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







Thursday, March 10, 2016

Need An Extra Hand Emptying Your Rain Gauge?

CoCoRaHS observers in the south have been making many pours from the outer cylinder into the inner measuring tube to measure their rain this week. Six observers in Louisiana measured more than 10 inches inches of rain in their gauges Wednesday morning, necessitating at least 11 pours to get the measurement. Measuring several inches of rain takes a little work and some extra attention. When the outer cylinder is full, or even half full of water it gets to be heavy and just a bit awkward to handle. In order to carefully pour from the outer cylinder into the inner measuring tube it usually requires using two hands to hold the cylinder. So what do you do with the inner tube and funnel?

Here is a "third hand" that will make your job of measuring multiple inches of rain a lot easier. This simple stand to hold the inner tube and funnel and allows you to use both hands to carefully pour into the measuring tube.

 
This is a scaled down version of a stand I made to hold the measuring tube of the standard 8-inch rain gauge. 



The idea came after I witnessed our Cooperative Station weather observer pouring water from the 8-inch steel can into the funnel on top of the free-standing tube. This was a disaster waiting to happen. Should the top-heavy tube and funnel topple over, say goodbye to that precipitation measurement. The steel can itself is heavy enough without any water in it. So, a little time in my shop with some scrap wood and I came up with the tube support stand for the 8-inch gauge. 

8-inch rain gauge can (r) with measuring tube in the tube support stand (l)


After I became active in CoCoRaHS I realized that a scaled-down version of  the large stand would be helpful when pouring from the outer cylinder into the measuring tube. An an extra precaution to spilling any water, you could place the stand with tube and funnel in a dishpan or other container, "just in case".



You can make the tube support stand with as little as one square foot of 3/4 inch plywood. Lay out the parts to maximize wood use. You can also make it out of standard construction lumber. I have created plans for the stand with all the dimensions which can be downloaded here. If you run into any problems or have questions about its construction just drop me an email.

Tuesday, March 8, 2016

Cry Me A (Atmospheric) River

The general weather pattern that has set up over the U.S. the last few days has brought mild weather to much of the central and eastern part of the country. More ominously, it has established two narrow regions of concentrated atmospheric moisture that will bring a myriad of problems to California and the central U.S. These atmospheric rivers are not uncommon, and they are responsible for much of the transport of atmospheric moisture outside of the tropics. These atmospheric river setups are often responsible for heavy rain and flooding when they impact the U.S. You have probably heard about the Pineapple Express, which is one of these atmospheric rivers.

The strong southern jet stream across the Pacific Ocean, a manifestation of the warm Pacific waters associated with El Niño, is funneling moisture from eastern Asia to the west coast of the U.S. Precipitable water is a measure of the moisture vapor in the atmosphere that, if condensed, would accumulate as rain. In the image below you can see the stream of moisture extending across the Pacific, and another plume of moisture extending from the southern tip of Mexico north through the Midwest.

Precipitable water for 6:00 p.m. CST Tuesday, March 8, 2016.

The Pacific atmospheric river is feeding into a strong trough of of the west coast, combining to bring rain to California and heavy snow the Sierras. A cutoff low developed in the trough late yesterday, with strong southerly winds on the east side of the trough.


500 millibar map for 6:00 a.m. CST Tuesday, March 8, 2016. Shaded areas indicate wind speeds.

As this system moves slowly east the moisture over the Pacific will wrap around and enhance the flow of moisture into the central U.S from the Gulf of Mexico.  The atmospheric river in the Pacific, as well as the one developing from Mexico to Texas into the central U.S. (dubbed the Maya Express) are clearly evident on the precipitable water maps for today (above) and the forecast map for Thursday morning.



Precipitable water forecast map for 6:00 a.m. CST Thursday, March 10, 2016

The precipitation forecasts for the next several days are impressive. Over the next three days the heaviest precipitation is expected to occur in Texas and east along the Gulf coast with perhaps amounts up to 12 inches, and flooding is likely in many areas if this amount of rain does in fact occur. this is in addition to 2 to 6 inches of rain in Texas that occurred in the 24 hour period ending Tuesday morning.  Heavy precipitation is also expected in northern California. Much of California has received from one to two inches of rain in the past few days. While California needs the rain, this much rain will be causing flooding problems.


Quantitative precipitation forecast for the 72 hour period ending 6:00 p.m. CST Friday, March 11, 2016

Looking ahead the next seven days, more rain is expected beyond the three-day period as additional storms reach the coast. These will also add to the snowpack in the Sierras and Cascades, adding to the water storage for the warm season.


Quantitative precipitation forecast for the 7 day period ending 7:00 p.m. CDT Tuesday, March 15, 2016.
You can read more about atmospheric rivers in this Forbes article by Dr. Marshall Shepherd of the University of Georgia and host of WXGeeks on the Weather Channel.



Wednesday, March 2, 2016

Winter is Over, Yet Winter Continues

On March 1st we left the three-month meteorological winter behind us and stepped into spring. Unfortunately for some of those who are chomping at the bit for spring weather, winter is holding on for a bit longer.

This was was a winter where El Niño was figured to douse California with plenty of drought-reducing rain, where the Pacific northwest to the central U.S. were expected to be normal to dry and warm, and Texas and the Gulf coast states were expected to be cool and wet. However, this wasn't your typical El Niño and the winter season had many surprises.

First, it was warmer than normal across just about the entire country. The exception was in the southwestern third of the country, where temperatures were near to cooler than normal. The Northern Plains were as much as 8°F to 10°F above normal which is fairly typical occurrence with a strong El Niño. Cooler than normal weather expected from Texas to the Southeast U.S. with a strong El Niño didn't happen this time around. It was also very warm in the Northeast, especially from northern New York through Vermont into Maine. Albany, NY notched their warmest and least snowiest winter on record. Albany's winter temperature was 7.8°F above normal, and snowfall for the three months was just 10.7 inches, with most of that occurring in December. Average for the three-month winter period in Albany is 43.7 inches.



Overall, contrary to the "typical" strong El Niño pattern, most of the country was wetter than normal during the winter. The Pacific Northwest received from 150 to more than 200 percent of normal precipitation, as did part of the Central Plains and western Great Lakes. The Northern Plains, especially North Dakota were drier than normal. The area from southern California into central Texas received generally from less than 50 percent to 75 percent of normal precipitation. This was one area that was expected to have a greater probability for a wet winter this year. Most of the eastern third of the country received normal to above normal precipitation, with southern Florida taking the prize for the wettest area in the East and Southeast.



As might be expected with the extent of the warm weather, snowfall was hard to come by in many areas. The winners with snow were the central and southern Rockies, The Sierra Nevada, and the Central Plains. The footprint of several storms that crossed into the mid-Atlantic states, including the Blizzard of 2016, is clear on the map. Also clear is the lack of snow from the Midwest and Great Lakes eastward through New York and most of New England.




It appears there are some changes coming which will make those who are ready for spring a little happier. We'll first have to deal with a weather system that will bring snow to parts of the Midwest and the Mid-Atlantic Thursday.

Forecast map for Thursday, March 3, 2015.

 Another clipper system will bring snow to the upper Midwest this weekend. Once that system is out in the Atlantic, high pressure will set up along the eastern seaboard and begin pumping warm, springlike air into much of the eastern two-thirds of the country.

Forecast surface map for Sunday, March 6, 2015 at 6:00 a.m. CST.


Maximum temperature forecast for Sunday, March 6, 2015
That air will eventually be accompanied by moisture and rain for the central U.S. next week.  Meanwhile, a series of storms are expected to move off of the Pacific into California this weekend, bringing some much needed rain to California.

Precipitation forecast for the period from 6:00 p.m. Saturday, March 5 through 6:00 p.m. Monday, March 7, 2015