Wednesday, August 20, 2014

Deluge in the Valley of the Sun

Last week exceptional rains occurred in the central and eastern U.S.  This week the weather eye turns to, of all places, the Desert Southwest. For most of the year "rain" and Arizona" are two words rarely heard in the same sentence.  However, the summer monsoon is in full swing, and on Tuesday the Phoenix area received some impressive rain amounts which caused major flooding on some creeks and rivers as well as flooding of many roads, including portions of Interstate 17.

The thunderstorms were set off by an approaching trough over the western U.S. With ample moisture available from the monsoonal flow and southwesterly winds forcing the air up over higher terrain, thunderstorms quickly developed and intensified.

The first round of storms occurred early Tuesday morning near Carefree and Cave Creek, located at the northeast corner of the metro area.
Radar reflectivity for 6:02 a.m. PST Tuesday, August 19. Storms are intensifying west and north of Phoenix.

By 7:00 a.m. very heavy rain was falling northeast of Phoenix over the higher terrain of Maricopa County.

Radar reflectivity for 7:02 a.m. PST Tuesday, August 19.  Heavy rain is falling east of I-17 and north of Phoenix.

The CoCoRaHS observer at Cave Creek 2.9 SSW (AZ-MR-17) reported 0.75 inch of rain in 15 minutes (that's a rate of 3 inches per hour) as did the observer at Carefree 2.1 E (AZ-MR-31). Both stations ended up with about 2 inches of rain. However, the heaviest rain was located north of our CoCoRaHS observers.

Initially floodwaters and debris covered the northbound lanes of Interstate 17, but by 10:00 am Interstate 17 four miles north of New River was totally under water and many cars were stranded, necessitating rescues.

Water covers the northbound lanes of I-17 Tuesday morning.
Credit: ABC15 Facebook page

A second round of heavy storms moved over the same area northeast of Phoenix during the early afternoon, and additional thunderstorms moved through the region though the afternoon and evening. However, most of the rain fell in these first two rounds.

Radar reflectivity at 12:02 p.m. PST on Tuesday, August 19.
A second round of heavy storms is moving through the Phoenix area.

Measured ainfall totals for this event reached 5.51 inches at Tower Mountain north of Phoenix, but there was a wide swath of  4-inch plus rainfall that extended to the southeast from there. The highest amount reported by a CoCoRaHS observer was 2.86 inches at Cave Creek 2.4 N (AZ-MR-163).

24-hour rainfall map for period ending the morning of August 20.
Area outlined in orange encloses rainfall amounts of 4 inches or more.
Base map credit: Flood Control District of Maricopa County

The National Weather Service Office in Phoenix has a web page with more information on this event, including photos of the flooding.

Thursday, August 14, 2014

Too Much Rain

Spectacular flooding rains occurred in several parts of the country in the past few days. On Friday night and early Saturday morning two rounds of heavy thunderstorms hit Kearney, NE. CoCoRaHS observers in and around Kearney reported from, 2.35 to 3.89 inches of rain for the 24 hours ending Saturday morning. Most of it fell in a span of a little more than three hours from 11:00 p.m. to 2:00 a.m.

Radar reflectivity for 11:14 p.m. CDT August 9 (left) and 1:18 a.m. August 10 showing storms approaching
Kearney, NE (inside white circle)

The rain caused flash flooding as you might expect as the amount of water exceeded the capacity of storm sewers to drain it away. You probably have seen this video - it's making the rounds on the Internet, but just in case you haven't I've included it here. A security camera at Good Samaritan Hospital in Kearney captured flood waters bursting through windows on the lower level of the hospital flooding the cafeteria and other areas.

Surface weather map for 8:00 EDT on Monday, August 11, 2014
On Monday, August 11 a nearly stationary low pressure wave along a front in the Midwest helped generate thunderstorms that dumped more 4 to 6 inches of rain across southeastern Michigan in a matter of four hours. One of the worst hit areas was the Detroit Metro area. The Detroit Airport received 4.57 inches of rain, setting a new daily record. The old record was 2.06 inches in 1964. This was also just 0.14 inches short of the all-time daily rainfall records. Daily rainfall records were also set at Flint and Saginaw. The flash flooding from these thunderstorms quickly inundated roads and highways. There were 17 CoCoRaHS reports of rain 4 inches or more with the highest amount 6.31 inches at MI-WY-7 Dearborn 3.5 NE. The water was so deep in some areas that the Michigan State Police sent divers to check flooded freeways in the Detroit area to make sure no one was trapped in vehicles. More information on this event, including many photos, can be seen on the August 11, 2014 Historic Rainfall Event page on the NWS Detroit/Pontiac web site.

Then, Tuesday through early Wednesday morning very heavy to extreme rainfall occurred from the mid-Atlantic into southern New England. This rain resulted from the same system that affected Michigan on Monday. The cold front extended along the Appalachians Tuesday evening, with warm, very moist air feeding into it from the Atlantic and the surface low moving east through northern Virginia.

Surface map for 2:00 a.m. EDT, August 13, 2014
In the upper atmosphere, an unseasonably strong trough extended south from the eastern Great Lakes. This trough produced strong upward motion of the air ahead of it, and all of these components came together to produce inches and inches of rain. This water vapor satellite image shows setup Tuesday night night.

Water vapor satellite image.  Green arrows indicate the wind flow. The divergent wind flow from the mid-Atlantic up through New England helped produce the strong lift needed to generate the heavy rain.

Here is the 500 millibar chart for about the same time;

500 millibar chart for 2:00 a.m. EDT August 13, 2014

In the Baltimore-Washington area flash flooding snarled traffic and caused numerous other problems. Rainfall amounts were in the 3 to 5 inch range around Washington DC, but in Maryland amounts were higher. Several Maryland CoCoRaHS observers reported more the 7 inches of rain,and the highest amount reported was 10.32 inches near Greenhaven in Anne Arundel County.

The band of heavy rain extended from Baltimore-Washington northeast across southern New Jersey and across western Long Island. For more information on the rain in New Jersey, see this special report by the New Jersey State Climate Office. The band was quite narrow, and amounts rapidly fell off as you went north and west from the rain band.

Radar estimated precipitation from the radar at Upton, NY from about midnight on August 12 to noon EDT on August 13.

Long Island showing location of Islip
and CoCoRaHS precip reports
The 24-hour rainfall at the Islip Macarthur Airport on Long Island was 13.57inches as of 9:30 a.m. this morning, and is a new 24-hour New York state precipitation record. The previous record was 11.60 inches at Tannersville, NY on August 27-28 during Hurricane Irene. The daily record was also set at Islip, with 13.51 inches. This amount obliterated the old record of 0.91 inch. The total for the month (so far) for Islip is 14.03 inches, a new record.  The old record was 13.78 inches in August 1990.

On Wednesday the heavy rain shifted further northeast into southeast Maine, where upwards of 6 inches of rain accumulated by this morning.

CoCoRaHS map for Cumberland County, Maine for August 14, 2014.

If you would like some quick summary information on flooding and its causes and costs,Weather Underground has a nice "poster type" web page on floods.

Wednesday, August 6, 2014

Hawaii Braces for Tropical Punch

The Atlantic may be quiet as far as tropical activity is concerned, but that is not the case in the Pacific. There are currently three systems in the central Pacific, and two of them are posing a threat to the Hawaiian Islands.

Genevieve, the furthest west storm and the strongest of the three with sustained winds of 135 mph, is located about 1680 miles west southwest of Hawaii. All eyes, however are on Hurricane Iselle and Hurricane Julio.  As of 5:00 p.m. HST  (11:00 p.m. EDT) Hurricane Iselle was was 510 miles east-southeast of Hilo. The maximum sustained winds in Iselle were 90 mph, and it's forward speed was to the west-northwest at 18 mph. At this rate Hurricane Iselle will reach the Big Island on Thursday. Iselle is expected to maintain strength.  A hurricane warning is in effect for the Big Island and surrounding waters, and a tropical storm warning is in effect for all the remaining islands except Niihau and Kauai, where tropical storm watch is in effect. Although the exact storm track is far from certain, heavy rain, heavy surf, and damaging winds are expected to spread across the island tomorrow and Friday.

5-day forecast track for Iselle as of 5:00 p.m. HST (11:00 p.m. EDT) today.
If you look at the satellite image of the storms above and the location of the Hawaiian Islands, it's not hard to realize that hurricanes are relatively rare in Hawaii. It's a small target in a very big ocean. The average climatological hurricane track in the central Pacific is located to the south of Hawaii. In addition tropical systems that approach Hawaii from the east tend to weaken because of unfavorable wind shear and cooler sea surface temperatures. The last time a hurricane made landfall in Hawaii was in 1992 when Hurricane Iniki made landfall in Kauai in early September. This category 4 storm took 6 lives and caused $1.8 billion in damage.

Satellite image of Hurricane Iniki on September 11, 1992.

So, right now we have one hurricane bearing down on the islands, and a second one following close behind. It's unusual enough to have two tropical systems to hit Hawaii in one season, much less in the span of three days.

At 5:00 p.m.. HST (11:00 p.m. EDT) Hurricane Julio was located 1555 miles east of Hilo, moving to the west northwest at 16 miles per hour. Residents of Hawaii will get about 24 hours or so to get ready for Julio after Iselle passes through. The present forecast brings the center of Julio just to the north of the islands late Saturday and Sunday.

5-day forecast track for Hurricane Julio issued at 11:00 p.m. EDT.
Of course, the exact track of both storms is still uncertain at this time, but it appears that Hawaii is in store for a one-two tropical punch over the next four days.

Information on Hurricane Iselle can be found on the NWS Central Pacific Hurricane Center web site. Information on Hurricane Julio is available on the National Hurricane Center web site   Once Julio moves west of 140°W longitude, the Central Pacific Hurricane Center will take over monitoring and forecasting responsibilities for this storm.

The latest watches, warnings, and advisories for the Hawaii can be found on the NWS Honolulu web site. The information I've included about the storms is valid as of late on August 6, so be sure to check these web sites if you are interested in the latest updates.

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.

Tuesday, June 24, 2014

What in the World Cup is the Wet Bulb Globe Temperature Index?

Location of Manuas in Brazil
The World Cup match between the U.S. and Portugal this past weekend was played in Manaus, the capital city of the state of Amazonas in northern Brazil. It is located in the middle of the Amazon rainforest. It has a tropical monsoon climate, with temperatures fairly constant and of course tropical humidity.  The U.S.-Portugal game began about 6:30 p.m. local time.  The temperature at game time, about 30 minutes after sunset, was about 85°F and the dewpoint was a stifling 73°F. You may have read or heard a reference to the Wet Bulb Globe Temperature index this weekend in relation to the U.S.-Portugal game and anticipated conditions. To be honest this was the first time I had heard of this, and that sent me searching for some information.

Most of us are familiar with the Heat Index used here in the U.S. to describe the combined effects of temperature and humidity as an "equivalent temperature".  The Wet Bulb Globe Temperature index (WBGT)is a more elaborate and complex method of measuring heat stress. It measures heat stress in direct sunlight which takes into account temperature, humidity, wind speed, sun angle and solar radiation.

There are three temperature measurements that are part of the WBGT calculation. The wet-bulb temperature is measured using an exposed thermometer with its bulb covered by a cotton wick wetted by distilled water. This measures evaporative cooling which in turn is affected by wind, humidity, and solar radiation.

The second measurement is the black globe temperature. This consists of a thermometer centered in a 6-inch black globe. This measurements represents the integrated effects of solar radiation and wind.

The third measurement is the standard air temperature measured by a shielded thermometer (in a radiation screen). This represents the temperature "in the shade" and is the standard air temperature most of us are familiar with.

These three measurements are used to calculate the WBGT as follows:

Tw = wet-bulb temperature
Tg = black globe temperature
Ta = air temperature

WBGT = (0.7 × Tw) + (0.2 × Tg) + (0.1 × Ta)

Indoors, or when solar radiation is not a factor such as at night, the following formula is used:

WBGT = (0.7 x Tw) + (0.3 x Tg)

Since the formula just weights the contribution of the respective temperatures, temperatures can be in either Celsius or Fahrenheit.

The WBGT was developed in the late 1950s by the U.S Department of the Navy in response to heat stroke cases at the U.S. Marine Corps Recruit Depot at Paris Island, South Carolina. It was recommended as an international standard to measure workplace heat stress in 1989.

One reason we haven't heard much about the WBGT is that it is not easily measured. There are ways to calculate it using wind, solar radiation, temperature and humidity measurements, but wind and especially real-time solar radiation measurements aren't readily available. The Heat Index used here in the U.S. uses temperature and humidity, two easily and regularly measured parameters. However, it represents conditions "in the shade" and does not account for wind and sunshine, both of which can make a significant difference on the heat stress on the body.

WBGT values are comparatively lower than corresponding heat index values. Here is a table of comparisons between the WBGT and the Heat Index. Note how when the wind is stronger the WBGT is a little lower, accounting for the cooling effect of the wind. Solar radiation is approximated by percent of sky cover.
Comparison of WBGT and Heat Index for various weather conditions.
Credit: NWS Tulsa

When the WBGT is 80 to 85°F, working or exercising in direct sunlight will stress the human body after 45 minutes.When the WBGT is above 90°F, heat stress will occur after only 15 minutes.
There are commercially available instruments available to measure WBGT.  They run about $200 and up, a relatively small expense for the workplace or for a large sporting event such as the World Cup. The Occupational Safety and Health Administration (OSHA) has guidelines for workplace heat stress using WBGT.

Devices for measuring WBGT

Thursday, June 19, 2014

Large Tornadoes, Heavy Rain, and Nebraska Twin Tornado Update

The severe weather on Monday from Nebraska eastward to southern Wisconsin was far from the last of it this week. The day after one of the "twin tornadoes" in northeast Nebraska leveled the town of Pilger, another supercell Tuesday evening spawned at least two huge tornadoes. These were unusual in several respects. They were both "wedge" tornadoes - menacing looking storms that were perhaps a half mile wide at times. The most unusual aspect was that a second large tornado (there probably were several tornadoes generated by this cell over its four hour lifetime) was on the ground almost spinning in place for well over an hour. It first set down north of Laurel, NE in Cedar County, and moved very slowly to the southeast. Here are few radar images of the storm when this tornado was on the ground.

The base reflectivity image at 10:07 p.m. CDT (left) from the Sioux Falls, SD radar. The hook echo pattern is evident on the west side of the cell. The right image is the storm relative velocity display at 10:02 p.m. A circulation is located where the red/yellow and green/blue colors are adjacent to each other.

This is a wider view of the radar at 10:19 p.m. The hook echo is clearly evident on this image. The red box is the tornado watch that was in effect.

Storm relative velocity image at 10:51 p.m. CDT. The circulation is still evident but is weaker. The violet polygon is a tornado warning and the green polygon is a flash flood warning.

Tornado on the ground near Coleridge, NE.
Photo: Matthew Higgins via Twitter

The National Weather Service in Omaha is still in the process of completing a survey of this storm and noted that several tornado tracks were found. The good news, if you can call it that, is this storm stalled over a sparsely populated area or otherwise damage could have been much worse.  It will take a few days to determine the tracks, times, and number of tornadoes that occurred.Here is a preliminary assessment they released today.

For more information and future updates, see the Coleridge Area June 17 tornado page on the NWS Omaha/Valley, NE web site.

The NWS Weather Prediction Center put together this excellent video which shows the development of the storms on satellite, overlaying the surface weather and radar images.

Earlier in the afternoon another large tornado touched down in southeastern Montana in Carter County near the South Dakota border. This had the distinction of being the first ever EF-3 tornado in southern Montana.  What caught my eye about this storm was this spectacular wide shot by Roger Hill, a professional photographer. The tornado can be seen just to the right of the bright area on the left side of the storm.

Carter County, MT tornado on June 17. 
Photo by Roger Hill via Facebook
More information on this storm can be seen at the NWS Billings, MT web site.

On Wednesday there were additional tornadoes in South Dakota, and damaging winds from a complex of storms from southern Wisconsin to western Pennsylvania.  However, heavy rain was a major occurrence from southeastern South Dakota on Tuesday to Minnesota, Wisconsin, and northern Illinois on Wednesday and Wednesday night.  On Monday the slow-moving storms in northeast Nebraska and southeastern South Dakota produced torrential rain. Canton, SD measured 8.43 inches of rain as of 6:00 a.m. CDT Tuesday, and many more locations in southeastern South Dakota measured from 3 to more than 5 inches of rain.  24-hour rainfall amounts in southern Minnesota topped 3 inches in many areas, and totals for the past four days have exceed 6 inches in some locations. Minneapolis set a new calendar record for rainfall for any day in the month of June today (June 19) with 3.95 inches of rain as of 1:30 p.m. There was more rain after that time.

24-hour rain fall for June 18 (L) and June 19 (R) ending at 7:00 a.m. CDT
Finally, the NWS released an update on the "twin tornadoes" in northeastern Nebraska on Monday. There were five tornadoes in northeastern Nebraska.  Four tornadoes, including the one that hit Pilger, NE were rated EF-4, and the remaining tornado was rated an EF-0. A summary of these tornadoes can be seen at the NWS Omaha/Valley web site.

Monday, June 16, 2014

Twin Tornadoes in Nebraska Highlight Another Day of Severe Weather in the Plains

It has been a turbulent few days in the Central Plains. For the second time in three days severe thunderstorms have hammered the area. On June 14 numerous storms producing damaging winds and a few tornadoes swept across southern Nebraska and northern Kansas. 

Storms returned again today, but this time the focus was on eastern Nebraska across northern Iowa.

The surface weather map this morning had a warm front extending southeast from a low over northern Wyoming along the Kansas-Nebraska border and east across northern Missouri. South of the front was very warm and humid air and strong south to southwest winds aloft. The area along and north of a warm front like this is often a prime location for severe storm development.
Surface weather map at 7:00 a.m. CDT, June 16, 2014
 In anticipation of the potential for significant severe weather, the Storm Prediction Center placed an area from eastern Nebraska across northern Iowa in a Moderate Risk for severe weather today.

Convective outlook issued at 8:00 a.m. June 16, 2014
At 7:00 a.m. this morning the upper air sounding, which profiles temperature, moisture, and wind vertically through the atmosphere indicated weak instability. Below is what is called a Skew-T plot of the sounding. The temperature is the red line, dew point the green line, and the brown dashed line (generally to the right of the temperature line in this plot) indicates the ascent of a saturated parcel of air. In this plot the line indicates that the saturated parcel is warm than the environment aoound, meaning it is more bouyant and will continue to rise. The area between the brown dashed line and the temperature line represents CAPE, or the Convective Available Potential Energy, or CAPE. In basic terms, CAPE is a measure of instability in the atmosphere, expressed in Joules per kilogram of air. The higher the CAPE, the more unstable the atmosphere.  The CAPE in this sounding (based on a parcel of air rising from the surface) was 468 J/kg, indicating weak instability at that time. However, the atmospheric profile was expected to modify during the day and conditions were expected to become more unstable.

Skew-T plot for 7:00 CDT sounding from Omaha, NE
At 1:00 p.m. CDT the warm front extended from north-central Nebraska to just south of Omaha.

Surface map at 1:00 p.m. CDT June 16, 2014

The National Weather Service in Omaha launched a special extra upper air sounding at 2:00 p.m. to determine the latest atmospheric profile. As expected the atmosphere had become very unstable, with surface CAPE at 5765 J/kg, well into the extreme instability category. Note also how the winds turn from the SSE at the surface to west at 10 kilometers. This is directional shear.

Skew-T plot of 2:00 p.m. upper air sounding at Omaha, NE

At 3:10 p.m. the Storm prediction Center issued a Tornado Watch for northeastern Nebraska. This was a PDS watch (Particularly Dangerous Situation), indicating the possibility for supercells with large hail and damaging tornadoes.

A supercell developed southwest of Norfolk, NE between 2:00 and 3:00 p.m. and moved northeast. The first tornado was reported south of Norfolk about 3:40 p.m. The storm moved east of Norfolk and approached Stanton and Pilger, NE. Two distinct and separate tornadoes were on the ground about a mile apart (according to the National Weather Service). Here is the radar image at about the time the tornado struck Pilger, NE.

Radar image at 4:04 P.m. CDT from Omaha, NE. The left image is the base reflectivity, and the image on the right is the Storm Relative Velocity image. The location of the tornado circulation can be seen where the blue color is adjacent to the red.
Below are some images of the tornadoes, including a brief video (last).

June 16, 2014 twin tornadoes in northeast Nebraska.
Photo credit: Tony Laubach/Facebook
Twin tornadoes in Nebraska.
Screen capture of video from
View of twin tornadoes from Wisner, NE.
Photo credit: Taryn Vanderford on Twitter

One of the tornadoes destroyed 50 to 75 percent of Pilger according to the county sheriff.  As of the time of this post there has been one fatality reported and more than 16 people injured, many of them critically. Tornadoes also caused damage in Cuming and Wayne counties.

Here is the map of storm reports for June 16 as of  8:50 p.m. CDT.