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 18 2020

Week 3-4 Forecast Discussion Valid Sat Oct 03 2020-Fri Oct 16 2020

La Nina conditions continue to persist in the Pacific, while an active Madden-Julian Oscillation (MJO) event is currently over the Maritime Continent. The MJO is forecast to push eastward over the course of the next two weeks, and may destructively interfere with the base state by the time it reaches the Pacific near the beginning of October. The present Weeks 3-4 outlook finds its basis in dynamical model guidance, coupled with secondary consideration given to long-term trends and typical La Nina conditions.

The forecast circulation from model guidance generally features broad anomalous ridging across much of North America, particularly from the JMA model. The model featuring the most impactful anomalous troughing for North America is the CFS which forecasts a trough over the Gulf of Alaska throughout the forecast period, in addition to slightly negative height anomalies over the Tennessee Valley. While lost in the Weeks 3-4 mean, the ECMWF does develop anomalous troughing over the Gulf of Alaska during Week-4. The CFS and JMA both forecast anomalous ridges over the West during Weeks 3-4. The ECMWF has this ridge as well, but instead it is part of the western edge of an elongated region of positive height anomalies that is instead anchored over Eastern Canada. The SubX multimodel mean closely aligns with the CFS perspective, which is unsurprising as this model makes up roughly a quarter of its members.

With broad anomalous ridging generally forecast in the models, coupled with positive decadal trends in temperatures, probabilities tilt toward above-normal temperatures across the CONUS. Weakest probabilities for anomalous warmth exist across the Southeast and southern Alaska tied to the potential for anomalous troughing as mentioned in the previous paragraph among some guidance. Highest probabilities for above-normal temperatures exist across Arizona where trends and anomalous ridging overlap, and to a lesser extent across much of the West and Great Plains as tied to forecast ridging over the West in the CFS, JMA, and Week-3 of the ECMWF. Long-term trends across Alaska also result in above-normal temperatures to increase poleward.

With a forecast of positive height anomalies across much of the country, a relatively dry forecast is the result for many areas. Exceptions for regions favoring below-median precipitation over the Lower 48 states include the West where forecast anomalous troughing over the Gulf of Alaska is likely to increase onshore flow to the Pacific Northwest and increase odds for above-median precipitation, with this also being consistent with typical La Nina conditions. Equal chances exist across much of California and into the Desert Southwest given the climatologically arid time of year favoring little to no precipitation. Equal chances also exist across portions of the East Coast tied to robust forecast wet signals over the Bahamas and the ECMWF model suggesting an active two week period for possible tropical cyclone activity recurving near the region. Highest probabilities for below-median precipitation exist over the Great Lakes in line with the forecast anomalous ridging from the ECMWF model which closely aligns with La Nina composites (although interestingly, not if also accounting for trends from the 1950s through recent). The Alaska Panhandle and portions of the southern coast see a slight tilt toward above-median precipitation tied to forecast troughing over the Gulf of Alaska. Below-median precipitation is favored with increasing confidence while heading northward across Alaska given topographic orientation and the lack of any source region for moisture with the forecast circulation.

Hawaii continues to be surrounded by above-normal sea surface temperatures, with the greatest positive anomalies surrounding western parts of the state. This supports high confidence for above-normal temperatures across the archipelago, with the largest chances for western points. SubX guidance consistently features dry signals for Hawaii, resulting in a tilt toward below-normal precipitation being favored.

Temperature Precipitation

FCST FCST

Hilo A90 B60

Kahului A80 B60

Honolulu A70 B60

Lihue A70 B60

Forecaster: Daniel Harnos

The next week 3-4 outlook will be issued on Friday, Sep 25, 2020

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