Climate Prediction Center - Week 3-4 Outlook

Prognostic Discussion for Experimental Week 3-4 Outlook
NWS Climate Prediction Center College Park MD
300PM EST Fri Jan 13 2017

Week 3-4 Forecast Discussion Valid Sat Jan 28 2017-Fri Feb 10 2017

Dynamical model ensemble guidance supports a substantial pattern change from Week-2 into the Week 3-4 outlook period. During Week-2 anomalous 500-hPa ridging is favored over Hudson Bay, with anomalous troughing stretching from Alaska down the Pacific coast, with an eastward extension along the border with Mexico. Beginning around 25 January ensemble guidance suggests ridging will begin to build southwestward across the Pacific Northwest, with troughing building in the east. The JMA is an outlier, with broad ridging forecast across North America. Some hesitation is needed with this forecast solution, as dynamical guidance has shown a tendency to abruptly adjust the general circulation pattern too quickly during the Week 3-4 period. Nevertheless, the consistent solution between the CFS and ECENS does make this solution the basis for the present outlook; however, probabilities are damped based on recent performance in pattern change scenarios.

Low frequency dryness continues to be observed in the Central Pacific, while a prolonged period of inactivity from the Madden-Julian Oscillation is apparent over approximately the last month. The RMM index currently indicates a marginal signal over the far West Pacific, but GEFS, ECENS, and UKMET guidance each indicate an abrupt shift to a signal centered over the Atlantic in Week-1 that subsequently grows in amplitude and exhibits eastward propagation. While an emerging intraseasonal signal in Phase 8/1 over the Western Hemisphere could be leveraged for the Week 3-4 outlook, these states tend to be transitional at lags of 15-28 days, evidenced by statistical guidance having poorer retrospective performance in these phases and mixed signals among lagged composites. As such, calibrated dynamical model guidance and the anticipated 500-hPa circulation patterns are the primary basis for the present outlook, with some deference to decadal trends and antecedent conditions.

Above-normal temperatures are favored across the Western U.S. and much of Alaska associated with expectations of 500-hPa ridging building across the region. Some equal-chances areas are carved out in the Great Basin, where antecedent snowfall in this region could result in lingering cold temperatures across the valley regions as nocturnal temperature inversions set up. Anomalous 500-hPa troughing east of the Appalachians results in a region of below-normal temperatures favored from the Southern and Central Plains eastward through the Mid-Atlantic and Southeast, with the highest probabilities for below-normal temperatures in the Central Appalachians. Differences in the magnitude and positioning of the anomalous ridging and troughing at 500-hPa lead to equal chances being forecast across the Rockies, northern tier of the CONUS, and North Slope of Alaska.

Below-median precipitation is favored along the Pacific Coast of the Lower-48 associated with the ridging forecast to build near the coastline. An extension of this below-median precipitation is possible for portions of central Alaska associated with a continental flow pattern indicated by the ridge positioning. Below-median precipitation is also favored from the eastern Great Lakes through the Lower Mississippi Valley behind the anticipated 500-hPa trough axis. Above-median precipitation is possible across the Northern Plains tied to possible upslope flow out of the east.

Ensemble guidance supports Hawaii being forecast to have slightly enhanced probabilities of above-normal temperatures and below-median precipitation.

Temperature Precipitation

FCST FCST

Hilo A55 B55

Kahului A55 B55

Honolulu A55 B55

Lihue A55 B55

Forecaster: Daniel Harnos

The next week 3-4 outlook will be issued on Friday, Jan 20, 2017

These outlooks are based on departures from the 1981-2010 base period

These are experimental two category outlooks and differ from official operational 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.