in the PNW through Tue 30 Jan.
The second AR is forecast to make landfall across the USWC later on Tue 30 Jan and progress down the USWC through Fri 2 Feb.
There is likely development of a ridge over the Northeast Pacific following the second AR, leading to a break in AR conditions for the USWC.
Both ARs are forecast to bring precipitation to the USWC, with the heaviest rainfall expected from the second AR over Northern CA and heavy snowfall over the Sierra Nevada.
The WPC Excessive Rainfall Outlook indicates a Slight Risk (level 2 of 4, or at least 15% chance) for flash flooding in Northern CA for the 24-hour period ending 4 AM PT Thu 1 Feb and in Southern CA for the 24-hour period ending 4 AM PT Fri 2 Feb with the second AR.
Although there is potential for flooding in areas discussed in this outlook, the forecasts are not predicting anything in the magnitude of the ARkStorm scenarios developed by Porter et al., 2011 or Huang et. al., 2022

Click images to see loops of GFS IVT and IWV forecasts Valid 1200 UTC 29 January 2024 – 1200 UTC 2 February 2024

Summary provided by M. Steen, S. Bartlett and J. Kalansky; 29 January 2024

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CW3E Publication Notice: Advancing atmospheric river science and inspiring future development of the Atmospheric River Reconnaissance Program

CW3E Publication Notice

Advancing atmospheric river science and inspiring future development of the Atmospheric River Reconnaissance Program – in the January 2024 issue of Bulletin of the American Meteorological Society

January 29, 2024

A new paper titled “Advancing atmospheric river science and inspiring future development of the Atmospheric River Reconnaissance Program” by David A. Lavers (ECMWF), Anna Wilson (CW3E), F. Martin Ralph (CW3E Director and AR Recon PI), Vijay Tallapragada (NOAA/NCEP, AR Recon Co-PI), Florian Pappenberger (ECMWF), Carolyn Reynolds (NRL), James D. Doyle (NRL), Luca Delle Monache (CW3E), Chris Davis (NCAR), Aneesh Subramanian (University of Colorado, Boulder), Ryan D. Torn (University at Albany), Jason M. Cordeira (CW3E), Luca Centurioni (SIO), and Jennifer S. Haase (SIO), was recently published in Bulletin of the American Meteorological Society. On June 27-29 2023, forty collaborators from research, operational, and academic centers met at the Atmospheric River Reconnaissance Workshop 2023 at ECMWF in Reading, UK. This paper summarizes the latest research, science questions raised, and future campaign plans presented at the AR Recon Workshop. Holding and reporting on this workshop contributes to CW3E’s 2019-2024 Strategic Plan Atmospheric River Research and Applications Priority Area, both by using AR Recon to further understand AR dynamics, and to enhance global AR monitoring.

Presentations, posters, and discussions were grouped under the following titles: (1) AR Recon in water year 2023; (2) Modeling, data assimilation, and impact studies; (3) Scientific advances in physical process understanding; and (4) the future of AR Recon. During the workshop, the multiple science questions were raised and suggested for further study, possibly through working groups. Topics included ways to refine and coordinate on impact studies with the various modeling centers, proposals for new verification metrics, exploring new sampling strategies, and leveraging new observational platforms (e.g., the NOAA G550 aircraft that will be available in coming years).

Recommendations for the future included the spatial expansion of observations to increase short and medium range forecast skill, more effective use of increased quantity and diversity of observations, and exchange of visiting scientists between institutions. The current AR Recon season began in November 2023 and the next AR Recon Workshop will be held in the fall of 2024.

The full meeting agenda, recording of each talk, copies of presentation slides, and posters presented at the meeting can be found here, on ECMWF’s website for the event.

Figure 1: Figure 2 from Lavers et al., 2024: A schematic showing the observing platforms currently used in the AR Recon program. This is an updated schematic that was first published in Zheng et al. (2021).

Lavers, D. A., Wilson, A. M., Ralph, F. M., Tallapragada, V., Pappenberger, F., Reynolds, C., Doyle, J. D., Monache, L. D., Davis, C., Subramanian, A., Torn, R. D., Cordeira, J. M., Centurioni, L., & Haase, J. S. (2024). Advancing atmospheric river science and inspiring future development of the Atmospheric River Reconnaissance Program. Bulletin of the American Meteorological Society, 105, E75-E83. https://doi.org/10.1175/BAMS-D-23-0278.1

CW3E Subseasonal Outlook: 26 January 2024

January 26, 2024

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Summary provided by Z. Yang, C. Castellano, J. Wang, M. DeFlorio, and J. Kalansky; 26 January 2024

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CW3E AR Update: 26 January 2024 Outlook

January 26, 2024

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Several Strong ARs Forecast to Impact US West Coast Continuing Active Weather Period

An active weather pattern for the US West Coast is forecast to continue through Wed 31 Jan and potentially beyond.
The first AR made landfall early Fri 26 Jan into the PNW. This AR kicks off a period of AR conditions in the PNW and Northern California expected to continue through Wed 31 Jan.
The second AR, stronger than the first, is forecast to make landfall early Sun 28 Jan into the PNW and British Columbia, continuing AR conditions in the region.
The third, strongest AR is forecast to make landfall into British Columbia and the PNW toward the end of the second AR period on Tue 30 Jan.
All three ARs are forecast to bring precipitation to the USWC, with the heaviest rainfall expected from the first and second ARs over CA/OR border and Olympic Peninsula and in N. CA with the third AR.
The WPC Excessive Rainfall Outlook indicates a Slight Risk (level 2 of 4, or at least 15% chance) for flooding along the CA/OR border for the 24-hour period ending 4 AM PT Sat 27 Jan with the first AR.
The third AR is forecast to progress down the USWC and potentially bring impacts to much of the Western US.
The Climate Prediction Center (CPC) has already indicated a moderate risk for heavy precipitation, heavy snow, and high winds for regions in the Western US and possible flooding along the CA coast and in AZ for Feb 2 through Feb 5.
The British Columbia River Forecast Center has issued Flood Watches for Vancouver Island, the South Coast and Fraser Valley as the second and third ARs are forecast to bring as much as 10 inches of rain to the region.

Click images to see loops of ECMWF IVT and IWV forecasts Valid 1200 UTC 24 January 2024 – 1200 UTC 1 February 2024

Summary provided by M. Steen, C. Castellano, S. Bartlett, J. Kalansky; 26 January 2024

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*Outlook products are considered experimental

CW3E AR Update: 24 January 2024 Outlook

January 24, 2024

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Active Weather Forecast to Continue into Early February for US West Coast

An active weather pattern for the US West Coast is forecast to continue through Tue 30 Jan and potentially beyond.
The first AR period begins early Fri 26 Jan as an AR makes landfall into the PNW. This AR kicks off a period of AR conditions in the PNW and Northern California expected to continue through Tue 30 Jan.
The second AR is forecast to make landfall early Sun 28 Jan into the PNW, continuing AR conditions in the region.
The third, strongest AR is forecast to make landfall into British Columbia and the PNW toward the end of this AR period on Tue 30 Jan.
All three ARs are forecast to bring precipitation to the USWC, with the heaviest rainfall expected from the first and second ARs over CA/OR border and into the PNW with the third AR.
The WPC Excessive Rainfall Outlook indicates a Marginal Risk (level 1 of 4, or at least 5% chance) for flooding in days 3 through 5 (24-hour periods ending 4 AM PT Sat 27 Jan, Sun 28 Jan and Mon 29 Jan) along the PNW coast and over the Olympic Peninsula.
The third AR is forecast to progress down the USWC and potentially bring impacts to much of the Western US. The Climate Prediction Center (CPC) has already indicated a moderate risk for heavy precipitation, heavy snow, and high winds for regions in the Western US and possible flooding along the CA coast and in AZ for Jan 31 through Feb 5.

Click images to see loops of GFS IVT and IWV forecasts Valid 1200 UTC 24 January 2024 – 0000 UTC 1 February 2024

Summary provided by M. Steen, C. Castellano, S. Bartlett, S. Roj and P. Iniguez; 24 January 2024

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*Outlook products are considered experimental

CW3E Publication Notice: Subseasonal prediction of impactful California winter weather in a hybrid dynamical-statistical framework

CW3E Publication Notice

Subseasonal prediction of impactful California winter weather in a hybrid dynamical-statistical framework

January 23, 2024

A new paper titled “Subseasonal prediction of impactful California winter weather in a hybrid dynamical-statistical framework” was recently published in Geophysical Research Letters by Kristen Guirguis (CW3E), Alexander Gershunov (CW3E), Benjamin Hatchett (Desert Research Institute), Michael DeFlorio (CW3E), Aneesh Subramanian (University of Colorado, Boulder), Rachel Clemesha (CW3E), Luca Delle Monache (CW3E), and Marty Ralph (CW3E). This research was sponsored by the California Department of Water Resources (CA DWR) Atmospheric River Program, and contributes to the Subseasonal-to-Seasonal (S2S) Prediction of Extreme Weather Priority Area in CW3E’S 2019-2024 Strategic Plan.

The paper describes a methodology for subseasonal prediction of impactful winter weather in California, focusing on atmospheric rivers (ARs), Santa Ana winds (SAWs), and heat waves. The hybrid approach combines output from dynamical forecast models with a statistical model that relates large scale atmospheric circulation patterns to impacts in California. The approach builds on earlier work (Guirguis et al. 2018, 2020, 2022) linking atmospheric weather patterns to ARs, extreme precipitation, historic California floods, SAWs, heat extremes, and Southern California wildfires. The forecast system was evaluated over a 20-year period using ensemble hindcasts from the European Center for Medium-Range Weather Forecasts (ECMWF) model, along with several observational datasets, and was shown to have skill beyond climatology at subseasonal timescales. These results suggest its potential application in decision support for water, health, land, and fire management. We anticipate that these findings will serve as the foundation for developing real-time forecast tools, which could find applications in early warning systems to support decisions across diverse sectors. An experimental S2S forecast product based on this methodology is currently tested internally at CW3E.

Figure 1: (modified version of Figure 2 from Guirguis et al. 2023) Hindcast skill assessment of (a) heat waves in three regions, (b) SAWs in Southern California, and (c) ARs at four coastal latitudes. The y-axis shows observed event frequency following three forecast categories: low (blue), above normal (orange), and much above normal (red) probability. The sign of the Brier Skill Score (+/-) is shown along the bottom, highlighted in yellow. The gray shaded area gives the 10^th-90^th percentiles of the resampled distribution over the hindcast period (2001-2020). Red, orange, and blue markers are weighted by the log of the sample size (n=10 to 451), and filled markers indicate statistically significant skill (90% level using resampling).

Guirguis, K., Gershunov, A., Clemesha, R. E. S., Shulgina, T., Subramanian, A. C., & Ralph, F. M.(2018). Circulation drivers of atmospheric rivers at the North American West Coast. Geophysical Research Letters, 45, 12–576. https://doi.org/10.1029/2018GL079249

Guirguis, K., Gershunov, A., DeFlorio, M. J., Shulgina, T., Delle Monache, L., Subramanian, A. C., et al. (2020). Four atmospheric circulation regimes over the North Pacific and their relationship to California precipitation on daily to seasonal timescales. Geophysical Research Letters, 47, e2020GL087609. https://doi.org/10.1029/2020GL087609

Guirguis, K., Gershunov, A., Hatchett, B., Shulgina, T., DeFlorio, M. J., Subramanian, A. C., et al. (2022). Winter wet–dry weather patterns driving atmospheric rivers and Santa Ana winds provide evidence for increasing wildfire hazard in California. Climate Dynamics, 60(5–6), 1729–1749. https://doi.org/10.1007/s00382-022-06361-7

Guirguis, K., Gershunov, A., Hatchett, B. J., DeFlorio, M. J., Subramanian, A. C., Clemesha, R., et al. (2023). Subseasonal prediction of impactful California winter weather in a hybrid dynamical-statistical framework. Geophysical Research Letters, 50, e2023GL105360. https://doi.org/10.1029/2023GL105360

CW3E AR Update: 18 January 2024 Outlook

January 18, 2024

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Active Weather Pattern Continues in Pacific, Bringing Precipitation to US West Coast

An active weather pattern for the US West Coast is forecast to continue through Wed 23 Jan and potentially beyond.
The first AR begins on Thu 18 Jan as a low-pressure system drives it up the USWC.
The first AR is forecast to bring heavy precipitation to the PNW, including snowfall in the Northern Cascades and freezing rain in the Portland Metro area and regions along the WA/OR border through early Fri 19 Jan.
The second AR period begins late Sun 20 Jan as a large AR begins to make landfall across the USWC.
A potential third AR follows shortly behind with a short burst of IVT into the PNW on Tue 23 Jan.
The second and third ARs are forecast to bring precipitation to the USWC, with the heaviest precipitation expected from the second AR over the CA coast and in the Sierra Nevada, where heavy snowfall (>12”) is forecast through Tue 22 Jan.
Fresh snowpack and moist soils from previous events that impacted the USWC present the risk for rain-on-snow flooding.
The WPC Excessive Rainfall Outlook indicates a Slight Risk (level 2 of 4, or at least 15% chance) for flooding in days 4 and 5 (24-hour periods ending 4 AM PT Mon 22 Jan and Tue 23 Jan) for much of the CA coast.
There is potential for a fourth AR to make landfall in the PNW indicated by the ECMWF EPS. The Climate Prediction Center (CPC) has already indicated a slight risk for heavy precipitation, heavy snow, and high winds in the PNW for Jan 25-27 when it may make landfall.

Click images to see loops of GFS IVT and IWV forecasts Valid 1200 UTC 18 January 2024 – 0000 UTC 25 January 2024

Summary provided by M. Steen, C. Castellano, S. Bartlett and P. Iniguez; 18 January 2024

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CW3E Subseasonal Outlook: 17 January 2024

January 17, 2024

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Summary provided by C. Castellano, J. Wang, Z. Yang, M. DeFlorio, and J. Kalansky; 17 January 2024

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*Outlook products are considered experimental

CW3E AR Update: 16 January 2024 Outlook

January 16, 2024

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Active Weather Pattern to Bring Precipitation to US West Coast

An active weather pattern for the US West Coast is forecast to begin late Tue 16 Jan, when the first of three AR periods begins.
The first AR period begins late Tue 16 Jan and continues through Wed 17 Jan. The second AR follows close behind on Thu 18 Jan.
The first and second ARs are forecast to bring heavy precipitation to the PNW, including heavy snowfall (>12”) in the Northern Cascades and freezing rain in the Portland Metro area and regions along the WA/OR border through early Fri 19 Jan.
The third AR period begins late Sun 20 Jan as a large AR begins to make landfall across the USWC.
The third AR period is forecast to bring precipitation to the USWC, with the heaviest precipitation expected over the CA coast and in the Sierra Nevada, where heavy snowfall (>12”) is forecast through Tue 22 Jan.
The NWS Weather Prediction Center is forecasting > 6 inches of precipitation in the Cascades, Sierra Nevada and for regions along the WA, OR and CA coasts over the next 7 days.
Fresh snowpack and moist soils from previous events that impacted the USWC present the risk for rain-on-snow flooding.
The WPC Excessive Rainfall Outlook indicates a Marginal Risk (level 1 of 4, or at least 5% chance) for flooding in days 2 through 5 (24-hour periods ending 4 AM PT Thu 18 Jan through Sun 21 Jan).