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 20 2020

Week 3-4 Forecast Discussion Valid Sat Dec 05 2020-Fri Dec 18 2020

Substantial tropical contributions to the midlatitude pattern during the Week 3-4 period are likely, with both an active La Nina and MJO signal in place at this time. The atmospheric response to La Nina continues to be robust, with enhanced trades continuing across the tropical Pacific, and anomalous high pressure aloft north of Hawaii. The MJO active phase is currently over the Indian Ocean, which teleconnects well with above-average mid-level heights over the central and eastern CONUS by Week-3 and may counteract the La Nina favored cold signal along the northern tier. Due to these active tropical signals, the multiple linear regression statistical tool based on ENSO, MJO, and long term trend exhibits fairly high probabilities in its temperature and precipitation outlooks, and the pattern is similar to the dynamical model forecasts.

Dynamical model 500-hPa height anomaly forecasts exhibit remarkable consistency among the various ensembles. The CFS, JMA, ECMWF, and Canadian all depict strong troughing in the vicinity of Alaska during Week 3-4, with the trough axis shifting slightly eastward of its forecast position in Week-2. Downstream over the CONUS, the models all favor above-average height anomalies. Overall, the pattern is indicative of a positive AO phase with strong Pacific flow into western Canada, which limits the potential for any significant arctic airmass development over North America. Therefore, while minor differences in the forecast height pattern result in variations among the tools regarding where the highest potential for above-normal temperatures sets up, all of the models depict a warm scenario for most of the CONUS.

The temperature and precipitation outlooks are based on a consensus of the dynamical and statistical models, which are fairly consistent with the canonical response to tropical forcing. Despite the potential for deep troughing over Alaska, the pattern appears to be transient, with southerly flow early in the period gradually transitioning to northerly flow. Despite the potential for northerly flow, the anticipated warmth early in the period and a lack of significant sea ice across the Chukchi Sea presents a limiting factor for substantial cold during the Week 3-4 period. Therefore, equal chances for above- and below-normal temperatures are maintained. Across the CONUS, equal chances are maintained for the Northwest, where dynamical model probabilities for above-normal temperatures are the weakest, and the La Nina pattern suggests a potential for below-normal temperatures. Above-normal temperatures are favored across the rest of the CONUS, with the highest probabilities extending across the northern Plains, upper-Midwest, and the Northeast.

The precipitation outlook is more complex. The CFS, JMA, and Canadian all favor above-average precipitation across the Pacific Northwest and Northern Rockies, while the ECMWF keeps the storm track to the north over western Canada and the Alaska Panhandle. Differences are also apparent over the Northeast, where most tools favor above-average precipitation, but the ECMWF again depicts dryness. Given the agreement among most of the guidance, above-average precipitation is favored for Washington State and the northern Rockies as well as New England, but probabilities are kept low in deference to the ECMWF. Elsewhere, the guidance fairly consistently favors below-median precipitation, with the highest probabilities along the southern tier, which is consistent with La Nina conditions as well as the lagged response to a robust Indian Ocean MJO. Below-median precipitation is also favored across the central CONUS, consistent with the anticipated above-average heights.

The outlook favors above normal temperatures for the Hawaiian Islands due to positive SST anomalies in the vicinity and ridging to the north. Guidance from the Subseasonal Experiment (SubX) multi-model ensemble suggests enhanced probabilities for below-median precipitation, though some of the individual model ensembles show enhanced rainfall.

Temperature Precipitation

FCST FCST

Hilo A70 B55

Kahului A70 B55

Honolulu A80 B55

Lihue A80 B55

Forecaster: Adam Allgood

The next week 3-4 outlook will be issued on Friday, Nov 27, 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