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 Nov 15 2024

Week 3-4 Forecast Discussion Valid Sat Nov 30 2024-Fri Dec 13 2024

Both tropical and extratropical climate signals could impact the forecast period, but there is a high degree of uncertainty due to the transient nature of the large-scale circulation across the two week period.This is evident in the Week 2 dynamical model forecasts of the Northern Hemisphere 500-hPa height anomaly field, with zonal wavenumber 6 being evident, indicative of a fast, transient, and chaotic flow leading up to the Week 3-4 period. In the tropics, the convection related to the Madden Julian Oscillation (MJO) has exhibited a clear eastward propagation, particularly in the velocity potential field. Dynamical models indicate that the MJO signal will continue for the next 1-2 weeks. The extratropical North Atlantic Oscillation (NAO) is currently in the negative phase. Both GEFS and ECMWF favor a continued negative phase though the amplitude does weaken. These signals are taken into account for the forecast in addition to the dynamical model suite.

A manual blend of dynamical and statistical guidance indicates that the large-scale circulation is dominated by anomalous troughing over Canada and the Northern Plains juxtaposed by positive height anomalies across the Southeast as well as a strong Aleutian ridge. However, it’s not clear how representative this pattern is of the variability across the forecast period. It projects strongly on Phase 1 of the MJO, but that signal is primarily evident in Week-3. The troughing extends deeper into CONUS during Week-3 before retreating into Canada in Week-4. Furthermore the strength of the Aleutian ridge is predicted to weaken in Week-4. Individual dynamical models are largely consistent with GEFS as the key outlier. The GEFS solution favors an overall weaker solution with more widespread anomalous ridging across CONUS and troughing centered over eastern Canada.

The Week 3-4 Temperature Outlook tilts toward above normal temperatures across much of eastern CONUS with strongest model agreement in the Northeast. As mentioned above, the progression of the troughing over western CONUS adds some uncertainty to the temperature forecast for that region. The extension of the anomalous troughing in western CONUS during Week-3 is reflected in the stronger below normal temperatures. By week-4, weak positive height anomalies are forecast for the West. Many dynamical models, particularly the JMA and to a lesser extent ECMWF, favor a shift from below normal to above normal temperatures during Week-4. As a result of this, and uncertainty in the timing of these changes, Equal Chances (EC) of above or below normal temperatures is forecast for much of the Central Plains and western CONUS except the Northern Plains and the Alaska Panhandle where below normal temperatures are consistently indicated.

The Week 3-4 Precipitation Outlook tilts toward above median for much of the Northern Plains and Pacific Northwest into the Alaska Panhandle under the anomalous troughing. A wetter signal is also favored for the Midwest and Mississippi Valley related to the baroclinic zone that could set up in the region. Dynamical models consistently indicate a weak dry signal for the desert Southwest. Elsewhere across CONUS and Alaska, model signals are weak and inconsistent so EC is forecast.

Sea surface temperatures around Hawaii are above normal promoting elevated probabilities of above normal temperatures for most of the islands. Model guidance, while consistent on temperatures, is more subtle regarding precipitation so a slight tilt toward below normal is indicated for Hilo and EC for the other islands.

Temperature Precipitation

FCST FCST

Hilo EC B 6

Kahului A60 EC

Honolulu A60 EC

Lihue A60 EC

Forecaster: Laura Ciasto

The next week 3-4 outlook will be issued on Friday, Nov 22, 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