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 Sep 17 2021

Week 3-4 Forecast Discussion Valid Sat Oct 02 2021-Fri Oct 15 2021

The Madden-Julian Oscillation (MJO) signal, while more coherent for last week's forecast, exhibits more uncertainty this week based on the broad disagreement among the members of the dynamical model guidance. However ultimately, the ensemble means forecast an inactive MJO for the weeks 3-4 forecast period. The week 3-4 temperature and precipitation outlooks are based on blends of the dynamical model guidance including the GEFS, CFS, Canadian, ECMWF, JMA, the SubX multi-model ensemble (MME), and statistical guidance.

The models broadly agree on a longwave pattern in the anomalous 500hPa height field that starts with ridging over the central North Pacific Ocean followed by troughing over Alaska and ridging over CONUS. This longwave pattern appears to be an evolution of a similar pattern from week-2 and is forecast by an equal-weighted blend of the dynamical and statistical model tools. The positive height anomalies over much of CONUS support probabilities of above normal temperatures for much of the area, while consistent model agreement supports elevated probabilities from the Central Plains to the Great Lakes region. The forecasted troughing over Alaska stretches southward hugging the coastline from the Alaskan Panhandle to the southern California coast. This area is forecast with low probabilities of alternating above and below normal temperatures. Across Alaska, forecast guidance supports below average temperatures along the northern slope and above average temperatures to the south, although the probabilities are low. Equal chances of above or below normal temperatures are forecast for the U.S. Southeast Gulf states with a slight tilt toward above normal temperatures in southern Florida.

Given the troughing pattern, and proximity to the ridging pattern, the most likely areas for above normal precipitation include the Central Rockies, the Northwest, and the Alaskan Panhandle. Forecast guidance indicates that most of this precipitation will be in week 3. Based on the predicted inland placement of the ridge axis, below normal precipitation is favored for much of the CONUS east of the Rockies. Storm track activity normally seen in the Gulf may be steered out toward the Atlantic by the high pressure system over CONUS, also contributing to the drier than average forecast.

Near normal temperatures and slightly above average precipitation are favored for Hawaii, based on guidance from the SubX MME and near average sea surface temperatures.

Temperature Precipitation

FCST FCST

Hilo EC EC

Kahului EC A55

Honolulu EC A55

Lihue EC EC

Forecaster: Emerson LaJoie

The next week 3-4 outlook will be issued on Friday, Sep 24, 2021

These outlooks are based on departures from the 1991-2020 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