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 Dec 11 2020

Week 3-4 Forecast Discussion Valid Sat Dec 26 2020-Fri Jan 08 2021

A weak Madden-Julian Oscillation (MJO) event is located over the Maritime Continent with most of its active convection over northern Australia. Its RMM projection is enhanced by an equatorial Rossby wave, which both the CFS and ECMWF forecast to quickly move into the Indian Ocean by the middle of Week-1. At that point we expect the RMM index to weaken again as the MJO shifts eastward and interacts with the anomalously cold central Pacific sea surface temperatures (SSTs). Since the MJO isn't expected to play a major role in the Week 3-4 pattern, this forecast is based primarily on a combination of dynamical models and an expected La Nina contribution.

There is good agreement among the ECMWF, JMA, and SubX multi-model ensemble means regarding the predicted 500-hPa height pattern. Positive height anomalies are forecast over the southwestern U.S. and New England while negative height anomalies are forecast over Alaska. Negative height anomalies also extend over the Southeast, but they are significantly weaker than those found over Alaska and the predicted temperature fields don't align well with this southeastern extension.

The expected height pattern is often favorable for Nor'easter cyclones along the East Coast and associated above normal precipitation is predicted throughout the Northeast and mid-Atlantic. Precipitation odds tilt below normal over most of Alaska, with the highest probabilities of below normal precipitation in the westernmost regions of the state. Chances increase for above normal precipitation associated with the aforementioned troughing off the Pacific Northwest coast as well. Dry conditions are favored along the southern tier of the U.S., from southwestern California to Florida. This pattern is common during La Nina winters and is also predicted in most dynamical model guidance. Warm SSTs and consistency amongst the dynamical models suggest weak probabilities of above normal rainfall over Hawaii. A 55-60% chance is forecast throughout the state.

Temperatures are favored to be above normal for most of the Lower 48, with equal chances of above and below normal over much of the Southeast. The warmest temperatures are likely to be west of the Rockies, where there is a 60-70% chance of above normal temperatures. Below normal temperatures are expected over most of Alaska, with probabilities of 60-70% extending westward to the Aleutians. Elevated above normal temperatures probabilities are also forecast uniformly throughout Hawaii as SSTs remain above normal throughout the region.

Temperature Precipitation

FCST FCST

Hilo A60 A55

Kahului A60 A55

Honolulu A70 A55

Lihue A70 A55

Forecaster: Kyle MacRitchie

The next week 3-4 outlook will be issued on Friday, Dec 18, 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