December 2020 HDD Over-achieved Considering Mild U.S. Climate

U.S. Cities HDD Report for December 2020: A warm December BUT Midwest/East HDD over-achieved on the cold side.

Discussion: A review of DECEMBER 2020 selected cities heating degree day (HDD) verification reveals that New England was surprisingly at normal demand. Boston finished the month at 894 HDD which is slightly higher than the 30-year normal (887 HDD) and much colder than the 10-year normal (866 HDD). Albany, NY was similar (1079 HDD vs. 1076 HDD 30-year normal). Other locations such as Hartford, CT to Rochester, NY were a little warmer – closer to the 10-year normal.

To the south, the large Northeast Corridor metro areas New York City, Philadelphia, Baltimore and Washington, DC observed colder-than-expected HDD values. NYC was at 740 HDD which is close to the 10-year normal (747 HDD). However, Philadelphia was a cold spot (808 HDD compared to the 30-year normal 796 HDD). Washington was also close (723 HDD) to the 30-year normal (728 HDD).

In Virginia, Richmond (also) over-achieved on the cold side producing 724 HDD somewhat colder than the 30-year normal (697 HDD). Coastal areas (Norfolk) were similar to the warmer 10-year normal. Similar to Richmond, Raleigh, NC was somewhat colder (661 HDD) than the 30-year normal (630 HDD).

In summary, despite the generally warmer-than-normal weather/climate guidance heading into December the month produced near to slightly colder than the conventional 30-year normal for the U.S. Northeast Corridor.

The Southeast U.S. observed a cold December 2020. In Atlanta, the December HDD was 576 HDD compared to a MUCH WARMER 10-year normal (489 HDD). Columbia and Charleston, SC were each 63-72 HDD colder than the 30-year normal.

Fig. 1: The preliminary U.S. temperature anomalies for December 2020 were robustly warmer than normal.

The Ohio Valley regime identifies where snow cover had an occasional cold effect on climate to push December 2020 HDD values to cold levels despite the anomalous warmth monthly departures (Fig. 1). In Pittsburgh, Detroit, Cleveland, Cincinnati and Columbus the December 2020 HDD values were as cold as 15 HDD colder-than-the 30-year normal in Pittsburgh to 21 HDD warmer-than-the 30-year normal in Cincinnati. However, not far to the west the lack of snow for Chicago drove a more representative monthly HDD (of the December warmth) producing 990 HDD which is far below the 30-year normal (1115 HDD).

The Midwest U.S. produced an interesting HDD profile. Minneapolis was much warmer than normal in December 2020 and the observed HDD (1226 HDD) was much warmer than the 30-year normal (1341 HDD). However, Des Moines (1099 HDD) was near the 30-year normal (1093 HDD) while Kansas City was (appropriately) much warmer than the 30-year normal (870 HDD vs. 983 HDD). Brief snow cover-driven cold unique to Des Moines pushed the inconsistent HDD for this region (despite the overall very warm month).

In Texas, December 2020 was somewhat warmer-than-normal. However, once again the HDD count doe not reflect that warm anomaly especially in Dallas where December produced 486 HDD which is right at the 10-year normal. Houston was a better HDD fit for the warm month of December where 297 HDD (vs. the 343 30-year normal) was observed.

The Pacific Northwest was a mild spot! Seattle and Portland each observed monthly HDD values that were about 100 HDD warmer than the 30-year normal. Daytime temperatures were cool but persistent temperature inversions lead to low diurnals and warmer than normal nights to help fuel the mild regime.

California observed a warmer than normal December 2020. HDD values support the warmth averaging about 35 to 55 HDD warmer than the 30-year normal across the state.

Summary: The first month of meteorological winter 2020-21 was much warmer than normal. HOWEVER, due to cold spikes related to increased snow cover or surface wind advecting over snow cover (and into the Southeast U.S.) plus above normal number of cloudy days much of the eastern third of the U.S. observed near to above normal heating degree days. The data is preliminary. If the data stays unchanged this may be the first December on record where the eastern U.S. monthly temperature anomalies were fairly robustly warmer than normal but HDD was near or colder than normal. Elsewhere HDD count for December 2020 was reasonably close to the supporting anomalous temperature for the month with a few exceptions such as Des Moines.