Sunday, January 3, 2016

A December to Remember

To say there was a lot going with the weather in December would be a gross understatement. The country seems to be catching  its collective breath this week after the conclusion of a truly amazing December. Unfortunately the frequency of events, a busy schedule, and the holidays made it tough to get any meaningful blogging done. Rather than concentrating on any one event this post will summarize the weather that made this a December to remember.

During the first weekend in December 10 to 15 inches of rain fell in southeast Florida, with nine to ten inches falling in 12 hours Miami-Dade County.

Heavy rain (and snow) also continued to fall in Washington and Oregon the first two weeks of the month. Rain totaled 16 to 25 inches in northwest Oregon and western Washington. This caused many flooding issues as might be expected. On the positive side, snowpack in the Cascades continued to increase, and by the end of December was well above normal across Washington and Oregon and near to above normal in the California Sierra.

A storm system that slowly lifted through the central U.S. the middle of December brought winter weather to the Rockies and central and northern Plains, while springtime warmth and rain spread across the central U.S. Three-day rainfall topped six inches in eastern Texas and Oklahoma and three to four inches in the upper Midwest.

A few days of fair weather followed this system. As a large high pressure system moved off the east coast strong southwesterly flow set up over the central U.S. on December 22. High temperatures reached A cold front helped trigger severe thunderstorms from Illinois south to the Gulf Coast. A number of tornadoes resulted from the severe storms, including one long track tornado in Mississippi which resulted in two fatalities.

The parade of storms continued over Christmas, with the next major system taking shape over the southern Rockies on December 26. This would end up being the defining storm of the month. Severe weather developed from Texas into Oklahoma, including tornadoes in the Dallas area. The communities of Garland and Rowlett were particularly hard-hit. A total of 11 people lost their lives in the storms, with eight killed when an EF-4 tornado tore through and interchange on Interstate 30. Severe weather continued on December 27 with tornadoes touching down in northeast Texas, southern Arkansas, and Louisiana.

Meanwhile, in the wake of the severe thunderstorms, blizzard conditions spread into eastern New Mexico and the Texas Panhandle. Winds and snow reduced visibility to zero and closed miles of Interstate, bringing post holiday travel to a standstill. The severe winter weather had a serious impact on cattle in the region, with an estimated 40,00 dairy cattle lost to the storm.

72-hour snowfall for the period ending 6:00 a.m. CST on December 29.

Back in the eastern half of the country, waves of rain were moving north through the Midwest as the storm slowly lifted north through Missouri and Illinois. Strong southerly flow pumped unseasonably warm and moist air into the eastern U.S.  In a three day-period in Missouri and Illinois rainfall topped 5 to 12 inches, with the heaviest rain in central Missouri.

The widespread heavy rain and saturated soils led to high runoff and severe flooding along many rivers in Missouri and Illinois. The Mississippi River in St. Louis reached its third highest crest on record, a phenomenon usually confined to the spring and summer. Record high temperatures were recorded from Florida to Maine.

Records Galore

There was so much going on this December that it's difficult to pick on specific thing to characterize it. However, all things considered it has to be the record warmth in the eastern half of the country. The warmth wasn't limited to the U.S., either. Eastern and central Canada were equally as warm.

December temperature anomalies for Canadian locations, in degrees Celsius

Temperature departures December temperatures averaged from 9 to 16 degrees above normal, a remarkable amount for a monthly average. There were more than 5,000 record daily high temperatures set in the U.S. and more than 6,700 record high minimum temperatures. To date 111 monthly record highs were set and 469 monthly record high minimums. Many of the old records were not broken by one or two degrees but several degrees and more.

Precipitation was much above normal in the eastern U.S. and the Pacific Northwest. The pattern was much different than in December 1997 during the last strong El NiƱo, and clearly there were other factors in play this year.

Comparison of precipitation between December 2015 and December 1997

December tornadoes put an exclamation point on what was otherwise a fairly quiet year for severe weather. Texas recorded 15 tornado fatalities in 2015, 11 of which occurred in the December 23rd tornado outside Dallas.

Flash flooding from the December rainfall resulted in multiple fatalities, and river flooding to record and near record levels is causing extensive damage in the Midwest.

The abundant snow in the west was good news not only for water supplies but also for ski resorts. This year was a welcome turnaround from the snow drought last year. In contrast, ski areas in the eastern U.S. suffered with little snow and little cold air to work with until the very end of the month..

I've only touched on the events in this post, and there is still a lot to be written about December 2015. The National Centers for Environmental Information (NCEI) will have more information compiled later this week.

It remains to be seen what the rest of winter 2015-2016 will bring, but it is bound to be interesting.

1 comment:

  1. Thank you for a very interesting summary and informative graphics.