Prognostic Discussion for Week 3-4 Temperature and Experimental Precipitation Outlooks
NWS Climate Prediction Center College Park MD
300PM EDT Fri Apr 30 2021
Week 3-4 Forecast Discussion Valid Sat May 15 2021-Fri May 28 2021
La Nina conditions continued but weakened across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) are below average from the west-central to eastern Pacific Ocean, as enhanced upper level westerly wind anomalies persist across most of the equatorial Pacific Ocean. The RMM index depicts an eastward propagation of the MJO from mid-March through early April, when the MJO became less coherent during mid-to-late April. The GEFS and ECMWF are in good agreement that the MJO resumes its eastward propagation over the Western Hemisphere during week-1, but uncertainty on its evolution increases later in week-2. The Week 3-4 temperature and precipitation outlooks are based primarily on dynamical guidance including the GEFS, CFS, Canadian, ECMWF, JMA, the Subseasonal Experiment (SubX) multi-model ensemble (MME) of experimental and operational ensemble prediction systems, with additional considerations for MJO, La Nina, decadal climate trends, and soil moisture conditions, as well as the predicted evolution of the pattern from Week-2 forecasts.
The forecast circulations are reasonably consistent among model guidance regarding the anomalous 500-hPa height patterns over the week 3-4 period. Dynamical model 500hPa height anomaly forecasts during week 3-4 show a fairly consistent evolution from the forecast state during Week-2. The CFS and ECMWF models feature near to above normal 500-hPa heights over the CONUS, while the JMA model favors above normal 500-hPa heights over the southern CONUS and near to below normal 500-hPa heights across the northern CONUS. The CFS and ECMWF models indicate anomalous ridging with above normal 500-hPa heights over Mainland Alaska, the Alaska Panhandle and the Aleutians. Near normal 500-hPa heights are likely over Hawaii.
Below normal temperatures are favored over parts of the Northern Plains, consistent with the CFS/ECWMF/JMA Equal Weighted and Manual Blend tools. Above normal temperatures are more likely across the remainder of the CONUS, under predicted near to above normal 500-hPa heights. The highest confidence for above-normal temperatures lies across the Southwest (greater than 70%), supported by most dynamical tools. Predicted ridging and above normal 500-hPa heights lead to predicted above normal temperatures over Alaska.
The dynamical model guidance is in reasonably good agreement on the spatial pattern of anomalous precipitation during the Week 3-4 period. Above normal precipitation is favored over parts of the Upper and Lower Great Lakes as well as the Ohio Valley, consistent with the CFS/ECWMF/JMA and CFS/ECWMF Equal Weighted as well as Manual Blend tools. Near to below normal precipitation is favored over the remainder of the CONUS, under the near to above normal 500-hPa heights, consistent with dynamical guidance. Anomalous ridging and above normal 500-hPa heights lead to enhanced probabilities of below normal precipitation over southern Mainland Alaska, the Alaska Panhandle, and the Aleutians.
Most dynamical model guidance, including the experimental SubX models, depict enhanced chances for above normal temperatures over the northwestern Hawaiian islands (Lihue and Honolulu). Equal chances of below or above normal precipitation are forecast for Hawaii, based on little to no signal among precipitation tools.
| Temperature | Precipitation |
| FCST | FCST |
| Hilo | EC | EC |
| Kahului | EC | EC |
| Honolulu | A55 | EC |
| Lihue | A55 | EC |
Forecaster: Luke He
The next week 3-4 outlook will be issued on Friday, May 07, 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