Prognostic Discussion for Week 3-4 Temperature and Experimental Precipitation Outlooks
NWS Climate Prediction Center College Park MD
300PM EST Fri Dec 01 2023
Week 3-4 Forecast Discussion Valid Sat Dec 16 2023-Fri Dec 29 2023
While the El Niño signature continues in the equatorial Pacific, several competing modes of tropical variability resulted in a short-lived weakening of the El Niño atmospheric pattern across the global tropics over the past week. Forecasts of the Madden Julian Oscillation (MJO) from the Global Ensemble Forecast System (GEFS) and European Centre for Medium Range Weather (ECMWF) models are in good agreement that an amplified MJO signal will propagate eastward into the Maritime Continent and the Western Pacific in the next few weeks, and more recent extended forecasts of the Realtime Multivariate MJO (RMM) Index support continued MJO activity into the end of December, reaching phases 7/8. As the MJO moves toward the dateline constructive interference with El Niño is possible, which could serve to strengthen the El Niño atmospheric pattern again. Recently, GEFSv12 and ECMWF forecasts of 60 degree N zonal wind anomalies at 10 hPa, which are used in prediction of a Sudden Stratospheric Warming (SSW), have begun trending toward negative which is indicative of a potential SSW. Though this has begun to be seen in forecasts, there is a high degree of uncertainty on if the SSW will occur, and when.
There is a high degree of variability in model forecasts of Week 3-4 500-hPa heights. The ECMWF model favors troughing over the western Contiguous United States (CONUS) and ridging over the Great Lakes and Northeast, while the Japanese Meteorological Agency (JMA) model favors ridging over the Northwest and troughing over the east. The Climate Forecast System v2 (CFS2) and GEFS models favor weak troughing over the west, and neutral or mixed heights over the East. This adds to uncertainty of the forecast, especially given the conflicting patterns in ECMWF vs. JMA. A manual blend of the dynamical models and a statistical Multiple Linear Regression (MLR) tool combining the influences of El Niño, MJO, and trend shows weak height anomalies over the CONUS, with some tilt toward below-normal heights over the West and more neutral over the East. As JMA was more of an outlier over the CONUS compared to our other models, we weighted ECMWF more highly. Over Alaska, models show mixed forecasts for 500- hPa heights as well, with the ECMWF favoring strong troughing, and JMA, CFS, and GEFS favoring positive or neutral heights.
Given the model differences in the 500-hPa pattern, the Weeks 3-4 Prediction of 2-meter temperature has modest probabilities to reflect the uncertainty in the forecast as well as the conflicting result from JMA. There is broad agreement among the models and support from warmer than normal local sea surface temperature (SST) anomalies on enhanced probabilities of above normal temperatures along the Northwestern coast and into the north central CONUS. CFS, JMA, ECMWF, and GEFS support a tilt toward above normal temperatures from the north central CONUS to the East Coast, however, probabilities are tempered given the cooler solution from JMA. The tilt toward above normal over this region is also supported by lagged MJO composites that favor above normal over the Mid-Atlantic. Models are mixed or inconsistent over the southern CONUS and northernmost parts of New England, and thus equal chances (EC) of above and below normal temperatures are indicated. Over Alaska, the CFS, GEFS, and JMA support increased probabilities of above normal temperatures, but ECMWF leans toward below normal. Despite the tilt toward below normal in ECMWF over Alaska, a manual blend of Weeks 3-4 Temperatures leans weakly toward above normal probabilities. EC is indicated for the northern parts of Alaska, and a weak tilt toward above normal is favored for the south where there was more agreement among models.
In contrast to the Weeks 3-4 Temperature Forecast, the Weeks 3-4 Precipitation forecast was much more consistent among the models and statistical tools. As noted in the most recent Climate Prediction Center (CPC) monthly forecast of precipitation, a wet start to December is expected over the Northwest due to an Atmospheric River event forecast for early December. However, by the Weeks 3-4 period, models are more inconsistent in signals with some models showing a tilt toward above median and some tilting toward below median precipitation. As such, EC is favored over the Northwest. Below median precipitation is favored across the Northern Great Plains and parts of the Great Lakes given model consistency. Finally, above median precipitation is indicated for parts of central and southern California, along the southern CONUS, and up the East Coast to parts of New England. The highest probabilities are over the Southeast where impacts from the ongoing El Niño are expected to remain the dominant driver.
Sea surface temperature anomalies remain elevated around Hawaii, but dynamical model guidance from the Subseasonal Prediction Experiment (SubX) multi-model ensemble on 2-meter temperatures is mixed, particularly over Hilo and Lihue leading to EC, while Kahului and Honolulu are forecast to weakly tilt toward above normal temperatures. The tilt toward below normal precipitation across Hawaii is mostly consistent among SubX models, with more certainty over Hilo.
| Temperature | Precipitation |
| FCST | FCST |
| Hilo | EC | B60 |
| Kahului | A55 | B55 |
| Honolulu | A55 | B55 |
| Lihue | EC | B55 |
Forecaster: Johnna Infanti
The next week 3-4 outlook will be issued on Friday, Dec 08, 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