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 Feb 26 2021

Week 3-4 Forecast Discussion Valid Sat Mar 13 2021-Fri Mar 26 2021

The evolution of the MJO and ENSO is especially uncertain during the next 3-4 weeks. The RMM index is a good proxy for the strength and location of the MJO when tropical convection projects well onto the RMM structures. However, this is often not the case during strong La Nina events or when higher frequency tropical variability is present. Recently, La Nina and equatorial Rossby wave activity have driven most of the variability in the tropics and the current RMM index is mostly the result of the superposition of these two modes. The GEFS and ECMWF forecast the RMM index to approach Phases 8 and 1 by the end of Week-2, but it is difficult to determine whether the models are forecasting a bonafide MJO event or the evolution of other tropical variability. Given this uncertainty and lack of confidence in the formation of a strong MJO event, we have decided to lean on the dynamical forecasts more heavily than the state of the tropics this week.

The ECMWF, GEFSv12, CFS, and JMA models are in especially good agreement this week regarding the 500-hPa height forecast over the Northern Hemisphere. Each model forecasts high amplitude positive height anomalies centered just south of the western Aleutians, negative height anomalies over much of mainland Alaska and the Northwest, and positive height anomalies over the eastern CONUS. Although the general patterns among the models are similar, the JMA forecasts a notably weaker ridge over the East Coast than the other models.

The surface temperature fields are as much in agreement among the models as we'd expect given the similar 500-hPa height forecasts. Our forecast, based closely on an equally weighted mean of the aforementioned dynamical models, is a tilt towards above normal temperatures from the Rockies to the East Coast. The highest probabilities of above normal temperatures, 70-80%, are throughout the central CONUS from the Great Lakes to the Gulf Coast. Probabilities of below normal temperatures are highest along the West Coast of the CONUS along with southern Mainland Alaska, 60-70%, and are located directly under the highest amplitude area of the forecast 500-hPa trough.

As usual, model agreement is weaker regarding the precipitation forecast than the temperature forecast. The area of closest agreement is a tilt towards dry conditions in the Southeast underneath the forecast ridge. The highest forecast probabilities in this region are 60-70% throughout Florida, most of Georgia, and eastern South Carolina. There is also a forecast preference for above normal precipitation along the West Coast of the CONUS along with southwestern Mainland Alaska, matching up closely with the aforementioned trough and extent of below normal temperatures.

We also forecast a slight tilt, 55-60%, towards above normal precipitation throughout the Midwest and Great Lakes. Since precipitation climatologies are relatively dry this time of the year throughout most of that region, even a weak cyclone can result in above normal precipitation. The CFS and the GEFSv12 are the strongest proponents of this forecast, although the JMA also hints at it during Week-3.

SSTs remain above normal around Hawaii underneath the southern extent of the anticipated 500-hPA ridge. Therefore, the forecast favors above normal temperatures and below normal precipitation for Hawaii during the forecast period.

Temperature Precipitation

FCST FCST

Hilo A60 B55

Kahului A60 B55

Honolulu A60 B55

Lihue A60 B55

Forecaster: Kyle MacRitchie

The next week 3-4 outlook will be issued on Friday, Mar 05, 2021

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