CW3E Publication Notice: Impact of atmospheric rivers on Arctic sea ice variations

CW3E Publication Notice

Impact of atmospheric rivers on Arctic sea ice variations

February 5, 2024

A new paper titled “Impact of atmospheric rivers on Arctic sea ice variations” led by Linghan Li with co-authors Forest Cannon (Tomorrow.io), Matthew Mazloff (UCSD), Aneesh Subramanian (University of Colorado Boulder), Anna Wilson (CW3E), and Fred Martin Ralph (CW3E Director), was recently published in the European Geosciences Union’s journal The Cryosphere. This paper examines how atmospheric rivers (ARs) contribute to variations in Arctic sea ice, which has been rapidly decreasing especially in the summer in recent decades. The research first focuses on physical processes in two case studies of ARs in 2012 and 2020, then expands to a larger statistical analysis for 1981-2020 over the entire Arctic Ocean. This research contributes to the Atmospheric River Research and Applications, and the Monitoring and Projections of Climate Variability and Change Priority Areas in CW3E’s 2019-2024 Strategic Plan by adding to the understanding of the global impacts of atmospheric rivers and the relationship with climate change in polar regions.

Li uses hourly data at 0.25° × 0.25° resolution from ERA5, the most recent atmospheric reanalysis from ECMWF, to study causal relationships between ARs and decreases in sea ice concentration. In August 2012 and July 2020, ARs associated with large cyclones triggered rapid sea ice melt through modulating turbulent heat fluxes and winds. In a statistical analysis on weather timescales, Li finds a significant negative correlation between atmospheric moisture content and the rate of changes in sea ice concentration over almost the entirety of the Arctic Ocean (Figure 1, Figure 10 from Li et al. 2024). Ice concentration changes are also shown to be negatively correlated with northward winds and with latent and sensible heat fluxes. The work demonstrates how conditions associated with ARs play an important role in the changing Arctic sea ice cover.

Figure 1: (Figure 10 from Li et al. 2024) (a) Rank correlation between anomalies of IWV and sea ice concentration tendency. Only significant correlations are plotted. (b) Rank correlation between anomalies of northward wind and sea ice concentration tendency. (c) Rank correlation between anomalies of latent heat flux and sea ice concentration tendency. (d) Rank correlation between anomalies of sensible heat flux and sea ice concentration tendency.

Li, L., Cannon, F., Mazloff, M. R., Subramanian, A. C., Wilson, A. M., & Ralph, F. M. (2024). Impact of atmospheric rivers on Arctic sea ice variations. The Cryosphere, 18(1), 121-137. https://doi.org/10.5194/tc-18-121-2024

CW3E AR Update: 2 February 2024 Outlook

CW3E AR Update: 2 February 2024 Outlook

February 2, 2024

Click here for a pdf of this information.

Strong Atmospheric River to Drive High-Impact Precipitation Event Over California

  • A strong AR and low pressure system are forecast to make landfall over Central CA Sun 4 Feb and progress down the CA coast through Mon 5 Feb.
  • There is uncertainty between the GFS and ECMWF deterministic models on the strength of the low pressure system and AR as well as the landfall location.
  • The GFS is forecasting the low pressure and AR to be stronger than the ECMWF and make landfall further north in CA.
  • The GEFS and West-WRF ensemble are forecasting AR3 conditions (based on Ralph et al. 2019 AR scale) over the central CA coast, with AR1/2 conditions in northern and southern CA.
  • This AR is forecast to bring significant precipitation to much of CA, including heavy rainfall along the central and southern CA coasts and heavy snowfall in the Sierra Nevada.
  • The WPC is forecasting greater than 6 inches of precipitation over the Sierra Nevada and Transverse Ranges over the next 7 days.
  • The WPC Excessive Rainfall Outlook indicates a Moderate Risk (level 3 of 4, or at least 40% chance) for flash flooding in Central CA coast for the 24 hour period ending 4 AM PT Mon 5 Feb and Los Angeles Metro Area for the 24 hour period ending 4 AM PT Tue 6 Feb.
  • NWS San Francisco has issued a high wind warning from the South Bay area to San Luis Obispo County for southerly wind gusts possibly exceeding 60 mph from 4am to 10pm Sunday 4 Feb

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 2 February 2024 – 1800 UTC 6 February 2024


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Summary provided by M. Steen, S. Bartlett and S. Roj; 2 February 2024

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*Outlook products are considered experimental

CW3E Publication Notice: Keeping Water in Climate-Changed Headwaters Longer

CW3E Publication Notice

Keeping Water in Climate-Changed Headwaters Longer

February 2, 2024

Figure 1. A parched landscape at Tuolumne Meadows, Yosemite National Park, captured in October 2011 highlights a growing concern for the Park Service: the recent dryness, a troubling trend over the past two decades, affecting mountain meadows across the West. Credit: Mike Dettinger.

In the recent essay “Keeping Water in Climate-Changed Headwaters Longer” published in the San Francisco Estuary and Watershed Science journal, CW3E authors Michael Dettinger, Anna Wilson, and Garrett McGurk delve into strategies for improving water retention in California’s headwaters affected by climate change. The article recommends more proactive measures in headwater regions to address the adverse impacts of climate change on water resources, to augment current downstream-focused adaptation strategies. This research contributes to the Monitoring and Projections of Climate Variability and Change Priority Area in CW3E’S 2019-2024 Strategic Plan, by discussing water management decision-making in scenarios including current and future extremes.

The article emphasizes that current water management practices are not sufficient to tackle the root problems caused by climate change, such as warmer temperatures, increased evapotranspiration, more intense winter storms, flashier flows, and drier summer conditions. It proposes upstream interventions like beaver repopulation, forest health treatments, and Forecast Informed Reservoir Operations (FIRO) as means to delay water movement to downstream systems, better mimicking historical hydrographs, which could help mitigate winter flood risks, reduce summer dryness and wildfire dangers, and improve groundwater recharge.

The authors highlight that even minor efforts to prolong water retention in headwaters could significantly benefit downstream water supplies, potentially reducing the need for extensive, and more invasive, water management adaptations across California to cope with the impacts of climate change on water availability. For a detailed exploration of the challenges and ideas to spark work towards real and sustainable solutions, the full paper is available here.

Figure 2. View east from the Tahoe Rim Trail, captured in July 2019, showcases a dense yet vibrant forest of the sort being addressed by state-led treatments for wildfire prevention and forest health, with the rain-shadowed Pinon Range and expansive Great Basin stretching into the distance. Credit: Mike Dettinger.

Dettinger, M., Wilson, A., & McGurk, G. (2023). Keeping Water in Climate-Changed Headwaters Longer. San Francisco Estuary and Watershed Science, 21(4) https://doi.org/10.15447/sfews.2023v21iss4art1. Retrieved from https://escholarship.org/uc/item/7mq8174f

CW3E Publication Notice: An Assessment of Dropsonde Sampling Strategies for Atmospheric River Reconnaissance

CW3E Publication Notice

An Assessment of Dropsonde Sampling Strategies for Atmospheric River Reconnaissance

January 31, 2024

A new article titled “An Assessment of Dropsonde Sampling Strategies for Atmospheric River Reconnaissance” By Minghua Zheng (CW3E), Ryan Torn (University at Albany), Luca Delle Monache (CW3E), James Doyle (Naval Research Laboratory), F. Martin Ralph (CW3E), Vijay Tallapragada (NOAA/NCEP/EMC), Christopher Davis (NCAR), Daniel Steinhoff (CW3E), Xingren Wu (NOAA/NCEP/EMC), Anna Wilson (CW3E), Caroline Papadopoulos (CW3E), and Patrick Mulrooney (CW3E) was recently published in the American Meteorological Society’s Monthly Weather Review. As part of CW3E’s 2019-2024 Strategic Plan to support Atmospheric River Research and Applications, CW3E seeks to enhance global AR monitoring through a transformative modernization of atmospheric measurements over the Pacific and in the western United States. In alignment with this goal, the study explores the impact of varying mission frequency, dropsonde spacing, and aircraft utilized during AR Reconnaissance (AR Recon) on the forecast skill of an AR-related heavy precipitation event that was sampled over a 6-day sequence of Intensive Observing Periods (IOPs) in 2021.

Throughout the 6-day IOP in late January of 2021, AR Recon aircraft sampled a series of ARs over the northeastern Pacific, resulting in heavy precipitation over coastal regions of California and the Sierra Nevada Mountains. Using these observations, data denial experiments were conducted with a regional modeling and data assimilation system to explore the impacts of different flight scenarios and dropsonde sampling strategies.

Results indicate that dropsondes significantly improve the representation of ARs in the model analyses and positively impact the forecast skill of ARs and quantitative precipitation forecasts (QPF), particularly for lead times > 1 day. Reduced mission frequency and reduced dropsonde horizontal spatial resolution both degrade forecast skill. On the other hand, experiments that assimilated only G-IV data and experiments that assimilated both G-IV and C-130 data show better forecast skill than experiments that only assimilated C-130 data, suggesting that the inclusion of two types of aircraft (G-IV and C-130s) is an effective strategy to enable the benefits of missions on a consecutive way.

This study suggests some promising guidance for flight planning in future operational AR Recon missions. The findings recommend that future operational AR Recon missions incorporate daily mission or consecutive back-to-back flights, maintain current dropsonde spacing, support high-resolution data transfer capacity on the C-130s, and utilize G-IV alongside C-130s.

Figure 1: Box plot of (a) the interest value, (b) the intersection area, and (c) the object size error for the coastal object validation in Figure 10 of Zheng et al. (2024). The box plots are calculated by combining all 19 lead times together, with the non-matched forecast object excluded in the corresponding lead time. The bottom and the top of each box represents the 25th percentile and the 75th percentile, respectively. The magenta line in the middle of the box is the median. The cyan asterisk is the mean value of each experiment. The magenta horizontal line is the median of each data. Panel (d) shows the p-value representing the degree of significance for the mean value differences between two experiments. The green shades in (d) correspond to that the 1st experiment in the parentheses has less errors for the three metrics while the red shades show the 2nd experiment has less errors. Bold values in the chart of (d) show two experiments are significantly different at the 80% confidence levels. TS stands for “temporal sampling”. This figure is modified from Figure 11 of Zheng et al. (2024).

Figure 2: Same as Figure 1 but for the SS (spatial sampling) experiments. This figure is modified from Figure 15 of Zheng et al. (2024).

Zheng, M., Torn, R., Delle Monache, L., Doyle, J., Ralph, F. M., Tallapragada, V., Davis, C., Steinhoff, D., Wu, X., Wilson, A. M., Papadopoulos, C., & Mulrooney, P. (2024). An Assessment of Dropsonde Sampling Strategies for Atmospheric River Reconnaissance. Monthly Weather Review (published online ahead of print 2024). https://doi.org/10.1175/MWR-D-23-0111.1

CW3E Subseasonal Outlook: 31 January 2024

CW3E Subseasonal Outlook: 31 January 2024

January 31, 2024

Click here for a pdf of this information.


 

 

 

 

 

 

 

 

 

 

 

 

Summary provided by J. Wang, C. Castellano, Z. Yang, M. DeFlorio, and J. Kalansky; 31 January 2024

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*Outlook products are considered experimental

CW3E Event Summary: 22 January 2024

CW3E Event Summary: 22 January 2024

31 January 2024

Click here for a pdf of this information.

Weak Atmospheric River Brought Heavy Rain to Southern California and Southern Arizona

  • On January 22, 2024, a weak (AR0 on the Ralph et al. 2019 scale) atmospheric river (AR) moved across southern California and into Arizona
  • Broad light to moderate rain accompanied the AR, bringing notable rainfall amounts to the Southwest
  • An area of heavy rain developed offshore from San Diego and persisted for a few hours across the region
  • While IVT was not particularly high, heavy rainfall was supported by strong low-level moisture flux and mesoscale forcing for ascent beneath the left exit region of an upper-level jet
  • San Diego/Lindbergh Field (KSAN) recorded 2.73” of rain, which set a new daily record (previous record was 1.57” in 1963
  • This ranks as the 4th highest daily rainfall amount on record (since 1850)
  • The rolling 3-hr precipitation (computed from 1-min data) peaked at 2.13”. Based on NOAA Atlas 14 data, this represents a return interval of 178 years (0.6% chance of occurring in a year)

Click images to see loops of GFS IVT/IWV analyses

Valid: 0000 UTC 21 January – 1800 UTC 23 January 2024

Click image below to see loop of infrared satellite imagery

Note: Full animation may take a moment to load

Click image below to see the regional Southern California radar loop

Note: Full animation may take a moment to load

Click image below to see the San Diego radar loop

Note: Full animation may take a moment to load

Click image below to see the Integrated Water Vapor Transport loop

Note: Full animation may take a moment to load


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Summary provided by P. Iniguez, M. Steen, S. Bartlett, S. Roj, and J. Kalansky; 31 Jan 2024

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CW3E Event Summary: 13-23 January 2024

CW3E Event Summary: 13-23 January 2024

29 January 2024

Click here for a pdf of this information.

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

CW3E AR Update: 29 January 2024 Outlook

January 29, 2024

Click here for a pdf of this information.

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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*Outlook products are considered experimental

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

CW3E Subseasonal Outlook: 26 January 2024

January 26, 2024

Click here for a pdf of this information.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental

CW3E AR Update: 26 January 2024 Outlook

CW3E AR Update: 26 January 2024 Outlook

January 26, 2024

Click here for a pdf of this information.

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

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental

CW3E AR Update: 24 January 2024 Outlook

CW3E AR Update: 24 January 2024 Outlook

January 24, 2024

Click here for a pdf of this information.

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

To sign up for email alerts when CW3E post new AR updates click here.

*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 10th-90th 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

CW3E AR Update: 18 January 2024 Outlook

January 18, 2024

Click here for a pdf of this information.

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

To sign up for email alerts when CW3E post new AR updates click here.

*Outlook products are considered experimental