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.

Monday, November 3, 2014

Wild Windy Wintry Weekend

A rapidly deepening upper level trough brought three days of wild weather to the eastern half of the country from the end of last week through this weekend.

An intensifying surface low and cold front brought cold, windy, and snowy weather to much of the eastern Midwest on Thursday and Halloween.

500 millibar map (left) and surface map (right) for 7:00 a.m. CDT October 31, 2014

Winds gusted to 69 mph in Gary, IN on Friday and 67 mph at the Chicago Harrison-Dever Crib and Michigan City, IN.  Wind damage was common, and the winds drove large waves from Lake Michigan onto Lake Shore Drive in Chicago and caused damage to Chicago's lakefront bike path. Heavy snow fell in northern Wisconsin and the Michigan U.P. and measurable snow was recorded as far south as northern Illinois and Indiana. Snow showers occurred even farther south through central Illinois and Indiana.

Snowfall in northern Wisconsin and part of the Michigan U.P. as of the morning of October 31
 The mid-lake south buoy, located about 43 miles east of Milwaukee recorded a wave height of 21.7 feet.  This was tied for the second highest wave height on record at the south buoy, after only 22.9 feet on September 30, 2011.

Snow in Asheville, NC
Photo by Mike Palecki
As the upper level trough deepened a second low developed over western South Carolina, with another low off the Virginia coast Friday night. Plenty of moisture and now cold air was available to the low over South Carolina and heavy snow fell in the Appalachians. Snow piled up to 22 inches in both Spring Creek, NC and Gatlinburg, TN. Measurable and often heavy snow accumulated in higher elevations of Kentucky, Tennessee, Georgia, South Carolina, North Carolina, Virginia, and West Virginia.

500 millibar map (left) and surface map (right) for8:00 a.m. EDT November 1, 2014
Snow in Morristown, TN.  Photo by the NWS Morristown

By midday Saturday the South Carolina low moved out over the Atlantic while continuing to intensify. By late afternoon it began to merge with a second low to its north. By 1:00 a.m. EST on Sunday morning the low was located east of Norfolk, VA at pressure had dropped to 988 millibars. Twelve hours later (1:00 p.m. EST) the strong storm was centered east of Nantucket, MA.

500 millibar map (left) and surface map (right) for 7:00 a.m. EST November 2, 2014

Some snow fell in Massachusetts, but the heaviest snow on Sunday (10 to 21 inches) fell in the eastern portions of Maine. Bangor (12 inches) and Caribou (10.1 inches), ME both set their record earliest day (November 2) with snowfall 10 inches or more. The largest snow accumulation was 21 inches in Cary, ME near the Canadian border. The heavy wet snow and howling winds brought down power lines cutting off power for 130,000 customers. Power may be out in some areas several days.

72 hour snowfall accumulation ending the morning of November 3.

Thursday, October 30, 2014

Fall Bird Migration Caught on Weather Radar

On Tuesday evening I noticed some peculiar radar returns developing on the Lincoln IL radar beginning about 6:45 p.m. CDT. The returns appeared to develop along the Illinois River, and my first thought was that it might be biological. I checked the satellite image for the region and there were a band of clouds over central Illinois, so I attributed it to precip, even though none was expected.

While working in my home office last night I glanced at the radar and similar returns were starting to show on the radar at the same time as the night before!. For a moment I thought that perhaps I was looking at an image from the previous night. The returns developed along the Illinois River, and then moved to the southeast. After some inquiries of friends and colleagues, it appears what we were seeing on radar was the fall migration of thousands of birds, probably mostly waterfowl. The birds took flight about 50 minutes after sunset.

Lincoln, IL radar images for Wednesday night, October 29.

Tuesday, October 28. Click on image to view a two hour loop of the radar.

Reports and forecasts of fall migration are available on BirdCast, part of the Cornell University Lab of Ornithology web site. Regarding the migration through the central U.S. on Wednesday BirdCast indicated that

"This night will surely represent one of the last large movements of the fall of both landbirds and waterbirds. Species on the move this week will include Hooded Merganser, American Tree Sparrow, Lesser Scaup, Common Loon, Red-breasted Merganser, Lapland Longspur, Canvasback, Bonaparte’s Gull, Fox Sparrow, Greater Scaup, Ring-necked Duck, Common Goldeneye, Redhead, Dark-eyed Junco, and Ruddy Duck."

Credit: U.S. Fish & Wildlife Service
A colleague of mine who is a birder reported that he had seen a report of thousands of waterfowl at Chautauqua National Wildlife refuge near Havana on the Illinois River. There are other National Wildlife Refuges along the Illinois River as well.

The Illinois River is a major flyway for migratory birds and the most striking radar images I found came out of Lincoln. However, birds taking flight were also captured by the Des Moines, IA and Kansas City, MO radars on Tuesday night.

Des Moines, IA radar at 7:11 p.m. CDT Tuesday, October 28. The dark blue and green returns are likely birds taking flight.
Kansas City, MO radar at 7:27 p.m. Tuesday, October 28. Note how the bird returns are along the Missouri River.

Birds use a variety of means to navigate during migration, including the sun, stars, and magnetic fields. It is thought that nighttime migration is advantageous for birds because of the lower temperature. Birds generate a lot of heat while flying, and lower temperatures (and lack of sun) help them maintain and optimum body temperature as they travel.

You can learn much more about bird migration at the following web sites:

Northern Prairie Wildlife Research Center
Cornell University Lab of Ornithology  

Tuesday, October 28, 2014

Big but Short-Lived Chill on the Way

Temperatures have been mild for much of October throughout the U.S., with near normal temperatures across the central U.S. and Southeast. The mild weather along with extended periods of dry weather have made for nearly ideal harvest conditions.

Freezing and sub-freezing temperatures have occurred across the Northern Plains, Minnesota, Iowa, and Wisconsin, but killing freezes so far have been restricted to the northern tier of states.

Lowest minimum temperatures since August 1.

The first freeze across the central U.S. is running later than normal, but that could change this weekend.

A strong upper level trough will deepen over the eastern U.S. at the end of this week and the resulting northwesterly flow on the west side of the trough will bring frigid air deep into the central U.S.

500 millibar map forecast for 7:00 a.m. CDT Saturday, November 1

 A strong surface high pressure system will drop out of Canada and by Saturday morning will be centered over the upper Great Lakes. Under clear skies and calm air temperatures will drop into the low 20s and perhaps lower over the upper Midwest, mid 20s over the central Midwest and below freezing as far south as the Ohio River. The Northern and Central Plains will remain on the periphery of the coldest air.

Minimum temperature forecast for Saturday morning, November 1.
Recovery from the cold air will be quick. By next Monday the amplified upper level pattern will be flattening out and relatively mild Pacific air will be streaming across the U.S.

500 millibar map forecast for Tuesday, November 4 at 12:00 CST
That means the first week of November is likely to be warmer than average throughout much of the country east of the Rockies.

Maximum temperature anomaly for Tuesday, November 4.
The outlook from the Climate Prediction Center is for a higher probability of warmer than normal temperatures through the first half of November for most of the U.S.  However, accompanying the mild weather is a likely wet pattern in the Pacific Northwest and in the eastern half of the country.

Wednesday, October 22, 2014

Changes to Severe Weather Outlooks

The Storm Prediction Center has instituted several changes to it's severe weather outlook products effective today.

If you are familiar with the convective outlooks that the SPC issues several times per day you know that the outlooks indicate where general thunderstorms are expected, where there is a low probability of severe weather (the "See Text" category), and where there is a Slight, Moderate, or High Risk of severe weather. Effective today there is one change and one additional category being used in the Day 1 through Day 3 outlooks.

There are real numbers behind the determination of each of the former and current risk areas. A 15 percent probability of a tornado (Moderate Risk category) may seem low, but the normal probability of a tornado, for example, during the peak of the season on May 13 in central Oklahoma is only about 1.5 percent. A Moderate Risk in  in this case would indicate about 10 times of the normal probability for a tornado.

The probability for a tornado changes by season.The probability of a tornado in central Oklahoma this week in October is normally about 0.15 percent, ten times lower than in mid-May.

Here are the new/revised categories and what they describe. The probabilities for Day 1 are more detailed than Days 2 and 3.

Day 1
General Thunderstorms
    10 percent or greater probability of non-severe or near severe thunderstorms

    - 2% tornado probability, or
    - 5% severe hail or severe wind probability.
    - 5% tornado probability, or
    - 15% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater.
Enhanced (the upper end of the former SLIGHT category)
    - 10% tornado probability WITH OR WITHOUT 10% or greater probability
      of an EF2 or greater tornado, or
    - 15% tornado probability, or
    - 30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater, or
    - 45% probability of severe hail or wind.
    - 15% tornado probability AND 10% or greater probability
      of an EF2 or greater tornado, or
    - 30% tornado probability, or
    - 45% severe wind probability AND 10% or greater
      probability of a wind gusts 75 mph or greater, or
    - 45% severe hail probability AND 10% or greater
      probability of hail 2 inches or greater in diameter, or
    - 60% severe wind probability, or
    - 60% severe hail probability WITH OR WITHOUT 10% or greater probability
      of hail 2 inches or greater in diameter.
    - 30% tornado probability AND 10% or greater probability
      of an EF2 or greater tornado, or
    - 45% or greater tornado probability WITH OR WITHOUT 10% or greater probability
      of an EF2 or greater tornado, or
    - 60% severe wind probability AND a 10% or greater
      probability of a wind gust 75 mph or greater.
Days 2 and 3
General Thunderstorms
      - 10% or greater probability of non-severe or near severe thunderstorms.

      - 5% total severe probability.

      - 15% total severe probability WITH OR WITHOUT 10% or greater probability
        of significant severe.

      - 30% total severe probability WITH OR WITHOUT 10% or greater probability
        of significant severe, or
      - 45% total severe probability.

      - 45% total severe probability AND 10% or greater
        probability significant severe, or
      - 60% total severe probability (Day 2 only).

High (Day 2 only)
      - 60% total severe probability AND 10% or greater
        probability of an EF2 or greater tornado or a wind gust 75 mph or greater.

The SPC has a number of examples of the new vs. old  categories on its web site. Here is an example of the difference in outlooks for the day of the Southeast tornado outbreak on April 27, 2011.

The Day 1 convective outlook for April 27, 2011 in the new format (left) and old format (right)

The Storm Prediction Center is also considering changes to the Day 4 to 8 outlook product. This is still in experimental mode and more information can be found here.

Although much of the country is currently enjoying pleasant fall weather, we have entered a ramp-up period to a secondary peak in severe weather season. October through December is a period of increased frequency of severe weather and tornadoes from far eastern Texas through Alabama, an area dubbed "Dixie Alley", with a peak in the frequency of tornadoes in mid to late November.

Mean number of tornadoes for October through December in Dixie Alley.

If you would like to explore the climatology of severe weather the SPC has an interactive web page where you can view the probabilities for tornadoes, significant wind, and significant hail for 52 one-week periods. You can animate the maps to see how the severe weather shifts through the country and how the probabilities change from week to week.

Thursday, October 9, 2014

Super Typhoon Vongfong - An Impressive Storm

Not only has the eastern Pacific tropical season been above normal, but storms have been frequent in the central and western Pacific as well. Super Typhoon Vongfong has been in the news lately partly because of some of the spectacular photos and satellite images that have been collected as this typhoon has crossed the Pacific but also because it is the strongest storm of 2014 in any basin.

Vongfong was born as a weak depression just south of the Marshall Islands on September 30. By October 3 it had strengthened into a tropical storm, and 30 hours later a typhoon. It reached super-typhoon strength on the morning of October 7 with winds of 155 knots (178 mph).  Winds are currently down to 130 kts, still a strong,dangerous, and massive storm. Gale force winds (>34 knots/39 mph)associated with this storm cover an area of 340,00 square miles.

The track of Super Typhoon Vongfong.
While here in the U.S. we are familiar with the terms tropical storm and hurricane, terminology differs in other parts of the world. A typhoon is the same as a hurricane west of the International dateline. A "super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Center for typhoons that reach maximum sustained 1-minute surface winds of at least 130 kt/150 mph). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin.

One of the more interesting and spectacular images of Super Typhoon Vongfong was this visible image from NASA illuminated only by moonlight . Note the thunderstorms on the west and northwest outer bands of the storms identified by the bubble-like cloud tops.

This image is one hour earlier than the last position labeled on the chart above when the winds were 178 mph.

The Capital Weather Gang at the Washington Post assembled images of Super Typhoon Vongfong for their column today. Rather than reproducing them here, visit their web page "Twelve incredible images of most powerful storm of the year, Super Typhoon Vongfong".

As you can see on this image of the current and projected track, Vongfong is forecast to weaken as it moves north. It will reach Okinawa as a category 3 equivalent storm, and by late this weekend will be entering southern Japan as a category 1 equivalent storm with top winds of about 90 mph.

Current and projected track of Super Typhoon Vongfong.

Tuesday, October 7, 2014

Pacific Tropical System Injects More Moisture into U.S.

It has been an active tropical weather season in the eastern Pacific.  As of today there have been 18 named storms, with 13 of those reaching hurricane strength. The eastern Pacific season is above normal so far this year. At this point in the season the average number of named storms is 13 and hurricanes seven.

The increase in storms has been a "good news, bad news" situation for the southwestern U.S. It has been good because there has been significant improvement in (but not elimination of) drought conditions in much of New Mexico, the southern half of Arizona, and western Texas since early July. The bad aspect is the amount of flooding and flash flooding that has occurred as a result of heavy rains from the enhanced moisture.

As of noon Tuesday Simon was a minimal tropical storm located just off the central coast of the Baja California peninsula, and by evening Simon was downgraded to a tropical depression. Over the next few days moisture associated with Simon will cross through the Desert Southwest and then eastward across the country.

Quantitative precipitation forecast for the 72-hour period ending 7:00 p.m. CDT on Friday, October 10.
A large high pressure system over the upper Midwest will keep most of the precipitation associated with this moisture in the southern half of the country. A wave of low pressure moving across the frontal boundary dividing cool dry air to the north from the warmer, moist air to the south will provide the trigger for widespread and potentially heavy rain. Heavy rain is expected across Arizona - again - , and presenting a threat for flash flooding the next couple of days. The threat for heavy rain will then shift to the Central Plains and Midwest.

Surface weather map forecast for 7:00 p.m. CDT Wednesday, October 8.

Surface weather map forecast for 7:00 p.m. CDT Thursday, October9

Unfortunately very little, if any of that rain will reach parched California, where severe to exceptional drought encompasses 83 percent of the state.

Status of California drought as of September 30.

The rain through southern southern half of the U.S. will slow up fall harvest which has been progressing very nicely with the long stretch of dry weather at the end of September.