Category

Extreme Local Storm Precipitation

Ozarks Hammered with Second Major Flood in Last 18 Months

By | Extreme General Storm Precipitation, Extreme Local Storm Precipitation, MetStorm | No Comments

This past weekend, the Ozark Mountains of southern Missouri and northern Arkansas experienced two days of near-continuous heavy thunderstorms. The sustained heavy rain over the steep hills of this region led to rapid runoff and a dramatic rise in local creeks and streams. Numerous towns and cities from central Arkansas to southern Indiana have been inundated over the past few days by floodwaters that, in some cases, have reached record heights. While floodwaters have begun to gradually recede in many locations, another ongoing (but fortunately less extreme) rain event will keep rivers elevated for several more days.

Figure 1. The town of Eureka, MO, a suburb of St. Louis, under record floodwaters from the nearby Meramec River. The river crested at an all-time high of 46.11′ on 2 May 2017.

As is the case with any extreme flooding event, numerous factors contributed to the disaster, but meteorological conditions are foremost. Late last week (on and around April 28, 2017), a sprawling low pressure system spun into life along the southern Rocky Mountains and drifted northeastward. This powerful system inspired a huge variety of spring weather across the middle portion of the United States, with severe and tornadic storms across the southern Plains, a record-strong late-season blizzard in the High Plains of Kansas and Colorado, and heavy rain across much of the lower Missouri and Mississippi River valleys (Figure 3). Below, a spectacular GIF of infrared satellite images from the new GOES-16 shows thunderstorms blossoming across the central part of the country before the main upper level low pushes through. (See also this YouTube video of the lightning from these storms, as captured by GOES-16.)

Figure 2. Thunderstorms blossom from the Ohio River Valley to Texas during the overnight hours of April 28-29, 2017. (Image credit: http://www.weather.gov/sgf/28-30AprilHistoricFloodingEvent).

Figure 3. Storm-total precipitation across the Ozarks region from the morning of April 28 to the morning of May 1. A large band of 6″+ of rain occurred over hilly terrain, leading to substantial flooding across the region.

While the tornadoes proved deadly and the blizzard may have damaged some sensitive vegetation, by far the most devastation from this storm was associated with the flooding. Figure 4 shows a snapshot of radar across the region from the evening of April 29, along with a selection of storm reports from the April 28-May 1 period. All kinds of severe weather occurred over this period, but note that flooding was widespread from MO/AR through the Ohio River Valley.

Figure 4. A snapshot of radar from the evening of April 29th and an assortment of local storm reports across the Ozarks and Ohio River valley. Image credit: https://nwschat.weather.gov/lsr/.

The most intense flooding was concentrated across the Ozarks of southern Missouri, northern Arkansas, and far northeastern Oklahoma, where 14 river gauges hit all-time record crests this week. Of course, factors such as floodwall and levee development along rivers can certainly affect the flood depths achieved — the more a river’s floodplains are restricted, the higher the water must go. However, we at MetStat like to examine the precipitation factor from a frequency perspective to determine just how extreme rainfall events are (Figure 5).

Figure 5. The Average Recurrence Interval (ARI) for the 72-hour rainfall from April 28 to May 1, 2017.

As the Average Recurrence Interval (ARI) analysis above indicates, some areas experienced rain on par with a 1-in-100 year event or more, particularly across the Ozarks and on into southern Illinois. This product compares the observed rainfall to rainfall frequency distributions in order to illustrate the relatively rarity of an event. On average, a 100-year ARI event would be expected to occur just once every 100 years for a given point. This event brought an impressively large area of 100- to 1000-year rains (reds to purples in Figure 5), which certainly played a leading role in producing such catastrophic flooding.

To some, this storm may seem like a case of déjà vu. A remarkably similar storm system brought rains of remarkably similar magnitude to this same region in late December 2015. As the comparisons below show, the rains in 2015 were slightly heavier, more widespread, and further northwest than the ones this time around:

Figure 6. Comparison between the 72-hour storm total precipitation from the December 26-29, 2015 event (left) and the April 28-May 1, 2017 event (right).

Figure 7. Comparison between the Average Recurrence Interval product for the storm of December 26-29, 2015 (left) and the storm of April 28-May 1, 2017 (right).

Clearly, while the 2015 heavy rain was more widespread, the 2017 event had more intensely concentrated pockets of heavy rain over hillier terrain, and this likely contributed to the extreme nature of recent floods. While these two very rare events occurred within a short time of each other, it must be kept in mind that recurrence intervals just represent a probability, and that large events can happen at any time. This past weekend’s event is the first major event of the season, and more are sure to follow. We are constantly monitoring extreme precipitation events all around the country, so monitor this space for updates from MetStat.

40th Anniversary of the Big Thompson Flood

By | Extreme General Storm Precipitation, Extreme Local Storm Precipitation, MetStorm | No Comments

An afternoon thunderstorm situated in just the right place can spark a chain of events that can completely change a community and how it learns to respond to a flooding disaster. Such was the case the evening of July 31st above the Big Thompson Canyon in Colorado, 60 miles northwest of Denver. The thunderstorms, that ultimately killed 145 people and resulted in $40 million of damages, dumped over a years-worth of precipitation in a very short amount of time.

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Many people are able to recall this horrific event, due in no small part to luck and quick-thinking. Outrunning a wall of water is an impossible feat in a river canyon, and the unfortunate truth to the Big Thompson flood is that those who attempted this mostly perished. Climbing the canyon walls to safety, however, gives one a much greater chance of survival.

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On the evening before Colorado celebrated its 100th birthday, a mass of storms began to set up and take root right above the Big Thompson Canyon. This relatively stationary storm system began its downpour directly over the river, and within a matter of hours completely changed the the surrounding landscape. Below is a mass curve plot, generated by MetStorm, that displays the incremental and accumulated precipitation in the area of heaviest estimated precipitation (a total of 15.6 inches of rainfall over a two-day period).

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West of Loveland, throughout the canyon, storm rainfall totals above 10″ stretched from Glen Haven to the north to the border or Rocky Mountain National Park to the south:

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This incredible amount of rainfall in such a short amount of time is undoubtedly a rare event. To assess just how rare this amount of rainfall was, an analysis of the average recurrence interval, or ARI, of the storm was performed by the MetStat team. Below, the MetStorm-generated ARI map for the 3-hour period of maximum rainfall for each point on the map shows that for much of the area in and around the Big Thompson Canyon, the amount of rainfall that pummeled the canyon has a less than one in one thousand chance of occurring in any given year.

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The Big Thompson flood remains Colorado’s deadliest and one of its most costly. The lessons learned from this event still resonate with Coloradans; and the communities within now know how to respond to such a disaster.

Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm® Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at here.

-MetStat Team



Stunning Microburst in Phoenix

By | Extreme General Storm Precipitation, Extreme Local Storm Precipitation, MetStorm, Uncategorized | No Comments

Phoenix, along with much of the rest of the country, has been battling with excessive heat for most of the summer. In the southwest this dry heat, combined with their summer monsoonal rainfalls, can create a virulent effect in the atmosphere that accompanies the rain, known as a microburst.

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Photo Credit: Bruce Haffner

Microbursts form as rain from thunderstorms enter hot, dry air underneath them. This air causes raindrops to evaporate, and in the same manner as hanging around after taking a dip in a pool can make you shiver, it cools the surrounding air. Already being relatively colder to begin with, this cooling by evaporation (or: evaporative cooling) makes the downdraft of rainfall under the storm accelerate. This is because cold air is denser than hot air, causing it to cascade towards the ground faster and faster as more rainfall evaporates. The picture above illustrates this effect perfectly underneath a large thunderstorm producing very heavy rainfall. Once this rush of precipitation and cold air hits the ground, it has nowhere else to go but out horizontally, which is also noticeable in this picture. The violent outflowing air can kick up dust and debris along the way, creating another weather phenomenon called a haboob.

A quick MetStorm analysis on the thunderstorm that produced this incredible display shows 1.42 inches of rainfall falling between 5 and 6pm the evening of the 18th. This is not an uncommon occurrence near Phoenix during the monsoon season. The average recurrence interval for 1.42 inches of precipitation falling in a one hour timespan in this location has a probability of occurring once in ten years.

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This storm is an excellent demonstration of natures fury when all the right ingredients come together and produce a visually stunning phenomena.

Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at here.

-MetStat Team



Southwest Utah storm of September 14-15, 2015

By | Extreme Local Storm Precipitation, MetStorm, Uncategorized | No Comments

Thunderstorms moving through southern Utah and northern Arizona caused deadly flooding Monday September 14 through Tuesday September 15th, 2015. These thunderstorms were associated with a shortwave trough moving through the Southwest. This trough initiated the movement of warm, moist air from the Gulf of California northward, providing the moisture and instability necessary for thunderstorms to develop.

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Topography was a major factor in this flooding event. Heavy precipitation fell in a short period of time over steep and somewhat impermeable terrain, causing the storm water to flow into the valley and overwhelm Short Creek.

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In a two day span along the boarder of Utah and Arizona the National Weather Service (NWS) in Salt Lake City issued seven flash flood warnings, the NWS in Flagstaff issued two flash flood warnings and a severe thunderstorm warning, and the NWS in Las Vegas issued two flash flood warnings. All of these warnings explained the dangers of flash floods and urged the public to take action by retreating to higher ground.

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MetStorm™ was used to analyse this storm and determine how rare of an event it was from a precipitation stand point. Below are the total storm map, showing the maximum estimated precipitation to be 4.5″ in the 48-hour period, and the Average Recurrence Interval (ARI) map which depicts the rarity of this event.

The ARI, or “return period” for this storm at a one-hour time period, for the location north-northeast of Hildale and Colorado City, was 258.36 years, or a 0.39% chance of occurring in any given year. This represents the likelihood of 2.47″ falling in a one-hour period at this location.

Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at http://metstat.com/contact-us/

Thanks for visiting today!

-MetStat Team

(edited: 9/25/2015 )



Average Recurrence Interval for Colorado, Wyoming, and Nebraska storm of July 21, 2015

By | Extreme General Storm Precipitation, Extreme Local Storm Precipitation, MetStorm, Uncategorized | No Comments

The weather conditions last Tuesday, July 21, 2015, over the borders of northern Colorado, southeastern Wyoming, and western Nebraska were ripe for the formation of thunderstorms. The Weather Prediction Center (WPC) surface analysis for 21Z, or 3pm MDT, shows a high pressure system situated over western Colorado with a stationary front over the Front Range. The thunderstorms associated with this stationary front produced significant rain which caused flooding in southeastern Wyoming. The radiosonde for the morning of the 21st shows winds at the surface coming from the south and winds aloft from the west. The surface streamlines also show southerly winds originating in the Gulf of Mexico and continuing through Colorado, Wyoming and Nebraska. The southerly winds provided the moisture necessary to produce the magnitude of rains that occurred.
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The local National Weather Service (NWS) office in Cheyenne issued a flash flood warning for southwestern Banner County and northeastern Laramie County at 1:39pm which was valid through 5:30pm. The warning was instigated by heavy rain indicated on Doppler radar. See full warning here.

 

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To capture this event and determine its rarity by using the state of the science POLARIS QPE, satellite data, quality controlled rain gauges, and innovative algorithms a MetStorm™ run was generated. MetStorm™ was run for a 48-hour period from July 21st at 8am through July 23rd at 8am.  MetStorm™ determined the maximum 1-hour rainfall to be an impressive 3.3″ and the maximum 24-hour raingfall to be 5.61″.

From a frequency perspective, this storm was statistically rare. MetStorm™'s Average Recurrence Interval (ARI) for the 1-hour max of 3.3" was 402.46 years, meaning that there is a 0.2% chance of receiving 3.3" in 1-hour in any given year at this location. The ARI for the 24-hour max of 5.61", on the other hand, has a return period of 728.92 years, or 0.1% of occurring on any given year. While the flooding produced by the rainfall of these storms was minor, it was still a very rare event to occur in this location. 

ARI_metstorm201527_1hr_max_ppt ARI_metstorm201527_24hr_max_ppt

Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at http://metstat.com/contact-us/.

Thanks for visiting our blog today!

-MetStat Team



Southeast Montana storm of June 5-6, 2015

By | Extreme General Storm Precipitation, Extreme Local Storm Precipitation, MetStorm, Uncategorized | No Comments

Conditions on Friday night into the early morning hours on Saturday were prime for thunderstorms to develop in eastern Montana. A north-south oriented trough coupled with a surface stationary front instigated these thunderstorms, which ended up producing heavy rain, hail, and strong winds in southeastern Montana (see Weather Prediction Center’s surface analysis map below). The Storm Prediction Center issued a severe thunderstorm watch over the eastern half of the state from 4pm-midnight, with the southeast corner of the state in a flood warning.

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MetStorm™’s 24-hour analysis estimated that an astounding 7.72″ of rain fell just to the east of Broadus, Montana. While there were no rain gauges in the storm center, Weather Decision Technologies’ (WDT) Polarimetric Identification System (POLARIS) Quantitative Precipitation Estimate (QPE) from mosaicked and quality controlled radar data (used in MetStorm™) along with the National Weather Service’s estimated precipitation indicate this amount was valid. Over 7 inches of precipitation in 24-hours is an exceptional amount of rain to fall in such a short time. We were left asking ourselves how rare this event actually was.

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MetStorm™’s Average Recurrence Interval (ARI) product can tell us just how uncommon this amount of precipitation was over this duration for this area. As of June 2015, MetStorm™’s maximum frequency is 1000-year, so if the maximum ARI for a storm is 1000.5 years, it indicates that the storm was over the 1000-year threshold MetStorm™ has. Below is the 24-hour ARI map for this storm. The storm cells to the east of Broadus and south of Powderville have over a 1000-year return period associated with them, meaning that they have less than 0.1% chance of occurring in any given year and that this storm was remarkably rare.

ARI_metstorm201518_24hr_max_ppt

Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at http://metstat.com/contact-us/.

Thanks for visiting our blog today!

-MetStat Team



Front Range, CO 3-hour Average Recurrence Interval for June 4-5, 2015

By | Extreme Local Storm Precipitation, MetStorm, Uncategorized | No Comments

The Front Range in Colorado has been very active this week with severe storms. In fact, there have been several severe thunderstorm, tornado, and flash flood watches and warnings issued since Wednesday of this week.

These storms were initiated from a surge of moisture and energy from remnants of Hurricane Andres meeting up with a shortwave trough and surface pressure system located over eastern Colorado (as seen in the WPC surface analysis below).

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The expedited moisture influx has resulted in very high rainfall rates and amounts for eastern Colorado and neighboring states. The mass curve plot below demonstrates just how intense this storm was, with nearly all of the precipitation falling within about 5-hours.

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From a climatological standpoint, this event has been determined to be fairly rare. In the Front Range of Colorado alone, the maximum 3-hour Average Recurrence Interval (ARI) for yesterday’s rainfall was around 164 years (see MetStorm’s ARI map below). In frequency terms, a storm of this magnitude for this area and duration has only a 0.61% chance of happening in any given year.ARI_metstorm201517_3hr_max_ppt

With more rain and thunderstorms in the forecast, the flood threat continues through the weekend. Make sure to check back to see any updates and more of our near real-time precipitation analyses from MetStorm!

Please note that the data presented here is preliminary and will be updated with final information as all data is available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at http://metstat.com/contact-us/.

Thanks for visiting our blog!

-MetStat Team



6-hour ARI for Rainfall that Produced Flash Flooding in Austin, TX and Surrounding Communities, May 23-25, 2015

By | Extreme Local Storm Precipitation, MetStorm, Uncategorized | No Comments

The previous week has been incredibly active for the state of Texas, including widespread flooding in Austin, TX and the surrounding area over Memorial Day Weekend.

The surface analysis at 00Z on May 24th (7 pm CDT on May, 23) shows very cold (high) cloud tops across nearly the entire state, with several squall lines embedded within, ahead of an advancing cold front on the Texas-New Mexico border.

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As seen by the mass curve for the storm center, located to the west between Austin and San Antonio, TX, a majority of the precipitation fell in a very short period of time starting during the evening on Saturday, May 23rd. Nearly 10.5 inches of rain fell over a 6-hour period from 2200 UTC on 5/23 to 03 UTC on 5/24 (5pm to 10 pm CDT)

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These high rainfall rates at the storm center are associated with a Maximum Average Recurrence Interval (ARI) of over 1000 years! This is to say that, statistically, an event of this magnitude has less than a 0.1% chance of occurring in any given year.  While total rainfall was not nearly as high over the metro areas of Austin and San Antonio, the city of Austin still has Max. ARI values nearing 50-years, which relates to a 2% chance of an event taking place any given year. It is important to note, however, that these ARI values are of the rainfall only and are not indicative of the recurrence interval with the associated flooding.

 

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Please note that the data presented here is preliminary and will be updated with final information as all data is available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at info@metstat.com or through our contacts page at http://metstat.com/contact-us/.

Thanks for visiting our blog today! Come back soon for updates to these latest MetStorm™ analyses and for more posts to come!

-MetStat Team

(edited: 6/3/2015 )