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 Dec 06 2019

Week 3-4 Forecast Discussion Valid Sat Dec 21 2019-Fri Jan 03 2020

The Week 3-4 temperature and precipitation outlooks are based on the evolving circulation pattern from Week-2, dynamical model forecasts from the NCEP CFS, ECMWF, and JMA, and SubX multi-model ensemble. Long-term trends and anomalous snow cover were also considered. ENSO-neutral conditions continue across the tropical Pacific Ocean. The Madden-Julian Oscillation (MJO) weakened during late November with the positive Indian Ocean Dipole (IOD) remaining dominant. Although some dynamical models indicate that the MJO resumes eastward propagation during the next two weeks, its amplitude is likely to be reduced by destructive interference with the positive IOD.

Poor consistency and agreement has plagued the dynamical models during a transition from a negative to positive Arctic Oscillation (AO) index during the past week. However, the latest GFS and ECMWF ensemble means have converged on the 500-hPa pattern during Week-2 with a weaker ridge over western North America. Although there is likely to be an outbreak of Arctic air across the north-central U.S. during the second week of December, the anomalously cold temperatures are expected to be short-lived without the persistence of a full latitude ridge upstream. During Week-3, the CFS model maintains a highly amplified ridge along the West Coast which seems to be the least likely outcome based on the evolving pattern later in Week-2. It appears that the CFS model is slower with a pattern change as its predicted 500-hPa height pattern eventually converges to the ECMWF and JMA models during Week-4. The ECMWF and JMA models are in good agreement that an upper-level trough develops over the West during Week-3 with an increase in 500-hPa heights along the East Coast. The members of the SubX multi-model ensemble feature a wide range of solutions which leads to only small 500-hPa height anomalies throughout the CONUS. However, at higher latitudes, the SubX multi-model ensemble mean is most similar to the JMA with large 500-hPa positive (negative) anomalies over mainland Alaska (eastern Canada). Also, there are hints of an upper-level trough extending into the Rockies. If there is any influence on the mid-latitude circulation pattern from the MJO, it would tend to favor the 500-hPa height pattern represented by the ECMWF and JMA models.

Above-normal temperatures are favored for much of the CONUS due to the most likely trough/ridge pattern and temperature tools from the ECMWF and JMA models, the evolving longwave pattern during the latter half of December, and any forcing from the MJO. Due to uncertainty on how quickly 500-hPa heights increase along the East Coast, only a slight tilt in the odds for above normal temperatures is forecast across the Southeast which tapers to equal chances for New England. Based on the expectation that an upper-level trough digs into the West later in Week-2, anomalous snow cover, and MJO considerations, equal chances in above or below normal temperatures are also forecast across the Great Basin, desert Southwest, and parts of California.

The precipitation outlook is consistent with the most likely longwave pattern featuring an upper-level trough extending into the West. Above normal precipitation is favored across the Four Corners region at the base of this trough. A broad area with a slight tilt towards above normal precipitation is forecast downstream of the upper-level trough axis with the highest probabilities centered over the Ohio and Tennessee Valleys. Periods of downslope flow favor below normal precipitation across the northern high Plains. Since the Week-3 height pattern is out of phase between the CFS and ECMWF/JMA models and precipitation tools vary during the Week 3-4 period, equal chances of below or above precipitation are forecast along the West Coast.

The temperature and precipitation outlooks for Alaska are based largely on the JMA model which features an upper-level trough axis from the Bering Sea south to the Aleutians and an upper-level ridge over mainland Alaska. This predicted longwave pattern yields increased chances for above normal temperatures throughout Alaska and above normal precipitation most likely across the Alaska Peninsula, Aleutians, and southwest Alaska.

Weekly sea surface temperature (SST) anomalies, centered on November 27, are running more than 1 degree C above normal surrounding Hawaii. Based on these SST anomalies along with dynamical model guidance, above normal temperatures are likely for Hawaii. Due to conflicting or weak signals among the precipitation tools, equal chances of below or above normal precipitation are forecast across Hawaii.

Temperature Precipitation

FCST FCST

Hilo A90 EC

Kahului A90 EC

Honolulu A90 EC

Lihue A90 EC

Forecaster: CPC Forecaster

The next week 3-4 outlook will be issued on Friday, Dec 13, 2019

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