Sharp Differences in Winter Forecast10/19/2018
The weather recordkeeping winter season, from December 2018 through February 2019, is about five weeks away. Ahead of winter, the U.S. National Oceanic and Atmospheric Administration (NOAA), is calling for above-normal temperatures over the northern and western U.S. — including the Plains and the northern and western Midwest — with “equal chances” for above, normal, or below-normal temperatures elsewhere.
There is no region of the contiguous U.S. with below-normal temperatures forecast in the NOAA outlook.
That call is not universal in the weather community. DTN’s winter forecast, issued in early October, contrasts sharply with NOAA’s prediction. The DTN forecast is for below-normal temperatures — a cold winter — over much of the U.S. east of the Rockies.
This large departure has to do with how the influence of a weak El Nino temperature pattern in the equatorial Pacific Ocean is factored into the forecast. NOAA Climate Prediction Center Deputy Director Mike Halpert credits El Nino influence developing. “We expect El Nino to be in place in late fall to early winter,” Halpert said. “Although a weak El Nino is expected, it may still influence the winter season by bringing … warmer, drier conditions to parts of the north.”
In contrast, DTN’s forecast view is based on the weak El Nino having very little influence on the winter pattern, according to DTN long-range forecaster Nathan Hamblin.
“If there is a moderate or strong El Nino, the chances increase markedly for warmer than normal temperatures in the Midwest. We don’t think that will happen, given the slow start to the development of this El Nino,” Hamblin said. “A weak El Nino increases the risk for cold intrusions (the appearance of the ‘Polar Vortex’) during the winter, especially later in the winter.”
With a weak El Nino likely, DTN’s method gave more weight to winter weather patterns in similar (analog) years. The winter seasons selected were: 1968-69; 1977-78; 1994-95; 2003-04; and 2009-10. There were, indeed, some cold winters in this array of seasons.
Hamblin also noted that well-known weather agencies in the European Union and the United Kingdom are closer to the DTN viewpoint than they are to the NOAA forecast. “We leaned toward the ECMWF and UKMET models, which were in better phase with the type of pattern the analogs were showing,” Hamblin said.
A key development will be how the Pacific El Nino evolves. A stronger El Nino would work in favor of the NOAA forecast call. “The stronger this El Nino gets, the better chance the (NOAA) CFS models will be correct,” Hamblin said. “The next 30 days will reveal a considerable amount of insight as we observe El Nino’s evolution.”
Bryce Anderson can be reached at firstname.lastname@example.org
Follow him on Twitter @BAndersonDTN
Source: Bryce Anderson, DTN