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.