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 22 2023

Week 3-4 Forecast Discussion Valid Sat Oct 07 2023-Fri Oct 20 2023

Given the time of year, the influence of climate signals on the weeks 3-4 period is mixed. El Niño continues to strengthen; the North Atlantic Oscillation (NAO) index hints at remaining negative during the forecast period; and the Madden-Julian Oscillation (MJO) is forecast to propagate across the Americas throughout the outlook period. While some experimental statistical guidance indicates that both El Niño and the MJO influence the temperatures across CONUS, dynamical model guidance does not seem as convinced. The Real-time Multivariate MJO (RMM) index is currently active over the Maritime Continent and is predicted to strengthen as it propagates over the next few weeks in some model guidance, potentially gaining strength from constructively interfering El Niño conditions. The Week 3-4 Temperature and Precipitation Outlooks are based primarily on operational dynamical guidance including the GEFSv12, CFSv2, ECMWF, JMA, and experimental guidance from the Subseasonal Experiment (SubX) multi-model ensemble (MME). The influence of El Niño and the MJO on temperature and precipitation and the predicted evolution of the pattern from the Week-2 forecast were also factors in the outlooks.

A blend of dynamical model 500-hPa height forecasts indicate a pattern that is currently in the week-2 forecasts, namely, troughing to the south of Alaska and along the West Coast and Intermountain West of the U.S. Ridging over the eastern half of Canada dips south to cover New England and part of the Great Lakes region. The height pattern over the Midwest, Southeast, and Southern Plains is forecast to be near normal for the outlook period.

The ridging over eastern Canada is expected to contribute to above-average temperatures for New England and the Great Lakes, while the troughing near the West Coast is expected to contribute to below-average temperatures across several states in the West. The highest probabilities of below-average temperatures are forecast for the lower half of California and Nevada. Above-average temperatures are also forecast for the Southern Plains and the Southeast with the highest probabilities located in the Gulf Coast States. The eastern half of Alaska is forecast to have above-average temperatures. Equal chances are forecast for the rest of Alaska due to low probabilities and mixed model guidance. Similarly, Equal Chances across central CONUS reflects the weak probabilities and mixed model guidance. It is worth noting that a review of weeks 3 and 4 independently showed some model consistency toward a below-average solution to reach the Central Plains during week 3, however, this below-average signal did not persist into week 4 and appears to be damped in the weeks 3-4 outlook.

While the troughing pattern during this outlook period likely contributes to the above-average precipitation that is forecast for the Northwest and Southern Plains, it is possible that the transient activity primarily forecast in week 3 contributes to the above-average signal in the Northeast along with the potential for enhanced, moist easterly flow from the ridging located to the north. Above-average rainfall is also forecast across southern Alaska, to include the Panhandle and the Aleutian Islands. Equal Chances reflects in this forecast, mixed guidance of low probabilities for both above and below climatology..

Above average sea surface temperatures are currently observed in the vicinity of Hawaii.The SubX MME probabilistic temperature forecast favors above-average temperatures over Hawaii during the Week 3-4 Outlook period with greater probabilities to the northwest. Dynamical model guidance from the SubX MME shows increased probabilities for below median precipitation over the islands, with greater probabilities to the southeast.

Temperature Precipitation

FCST FCST

Hilo A80 B70

Kahului A80 B55

Honolulu A90 B60

Lihue A80 B60

Forecaster: Emerson LaJoie

The next week 3-4 outlook will be issued on Friday, Sep 29, 2023

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