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 06 2024

Week 3-4 Forecast Discussion Valid Sat Sep 21 2024-Fri Oct 04 2024

El Niño Southern Oscillation (ENSO) neutral conditions are currently present in the Equatorial Pacific and are expected to continue for the next few months, though 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) has been largely coherent since early August, propagating from the Western Hemisphere into the Indian Ocean and strengthening. However, recent forecasts of the Realtime-Multivariate MJO (RMM) index from GEFS and ECMWF indicate a stalling of the MJO over the western Pacific due to destructive interference from the developing La Niña by the time the week 3-4 forecast period begins. Impacts of local SSTs, such as the anomalously cool SSTs off the southern and western coasts of Alaska, are also considered. Finally, the Weeks 3-4 Temperature and Precipitation Outlooks are based on the expected continuation of atmospheric patterns from the week-2 period, and dynamical model forecasts from the CFSv2, GEFS, ECMWF, JMA. A statistical Multiple Linear Regression (MLR) tool which includes input from ENSO, MJO, and decadal trends was also used, mainly for how influence of trend may impact the forecast given weak ENSO and MJO activity. Experimental weighted blends of dynamical models and MLR are also used.

The week-2 midlatitude circulation pattern features anomalous ridging across eastern Canada and generally positive heights over the north-central and northeastern Contiguous United States (CONUS). The week-2 pattern is more transient over the Western CONUS, but near-normal heights are predicted over the West and below-normal heights over western Mainland Alaska. The mid-level atmospheric pattern during the week 3-4 period is generally consistent with week-2, with some shift in location and strength of the ridge and trough features. During week 3-4, below normal heights are forecast over the Aleutians and southwestern Alaska, neutral heights are forecast over the West, and above normal heights over much of the north-central and northeastern CONUS with the location of strongest ridging over eastern Canada. Week 3-4 dynamical model forecasts have good agreement on this pattern, despite some differences in strength. 500-hPa heights are neutral to above normal over Hawaii, with variability between models.

Dynamical models are in fairly good agreement on the pattern of temperature probabilities. The Weeks 3-4 Temperature Outlook favors enhanced probabilities of above normal temperatures over the Southwest and eastern half of the CONUS beneath favored ridging and consistent with decadal trends. Equal chances of above and below normal temperatures are indicated over the western third of the CONUS where 500-hPa heights were neutral, and models showed weaker probabilities. The highest probabilities, reaching 70 to 80%, are over parts of New Mexico and western Texas where model agreement was strongest. Below normal temperature probabilities are forecast over southwestern Alaska, including the Aleutians, beneath troughing that is consistent across the dynamical models and owing to below normal coastal SSTs. Probabilities transition to above normal toward northern Alaska where neutral 500-hPa heights are indicated.

Precipitation is more uncertain in dynamical models comparatively to temperature and 500-hPa heights, particularly over the eastern third of the CONUS. Despite the eastern CONUS uncertainty, GEFS, JMA, CFSv2, and ECMWF all favor above median precipitation over northwestern CONUS and southeastern Alaska, as such, probabilities are strongest in this region reaching 60 to 70%. Models showed inconsistent signals over much of the eastern CONUS, however, a tilt toward below median precipitation is indicated over the Great Lakes and Northeast beneath mid-level ridging. Some models, such as JMA, brought the below median precipitation southward all the way to the Gulf Coast, however, a weak tilt toward above median precipitation is favored due to chances for late-season storminess associated with tropical cyclones and in line with GEFS and ECMWF.

Dynamical models show mixed 500-hPa forecasts over Hawaii, with anomalously warm SSTs over Lihue and more neutral SSTs over Hilo. Calibrated models favor weakly above normal temperatures over Hawaii but probabilities are lowered given the weak signal and support, and EC is indicated over Hilo where model disagreement was strongest. Precipitation is again more uncertain than temperature, though calibrated models mostly tilt very weakly toward below median precipitation. The exception is GEFS, which has a weak tilt toward above median precipitation. A tilt toward below median is favored across the islands, based on CFSv2, ECMWF, and GEFS forecasts, though probabilities are weak.

Temperature Precipitation

FCST FCST

Hilo EC B55

Kahului A55 B55

Honolulu A55 B55

Lihue A60 B55

Forecaster: Johnna Infanti

The next week 3-4 outlook will be issued on Friday, Sep 13, 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