Tuesday, December 9, 2014

Deluge Amidst Drought

December 2, 2014
The western U.S. has been suffering through drought for two years, but for the last year or more California has been the epicenter of the drought. Moisture conditions have been deteriorating in California for more than a year, and the most recent U.S. Drought Monitor paints all of California with a palette of red and orange. Almost 80 percent of the state is experiencing Extreme and Exceptional drought, with about 55 percent of the state in the Exceptional category. December 1 marks the start of the wet season in California, and typically about one-half of California’s annual precipitation can be expected to fall during December through February.

The northern portions of the state, at least, got a good start to the wet season last week. On December 2-3 a Pacific storm produced inches of rain over northern California. Three to almost six inches fell in the 24 hour period ending the morning of December 3 in central Marin County, north of San Francisco.

Marin County, CA precipitation for 24 hours ending December 3, 2014
December 2014 precipitation through December 9.

Percent of normal precipitation, December 2014 through December 9.

Three-day totals for December 2-4 from CoCoRaHS observers in northern California ranged from around 3 inches in San Francisco to 7.83 inches in Santa Clara County at CA-SC-3, Los Gatos 6.1 S. Rain fell across much of California with this system, but amounts dropped off to one to two inches and less in southern California.

The heavy rain in California caused, as might be expected, flash flooding in many areas. Even in southern California where rainfall amounts were modest by Midwest standards, the heavy rain and high rainfall rates caused flash flooding and mudslides. The rain occurred over saturated ground and in some cases barren ground from wildfires. Officials had to rescue dozens of people from vehicles caught in the water or mud, and homes needed to be evacuated in several areas due to the danger posed by mudslides. In northern California creeks and small rivers became raging torrents and sinkholes developed in some San Francisco streets.

During much of the fall and winter 2013-2014 a huge ridge of high pressure in the eastern Pacific deflected storms away from California. The pattern is quite a bit different this year, as the door is wide open to the Pacific. This steers storms and moisture into the western U.S.

500 millibar map for 4:00 a.m. PST December 9. Note the strong band of westerly winds in the eastern Pacific.

Another powerful storm is forecast to hit the coast the last half of this week. It will produce heavy rain, heavy (and much needed) snow in the Sierra Nevada, and high winds as it moves through. The main low pressure system will likely come ashore in northern California and southern Oregon early Thursday, but its effects will be felt along the entire west coast.

Forecast surface map for 4:00 a.m. PST Thursday, December 11, 2014.

Gale warnings are in effect are in effect for most of the California coast and storm warnings are in effect for the Oregon and Washington coastal waters. High wind warnings are in effect for inland areas of central and northern California. Blizzard warnings are in effect for the northern Sierra Nevada from Thursday night through Saturday morning.

Watches, warnings, and advisories in effect as of 7:20 p.m. PST December 9.

Rainfall (and water equivalent in the mountain snow) is expected to be heavy with this storm, perhaps in the range of 5 to 8 inches or more.

Quantitative precipitation forecast for the 72-hour period ending 4:00 p.m. PST Friday, December 12

The rain the last two weeks and the rain expected this week certainly helps, but don't look for any big changes to the Drought Monitor week to week. Snowpack in the Sierra Nevada, critical to western water supply, was only 25 percent of normal earlier this month. Reservoirs are far below capacity. Groundwater needs to be recharged as the rain percolates down through the soil. It will take a number of storms over the next several months to make an appreciable dent in the California drought. Whether or not that happens will depend on whether the "door" to the Pacific remains open, and for how long.

Monday, December 1, 2014

One Wild and Crazy November

We're back with the CoCoRaHS Blog after a four-week break while Steve was on medical leave.

Today marks the start of meteorological winter, though residents in much of the eastern half of the country no doubt feel like winter started a month ago, with good reason!

Most of the contiguous 48 states from along and east of the Rockies, from the U.S. Canadian border to the Gulf of Mexico experienced temperatures below normal in November. The core of the cold air was in the upper Midwest and Northern Plains where temperatures were more then 9°F below normal. California and Nevada experienced another month of warmer than normal temperatures.

The wild weather really kicked into gear with the outbreak of Arctic air which plunged through the U.S. the second week of the month.

Surface weather map for 6:00 a.m. CST November 12
This set the stage for days of lake-effect snow in Michigan, Wisconsin, and particularly western New York in and around Buffalo. There were actually three separate lake-effect events around Buffalo - November 12-14, November 17-19, and November 19-21. The first event on November 12-14 was with the initial cold air outbreak and produced snow downwind of Lakes Erie and Ontario.  You can read a summary of this event on the NWS Buffalo web site. The second lake-effect event on November 17-19 was historic. A band of heavy snow developed off of Lake Erie and settled south of Buffalo - and didn't move for 36 hours. Snowfall rates reached 6 inches per hour at times, and when it was all over on Wednesday morning 65 inch of snow had piled up in south Cheektowaga east of Buffalo. Snow off of Lake Ontario accumulated to 22 inches in Philadelphia, NY east of Watertown.

For more detail on this event see this summary from the NWS Buffalo. The third lake-effect storm occurred right on the heels of the previous storm. It affected much of the same area that had been clobbered by the previous storm with one to four feet of new snow. When it was over some areas had received more than seven feet of snow from the two storms. The water equivalency of the snowpack was high, with some five to six inches of water contained in three to four feet of densely packed snow. The heavy wet snow caused collapsed roofs and contributed to flooding once warmer weather returned the following week. A summary of the third storm can be found here.

Snowfall records tumbled in Michigan as lake-effect snow piled up.  Gaylord, MI accumulated 65.1 inches of snow,45.5 inches above normal. Sault Ste. Marie in the Upper Peninsula measured 65.4 inches of snow, 50.3 inches above normal. These amounts were also all-time monthly records for both locations. Grand Rapids, MI experienced its snowiest November on record with 31.0 inches, breaking the old record of 28.2 inches in 1895. Muskegon, MI had its second snowiest November on record with 24.5 inches.

While much of the attention was on the lake-effect snow in western New York and Michigan, the cold air had produced early season snow in many parts of the country, including the south.

On November 17, 50.4 percent of U.S. had snow on the ground, remarkable for mid-November.

Snow depth on November 17, 2014.
Source: National Operational Hydrologic Remote Sensing Center

Warming weather following the Arctic outbreak ate away much of that snow and as of today only 22 percent of the country has snow cover. Only three to four inches of snow remain over central Erie County, NY according to our CoCoRaHS observers. That's the natural snow - some of the mountains of snow that were plowed and piled will take a much longer time to melt.

There were thousands of daily temperature records set during November - more than 4800 low maximum temperature records alone. Here is the current summary from the National Climatic Data Center.

Daily record count as of November 30, 2014.
Source: National Climatic Data Center

While snow was plentiful in the lower 48 states, much of Alaska had a mild and not so snowy November. Fairbanks received its first November snow of the season on November 24. Normally they have 23.3 inches by that date. Anchorage received its first snow of November on November 25, a mere 0.1 inch. However, a large storm occurred across much of southeast and interior Alaska this weekend leaving 6 to 12 inches of snow in its wake.