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Summary: Major CA Winter Storm (4-6 Feb 2024)

CW3E Event Summary: 4-6 February 2024

14 February 2024

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Major CA Winter Storm (4-6 Feb 2024)

Overall summary:

A strong atmospheric river (AR) impacted much of California on 4-6 Feb 2024.
It was an AR3 in the Santa Barbara base on the Ralph (2019) AR scale.

Precipitation impacts:

Widespread precipitation of 1-5” fell across most of California with 5-10” in the coastal mountains and Los Angeles Basin. Some areas of the San Gabriel Mountains received 10-15” of precipitation.
Los Angeles recorded one of its wettest multi-day stretches on record.
Heavy rain resulted in hundreds of mudslides and high flows on area rivers.
Year-to-date percent of normal snowfall increased by 10-20% across the Sierra Nevada mountains.

Wind and power outages:

- Storm produced widespread winds of 60+ mph across northern California with local peak wind gusts around the San Francisco Bay Area of 80-100 mph.
- Strong winds and wet soils felled hundreds of trees.
- A reported 1.4 million customers were without power at various points across the state.

IVT across the northeast Pacific Ocean during this event

Click image for animation (9MB).

IVT near the US West Cost during this event

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Peak IVT values for this AR

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AR Recon IOP 31 map

CW3E Radiosondes

MSLP analysis of bomb cyclone

Click image for animation (27MB).

Wind gust analysis

Click image for animation (15MB).

Peak wind gusts

Total precipitation and share of annual rainfall

Chance in WYTD percent of normal precipitation

Chance in WYTD percent of normal precipitation LA area

Rainfall statistics for downtown LA

Snowfall analysis

Summary of impacts

Images of trees toppled

Images of landslides

Images of flooding

River gage data for the Los Angeles River

Images of snowfall

Example of FIRO operations

Summary provided by: P. Iniguez, C. Castellano, J. Cordeira, , J. Kalansky, S. Roj, and M. Steen.

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CW3E Event Summary: 26 January – 2 February 2024

13 February 2024

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Atmospheric Rivers Produce Heavy Precipitation from Alaska to Southern CA

A family of atmospheric rivers (ARs) brought heavy precipitation to portions of Alaska, British Columbia, the Pacific Northwest, and California during 26 Jan – 2 Feb.

The ARs:

AR #1 made landfall in Oregon on 26 Jan and produced at least 2–6 inches of precipitation in portions of western Washington and Oregon.
AR #2 made landfall in British Columbia and southeastern Alaska on 28 Jan and produced 6–12 inches of precipitation over Vancouver Island, the Coast Mountains, the Alaska Panhandle, and the St. Elias Mountains.
AR #3 produced AR4 conditions (based on the Ralph et al. 2019 AR Scale) along the southern Oregon coast and AR3 conditions in coastal Northern California.
AR #3 brought widespread precipitation to California, including 4–8 inches of rain in the Northern California Coast Ranges and western Transverse Ranges, and 1–3 feet of snow in the Sierra Nevada.
All three ARs were fed from a tropical moisture source referred to as a Tropical Moisture Export (TME).

Impacts:

Rain falling on moist soils caused minor-to-moderate riverine flooding in western Washington during the first AR.
The greatest hydrologic impacts occurred in British Columbia during the second AR, with significant flooding near Pemberton, BC.
Minor flooding and several landslides were reported in Northern California during the third AR
This family of ARs and nearby essential atmospheric features were sampled by the NOAA and the 53rd Weather Reconnaissance Squadron as part of the AR Recon field campaign.

Click images to see loops of West-WRF IVT/IWV analyses and forecasts Valid: 0000 UTC 26 January – 0900 UTC 2 February 2024

Summary provided by C. Castellano, S. Bartlett, J. Cordeira, P. Iniguez, J. Kalansky, M. Steen, and S. Roj; 13 Feb 2024

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CW3E Seasonal Outlook: 9 February 2024

February 9, 2024

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Summary provided by Z. Yang, C. Castellano, J. Wang, M. DeFlorio, and J. Kalansky; 9 February 2024

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*Outlook products are considered experimental

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 Subseasonal Outlook: 2 February 2024

February 2, 2024

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Summary provided by C. Castellano, J. Wang, Z. Yang, M. DeFlorio, and J. Kalansky; 2 February 2024

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*Outlook products are considered experimental

CW3E AR Update: 2 February 2024 Outlook

February 2, 2024

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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

January 31, 2024

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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

31 January 2024

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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)