Wednesday, July 30, 2014

Is Weather Radar for the Birds?

Radar is designed to detect "things" in the air. In meteorology those "things" are hydrometeors - rain drops, ice crystals, hail, and even clouds.   However, for a long time radar has been recognized as a tool to detect birds and insects in the atmosphere.

In the last couple of weeks there have been two cases where weather radar has captured bird and insect movement. The first of these was the detection of a massive mayfly emergence along the Mississippi River on July 20. This event received a lot of media coverage because of the size of the emergence and the photos of the swarms of mayflys. The mayfly emergence and its northward drift was detected by radar throughout the evening. By late evening reports were coming of the piles of mayflys in communities near the river.

Radar loop of mayfly emergence on July 20 along the Mississippi River.
The loop is for the period from 8:35 p.m. to about midnight CDT

The photo below was taken by Tim Halbach, a forecaster at the NWS La Crosse office and a CoCoRaHS observer, a little after midnight on July 21 several hours after the beginning of the hatch.  Tim decided to go out "mayfly chasing" after his shift ended at midnight.

Photo taken at 12:15 a.m. on July 21of mayflys covering the building and pavement.
Photo by Tim Halbach

Mayflys spend the better part of the year as aquatic nymphs in the mud. During the summer they emerge as adults to find a mate. While this year's major event on July 20 got a lot of attention, mayfly emergence occurs every year along the river.  It's not always this massive, or this gross.  The NWS La Crosse office has a page on its web site dedicated to the mayfly emergence events in that area.It includes radar images and many photos submitted by residents of the area.

You can read more about the program to monitor mayfly emergence at the U.S. Fish and Wildlife Service web site.

Another interesting although less spectacular detection by weather radar was that of a "roost ring" Tuesday morning by the Chicago weather radar. A roost ring is radar signature of birds as they leave their morning roosting site. This one occurred about 5:30 a.m. along the Kankakee River in northwestern Indiana.

A roost ring detected the NWS Chicago radar at 5:30 a.m. CDT on July 29.

Roost rings are typically seen in late summer and early fall leading up to fall migration when birds gather at large roosting sites. The rings occur when the birds take flight in the morning. Roost rings are typically seen in the morning when atmospheric conditions are favorable for the birds to be detected by radar. Often there is an inversion (a layer where temperature increases with height), and that has the effect of bending the radar beam back toward the earth and is more likely to detect objects close to the earth's surface..

The NWS Wilmington, OH office has a nice description of the occurrence of roost rings along with some radar images.   

 If you would like to read more about the use of radar in detecting birds and insects, check out Radar Technology - A Tool for Understanding Migratory “Aerofauna”   by the U.S. Geological Survey.

Thursday, July 24, 2014

Tropical Thoughts

Track of Hurricane Arthur
We are well into the tropical storm season, and things in the Atlantic have been very quiet so far. The only activity was Hurricane Arthur at the beginning of the month and a recent tropical depression that quickly dissipated. The eastern Pacific has been a little more active with two hurricanes and three tropical storms. NOAA's 2014 Atlantic Hurricane Season Outlook calls for a near-normal to below normal season, with an expected 8-13 names storms, 3-6 hurricanes, and 1-2 major hurricanes. The normal range is 12 named storms, 6 hurricanes, and 3 major hurricanes.


Of course, the outlook says nothing about land-falling storms, and coastal residents know that it just takes one storm to wreak havoc in an area, and it doesn't have to come ashore. Arthur was a good example of that. We are still several weeks away from the peak of the season so the lack of activity should not lead anyone to complacency. While the tropics are quiet, however, it's a good time to become familiar with a couple of new experimental products that have been introduced by the National Hurricane Center.

The first of these is a new 5-day Graphical Tropical Weather Outlook. This outlook indicates the formation potential of current and future disturbances during the next five days. Shaded areas will represent the potential tropical cyclone formation areas, similar to the current 2-day outlook. Yellow represents a low chance (<30 percent), orange a medium chance (30 to 50 percent), and red a high chance (> 50 percent) of tropical cyclone in the next five days.

You can view a video about the outlook at here.

The second product is a potential storm surge flooding map. This map product has been in development for several years and is designed to clearly depict the risk from storm surge by show the areas where flooding from storm surge could occur and how high above ground the water could reach. Flooding from storm surge accounts for about half the deaths associated with tropical cyclones, and many people do not understand the threat from storm surge. Four colors will represent flood levels from up to 3 feet, greater than 3 feet, greater than 6 feet, and greater than 9 feet. The map is produced using forecast and statistical models, and accounts for flooding from storm surge from the ocean, astronomical tides, land elevation, and uncertainties related to the forecast of the cyclone. It does not account for flooding from heavy rain, wave action, or overtopping or failure of levees. The maps can change as the storm track and intensity forecast changes.

Here is a sample map of flooding along the Texas Gulf Coast.

Experimental potential storm surge flooding map for a sample hurricane along the Texas Gulf Coast.

You can view a short video about the experiment storm surge map here.

The National Hurricane Center is accepting comments and feedback about both of these products, and you can find the feedback forms on the page describing the product

Tuesday, July 15, 2014

Two Sides of the Same Coin

Temperatures at 1:00 p.m. CDT July 15.
It was pleasantly cool here in the Midwest today. Too cool for swimming, but great for just about anything else outdoors. It's a nice break from the humid weather of the past several days.

Now that the hype about the "polar vortex" has died down, I thought I would try and describe what actually is happening across the country this week. While it is a bit unusual for mid-July, it isn't unprecedented by any means. The circumstances by which this pattern set up may be a little out of the ordinary, but unseasonably cool weather in the summer has occurred before and will occur again.

500 millibar chart for 7:00 a.m. CDT July 15
There is a strong upper level trough over the central U.S., and that is responsible for the cooler air dropping south through the U.S. However, it's not really the "polar vortex". When we have weather patterns such as we have this week it is important to look at the big picture, big as in continental, if not hemispheric.  Why?  While you may be enjoying cool weather this week, you can bet that somewhere to the west and east abnormally warm weather is occurring.  When there is a large, strong trough of low pressure over one part of the country, then it is likely to be sandwiched in between two corresponding large, strong ridges of high pressure to the west and east. Beneath those ridges of high pressure air sinks and warms. That is what's happening in the western U.S. this week as the central U.S. enjoys refreshingly cool weather. Five days ago temperatures 100°F and higher were confined to interior California and the southwestern deserts. Yesterday a broad swath of maximum temperatures100°F and higher occurred from Arizona north through Washington state.
Maximum temperatures for July 14. Temperatures 100°F and higher are shaded in red.

Meanwhile, the cooler air pushed south through the central U.S. The cold front has helped trigger flooding rains in the eastern U.S. The leading edge of the cool air is expected to push to just off the east coast by Thursday morning, but will likely stall because of the large high in the western Atlantic.

The development of the large trough over North American this week is attributed, in part, to Typhoon Neoguri which slammed into Japan late last week.  This was an immense storm, and it induced a "ripple" in the jet stream over the northern Pacific. The atmosphere is a fluid, and what happens upstream will have an effect downstream. This caused a trough to form over Alaska, with a ridge downstream over the western U.S. and the developing trough over the central U.S.

500 millibar map showing a trough over Alaska (red line), the ridge in the western U.S. (blue dashed arrows), and the trough over the central U.S. The chart is for 7:00 a.m. July 15.
Over the winter there was a persistent trough of low pressure over the central and eastern U.S.  That directed the cold air into those regions. Since there was a strong trough, there had to be a corresponding strong ridge.  The axis of the western ridge was located off the west coast and accounted for warmer than normal temperatures throughout the southwestern quarter of the U.S. Alaska experienced their 8th warmest winter on record as the ridge generally deflected intrusions of Arctic air. The axis of a flatter eastern ridge was located in the eastern Atlantic. The jet stream located between the cold air in North America and the warmer, tropical air over the eastern Atlantic directed many large and intense storms into northern Europe. 

The mean 500 millibar pattern (left) and the temperature departure from normal
for the period December 1, 2013 through March 31, 2014.

Very often, the large intense upper air trough/ridge patterns that develop during the winter span much more than the North American continent and their effects extend far beyond the U.S. and Canada.

Wednesday, July 2, 2014

Wet June Ends on Wild Note

The 30 days of June produced many days of rain and many inches of rain across a large part of the country. Rainfall was generally above normal from the mountains of Montana all the way into the lower Mississippi River Valley. In parts of the northern and central Plains and the Midwest June rainfall was five to six times normal. When all is said and done, this could end up being one of the wettest Junes on record in the corn and soybean belt, the area that extends from the Dakotas southeast into the Ohio Valley. Rain is good for the crops, but too much rain is not.

Percent of normal precipitation for June 2014.
Credit: NWS AHPS

In eastern South Dakota and western Iowa June precipitation records were shattered in many locations. Much of this area had been in a precipitation deficit for they year at the end of May, but no more. Sioux Falls, SD received 13.70 inches of rain in June, breaking the old record by more than 5 inches (8.43 inches in 1984). Sioux City, IA had 16.65 inches for June, breaking the old record by almost 8 inches (8.78 inches in 1967)! A number of other locations set records as well, including Canton, SD with 19.65 inches of rain for the month. This is a new state record  for the wettest month at any location in South Dakota. Canton has been an official National Weather Service Cooperative Observer location since 1896. Many locations received from 10 to 15 inches of rain.  The South Dakota CoCoRaHS observer at Beresford 0.2 E (SD-UN-5) measured 15.80 inches in June, and the observer at Garretson 6.9 W (SD-MH-56) measured 15.49 inches. One other observer reported more than 18 inches, which may be low since there were reports for only 7 days in the month. More on the heavy rain can be found on the NWS Sioux Falls, SD web site.

Compare the percent of normal map for this June with that of June 2013. Two significant features stand out. First, last June much of the country had below normal precipitation except for Montana, northern California,  and the eastern seaboard. The heavy rain along the eastern seaboard was largely due to Tropical Storm Andrea which moved up the coast the first week of the month.
Percent of normal precipitation for June 2013.
Credit: NWS AHPS
The other feature that probably caught your eye is the large blob of red in the southwestern U.S. Now, that area normally doesn't get a lot of rain during June, but they have received even less this month and most of the year to date. Drought conditions have overtaken all of California at the end of June, and the entire state is in Severe Drought, with 77 percent of the state in Extreme to Exceptional drought.

Drought status in California as of June 24, 2014.
Source: U.S. Drought Monitor

June closed out with some wild weather in the Midwest. Two derechoes tore through Iowa, northern Illinois, southern Wisconsin, and northern Indiana on Monday causing extensive amounts of straight-line wind damage. Power was still out in many areas as of Wednesday afternoon. Heavy rain with the storms caused widespread flash flooding, and  flood warnings are in effect for many rivers and streams in eastern Iowa and northwestern Illinois. Several smaller rivers have already reached record flood levels in Iowa.

Tracks of two derecho events in the Midwest on June 30.

 For more information on the derechoes, see the NWS Chicago web page on this event and this page for more on the severe weather that day.

 For information on the storm in Iowa, see the NWS Davenport/Quad Cities page on the severe weather.