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 EDT Fri Aug 23 2019

Week 3-4 Forecast Discussion Valid Sat Sep 07 2019-Fri Sep 20 2019

ENSO-neutral conditions are present across the equatorial Pacific Ocean. Equatorial sea surface temperatures (SSTs) are above average across the western and central Pacific Ocean and below average in the eastern Pacific. The MJO is weak and not forecast to strengthen in any meaningful way during the next couple of weeks. The most pertinent tropical activity would be a hurricane in the Atlantic during the Week 3-4 time period. Although there isn't one in the forecast now, today's Week 3-4 forecast covers mid-September which is climatologically the peak of the Atlantic Hurricane season and we can't rule out tropical activity.

Today’s forecast features above normal temperatures over the eastern and western thirds of the country, with below normal temperatures in the central U.S. Most dynamical guidance supports anomalous ridging over the eastern CONUS. The CFS and ECMWF both forecast weak to moderate positive height anomalies over the western CONUS. The ECMWF predicts height anomalies of about 20-30m more above normal over Alaska than the CFS, but both predict above normal temperatures at the surface. The CFS positions the negative height anomalies over the western U.S. further south than the ECMWF, especially during Week-3, which if true would result in a southwestward shift of the coldest temperatures relative to our forecast.

The precipitation forecast is especially tricky this week because of noisy model guidance and the potential for tropical cyclone activity. There is good agreement in the models of below normal precipitation over northwestern Alaska and the Pacific Northwest, as well as above normal precipitation in the center of the CONUS. Both the CFS and the ECMWF forecast anomalous dryness over the eastern U.S, and our official forecast reflects this. It is important to note, though, that our forecast does not take into consideration any potential tropical cyclone activity, and a single storm could easily affect the total Week 3-4 rainfall over the East.

Observed above normal sea surface temperatures and dynamical model guidance from the SubX suite strongly support above normal temperatures during the Weeks 3-4 period across Hawaii. A consensus of the SubX dynamical models predicts likely above normal precipitation for the Hawaiian islands.

Temperature Precipitation

FCST FCST

Hilo A80 A60

Kahului A80 A60

Honolulu A80 A60

Lihue A80 A60

Forecaster: Kyle MacRitchie

The next week 3-4 outlook will be issued on Friday, Aug 30, 2019

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