Prognostic Discussion for Week 3-4 Temperature and Experimental Precipitation Outlooks
NWS Climate Prediction Center College Park MD
300PM EDT Fri Oct 23 2020
Week 3-4 Forecast Discussion Valid Sat Nov 07 2020-Fri Nov 20 2020
La Nina conditions are present, as below-normal SSTs persist over the equatorial east-central Pacific alongside a coherent atmospheric response with enhanced trade winds and westerly wind anomalies aloft. The MJO has also become more active, with the enhanced convective phase propagating eastward across the Maritime Continent during the past week. Dynamical models generally favor continued eastward propagation of the intraseasonal signal, where it would begin destructively interfering with the La Nina response. As the boreal autumn season progresses, these tropical signals have the potential to teleconnect well with the extratropical pattern. Therefore, the Week 3-4 outlook favors statistical guidance based on tropical-extratropical interactions, particularly the multiple linear regression tool based on the RMM indices, ONI, and long term trend.
Dynamical model guidance features a wide variation of 500-hPa height anomaly patterns during the Week 3-4 period. The CFS, ECMWF, and JMA all depict a trough in the vicinity of the Aleutians, but while the ECMWF has ridging downstream over much of the northern CONUS, the CFS and JMA both extend the trough to the vicinity of the Pacific Northwest and have a much more amplified ridge near the eastern US. These variations result in considerable differences in the model temperature and precipitation outlooks, with the CFS and JMA showing enhanced precipitation over the Pacific Northwest and widespread above-normal temperatures across the East, while the ECMWF is warmer across the West and generally drier. The MLR shows a blended MJO/ENSO influence, with troughing over the Great Lakes and ridging over the West favoring a colder and wetter solution for the Great Lakes and upper-Midwest. The outlook is based strongly on a weighted blend of the MLR, CFS, ECMWF and JMA, with 70 percent contributions from the MLR and ECMWF. However, the outlook favors the MLR solution across the northeastern quadrant of the CONUS due to the robust tropical signals.
The Week 3-4 temperature outlook features enhanced chances of above-median temperatures across Alaska, much of the western US, and the Appalacians and Eastern Seaboard south of Maine. The highest probabilities of above-median temperatures exist over northern Alaska and the western CONUS extending from southern Washington to western Texas. Despite the blended guidance favoring warmth across much of the CONUS, an area of equal chances for below- and above-median temperatures is indicated for the north-central CONUS through northern New England, with below-median temperatures favored for the upper-Midwest and northern Great Lakes region. This solution most closely resembles the MLR, although with less cold in deference to the warmer dynamical model forecasts.
Enhanced chances for above-median precipitation extend across parts of the Northern Tier, with the highest probabilities across central Montana and northern New England. Consistent with canonical La Nina conditions, below-median precipitation is favored for the southern tier of the CONUS. Equal-chances are maintained across the Great Lakes, southern New England, and the mid-Atlantic states, as well as the Pacific Northwest. While some guidance depicts considerable wetness over the Pacific Northwest, given the increasingly wet climatology, there is insufficient guidance to warrant higher probabilities at this time. Below-median precipitation is favored for most of Alaska, while onshore flow favors enhanced precipitation for the southern Alaska panhandle.
The outlook favors above-median temperatures for the Hawaiian Islands due to positive SST anomalies in the vicinity, and the SubX MME forecasts show increased probabilities for below-median precipitation.
| Temperature | Precipitation |
| FCST | FCST |
| Hilo | A70 | B55 |
| Kahului | A80 | B55 |
| Honolulu | A80 | B55 |
| Lihue | A80 | B55 |
Forecaster: Adam Allgood
The next week 3-4 outlook will be issued on Friday, Oct 30, 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