Author: Chris Castellano

CW3E AR Update: 16 December 2024 Outlook

CW3E AR Update: 16 December 2024 Outlook

December 16, 2024

Click here for a pdf of this information.

Family of Atmospheric Rivers Forecast to Bring Heavy Precipitation to US West Coast

  • A series of atmospheric rivers (ARs) and low-pressure systems are forecast to develop over the North Pacific and propagate toward the US West Coast through the end of the week.
  • The first AR is forecast to make landfall early tomorrow and bring AR 2/AR 3 conditions (based on the Ralph et al. 2019 AR Scale) and heavy precipitation to coastal Washington and northern Oregon.
  • The second and third ARs are forecast to make landfall over British Columbia on Thu 19 Dec and Fri 20 Dec, with limited impacts expected over the US West Coast.
  • The fourth and fifth ARs are forecast to make landfall on Sat 21 Dec and Sun 22 Dec and potentially bring moderate-to-strong AR conditions (IVT > 500 kg m−1 s−1) and heavy precipitation to western Washington, western Oregon, and Northern California.
  • Extended range and subseasonal forecast products suggest that additional AR activity and wet conditions are likely to continue into next week, especially over the Pacific Northwest.
  • The NWS Weather Prediction Center is forecasting 10–20 inches of total precipitation over the Olympic Peninsula during the next 7 days.
  • Uncertainty in the forecast evolution of these ARs and shortwave troughs is driving uncertainty in forecast precipitation. In general, EPS is forecasting higher precipitation totals across coastal Washington and Oregon during the next 10 days compared to GEFS.
  • Numerous stream gages in western Washington and Oregon are forecast to rise above action/bankfull stage over the next 10 days. The combination of high freezing levels and heavy rain will likely increase runoff and flooding potential during the first AR.

Click images to see loops of GFS IVT and IWV forecasts

Valid 1200 UTC 16 December 2024 – 0000 UTC 23 December 2024

Summary provided by C. Castellano, J. Kalansky, S. Roj, and M. Steen; 16 December 2024

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

*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.

CW3E AR Update: 9 December 2024 Outlook

CW3E AR Update: 9 December 2024 Outlook

December 9, 2024

Click here for a pdf of this information.

Unsettled Weather Pattern Likely to Return to US West Coast This Week

  • A series of mid-level shortwave troughs and atmospheric rivers (ARs) are forecast to impact the US West Coast starting around mid-week and potentially continuing into next week.
  • The first AR is forecast to bring a brief period of weak AR conditions (IVT < 500 kg m−1 s−1) to California late Wed 11 Dec into Thu 12 Dec.
  • A second and stronger AR is forecast to make landfall over Northern California Fri 13 Dec into Sat 14 Dec, but there is still some uncertainty regarding the timing, location, and magnitude of AR conditions.
  • Looking further ahead, deterministic and ensemble models are showing potential for additional landfalling AR activity next week.
  • The 00Z GEFS and EPS control members are forecasting an AR 2 (based on the Ralph et al. 2019 AR Scale) over most of coastal Northern California in association with the second AR.
  • The 12Z National Blend of Models (NBM) is forecasting at least 2–5 inches of total precipitation in portions of Northern California, western Oregon, and western Washington during the next 7 days.
  • Uncertainty in the forecast evolution of these ARs and shortwave troughs is driving uncertainty in forecast precipitation. In general, GEFS is forecasting higher precipitation totals across Northern California, western Oregon, and western Washington during the next 10 days compared to EPS.

Click images to see loops of GFS IVT and IWV forecasts

Valid 0000 UTC 9 December 2024 – 1200 UTC 16 December 2024

Summary provided by C. Castellano, J. Kalansky, and S. Roj; 9 December 2024

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

*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.

CW3E Publication Notice: Broadening the scope of anthropogenic influence in extreme event attribution

CW3E Publication Notice

Broadening the scope of anthropogenic influence in extreme event attribution

December 2, 2024

A new paper titled “Broadening the scope of anthropogenic influence in extreme event attribution” led by Aglaé Jézéquel, with multiple co-authors including CW3E’s Anna Wilson, was recently published in Environmental Research: Climate. Jézéquel proposes a multidimensional framework for Extreme Event Attribution (EEA) built on the framework proposed by Bastos et al. (2023) to study compound eco-climatic events. The new framework aims to bridge the EEA and Disaster Risk Reduction (DRR) communities by considering the contributions of both anthropogenic climate change and the structure of human societies to the impacts of extreme weather events. Jézéquel uses the new framework in this paper to examine five case studies, including California droughts and the Forecast Informed Reservoir Operations (FIRO) system as an adaptation strategy. This supports CW3E’s 2019-2024 Strategic Plan by documenting the potential benefits of using the best available science to inform reservoir operations.

The broader framework proposed by Jézéquel et al. includes attributions of disaster impacts to exposure and vulnerability. In the case study involving FIRO, Jézéquel et al. discusses how a model could be used to compare Lake Mendocino storage, with and without FIRO, and with and without climate change (Figure 1, Figure 6 from Jézéquel et al. 2024). The counterfactual world without the FIRO adaptation strategy displays a lower water supply in the drought year of water year 2020. By using the proposed framework and synthetic weather forecasts, tests could be run to determine how inflows to Lake Mendocino would be impacted by climate change, and whether the FIRO system would be robust to a counterfactual world with higher concentrations of greenhouse gases. Jézéquel also discusses how the framework could be used for analyzing the 2021-2022 Kenyan drought; the 2013-2015 marine heatwave in the northeast Pacific; the 2017 forest fires in Portugal; and Acqua Alta (flooding) events in Venice and evaluation of adaptation strategies.

Figure 1. (Figure 6 from Jézéquel et al. 2024) Partial causality chart for the 2020 California drought modulated by the use of the FIRO system.

Bastos, A., Sippel, S., Frank, D., Mahecha, M. D., Zaehle, S., Zscheischler, J., & Reichstein, M. (2023). A joint framework for studying compound ecoclimatic events. Nature Reviews Earth & Environment, 4, 333-350. https://doi.org/10.1038/s43017-023-00410-3

Jézéquel, A., Bastos, A., Wilson, A. M., Ramos, A. M., Shepherd, T. G., Stuart-Smith, R., Kimutai, J., Moemken, J., Zscheischler, J., Faranda, D., Lehner, F., Le Grix, N., Sippel, S., Bevacqua, E., Rufat, S., D’Andrea, F., Lloyd, E. A., & Van Loon, A. F. (2024). Broadening the scope of anthropogenic influence in extreme event attribution Environmental Research: Climate, 3, 042003. https://doi.org/10.1088/2752-5295/ad7527

CW3E Publication Notice: Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers

CW3E Publication Notice

Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers

December 2, 2024

A new article titled “Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers,” led by CW3E Director F. Martin Ralph and coauthored by Matthew Simpson (CW3E), Sam Iacobellis (CW3E), Jay Cordeira (CW3E), Forest Cannon (Tomorrow.io), Alison Cobb (ECMWF), Allison Michaelis (Northern Illinois University) and Luca Delle Monache (CW3E) was published in the American Meteorological Society’s Monthly Weather Review. This study investigates the vertical structure of the stable marine boundary layer (SMBL) in atmospheric river (AR) environments and key modulating processes using data from more than 1000 dropsondes collected during the AR Reconnaissance program. By analyzing an extensive database of dropsonde observations over the Northeast Pacific, we aim to document the occurrence of AR-SMBL conditions in mid-latitude ARs and explore their origins and the decoupling process between the lower AR atmosphere and the underlying ocean surface. This work is well aligned with CW3E’s goals to advance scientific understanding of ARs and to improve extreme precipitation forecasting for Western North America.

Water vapor transport associated with ARs is focused in the lower troposphere where the combination of strong low-level winds and increased moisture produces strong horizontal fluxes. As a result, the behavior of the SMBL is potentially important in modulating AR strength (Ralph et al. 2017). In addition, accurately representing SMBL dynamics beneath ARs by numerical weather prediction (NWP) models can be critical for data assimilation. For example, Lavers et al. (2018) found the greatest initial condition errors in European Centre for Medium-Range Weather Forecasts (ECMWF) forecasts of ARs are near the top of the MBL.

We examined the hypothesis that, as relatively warm air parcels are advected poleward towards and within an AR over progressively cooler ocean waters, sensible heat is transferred from the air mass defining the AR into the ocean surface, thereby cooling the lowest levels of the atmosphere. This atmospheric cooling subsequently increases static stability and vertical wind shear within the MBL, effectively decoupling the lower layer of the AR from the ocean surface.

Simulated backward air parcel trajectories originating from dropsonde locations within the AR core were used to calculate the 24-hour change in SST experienced by an air parcel (DSST24) beneath each AR. The DSST24 varies from –13°C to +2°C and is directly related to the strength of the AR and its orientation relative to the SST gradient. The DSST24, therefore, distinguishes weak and strong decoupling regimes (WDR, SDR). In SDR cases, relative to WDR cases, the SMBL is characterized by stronger static stability, low-level jet, vertical wind shear, horizontal water vapor transport, and moderately shallower SMBL depth (Figure 1).

Figure 1. Composite profiles within the AR core for strong (SDR, solid line) and weak decoupling regime (WDR, dashed line) cases for (a) potential temperature, (b) wind speed (m s–1), (c) water vapor flux (g kg-1 * m s–1), (d) Bulk Richardson number (Rib).

SDR cases were associated with a greater sensible heat loss to the ocean relative to the WDR cases (Figure 2), leading to a cooling of the lower atmosphere and the increase in static stability observed in composite profiles. Sensible heat flux and IVT values are highly correlated in SDR cases while the WDR sensible heat flux exhibits an irregular structure relative to IVT contours.

Figure 2. ERA5-derived spatially centered and rotated composite analyses of sensible heat flux (W m–2; shaded according to scale) and IVT magnitude (contoured every 100 kg m–1 s–1 starting at 200 kg m–1 s–1) for the subset of IOPs containing a change in SST (a) less than –4°C (N=74) and (b) between –4°C and +4°C (N=25).

The study highlights the complex structure of the SMBL beneath ARs and documents the increase in air-sea decoupling due to advection of the warm air mass in ARs over cooler SSTs. This observation-based paper will act as a foundation for future work to improve NWP model representation of AR-SMBL dynamics though enhanced boundary layer parameterizations.

Lavers, D. A., Rodwell, M. J., Richardson, D. S., Ralph, F. M., Doyle, J. D., Reynolds, C. A., Tallapragada, V., & Pappenberger, F. (2018). The gauging and modeling of rivers in the sky. Geophysical Research Letters, 45(15), 7828-7834. https://doi.org/10.1029/2018GL079019

Ralph, F. M., Iacobellis, S. F., Neiman, P. J., Cordeira, J. M., Spackman, J. R., Waliser, D. E., Wick, G. A., White, A. B., & Fairall, C. (2017). Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers. Journal of Hydrometeorology, 18(9), 2577-2596. https://doi.org/10.1175/JHM-D-17-0036.1

Ralph, F. M., Simpson, M., Iacobellis, S., Cordeira, J. M., Cannon, F., Cobb, A., Michaelis, A. C., & Delle Monache, L. (2024). Dropsonde Observations of the Stable Marine Boundary Layer Beneath Atmospheric Rivers. Monthly Weather Review (published online ahead of print 2024). https://doi.org/10.1175/MWR-D-23-0207.1

CW3E AR Update: 18 November 2024 Outlook

CW3E AR Update: 18 November 2024 Outlook

November 18, 2024

Click here for a pdf of this information.

Long-Duration Atmospheric River Event to Bring Heavy Precipitation to California and Oregon

  • A strong atmospheric river (AR) is forecast to make landfall over the US West Coast tomorrow (19 Nov) in association with a rapidly intensifying low-pressure system.
  • After the initial AR landfall, the AR is forecast to stall over Northern California for several days.
  • The location of AR landfall has continued to trend southward in recent model runs, with the strongest AR conditions and heaviest precipitation now expected in Northern California.
  • Active weather is likely to continue across California this weekend into early next week, but there is considerable forecast uncertainty in the duration and exact location of AR activity beyond Fri 22 Nov.
  • AR 4 conditions (based on the Ralph et al. 2019 AR Scale) are likely in coastal Northern California.
  • Some locations may experience AR conditions for > 72 consecutive hours.
  • The NWS Weather Prediction Center (WPC) is forecasting > 10 inches of total precipitation in portions of Northern California during the next 7 days.
  • Some watersheds in Northern California could receive > 20% of their normal total water year precipitation during the next 10 days.
  • Heavy rainfall will likely result in hydrologic impacts over portions of Northern California and southern Oregon.
  • The WPC has issued a moderate risk excessive rainfall outlook for Del Norte and Humboldt Counties Thu 21 Nov into early Fri 22 Nov.
  • Major winter storm impacts are likely in the Klamath Mountains, Southern Cascades, and Northern Sierra Nevada.

Click images to see loops of West-WRF GFS IVT and IWV forecasts

Valid 0000 UTC 18 November 2024 – 0000 UTC 25 November 2024

Summary provided by C. Castellano, S. Bartlett, J. Cordeira, J. Kalansky, and M. Steen; 18 November 2024

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

*Outlook products are considered experimental

For any unfamiliar terms, please refer to the American Meteorological Society Glossary.