Subseasonal to Seasonal (S2S) Experimental Forecasts

A multi-institutional collaboration sponsored by California DWR

Subseasonal Outlooks (Weeks 1-6)

Dynamical Model Atmospheric River Activity Forecasts

A multi-model experimental forecast for AR activity (defined as the # of AR days per week) at week-1, 2, and 3 lead time is shown below for the NCEP dynamical model.

This product was developed in collaboration with NASA’s Jet Propulsion Laboratory.

Weeks 1-2: Shading indicates the odds of AR activity for each day. ARs are defined using the Guan and Waliser (2015) algorithm and probability is calculated by the number of ensemble members predicting an AR at each grid point at 00 UTC on the given forecast day. Click on a panel to open in a new tab, click on title to open seven day panel plot.

Week 3: The top row shows the forecast number of AR days during week-3; the middle row shows the climatological values of AR activity in each model’s hindcast record for the week-3 verification period; the bottom row shows the departure of the AR activity forecast for that same verification period (top panel forecast minus middle panel climatology). For this row, blue values represent higher than average AR activity predicted during week-3; red values represent lower than average AR activity predicted during week-3. Grey rectangles surround grid cells where >75% of forecast ensemble members agree on the sign of the AR activity anomaly with respect to climatology. These regions can be interpreted as having higher confidence in their prediction of week-3 AR activity. The hindcast skill assessment associated with the NCEP, ECMWF, and ECCC hindcast systems is described in DeFlorio et al. 2019b.

Week 1
Week 2
Week 3

Dynamical Model S2S Ridging Forecasts

An atmospheric ridge is defined as elongated area of relatively high atmospheric pressure (American Meteorological Society; 2020). Persistent ridging events play an important role in determining if and where precipitation falls. Ridging forecasts are shown below for lead-times of weeks 1 and 2, weeks 3 and 4 and weeks 5 and 6. These forecasts are for three different ‘ridge types’ that have been shown in recent research [Gibson et al. 2020a; more methodology information here] to strongly influence atmospheric river and precipitation likelihood across the US West Coast. The North-ridge type is typically associated with widespread dry conditions across the entire US West. The South and West-ridge types are typically associated with dry conditions across Southern California and the Colorado River Basin but can also increase the likelihood of wet conditions across the Pacific Northwest. The hindcast skill assessment associated with this outlook is provided in Gibson et al. 2020b.

This product was developed in collaboration with NASA’s Jet Propulsion Laboratory.

Interpretation: In these plots, the left panel shows the occurrence frequency of each ridge type (bars) compared to climatology (horizontal line) for each of the model ensemble members. If over 50% of the ensemble members predict more ridging than expected (for this time of year) then the right panel maps are displayed indicating the likelihood of wetter or drier conditions based on how these ridge types typically influence precipitation.

Forecast Period:  

Seasonal Outlooks (Beyond week 6)

The current seasonal JFM forecast based on December Pacific SST anomalies suggests strong likelihood of anomalous dryness continuing in the southern tier of the Southwest where skill is high. The outlook also suggests chances for normal precipitation in coastal Southern California and above-normal precipitation in Central/Northern California where skill is much lower. The forecast also suggests above-normal precipitation over the interior Northwest. La Niña signals typically teleconnect to this region most strongly in mid-late winter and early spring and our JFM outlook is broadly consistent with La Niña, but certainly not bound by her. We attribute the deviations from ENSO canon to strong and extensive warm anomalies in the north-eastern and western tropical Pacific, which have persisted since late summer. These anomalies are not known to be particularly conducive to canonical La Niña teleconnections. However, if the projected wetness, particularly over the Sierra and Coastal Ranges, materializes, we would expect it to be due to a handful of ARs hitting the mountains from the west (as opposed to from the southwest, i.e. less of a southerly component to IVT than is typical), which would be consistent with La Niña. In summary, fall 2020 and winter 2021 statistical seasonal forecasts have been consistently dry for California due to the influence of La Niña but there is a possibility of wetter than expected conditions materializing in Central and Northern California for the remainder of the wet season.

For additional products and more information visit the Odds of Reaching 100% of Normal Water Year Precipitation webpage.

All products displayed on this page are considered experimental.