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 2024

Week 3-4 Forecast Discussion Valid Sat Dec 21 2024-Fri Jan 03 2025

With ENSO-neutral conditions prevailing across the Pacific, a robust and long-lived Madden-Julian Oscillation (MJO) event has dominated the global tropical convective pattern through much of October, November, and early December. The enhanced convective phase of the MJO is currently over the Maritime Continent, where it is constructively interfering with equatorial Rossby wave activity and the low frequency base state, which includes a growing negatively phased Indian Ocean Dipole (IOD) event and an equatorial Pacific that seems to be trending slowly towards weak La Niña conditions. This superposition of various modes resulted in an exceptionally strong low-level westerly wind burst over the eastern Indian Ocean that triggered tropical cyclogenesis both north and south of the Equator and a trade wind surge across most of the Pacific basin. As the MJO attempts to propagate across the Pacific, there is considerable uncertainty regarding whether the signal will remain coupled to enhanced convection as it begins destructively interfering with the base state. The GEFS dynamical model solutions show a more robust MJO evolution, while the ECMWF weakens the signal while continuing its eastward propagation. This uncertain MJO evolution is likely to play a role in the Weeks 3 and 4 outlook, as a midlatitude response to western Pacific convection can contribute to a pattern change across North America.

A manual blend of dynamical and statistical model guidance results in a 500-hPa height anomaly field with troughing over the Aleutians, ridging over the western CONUS, and weak troughing over the Great Lakes region. This pattern is a fairly substantial change from the Week-2 guidance and is suggestive of mean enhanced southerly flow into Alaska and a potential for northerly flow into the central and eastern CONUS. This pattern increases the potential for cold air outbreaks; however, with a strengthened polar vortex feature, a positive AO signal, and a warm pattern antecedent to the Weeks 3 and 4 period, there may not be significant reservoirs of cold air in place to invade the Lower 48. Furthermore, the lack of a negative NAO pattern over the North Atlantic may provide a means for quick evacuation of any cold airmasses that do drop into the eastern US. Therefore, the Weeks 3 and 4 outlook contains considerable uncertainty, and the pattern during this time period may be marked by transitional features.

The Weeks 3-4 temperature outlook favors above-average temperatures for much of Alaska with enhanced southerly flow and above-average temperatures for the western half of the CONUS due to mean ridging. Persistent high pressure may result in localized cooling of valleys under inversion layers, but in general the pattern is anticipated to be warmer than average. Across the East, equal chances are maintained for most regions due to the considerable uncertainty, with the ECMWF maintaining a warmer pattern in the absence of stronger MJO forcing, while the GEFS favors a transition to a colder pattern by Week-4. Mean enhanced southerly flow over New England favors above-average temperatures.

Despite the anticipated ridging, the ECMWF and other dynamical models depict a wet pattern across the Pacific Northwest. Greater confidence for above-average precipitation exists across southern and eastern Alaska and the Alaskan panhandle due to enhanced onshore flow. Should cold high pressure centers move southward across the central CONUS, upslope flow may favor enhanced precipitation for the northern High Plains and Rockies. Elsewhere, below-average precipitation is favored for the Southwest given the mean ridging pattern and above-average precipitation is forecast for the Northeast due to a potential for a mean baroclinic zone providing a focus for storm development and mean onshore flow over New England.

Weak ridging is favored over the Hawaiian Islands, which increases the probability for above-normal temperatures. Dynamical model forecasts generally depict enhanced moisture across Hawaii, extending northward into southern Alaska and western Canada. Therefore, above-average precipitation is forecast for Hawaii in this Weeks 3-4 outlook.

Temperature Precipitation

FCST FCST

Hilo A55 A55

Kahului A55 A55

Honolulu A55 A55

Lihue A55 A55

Forecaster: Adam Allgood

The next week 3-4 outlook will be issued on Friday, Dec 13, 2024

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