Wednesday, June 15, 2016

The Backdoor Cold Front

At the beginning of this week there was a good example of what is called a backdoor cold front in the Midwest. What was striking about this particular front was the contrast between the air behind the front and ahead of it.

A cold front is the leading edge of a mass of cold air that is cooler and/or drier than the air it is replacing, and usually marked by a wind shift. Typically in North America cold fronts move from northwest to southeast, from west to east, or north to south. The system moving through the central U.S. today is your "typical" cold front. The front is moving from northwest to southeast across the Plains and Midwest.

Surface map for 1:00 p.m. CDT June 15, 2016

A backdoor cold front is a cold front that moves to the west or southwest (from the east or northeast), typically in the Great Lakes or along the Atlantic seaboard. These are commonly occur in the spring along the Atlantic coast when colder air is pushed inland as high pressure builds over new England or the North Atlantic. These fronts can also move south along the eastern seaboard as well.

The cold front that pushed in the backdoor of the Midwest early this week was pretty impressive.

In the sequence of surface maps below you can see how the cold front went from an west-east orientation in the first map to a northwest-southeast orientation on the west end of the front. The cool, very dry air pushed from Michigan and Wisconsin across northeastern Illinois, eventually orienting on a line roughly fronm Des Moines, IA to St. Louis, MO to Evansville, IN.

Surface maps for 10:00 p.m. CDT Saturday, June 11 (top), 7:00 a.m. CDT Sunday, June 12 (middle) and 10:00 p.m. CDT Sunday, June 12

Dewpoints in the air behind the front reached the low 30s, not something you see very often in June in the Midwest. Southeast of the front, dewpoints were in the muggy upper 60s and low 70s. Here is a graph of the temperature and dewpoint at Chicago's O'Hare Airport prior to, during, and after the cold front passage. The front moved through the station between 6:00  and 7:00 a.m. CDT on June 12, and in the next hour the dewpoint dropped 16 degrees, and 24 degrees in the first two hours.

Below is an animation of dewpoint maps every three hours that shows the push of the drier air from northeast to southwest.

Loop of surface dew point maps from 10:00p.m. CDT Saturday, June 11 to 10:00 p.m. CDT Sunday, June 12

I watched with anticipation as the cold front pushed south and west during on Sunday, hoping for some of that dry air to clear out the humidity. Winds shifted to the northeast during the late afternoon, but it was almost sunset before the really dry air pushed in. It was, however, only a brief respite from the high dewpoints, and by the morning of June 13 dewpoints were beginning to climb back into the 60s as the muggy air mass to the south replaced the retreating cooler, drier air.

Wednesday, June 1, 2016

Torrents in Texas

If you watch the CoCoRaHs national precipitation map each day no doubt you have noticed that most of the highest amounts in the past two months - anywhere from 8-16 inches - have occurred in Texas. One of the locations impacted by the heavy rain (by no means the only one) is Houston. The Houston area has received an astounding amount of rain in the past 60 days, largely from two big events. One of these occurred in April, and the other within the past week. Now, any location receiving 12 to 16 inches of rain in 8, 12, or even 24 hours would experience some type of flooding. What is it about Houston that seems to make it susceptible to severe flooding?

One reason is geography. The landscape around Houston is criss-crossed by web of bayous. Bayous are slow-moving rivers or streams generally in flat, low-lying areas. There are often associated with marshes or wetlands and are a tributary to a larger body of water. Houston was founded the Buffalo Bayou, 52-mile long waterway that winds through Harris County, in 1836. The waterways are an integral part of the landscape supporting wildlife, recreational activities, and providing drainage.

Map of the bayous and drainage areas in Harris County, TX.
Credit: Harris County Flood Protection District.

Another reason - the Houston area is becoming a bowl. In the past 100 years the withdrawal of groundwater, oil, and gas has caused the land to sink. The situation is particularly critical in northwest Houston, where wells are tapping the groundwater to supply new residential areas. In the area northeast of two major reservoirs (Addicks and Barker) the land has dropped seven feet since 1906 and continues to do so.

Population increase, population density, and its attendant urbanization is another reason contributing to flood vulnerability. The natural landscape was once dominated by marshes, prairies, and wetlands which helped buffer floods. Much of that has now been replaced by impervious surfaces - buildings, roads, and other paved surfaces - that increase runoff.

The heaviest rain amounts during the heavy rain event of April 17-18 in the Houston area were found north and northwest of the city. The storms were slow-moving and often training over the same areas. The rainfall amounts were bad enough, but the rainfall rates were astounding. A gauge in Pattison, TX in Waller County which lies at the head of Cypress Creek measured 23.50 inches of rain in only 14.5 hours.

Quantitative Precipitation Estimates for the 24 hr. period ending the morning of April 17 (left) and April 18, 2016 (right).

Rainfall amounts (light blue) and flooding in Harris County for the April 17-19, 2016 storm.

The rain gauge at TX-MG-49 Magnolia 10.6 ENE
with 8.47 inches of rain on May 27.
Final total was 11.35". (via Facebook)
This was a particularly challenging storm for CoCoRaHS observers for several reasons. The heavy rainfall occurred overnight, and many were sleeping when their gauges started to overflow resulting in a loss of an actual measurement. Those who attempted to empty the gauge before it filled to capacity were thwarted by nearly continuous lightning which made it extremely dangerous to venture outside. It's not really possible to even estimate the amount of rain once the gauge overflows. The gauge holds from 11.3 to 12 inches (depending on if the tube and funnel are in place), so the most we know from the overflow gauges is that at least 11 inches of rain fell.

The most recent heavy rain event on May 26-27 repeated the April scenario from 6 weeks ago, with the heaviest rain north and northwest of Houston.

Quantitative Precipitation Estimates for the 24 hr. period ending the morning of May 27 (left) and May 28, 2016 (right).

Flood warnings are still in effect from this rain. CoCoRaHS observers in Waller County had the highest two-day rainfall amounts with amounts from 16 to 22 inches. The highest was reported at TX-WA-17 Brenham 9.9 N with a two-day total of 22.41 inches, and TX-WA-24 Brenham 0.7 E with 20.97 inches.

CoCoRaHS 48-hour precipitation amounts for the period ending the morning of May 28, 2016.
Only amounts in excess of 12 inches are shown.

The automated station (AWOS) at Brenham recorded 16.62 inches of rain on May 26, making it the wettest day in the city's history by more than six inches. The Brazos River in Texas surged to a record high 54.76 feet early this evening at Richmond, TX, northwest of Houston. The river is at  4.46 feet above the previous record (50.3 feet) set on October 21, 1994.

Hydrograph for the Brazos River as of 9:20 p.m. CDT June 1. The river reached a record 54.76 feet about 6:15 p.m.

The flooding from this latest storm has resulted in six fatalities and damage to hundreds of homes and buildings. Seven people lost their lives in the April flood. Most of these were in vehicles.

Not a river, but a flooded road in Fort Bend County (west of Harris County).
Note the swimming alligator in the lower right hand corner.
Credit: Fort Bend Sheriff Office via Twitter.

There's no rest for the weary, either. Thunderstorms have been frequent the past several days, and thunderstorms rolled through Harris County and surrounding areas today. The outlook for the next five days paints a wet picture for the Houston area.

Quantitative Precipitation Forecast (QPF) for the 5-day period ending 7:00 p.m. CDT Monday, June 6.

Flooding is the most frequent and dangerous natural hazard in the Houston area, but they are prepared to deal with it. The Harris County Flood Control District continuously monitors stream flow and precipitation to asses the flood potential. It also controls the releases from the reservoirs that hold the runoff from storms, maintains the infrastructure, and develops and implements flood damage reduction plans.You can learn more about the flooding issues in the Houston area on their web site  and view the real-time Harris County Flood Warning System at

Screen capture of the Harris County Flood Warning System web page.

Monday, May 9, 2016

Ingredients for a Wildfire - Fort McMurray, Alberta

The Fort McMurray fire in Alberta, Canada ignited sometime on May 1, very likely human-caused. It was quickly news on social media, but it was a couple of days before it was widely reported in the news. Spectacular video taken by evacuees quickly went viral on the Internet.

The spring fire season in the boreal forests in British Columbia and Alberta got off to an early start this year. A dry winter and early spring was one of the main factors.

This map shows the status of drought across Canada. Higher values indicate drier conditions

The very warm spring resulted in a faster than normal spring melt. In a normal spring the snow melts gradually keeping the forest floor moist. With the warm dry winter the forest floor was covered in dry leaves and pine needles. The premature spring warmth and low humidity dried things out even further.Deciduous trees, such as aspens, hadn't yet sprouted leaves. Leaves transpire moisture into the forest and help cool the forest environment.

The end of April and early May was unseasonably hot. The maximum temperature last Wednesday (May 4) in Ft. McMurray reached 91°F (32.6°C), with the humidity about 15 percent. Ft. McMurray is located at 56.6° North, about the same latitude as the Aleutian Islands. Several dozen Canadian record high temperatures were set that day. The hot weather last week was caused by an upper ridge of high pressure that extended through western Canada and into the Arctic Circle. High temperatures reached the 80s to low 90s from the Dakotas north through Alberta and Saskatchewan.

500 millibar map for Wednesday, May 4, 2016

With plenty of tinder dry fuel, hot dry weather, and strong winds it was no surprise the Fort McMurray fire took off like it did. It was far from the first fire of the season. Fire activity across British Columbia and Alberta was already well above average.

Number of Canadian wildfires by province in 2016 compared to normal

The Fort McMurray fire was intense and produced smoke that turned day into night. It also generated pyrocumulus clouds with lightning.

This video gives you a sense of what it was like evacuating from Ft. McMurray. It's both frightening and mesmerizing. Note that this was recorded during the middle of the afternoon.

This animation shows the extent of the Fort McMurray fire as it spread over the past nine days. As of today the extent of the area affected by the fire was more than 200,000 hectacres (2000 sq km). The good news is that latest reports indicate that about 85 to 90 percent of the structures in Fort McMurray were saved. Unfortunately, neighborhoods in the path of the fire, such as Beacon Hill in the video above, were largely destroyed. It's estimated that the fire destroyed 2,400 structures.

The fire danger last week across most of Alberta and southern Saskatchewan was Extreme.

Fire Danger across Canada on May 5, 2016

Conditions improved yesterday and last night as cooler weather and rain fell across the province.

Fire Danger on May 9, 2016

Amounts were light (5 to 10 mm, about 0.25 to 0.30 inch) but most welcome. One CoCoRaHS location on the Alberta/NWT border (CAN-NT-7) reported 34.3 mm (1.35 in.) of rain indicating a "steady cold rain all day and evening".

Wildland fire activity in Alberta has been well above average the last three years and is well on its way to another above average year. Last year fires burned 491,000 hectacres of land, well over twice the ten-year average of 179,000 hectacres. The Fort McMurray fire alone has already exceeded the average.

Thursday, April 21, 2016

Large Impacts from Slow-Moving System

The low pressure system on the weather maps today, both on the surface and aloft, has been taking its sweet time moving through the U.S. The upper level system moved into the Pacific Northwest a week ago (April 14), and this morning was located over Iowa.

500 millibar map for 7:00 a.m. CDT April 21
 Over the last week this system wreaked havoc on the central Rockies with heavy snow, and in southern Texas with record heavy rain. The slow progression of the low was due to what is called and omega block in the upper atmosphere. That effectively parked the upper low over the Great Basin and Rockies for several days. As is typical in these situations this was a good news, bad news situation. The good news was sunny, warm spring weather over the eastern half of the country. The bad news was a heavy spring snowstorm in the Rockies, and record heavy rainfall and flooding in southern Texas, particularly in the Houston area.

500 millibar map for 7:00 a.m. CDT April 17.

With the upper low parked over the Rockies, southerly winds on the east side of the low funneled copious amounts of moisture  northward through Texas and into the central Rockies and western Plains. Very cold air aloft, ample moisture, strong upward motion, and the fact that the system was barely moving resulted in an extended period of snow from Wyoming south through Colorado into northern New Mexico last weekend. By late Sunday more than 4 feet of snow - heavy, wet snow - had fallen in some locations on the east side of the front range above 9000 feet. Two to three feet was common in the Front Range foothills.

72 hour snowfall ending the morning of April 18.
4-day CoCoRaHS snow totals for locations in Colorado

Denver (Stapleton Coop site) picked up 8.4 inches of snow from the storm, but amounts varied from 6 to 12 inches across the metro area. This latest storm boosted Denver's snow for the season to 71.4 inches (11.4 inches for the storm and a season total of 69.3 inches at Denver International). That was enough to bump the Accumulated Winter Season Severity Index (AWSSI) for Denver back into the severe category for this year.

Far bigger problems were in store for Texas. A cold front trailing south from the surface low in Kansas stalled out and provided a focus for the development of heavy showers and thunderstorms. Southerly winds fed moisture laden air with dewpoints in the low 70s into southern Texas. This air collided with the colder, drier air north and west of the front and was forced upward, helping sustain heavy thunderstorms from Dallas south to Houston.

Surface map for 4:00 a.m. CDT April 18 for the southwest U.S.

The rain was heaviest in the Houston area, and was enhanced by an outflow boundary from thunderstorms that helped further sustain the rainfall. Thunderstorms regenerated and trained repeatedly over the same area. Houston's Intercontinental Airport set a one-day rainfall record of 9.92 inches on April 18, breaking the old record by almost two inches (8.16 inches in 1976). However, 10 inches was far from the highest amounts recorded. Those occurred in the northwest quadrant of the metro area. Measured rainfall amounts were in excess of 15 inches for the 24-hour period, and radar estimates were as high as 20 inches.

Quantitative Precipitation Estimate for southern Texas for the 24-hour period ending at 7:00 CDT April 18.

48-hour precipitation for the period ending 7:00 a.m. CDT April 19.
Source: Harris County Flood Warning Service

A number of CoCoRaHS observers recorded 12 inches or more on the morning of April 18. Much of this fell in a 12 to 13 hour period.

Southern Texas CoCoRaHS observations on April 18.
Flooding was rapid and widespread. Despite warnings to stay off the streets and not drive into flooded roads, eight people lost their lives after being trapped in floodwaters. Unfortunately the rain didn't quit completely, and another two to three inches fell in the Houston area through this morning.

Total precipitation for the 72-hour period ending at 8:00 a.m. April 20

Flooding is going to remain a problem in Houston for a number of days. Two dams in the area are considered "extremely high risk" by officials and are being closely monitored. The reservoirs behind them are at about 80 percent capacity.

Houston flooding on April 18.
Credit: Reed Timmer via Twitter

The rain wasn't limited to the Houston area. Five to eight inches of rain fell in the Dallas-Ft.Worth area resulting in flooding in northern Texas.

The pesky upper low responsible for this week of stormy weather will finally weaken and move out into the Atlantic late Saturday. Then, we'll turn our attention to the next system in the Pacific Northwest which may mean more snow for the Rockies and an unsettled week in the Plains and Midwest.

Wednesday, April 13, 2016

One Hail of a Week in Texas

Map of Wylie, TX
There's a saying that "everything is bigger in Texas", and this week Texans saw some big big hail. On both Monday and Tuesday severe thunderstorms pounded areas with large hail and strong winds, a very damaging combination. Insurance adjusters are going to be very busy in Texas for awhile.

On Monday, April 11 severe thunderstorms pounded the area around Wylie, TX, just east of Plano and northeast of Dallas, with 2 to 5 inch diameter hail.

The storm hit during the late afternoon/early evening and moved ESE through the region. 

Radar reflectivity in northern Texas  at 4:40 p.m. CDT on Monday, April 11, 2016

Winds gusted to 60-70 mph at times with these storms, and that turned golf-ball to baseball-size hail into destructive and deadly missiles. The hail produced extensive damage to vehicles that were outside, but also caused large amounts to homes as the wind-driven hail shattered windows, pockmarked siding, and even punched through roofs and ceilings. This wasn't just 30 seconds of hail - in many cases it lasted for several minutes.

Softball-size hail (4.5") in Wylie.
Credit: Kristin Baxter via Twitter

Holes punched through a roof  (left) and damage to an exterior wood door (right) from large hail  in Wylie, TX.
Source: Twitter

Here is a video of hail smashing in windows and blinds in a home in Wylie.

The damage was extensive enough that the Wylie school district canceled Tuesday classes at all 19 campuses. Damage occurred not only to school buses but to many of the school buildings as well.

This image is of a product named MESH (Maximum Expected Size of Hail) uses multiple radar data parameters to depict the swath of hail and the size expected.

Hail swath as of 6:50 p.m. CDT April 11, showing the largest hail over Wylie, TX
The Forth Worth office of the National Weather Service provided this 3-D depiction of the storm as the largest hail was falling on Wylie.

On Tuesday, April 12 the hail threat shifted into south Texas. Storms developing along the Rio Grande moved east dropping hail 2 to 3.5 inches in diameter.

Hail swaths associated with the severe thunderstorms in Texas on April 12. San Antonio is located in the center upper third of the image (labeled SAT).

 The storm with the largest hail hit near and in San Antonio. Again, many vehicles were damaged, and one report indicated that one hundred to perhaps as many as 300 luxury vehicles were damaged at a BMW dealer lot in San Antonio.

Hail that fell on San Antonio on April 12.

This is not the first time this season that severe hail has pummeled Texas. On March 17 a storm rolled through the Fort Worth area dropping 1 to 2.5 inch hail, causing an estimated $300 million damage to vehicles alone. Damage estimates for Wylie, when finally compiled, could be staggering. Many vehicles were totaled and there was extensive damage to homes. The storm in San Antonio Tuesday may end up being the costliest on record there, with preliminary damage estimates of $125 million. Texas hail events in March, along with these latest two storms are likely to push damages in the past month alone to in excess of $2 billion.

Measuring hail is a core mission of CoCoRaHS, and the separate hail reports on the CoCoRaHS web site allow you to submit your hail information. There are a few things you need to know before measuring hail, and you can find that information in our "Measuring Hail" training animation. Here is a hail size reference and measuring guide you can download, print, and laminate for use. You can download it here.

Tuesday, March 29, 2016

Winter and Spring Continue to Battle for Position

As mild as the month of March  has been to date across most of the country (and most of the winter, for that matter), it's somewhat surprising how tenacious winter has been in many areas. In most areas affected the winter weather has been tucked in between extended mild periods, but in the Rockies winter has returned for a longer stay.

During the past week a large portion of the country received snow, mostly in the west. However, snow also fell in the Northeast and New England, and today northern Maine was experiencing a good dose of winter with 3 to 6 inches of snow and 40 mph winds.

Total snow (L) and percent of normal snow (R) for the period March 22-29
Last week the storm that dumped a foot or more of snow on the Denver area affected the Plains and northern Midwest with snow sleet, and freezing rain.

72-hour snowfall for the period 7:00 a.m. CDT March 25, 2016

This storm also extended its influence into Canada. Southern Ontario was affected by freezing rain, sleet and snow, with ice accumulations of nearly an inch in some locations. In the Toronto area more than 38,000 customers lost power last week as power lines and trees collapsed under the weight of the ice.

Ice-coated trees in Alliston, Ontario on March 25.
Photo credit Melanie dePrinse via Twitter.

Today, another strong storm system has been spinning up over the Rockies. Snow is falling from west of Denver north into Wyoming, and extends west into Utah and Nevada. 

Forecast surface map for 12:00 a.m. MDT March 30, 2016.
Winter storm warnings extend from eastern Nevada into western Utah, southern Idaho, and much of Wyoming. Blizzard warnings are in effect from midnight tonight through 6:00 p.m. MDT tomorrow for parts of north-central Wyoming where heavy snow and north winds from 30-40 mph will produce whiteout conditions. Outside of the warning areas, winter weather advisories extend into South Dakota and Nebraska. This storm system is likely to continue the spread of winter weather eastward through the upper Midwest and into Ontario and Quebec through Friday.

Watches, warnings, and advisories as of 5:48 p.m. MDT March 29

Probability of snow accumulation of 2 inches or more during the period
7:00 p.m. CDT March 29 through 7:00 p.m. CDT Friday, April 1.

This may not be the last of wintry weather, either, at least not for the eastern U.S. The 6-10 temperature outlook from the Climate Prediction Center indicates a higher probability of below normal temperatures in the northeastern third of the country, with a very high likelihood of below normal temperature in the northeast and New England. In the remainder of the country spring will get a good foothold.

Wednesday, March 23, 2016

March Storm Hammers Denver, Heads for Central U.S.

Earlier this week there was a lot of attention on the potential for snow in the Plains and upper Midwest, not really an unusual occurrence for this time in March. There was also some attention being given to the potential for severe weather today into Thursday from eastern Texas into Missouri and Illinois. Late on Tuesday, though, attention turned to the Rockies as blizzard warnings were issued for parts of Colorado, including the Denver area, Wyoming, southwestern Nebraska and northwestern Kansas.

Denver and the central Rockies enjoyed mild weather on Tuesday with highs in the 70s. Rain overnight changed to snow early this morning as a storm intensified over the central Rockies.

Surface weather map at 12:00 noon MDT March 23, 2016

By 7:00 a.m. observers in Fort Collins were reporting 7 to 10 inches of wet snow, while further south in Boulder snowfall ranged from 6 to 7 inches in Boulder to more than 18 inches in higher elevations west of the city.

Snowfall as of 6:00 a.m. MDT March 23, 2016

Traffic cam photo from the morning of March 23 on I-70
outside of Denver
By midday snow totaled more than 20 inches in some locations. The snow was whipped around by winds up to 50 miles per hour reducing visibility to near zero. The heavy wet snow quickly snarled highway traffic. By early afternoon Denver International Airport threw in the towel and closed until conditions could improve. All Interstates in and out of Denver were closed, and most other highways were closed as well. Chris Spears, a meteorologist with KCNC-TV, the CBS affiliate in Denver and a CoCoRaHS observer, stated that the storm in Denver was "the worst weather I've ever personally witnessed in 38 years of living!!"

The heavy, wet snow (snow-to-water ratios as low as 7 to 1) coated trees and power lines, and with winds regularly gusting in excess of 45 mph it didn't take long before power outages began to develop. More than 135,000 customers were without power by late Wednesday morning in the Denver area.

Denver endured nine consecutive hours of blizzard conditions by mid-afternoon. Thundersnow was reported from Denver into Kansas and Nebraska. At 6:00 p.m. MDT the U.S Cooperative station in Boulder reported 16.8 inches of snow with 2.40 inches of water equivalent. By this time snow was tapering off and breaks were appearing in the clouds.

In the warm sector SE of the low center conditions were ripe for thunderstorms this afternoon. A tornado watch was issued for southeastern Nebraska, southwestern Iowa and northeastern MO until 10:00 p.m. tonight. The severe weather outlook indicated an area of Slight Risk of severe storms from northeast Texas to southern Iowa.

The strong winds circulating around the strong low pressure system complicated efforts to control a huge wildfire along the Kansas-Oklahoma border.  The fire covered 75 square miles, and the plume of smoke from the fire was clearly visible on radar and satellite today.

The smoke plume from a wildfire in Kansas is visible on the Dodge City radar

The plume from the wildfire is clearly visible on this satellite image taken at 3:00 p.m. CDT

Early this evening Winter Storm Warnings were in effect from southeast Wyoming eastward across southern Minnesota, northern Iowa, central Wisconsin, and the northern half of lower Michigan. Nine to 13 inches of snow are expected along the path of this storm along with high winds. Seven counties surrounding Green Bay, WI are in a Blizzard Warning from late tonight through early tomorrow afternoon as strong NE winds off of Lake Michigan combine with the snow to lower visibility.

Watches, warnings, and advisories in effect as of 8:55 p.m. CDT March 23.
The severe weather threat will shift east into the Ohio Valley and southeastern U.S. on Thursday, while colder air spills into the central U.S. behind the low pressure system as uit lifts northeast through the Great Lakes.

Convective outlook for Thursday, March 24, 2016 issued at 12:30 p.m. CDT