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 20 2024

Week 3-4 Forecast Discussion Valid Sat Jan 04 2025-Fri Jan 17 2025

The Weeks 3-4 Temperature and Precipitation Outlooks are for January 4-17, 2025, near the heart of meteorological winter. In the tropics, La Niña is slowly emerging, with Niño 3.4 currently at -0.6 degrees C. There is also evidence of an enhanced Walker Circulation, indicative of atmosphere-ocean coupling. However, the typical mid-latitude teleconnections associated with La Niña - ridging over the Aleutians and Southeast - have largely been absent thus far in December. The pattern instead has been highly transient and variable. This transient pattern appears likely to continue as we close out December and enter 2025.

With the apparent weak La Niña forcing on the mid-latitude teleconnection, this allows subseasonal tropical convection variability in the form of the Madden-Julian Oscillation (MJO) to play a more important role. Currently, the MJO is entering the Western Pacific and is forecast by the ECMWF and GEFS to propagate eastward over the next week into the Western Hemisphere. This propagation is most apparent when examining maps of velocity potential, which show negative anomalies strengthening over the Americas as we progress into Week 2. Historically, this evolution of the MJO tilts the odds in favor of a warm Week 2 across much of the contiguous U.S. (CONUS) that transitions to broad, colder than normal conditions during Weeks 3-4.

Other physical drivers considered for this outlook include the Northern Hemisphere cryosphere, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and the Pacific North American (PNA) pattern. Northern Hemisphere sea ice and North America snow cover are currently below normal. This suggests cold air intrusions into CONUS during Weeks 3-4 could modify toward warmer values. The NAO/AO is forecast by the dynamical models to be negative at the onset of Week 3 but trend toward neutral conditions by the end of Week 4. The GEFS forecast is notably more negative than the ECMWF, which implies colder conditions across CONUS. The PNA is also forecast to be neutral by Week 3. Currently it is positive, with a ridge axis just inland of the western coast of North America. This axis is forecast to retrograde westward over time to the North Pacific, more in quadrature with the canonical PNA pattern.

Taken in their entirety, the aforementioned physical drivers suggest a significant pattern change from Week 2 to Weeks 3-4 in CONUS from much above normal temperatures to below normal temperatures, particularly east of the Rockies. This transition is well-supported by the 500-hPa ensemble mean anomalies as forecast by the ECMWF, GEFS, CFS, JMA, and Canadian dynamical models. The agreement between the physical drivers and the dynamical models provides a medium- to high-degree of confidence in the outlook, which is evident in the elevated forecast probabilities for temperature and precipitation. Furthermore, the most anomalous troughing is forecast during the middle of the Weeks 3-4 period, which helps alleviate timing concerns, especially given how transient the pattern has been thus far this meteorological winter. One caveat to note is that if the current lack of North America snow cover continues, then the impact of any cold air outbreaks may be tempered.

The models' Week 3-4 depictions of the 500-hPa height pattern all show an anomalous ridge axis off the West Coast that extends northward to Alaska with anomalous troughing in eastern CONUS. Consistent with this height pattern, below normal temperatures are favored east of the Rockies, with the highest probabilities, reaching 60 to 70%, centered over the Southeast. Above normal temperatures are favored over Maine, where significant ridging in southeastern Canada forecast for Week 2 may extend into Week 3. In the Southwest, above normal temperatures are favored. There is some uncertainty in the Pacific Northwest where Equal Chances (EC) of above or below normal temperatures are forecast. With the ridge axis offshore, it is plausible that easterly winds will filter colder air into the region, as surface high pressure systems drop southward, east of the Canadian Rockies. In fact, surface high pressure in the West can produce inversions, often associated with below normal temperatures in the valleys during the heart of winter.

With respect to precipitation, much of CONUS is favored to receive below normal precipitation. This includes the West Coast, where mid-level offshore ridging will reduce moist, onshore flow. It also includes a broad region from Texas to the Mid-Atlantic and Northeast, where dry, northwest flow is expected to dominate the Weeks 3-4 period. Above normal precipitation is anticipated along the eastern slopes of the Northern and Central Rockies, including portions of the High Plains. The pattern is conducive to upslope flow, as Canadian high pressures drift southward. Above normal precipitation is also favored in southern Georgia and northern Florida, where cold frontal boundaries may stall and become stationary. Finally, EC is forecast for precipitation downwind of the Great Lakes, where lake effect snow regimes may manifest.

In Alaska, above normal precipitation and temperatures are forecast given strong agreement in the models and the potential for a southerly mid-level flow into the state. Above normal temperatures and precipitation are also forecast for most of Hawaii, also in agreement with the dynamical models, with the potential for mid-level troughing in the area.

Temperature Precipitation

FCST FCST

Hilo A60 EC

Kahului A60 A55

Honolulu A60 A60

Lihue A60 A60

Forecaster: Cory Baggett

The next week 3-4 outlook will be issued on Friday, Dec 27, 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

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