Author: cw3e-web

CW3E AR Update: 27 December 2023 Outlook

December 27, 2023

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Pair of Atmospheric Rivers Forecast to Make Landfall along US West Coast

• A pair of Atmospheric Rivers (AR) are forecast to make landfall in the Pacific Northwest (PNW), with the first having already made landfall early Wed 27 Dec. AR conditions are forecast to continue through Sat 30 Dec.
• The GEFS control run is forecasting AR1 to AR3 conditions (based on Ralph et al. 2019 AR scale) for both ARs for the PNW.
• The GEFS control run is forecasting AR1 conditions for the first AR and AR1 to AR2 conditions for the second AR for Northern CA.
• There is uncertainty in the duration of AR conditions and possible break between ARs in the GFS and ECMWF model ensembles.
• The National Weather Service (NWS) Weather Prediction Center (WPC) is currently forecasting 3-day precipitation totals ≥ 2” with highest precipitation totals over the Northern CA coast and CA/OR border.
• The WPC Excessive Rainfall Outlook (ERO) indicates a Marginal Risk (level 1 of 4, at least 5%) of exceeding flash flood guidance for Northern CA and the Southern OR coast for the 24-hour periods ending at 18Z on 30 Dec

Click images to see loops of GFS IVT and IWV forecasts Valid 1200 UTC 27 December 2023 – 0600 UTC 31 December 2023

Summary provided by M. Steen and S. Roj; 27 December 2023

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*Outlook products are considered experimental

CW3E AR Update: 22 December 2023 Outlook

December 22, 2023

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Pair of Atmospheric Rivers Forecast to Make Landfall along US West Coast

• A pair of Atmospheric Rivers (AR) are forecast to make landfall in the Pacific Northwest (PNW) early Mon 25 Dec and continue through Wed 27 Dec.
• The GEFS control run is forecasting AR1 to AR2 conditions (based on Ralph et al. 2019 AR scale) for the first AR period and AR2 to AR3 conditions for the second AR period for the PNW.
• The GEFS control run is forecasting up to AR1 for the first AR period and AR1 to AR2 conditions for the second AR period for Northern CA.
• There is uncertainty in the AR landfall timing, duration and possible break in AR conditions in the GEFS, ECMWF EPS and West-WRF Ensembles.
• The National Weather Service (NWS) Weather Prediction Center (WPC) is currently forecasting 3-day precipitation totals ≥ 2” with highest precipitation totals over the Olympic Peninsula and CA/OR border.
• The WPC Excessive Rainfall Outlook (ERO) indicates a Marginal Risk (level 1 of 4, at least 5%) of exceeding flash flood guidance for the Olympic Peninsula for the 24-hour period ending at 4 AM Tue 26 Dec.
• The NWS Climate Prediction Center’s Day 8-14 Hazard Outlook shows risks of heavy precipitation for Northern and Central CA from 29 Dec 2023 through 2 Jan 2024 with the surface low and the associated frontal boundary and potential third AR at the end of next week.

Click images to see loops of GFS IVT and IWV forecasts Valid 1800 UTC 24 December 2023 – 1800 UTC 28 December 2023

Summary provided by M. Steen, P. Iniguez, S. Roj, S. Bartlett and J. Kalansky; 22 December 2023

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CW3E AR Update: 20 December 2023 Outlook

December 20, 2023

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Atmospheric River Forecast for US West Coast Landfall on Christmas

• An Atmospheric River (AR) is forecast to make landfall in the Pacific Northwest (PNW) early Mon 25 Dec and continue through Wed 27 Dec.
• The GEFS control run is forecasting AR3 to AR4 conditions (based on Ralph et al. 2019 AR scale) for the PNW.
• Due to the long lead time with this event, there is uncertainty in the AR landfall timing and duration in the GEFS, ECMWF EPS and West-WRF Ensemble.
• The NWS Weather Prediction Center (WPC) is currently forecasting 3-day precipitation totals ≥ 2” with highest precipitation totals over the Olympic Peninsula and CA/OR border.
• The NWS Climate Prediction Center’s Day 8-14 Hazard Outlook shows risks of heavy precipitation, heavy snow and high winds for Northern CA from 12/27 through 12/30.

Click images to see loops of GFS IVT and IWV forecasts Valid 0600 UTC 24 December 2023 – 0000 UTC 28 December 2023

Summary provided by M. Steen, S. Bartlett, and S. Roj; 20 December 2023

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*Outlook products are considered experimental

CW3E AR Update: 13 December 2023 Outlook

December 13, 2023

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Cut Off Low Forecast to Bring Atmospheric River to California

• A mid-level trough currently just south of the Aleutian Islands is forecast to propagate into the Northeast Pacific and continue to deepen, eventually becoming cut off.
• The cut off low pressure system interacts with elevated moisture in the NE Pacific leading to the formation of an AR.
• The low pressure system is forecast to progress toward the USWC and bring AR conditions to CA beginning Sun 16 Dec.
• AR2 conditions (based on Ralph et al. 2019 AR scale) are forecast for this event.
• There is uncertainty in the AR landfall timing and duration in the GEFS, ECMWF EPS and West-WRF Ensemble.
• The NWS Weather Prediction Center (WPC) is currently forecasting 7-day precipitation totals ≥ 3” for much of the CA Coasts as well as the Sierras with isolated areas ≥ 6”.
• The NWS Climate Prediction Center’s Day 8-14 Hazard Outlook shows slight risks of heavy precipitation and high winds for the CA coasts and heavy snow in the Sierras 12/20-12/21 and 12/23-12/26.

Click images to see loops of GFS IVT and IWV forecasts Valid 0600 UTC 14 December 2023 – 0000 UTC 19 December 2023

Summary provided by M. Steen, S. Bartlett, C. Castellano and J. Kalansky; 13 December 2023

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*Outlook products are considered experimental