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 EST Fri Nov 13 2020

Week 3-4 Forecast Discussion Valid Sat Nov 28 2020-Fri Dec 11 2020

La Nina conditions currently are present across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) are below average from the west-central to the eastern Pacific Ocean with enhanced trade winds and westerly wind anomalies aloft. The RMM-based Madden-Julian Oscillation (MJO) index indicates fairly robust propagation of the MJO signal across the Pacific over the past several days. Both the ECMWF and GEFS feature the MJO continuing to propagate eastward across the Western Hemisphere and then weaken over the Indian Ocean. The Week 3-4 temperature and precipitation outlooks are based primarily on dynamical model forecasts from the NCEP CFS, ECMWF, and JMA, and the Subseasonal Experiment (SubX) multi-model ensemble (MME) of experimental and operational ensemble prediction systems with additional considerations for La Nina and long-term trends, as well as the predicted evolution of the pattern from Week-2 forecasts.

Dynamical model 500hPa height anomaly forecasts during week 3-4 show a fairly consistent evolution from the forecast state during Week-2. Models feature anomalous ridging over the North Pacific to the south of the Aleutians. Below-normal heights are forecast from Alaska extending southeastward to the west coast of North America, although models vary on how far south and inland this troughing progresses, with the CFS being the most bullish on both outcomes. Elongated anomalous ridging extends from the southwestern U.S. eastward through the Great Plains to the Northeast.

The Week 3-4 temperature outlook features enhanced chances of above-normal temperatures across much of central and eastern CONUS, under anomalous ridging and above normal 500-hPa heights. The greatest confidence for above-normal temperatures over the Lower-48 is from the Central Plains to the Upper Mississippi Valley, aligned with predicted above-normal 500-hPa heights throughout much of the period. Equal chances are indicated over the western CONUS, tied to uncertainty of impacts from the aforementioned troughing. Below-normal temperatures are favored for much of Alaska behind the anticipated trough axis, although long-term trends and the possibility of ridging nosing in from the west result in a shift toward above-normal temperatures being favored for the Aleutians, western mainland, and North Slope.

Predicted troughing over the west coast is likely to enhance onshore flow into the region, leading to elevated above-median precipitation chances. Below-normal precipitation is forecast across much of the remainder of the Lower-48 states tied to a forecast of broad mid-level ridging. Equal chances exist across the Northeast tied to model forecasts showing above-median precipitation, although the associated circulation field is not particularly conducive for precipitation. Below-median precipitation is favored across most of Alaska behind the forecast mean trough axis.

The outlook favors above normal temperatures for the Hawaiian Islands due to positive SST anomalies in the vicinity. Above-normal rains are favored across the islands by model guidance, as is typical of La Nina.

Temperature Precipitation

FCST FCST

Hilo A80 A65

Kahului A80 A60

Honolulu A80 A55

Lihue A80 A55

Forecaster: Luke He

The next week 3-4 outlook will be issued on Friday, Nov 20, 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