December 2021 – Near Record Warmth and Extremely Dry Conditions Across Delaware

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 44.6o F, which is 5.1o F above the 1991-2020 mean value of 39.5o F (Figure 1).  December’s average temperature was the second highest since observations began in 1895

December Precipitation

Delaware’s statewide precipitation for December 2021 averaged 1.03 inches, 2.82 inches below the 1991-2020 mean (Figure 2).  This placed this month’s precipitation as the 5th lowest December precipitation since records began in 1895.

Statewide Spatial Anomalies

Data from the Delaware Environmental Observing System (DEOS) show that temperature departures were uniformly above normal across the entire State, with many stations seeing positive temperature anomalies of between 6o F and 7o F (Figure 3).  Precipitation was below normal across the entire state with some stations reaching negative anomalies of greater than 3 inches (Figure 4).

National Anomalies

Temperatures were above normal across most of the United States, with the largest positive temperature anomalies found across Texas and the lower Mississippi Valley.  Large areas of the country saw positive anomalies of 10o F to 13oF   Only the Pacific Coast and portions of Montana and North Dakota saw below normal temperatures for the month (Figure 5).  Precipitation anomalies varied greatly across the country (Figure 6), with excessively dry conditions across the High Plains of Texas, Oklahoma, and Kansas.  The 500 hPa height pattern for December shows a strong jet stream ridge located over eastern North America and a strong jet stream trough over northwestern North America contributing to the temperature extremes across those regions (Figure 7).

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December 2020 – Continued Warm and Wet Across Delaware

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 40.0o F, which is 1.9o F above the 1981-2010 mean value of 38.1o F (Figure 1).  December 2020 was tied with two other years as the 24th warmest since observations began in 1895, placing it in the top one-third of all Decembers.

December Precipitation

Delaware’s statewide precipitation for December 2020 averaged 6.11 inches, 2.54 inches above the 1981-2010 mean (Figure 2).  This placed this month’s precipitation total as the 8th highest of all December’s since 1895

Statewide Spatial Anomalies

Data from the Delaware Environmental Observing System (DEOS) show that temperature departures were positive at nearly all stations, with the largest anomalies found across Kent and New Castle Counties (Figure 3).  Precipitation was uniformly above normal throughout the State, with departures of greater than four inches in portions of Sussex County (Figure 4). 

National Anomalies

Temperatures were above normal across the majority of the United States, with the largest temperature anomalies (5o F – 10o F) found across the north-central portion of the country (Figure 5).  Only the southeastern United States saw below normal December temperatures.  Precipitation anomalies varied greatly across the country with particularly heavy rainfall along the Atlantic coast from the Carolinas to Maine, while the Pacific coast saw very dry conditions (Figure 6).  The 500 hPa height pattern for December shows a northerly displaced jet stream across the North American continent resulting in the warm conditions across the country (Figure 7).

Figure 1. Delaware statewide mean December temperature (oF) 1895-2020. Black line indicates 1981-2010 normal (NOAA, NCEI, Climate at a Glance: Statewide Time Series).
Figure 2. Delaware statewide December precipitation (inches) 1895-2020. Black line indicates 1981-2010 normal (NOAA, NCEI, Climate at a Glance: Statewide Time Series).
Figure 3. December 2020 average temperature departures (oF) from the 1981-2010 mean based upon DEOS station data.
Figure 4. December 2020 precipitation departures (inches) from the 1981-2010 mean based upon DEOS station data.
Figure 5. National daily mean temperature anomalies for December 2020 (from the PRISM Climate Group, OSU).
Figure 6. Total Precipitation Anomaly for December 2020 (from the PRISM Climate Group, OSU).
Figure 7. 500 hPa geopotential height anomalies for December 2020. Negative (blue) anomalies indicate the presence of an anomalous jet stream trough while positive (red) height anomalies indicate the presence of an anomalous jet stream ridge.

water sunset

December 2019 – Warm and Moist Across Delaware

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 40.2°F, which is 2.1°F above the 1981-2010 mean value of 38.1°F (Figure 1). This places December 2019 as the 23rd warmest since records began in 1895.

December Precipitation

Delaware’s statewide precipitation for December 2019 averaged 4.26 inches, 0.69 inches above the 1981-2010 mean (Figure 2). This placed the December 2019 precipitation amount in the top one-third of months since 1895.

Statewide Spatial Anomalies

Data from the Delaware Environmental Observing System (DEOS) show that temperature departures were uniformly above normal throughout the State (Figure 3), with most anomalies ranging from 1° to 2° F. Precipitation was above normal at all stations across the State (Figure 4), except for the extreme southeast. The largest positive precipitation departures were found across northern New Castle County.

National Anomalies

Temperatures were above normal throughout the majority of the U.S, with some departures reaching 7° to 10° F in the Midwest (Figure 5). Precipitation varied greatly across the U.S. with very wet conditions across the northcentral States and much of the eastern one-third of the country (Figure 6). The 500 hPa height pattern for December showed a ridge dominating the northeastern two-thirds of the U.S. (Figure 7), bringing generally warm and moist conditions to our region.

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December 2018: Above Normal Temperatures and Precipitation Across Delaware

 
 

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 40.6°F, which is 2.5o°F above the 1981-2010 mean value of 38.1°F (Figure 1). December 2018 was the 20th warmest December since records began in 1895.

December Precipitation

Delaware’s statewide precipitation for December 2018 averaged 4.71 inches, 1.14 inches above the 1981-2010 mean (Figure 2). The December 2018 precipitation amount was within the top one-third of precipitation totals since 1895. Most precipitation fell across the State in the liquid form, with only a single snowfall event across the southern portions of the State on December 9th.

Statewide Spatial Averages

Data from the Delaware Environmental Observing System (DEOS) show that temperature departures were positive throughout the State (Figure 3), but varied in magnitude depending upon location. Precipitation amounts varied across the State, with the largest precipitation departures found across northern New Castle County and the smallest across Sussex County (Figure 4).

 

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A small bird perches itself upon a tree branch

December Climate Conditions – Cool and Dry Conditions Observed Across Delaware

 

 

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 36.7°F, which is 1.4°F below the 1981-2010 mean value of 38.1°F (Figure 1). This was Delaware’s coolest December since 2010.

December Precipitation

Delaware’s statewide precipitation for December 2017 averaged 1.08 inches, 2.49 inches below the 1981-2010 mean (Figure 2), making it the 5th driest December since 1895, and the driest in Delaware since 1988. Although the total liquid precipitation was much below normal for the State, the Wilmington – New Castle County Airport recorded 7.8 inches of snowfall during the month, more than twice the normal for December.

Statewide Spatial Averages

Data from the Delaware Environmental Observing System (DEOS) show that temperature departures were uniformly below normal across the entire State (Figure 3), with some departures as large as 4o F below the 1981-2010 mean. Precipitation departures were below normal at each station (Figure 4), with most stations seeing departures of 2 – 3 inches below normal.

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Dry plant with muted tan colors

Very Warm and Dry Conditions Characterize the 2016-2017 Winter Season Across Delaware

 
 

Winter Temperatures

Preliminary data indicates that winter temperatures were far above the 30-year “normal” during the winter season. The statewide mean winter temperature of 40.8°F was 4.5°F above the 1981-2010 mean value of 36.3°F (Figure 1). This made winter 2016-2017 the 5th warmest since records began in 1895, and the second winter in a row in the top 5 warmest on record.

Winter Precipitation

The statewide mean precipitation of 7.19 inches was 2.57 inches below the 1981-2010 normal of 9.76 inches, and was the 17th driest winter season since records began in 1895 (Figure 2).

Statewide Spatial Averages

Data from the Delaware Environmental Observing System (DEOS) show that winter temperatures across the entire State were above normal, with the largest anomalies found in northern Sussex and northern New Castle counties in Delaware and in Chester County, PA (Figure 3).

Winter precipitation totals were significantly below normal across the entire State, with the driest conditions along the Sussex County coast (Figure 4). At the New Castle County Airport, only 5.6 inches of snow fell during the December through February winter season, a value far below the 1981-2010 mean of 17.7 inches. During March, another 3.4” of snow fell in northern Delaware bringing the total for the entire snow season (November through April) to 9.0 inches.

 

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Blue green pines of a plant

Warm Conditions Persist Across Delaware During December 2016

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 39.3°F, 1.2°F above the 1981-2010 normal of 38.1°F (Figure 1). December was the sixth month in a row with temperatures appreciably above normal across the First State. Even though Delaware experienced above normal temperatures during December 2016, it was still 11.5°F cooler than the extreme warmth experienced across the State during December 2015.

December Precipitation

Delaware’s statewide precipitation for December 2016 averaged 3.11 inches, 0.46 inches below the 1981-2010 mean of 3.57 inches (Figure 2). December 2016 was the third month in a row with below normal precipitation.

Statewide Spatial Averages

Data from the Delaware Environmental Observing System (DEOS) shows that temperatures were uniformly above normal across northern Delaware and Chester County, PA during December (Figure 3). Temperature anomalies were generally small and varied between positive and negative values across the rest of the State. Precipitation was below normal at each DEOS station, with the largest negative anomalies found across northern New Castle County (Figure 4).

 

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Raindrops on glass

Very Warm and Moist Conditions Characterize the 2015-2016 Winter Season Across Delaware

 
 

 

Winter Temperatures

Preliminary data indicates that winter temperatures were far above the 30-year “normal” during the winter season. The statewide mean winter temperature of 41.1°F was 4.8°F above the 1981-2010 mean value of 36.3°F (Figure 1). This made winter 2015-2016 the 4th warmest since records began in 1895.

Witer Precipitation

The statewide mean precipitation of 11.47 inches was 1.71 inches above the 1981-2010 normal of 9.76 inches, and was the 3rd year in a row with above normal winter precipitation (Figure 2).

Statewide Spatial Averages

Data from the Delaware Environmental Observing System (DEOS) show that winter temperatures across the entire State were above normal, with the largest anomalies found in northern Sussex and northern New Castle counties in Delaware and in Chester County, PA (Figure 3).

Winter precipitation varied significantly across the State depending upon location. In general, the heaviest precipitation fell across interior Sussex County, DE (Figure 4). At the New Castle County Airport, 22.7 inches of snow fell during the December through February winter season, a value slightly above the 1981-2010 mean.

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Drops of water on bright green grass

Temperature and Precipitation Both Above Normal Across Delaware During 2015

 
 
 

Annual Temperatures

Statewide mean annual temperature in 2015 was 56.4°F according to preliminary data from the National Climatic Data Center (NCDC). This was 1.0°F above the 1981-2010 normal of 55.4°F (Figure 1) and was the 11th warmest year in Delaware since 1895.

Monthly temperatures varied greatly throughout the year (Figure 2). The first three months of 2015 saw much below normal temperatures, including the 6th coldest February since 1895. The last nine months of the year saw above normal temperatures, except for October, which was slightly below normal. Tremendous warmth ended the year with December temperatures averaging 12.4°F above the 1981-2010 mean. December 2015 was the warmest December on record, smashing the old record by 6.1°F.

Annual Precipitation

Statewide precipitation in 2015 averaged 46.37 inches, 2.04 inches above the 1981-2010 mean of 44.33 inches (Figure 3).

Monthly precipitation anomalies varied between positive and negative throughout the year, with seven months of below normal and five months of above normal precipitation (Figure 4). June saw the largest precipitation anomaly of 5.30 inches above the 30-year normal, helping to push the annual value above the long-term mean.

Statewide Spatial Averages

According to data from the Delaware Environmental Observing System (DEOS; deos.udel.edu), mean annual temperature anomalies varied across Delaware (Figure 5). Northern New Castle County, much of Sussex County, southern Kent County and Chester County, PA all experienced above normal temperatures for the year. Coastal Sussex County and western portions of News Castle and Kent counties saw below normal mean annual temperatures.

Precipitation across the region varied substantially in 2015, ranging from just under 40 inches to greater than 50 inches at several locations across the State (Figure 6).

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A very bright Sun shines on an autumn leaf

December 2015: Extraordinary Warmth and Above Normal Precipitation Across Delaware

 

December Temperatures

Preliminary data indicates that the statewide average temperature in December was 50.5°F; this was 12.4°F above the 1981-2010 normal of 38.1°F (Figure 1), making December 2015 the warmest on record since 1895. This December was 6.1°F above the second warmest December on record that occurred in 2001. To put this in perspective, December 2015 was nearly 3.0°F above the November long-term average temperature and 6.0°F above the long-term average March temperature for the First State.

December Precipitation

Delaware’s statewide precipitation for December 2015 averaged 4.68 inches, 1.11 inches above the 1981-2010 mean (Figure 2). December 2015 saw no snowfall accumulations anywhere in the State.

Statewide Spatial Averages

Data from the Delaware Environmental Observing System (DEOS) shows the unprecedented warmth across all of Delaware and Chester County, PA (Figure 3). Several stations in northern New Castle County, DE and Chester County, PA saw anomalies between 13°F and 14°F above the 30-year normal for December.

Precipitation was above normal across the entire State, with the largest positive departures found across southern New Castle County (Figure 4).

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