Climate Prediction Center - Week 3-4 Outlook

Prognostic Discussion for Week 3-4 Temperature and Experimental Precipitation Outlooks
NWS Climate Prediction Center College Park MD
300PM EDT Fri Aug 07 2020

Week 3-4 Forecast Discussion Valid Sat Aug 22 2020-Fri Sep 04 2020

The Madden-Julian Oscillation is active with convection centered over the Maritime Continent. While conditions over the Tropical Pacific reflect ENSO-neutral conditions overall, convection has been persistently suppressed near the Date Line. While propagation of the MJO signal may interact with suppressed convection near the Date Line, negative velocity potential anomalies are predicted by dynamical models to emerge over the East Pacific and the Tropical Atlantic by week 3, leading to an active East Pacific and Tropical Atlantic during the week 3-4 period. The week 3-4 outlook is based primarily on dynamical model forecasts from the operational NCEP CFS, ECMWF, and JMA ensemble prediction systems, as well as multi-model ensemble (MME) forecasts from the Subseasonal Experiment (SubX), combining both operational and experimental model systems. Consideration was also given to the potential evolution of the week 2 dynamical model forecasts, as well as the impact of decadal climate trends.

Dynamical models indicate greater than average uncertainty in the 500-hPa height forecasts for the week 3-4 period. Most dynamical models indicate persistence of negative 500-hPa height anomalies over parts of western North America and positive 500-hPa height anomalies over eastern Canada at high latitudes. The manual blend of dynamical model 500-hPa height forecasts predicts troughing and negative 500-hPa height anomalies over parts of eastern Alaska and northwestern Canada, while ridging is predicted over the west-central CONUS, similar to week 2 forecasts, and slightly positive 500-hPa height anomalies are predicted over the entire CONUS. The MLR statistical forecast based on current MJO and ENSO indices predicts a negative 500-hPa height anomaly over the Aleutians and was discounted in the manual blend.

Above normal temperatures are likely over almost the entire CONUS, under anomalous ridging and slightly above normal 500-hPa heights. Probabilities exceed 70 percent over parts of the west where conditions are persistently warm and dry in dynamical model forecasts. Probabilities of above normal temperatures are moderated to just over 50 percent in parts of the Southwest monsoon region, where active East Pacific convection may increase moisture flow into the region. While the ECMWF predicts any potential for below normal temperatures to be confined to parts of the western Gulf Coast in week 3-4, the MME consensus forecast from the SubX predicts near to below normal temperatures over an area stretching from the western Gulf Coast into the Central Mississippi Valley. Equal chances of below and above normal temperatures is predicted for this region in the week 3-4 outlook. Above normal temperatures are likely for southwestern Alaska, under positive 500-hpa height anomalies, and along the south coast of Alaska into the Panhandle, as indicated by most dynamical model forecasts. Above normal temperatures are also likely for northern Alaska, related to decadal trends. Above normal temperatures are likely for the Hawaiin Islands due to persistent above normal sea surface temperatures in the region.

Under a predicted ridge, below median precipitation continues to be likely for much of the western CONUS. Equal chances of below and above median precipitation is predicted for southwestern areas of the monsoon region, as moisture flow into this region may increase with an active East Pacific. Above median precipitation is likely for the western and central Gulf Coast region, as supported by most dynamical models, including the ECMWF, CFS, and the SubX MME. Equal chances of above and below median precipitation is predicted for most other areas of the eastern CONUS, where there is a potential for increased precipitation related to an active Atlantic, however there is much uncertainty among model forecasts. Equal chances of above and below median precipitation is also predicted for the Hawaiian Islands, where there is no consistent signal in dynamical models.

Temperature Precipitation

FCST FCST

Hilo A70 EC

Kahului A70 EC

Honolulu A70 EC

Lihue A70 EC

Forecaster: Dan Collins

The next week 3-4 outlook will be issued on Friday, Aug 14, 2020

These outlooks are based on departures from the 1981-2010 base period

These are two category outlooks and differ from official current three category outlooks currently used for the monthly and seasonal forecasts.

The shading on the temperature map depicts the most favored category, either above-normal (A) or below-normal (B) with the solid lines giving the probability ( >50%) of this more likely category (above or below).

The shading on the precipitation map depicts the most favored category, either above-median (A) or below-median (B) with the solid lines giving the probability ( >50%) of this more likely category (above or below).

In areas where the likelihoods of 2-week mean temperatures and accumulated precipitation amounts are similar to climatological probabilities, equal chances (EC) is indicated.

As of May 19, 2017, the temperature outlook is operational, while the precipitation outlook is still experimental