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 13 2024

Week 3-4 Forecast Discussion Valid Sat Sep 28 2024-Fri Oct 11 2024

Although the El Niño Southern Oscillation (ENSO) conditions are currently neutral in the equatorial Pacific, a La Niña Watch has been issued. Equatorial sea surface temperatures (SSTs) are above average in the western Pacific and near-to-below-average in the eastern Pacific Ocean. The Madden Julian Oscillation (MJO) propagated eastward over the Maritime Continent recently and is predicted to strengthen and propagate into the Western Pacific by dynamical models in the next week or two. However, dynamical model forecasts show the MJO signal weakening and becoming increasingly uncertain after about two weeks. Impacts of the MJO were only considered in the Week 3-4 Outlook to the extent that dynamical model forecasts of temperature and precipitation reflect MJO teleconnections. Propagation of an MJO signal from the current phase over the Maritime Continent would lead to a tendency for negative temperature signal over the northern central CONUS based on a multivariate linear regression (MLR) that uses MJO, ENSO and decadal trend predictors. Impacts of local SSTs, such as anomalously cool SSTs off the west coast of Mainland Alaska, are also considered. The Week 3-4 Temperature and Precipitation Outlooks are based primarily on the predicted evolution of the circulation, temperature and precipitation patterns from the week-2 period by dynamical model forecasts from the CFSv2, GEFS, ECMWF and JMA ensemble prediction systems, as well as blends of calibrated model temperature and precipitation forecasts.

Week-2 dynamical model forecasts of the mid-level circulation predict a trough west of Mainland Alaska extending southeastward along the Pacific coast of North America towards the Pacific Northwest. An amplified positive 500-hPa height anomaly is predicted over eastern Canada extending southward over most of the eastern contiguous United States (CONUS), with slightly negative 500-hPa height anomalies over parts of the Southeast. Dynamical model forecasts for the week 3-4 period largely predict persistence of this pattern with some variations between model forecasts on the strength and location of a trough stretching from southwestern Alaska potentially into the northwestern CONUS. Most models and the manual blend predict persistence of the strong positive 500-hPa height anomaly over eastern Canada and the northeastern CONUS with some extension of positive height anomalies southwestward into much of the western CONUS in week 3 and week 4. The manual blend of dynamical model forecasts predicts negative 500-hPa height anomalies over the Bering Sea, southwestern Mainland Alaska, and the Gulf of Alaska and troughing further down the coast of North America.

Below normal temperatures are likely for parts of the west coast of Mainland Alaska, the eastern Aleutian Islands, and southwestern Mainland Alaska, under the predicted trough. Model forecasts consistently predict above normal temperatures for much of northeastern Alaska, under anomalous southerly flow. Dynamical model forecast tools are inconsistent on the temperature forecast for Southeast Alaska and much of the western CONUS and generally predict weak signals, where an equal chance (EC) of below and above normal temperatures is forecast. Above normal temperatures are favored over much of the Southwest and the eastern half of the CONUS, under predicted positive 500-hPa height anomalies. Probabilities for above normal temperatures are enhanced for much of the Southwest and much of the Great Lakes region and Northeast, consistent with dynamical model forecasts. Weaker probabilities favoring above normal temperatures are indicated over parts of the Southeast, consistent with potentially cooler temperatures predicted by most dynamical models, such as the ECMWF, partly related to forecasts of precipitation over the next several weeks for this area. Above normal temperatures are favored across Hawaii, consistent with most dynamical model forecasts.

Above median precipitation is favored for southeastern Mainland Alaska and Southeast Alaska, ahead of the predicted trough and consistent with dynamical model precipitation forecasts. An enhanced probability of above median precipitation is indicated over the Pacific Northwest, consistent with most precipitation tools and the manual blend. Below median precipitation is slightly favored over a large area of the CONUS, including enhanced probabilities over the Southwest and over the western Great Lakes region, under predicted positive 500-hPa height anomalies. Below median precipitation is slightly favored for the Southern Plains, most of the Mississippi Valley, the Ohio Valley, and parts of the Northeast to the west of the Appalachian Mountains, consistent with the manual blend and most dynamical model forecast tools. Above median precipitation is favored for the Southeast Atlantic coast from Florida to southern Maryland, consistent with the manual blend of calibrated dynamical model precipitation tools. Below median precipitation is favored across Hawaii, as predicted by dynamical model forecasts.

Temperature Precipitation

FCST FCST

Hilo A55 B55

Kahului A55 B55

Honolulu A55 B55

Lihue A60 B55

Forecaster: Dan Collins

The next week 3-4 outlook will be issued on Friday, Sep 20, 2024

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

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