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From Hurricanes to Thunderstorms: Louisiana’s Storms in Perspective.

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

Louisiana is no stranger to natural disasters. From droughts to flooding, since the year 2000 Louisiana has endured 8 natural disasters with over $200 million in estimated economic impact, and 22 FEMA major disaster declarations. To no surprise, the most famous, costliest, and deadliest natural disaster to hit Louisiana was 2005’s Hurricane Katrina. The most expensive hurricane to ever hit the United States, Katrina resulted in 1,833 deaths and economic impacts around $150 billion. At the time of landfall, Hurricane Katrina was a category 3 hurricane with sustained hurricane-force winds extending 120 miles out from its center. Aided by heavy wind and rain, the massive storm surge created by Katrina breached multiple levees and left large swaths of New Orleans underwater within moments of the initial breaches. With that in mind, it is understandable just how extensive the sheer amount of damage was.

Given these impressive stats, it’s hard to imagine another non-hurricane natural disaster that could even come close to having such an impact within the state of Louisiana. Earlier this month, a much less exciting weather phenomena – in the form of a broad area of low pressure – settled in over the American Southeast. This allowed consistent thunderstorm development from August 11th to the 14th. This slow tide of steady rainfall dropped well over 20 inches of rain throughout Louisiana (compare this to the roughly 10″ totals from Katrina) and ultimately lead 13 deaths and has left tens of thousands homeless. In what has been called the worst disaster since Hurricane Sandy, the onslaught of thunderstorm rainfall created flooding virtually unheard of even within a state that by some measures is the wettest state in the country.

aftermath
Aftermath of Hurricane Katrina (left), compared with recent flooding near Baton Rouge (right).

How did a low pressure system, spinning up thunderstorms that are seemingly mere ordinary afternoon storms, dump 2 to 3 times as much rainfall as Katrina in only a matter of days? The answer lies in its organization. A mesoscale convective system, or MCS, is an intricate structure of thunderstorms that allows each individual storm to become part of a system larger than itself. This organization can take many forms, and usually means that the system as a whole is large and long-lived. Check out the radar reflectively loop over the southern Mississippi Valley in the days of heavy rainfall:

radar

This organization of storms rotated around itself and continually dropped rainfall in both the Baton Rouge and Lafayette areas. Below is a plot generated by MetStorm showing the total rainfall over the 96-hour lifespan of the MCS.

P_allsites_2016081206_googlemap

A mass curve plot was also generated by MetStorm, for the area that received the highest total rainfall within the analysis time. Note that for multiple hours across the first two days of the storm event rainfall values exceeded 1″ per hour, and that even after the largest storms had passed, the area still received steady rain for almost another 48 hours.

mass_curve_30.5587-90.9537_2016081206_Zone1

Of course, flooding is not only apparent in precipitation data, but in river gauges as well. The first plot below shows river height in feet of the Mississippi River near Baton Rouge. Over the course of about 24 hours, the Mississippi rose roughly five feet. For comparison, levee breaches and heavy rain during Katrina rose the Mississippi river at New Orleans by nearly 16 feet in less than 12 hours. Smaller rivers, like the Comite River also plotted below, were subject to the largest increases in river height. In the same 24 hour span in which the Mississippi increased, the Comite River rose from just a foot or two to over 25 feet, shattering the previous gauge height record set in 1961 by over a foot. The large increase in river heights also correspond to the hours with the largest amounts of precipitation, seen in the mass curve plot above.

gauge2
gauge

Finally, in assessing the rareness of this flooding event, we calculated the average recurrence interval of the maximum amount of rainfall at each grid point for both 1- and 24-hours. Diagnosing the maximum ARI value over a 1-hour time span reveals a maximum grid cell value of 94.81 years (i.e. the one hour rainfall maximum has a ~1 in 95 chance of occurring in a given year). While rare, this value is not exceptional in terms of causing such an extreme flooding event in Lousiana. However, paired with the 24-hour ARI analysis, you’ll notice the number of areas that had 24-hour rainfall totals so high that they would only be expected less than once every 1000 years. A single thunderstorm (usually producing rainfall in a fixed location for less than an hour) did not make this event what it was, but rather the large MCS that organized thunderstorms to produce lasting, steady rainfall for days on end in the same locations in the state of Louisiana.

ARI_metstorm2016081206_1hr_max_ppt

ARI_metstorm2016081206_24hr_max_ppt

The aftermath of the Louisiana floods have given pause to many residents in the state, and determining how to rebuild after another major flood will be a difficult challenge. In the weeks to come, there will likely be a lot of tropical storm activity in the Atlantic, and we hope that Lousiana is spared from any major tropical storm that finds its way into the Gulf Coast.

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.

CnuLXZRUAAAbkHd
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.

Rainfall

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



Extreme Rainfall in Texas and Oklahoma

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

More heavy rain fell in the south plains last weekend, continuing a rather long cycle of flooding and dangerous storms across the southern plains over the past couple of months (take a look at our previous MetStorm analyses for April storms in Texas, as well as this excellent NASA write-up of widespread rainfall in Texas and Oklahoma from late-May to early-June). Radar composite imagery for Texas and Oklahoma over the course of June 12-13th is shown below.

radar
radar images via http://www2.mmm.ucar.edu/imagearchive/

These storms were oftentimes slow-moving, especially in Oklahoma, and frequently went through dissipating and re-development stages. South of the Dallas/Fort Worth radar station (you can easily spot this radar station as the black dot in the center of the circular area of radar “clutter” in the north-Texas region near Dallas/Fort Worth), observe the line of thunderstorms that seemingly remain stationary from about 6 to 11 UTC (1 to 6 am central time) the morning of the 13th. This area experienced what is known as “training” in meteorology, in which thunderstorms consistently develop in the same area and then move in a similar direction as they mature and eventually dissipate. Areas underneath training thunderstorms thus see significant amounts of precipitation, often in a relatively short amount of time, compared to nearby areas. The implications of this training event are discussed below.

Storm rainfall totals for both Texas and Oklahoma exceeded 10 inches over the course of these two days, as shown below in the MetStorm Storm Total Precipitation map. The two regions that experienced the most amount of rainfall were the areas over and just east of Lawton, Oklahoma, and south of the Dallas/Fort Worth metropolitan area in northern Texas. On top of the previous south plains storms already mentioned, this large amount of rainfall over a two-day time span spelled disaster for homes and infrastructure, particularly for Lawton, where homes needed to be evacuated and extensive road closures occurred throughout the area. An eight-mile stretch of I-45 south of Dallas was closed Monday morning due to storm waters. Check out the MetStorm map to see that one of the core areas of precipitation in Texas fell directly over I-45: the area under the training thunderstorms mentioned above.

P_allsites_2016061106_googlemap

Situations like these are often difficult to forecast and are a complicated entity from a disaster management perspective. Below are MetStorm mass curve plots of incremental and accumulated precipitation plotted for the storm centers in both Oklahoma near Lawton (above) and south of Dallas along I-45 (below). The vast majority of the rainfall near Lawton fell approximately 12 hours before the rainfall in Texas occurred, each storm system producing the most rainfall at roughly 10 UTC, or 5am central time, on their respective days. Overnight and early morning flooding events such as these are quite dangerous, as they usually catch communities at their most vulnerable times, and similar events in the south plains this year have resulted in numerous deaths.

mass_curve_34.8123-97.3927_2016061106_Zone2

mass_curve_31.8663-96.3762_2016061106_Zone1

The heavy rainfall is also reflected in river flow and discharge data acquired from the USGS National Water Information System for nearby river basins. As an example, below is a time series of water discharge along the Neches River near Neches, Texas, from mid-May to present. Observe that in the hours overnight from the 12-13th of June the rate of water discharge surged from 2,000 cubic feet per second to about 5,500 cubic feet per second. During this very short span of time, the river height rose nearly 3 feet from 13 to 16 feet. Also note past surges in water discharge in late May/early June associated with other heavy rainfall events in the area.

USGS.08032000.07.00060..20160517.20160617.log.0..pres
data via http://waterdata.usgs.gov/tx/nwis/uv?site_no=08032000&format=gif&period=31

The final MetStorm product for this storm event is a determination of the relative rareness of a rainfall event such as this. This is accomplished through the calculation of an Average Recurrence Interval, or ARI. Simply, the ARI is the probability of the occurrence of the total recorded rainfall amount over a specified duration in any given year. Here we have plotted 6-hour ARI values over our area of interest. In Oklahoma, near Lawton as well as south of Norman near I-35, 6-hour ARI values exceeded 500-year occurrence. And in Texas south of Dallas along the I-45 corridor, the maximum ARI was over 1000-years. In other words, this stretch of I-45 saw heavy rainfall over a 6-hour duration that was so large that the probability of its occurrence in any given year is only one in one thousand.

ARI_metstorm2016061106_6hr_max_ppt

Sunnier and drier days are in the forecast for the southern plains for the days to come, as is most of the rest of the continental United States as a large upper-level ridge settles itself in for the long-run. With June halfway over we’re entering the thick of summer, which will likely be a welcome change of pace after what was an unusually eventful spring and early summer.

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 email us or send us a message though our contacts page here.

-MetStat Team


Houston Storm of April 2016

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

A slow moving frontal system resulted in heavy rains and catastrophic flooding in Texas and Louisiana last week. A north-south oriented front stalled over central Texas, continuously siphoning warm moist gulf air northwestward to the Gulf coast states. This moist air and the instability from the front provided the necessary ingredients for this wet event.

sfc_analysis
streamlines

A 48-hour MetStorm® analysis was performed to provide the public with a high resolution and quality analysis of this major event. The following is the total storm map and mass curve plot detailing the spatial and temporal extent of this storm.

P_allsites_2016041806_googlemap

mass_curve_29.8612-96.0337_2016041806_Zone1

Over 17 inches of rain in less than 48 hours is an incredible amount of precipitation in a short period of time. A recent report from the Houston/Galveston National Weather Service outlined several new records from this event:

“…APRIL 2016 IS NOW THE WETTEST APRIL ON RECORD FOR THE CITY OF HOUSTON (KIAH). THIS
TOTAL WILL LIKELY GROW BY THE END OF THE MONTH. THE RAINFALL ON THE 18TH WAS ALSO
THE SECOND WETTEST DAY ON RECORD FOR THE CITY…TRAILING ONLY THE 10.34 INCHES OF
RAIN THAT FELL ON JUNE 26TH 1989. THE CITY OF KATY HAS ALSO ESTABLISHED A NEW
MONTHLY RAINFALL RECORD BESTING THE PREVIOUS RECORD OF 12.22 INCHES ESTABLISHED
DURING APRIL 2009. SUGAR LAND ALSO SET A NEW MONTHLY RAINFALL RECORD BEATING LAST
YEARS RAINFALL RECORD OF 9.76 INCHES…”

Another way to put this event into perspective is to determine the frequency of occurrence. This is done by calculating the average recurrence interval (ARI) or the return period of the precipitation that fell. An ARI was computed over the maximum 6-hour period for every grid cell in the storm domain. The following map provides the spatial extent and magnitude of this storm:

ARI_metstorm2016041806_6hr_max_ppt

There are multiple locations west of Houston that had ARI values of over 1000 years; meaning the frequency of a storm of this magnitude in a 6-hour period in this location has less than a 0.1% chance of occurring in any given year. From a frequency perspective, this rain event proved to be a very rare case.

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 email us or send us a message though our contacts page here.

Check in soon for more exciting news about MetStorm!

-MetStat team



Heavy rainfall and potential flooding for the Lower Mississippi River Valley

By | Extreme General Storm Precipitation, Uncategorized | No Comments

MetStat is watching the approaching major rainfall event for the Lower Mississippi River Valley. As of last night, the Weather Prediction Center 7-day forecast (left panel) called for totals in excess of 12 inches in parts of Louisiana and more than 6 inches over a wide swath of Arkansas , Louisiana, and Texas. The Average Recurrence Interval (right panel) for 7-day rainfall totals generally range in the 25 to 50 year range with a maximum of 53 years over northwestern Louisiana. The predicted long-duration heavy rains could result in flooding concerns in the region. MetStat will provide a preliminary post-storm MetStorm analysis for this event if forecasts verify. You can monitor official National Weather Service forecasts for precipitation (http://www.wpc.ncep.noaa.gov) and river stages/flows (http://www.weather.gov/lmrfc) in the upcoming days.

7dayQPFandARI_WPC20160307_00Z

Update 03/16/2016

MetStat has run a 72-hour MetStorm analysis on this event to provide the weather community with a high-detailed depiction of the storm precipitation. The models captured the spatial pattern quite well as can be seen in the output from MetStorm; although the exact location and magnitude were slightly askew.

P_allsites_2016031006_googlemap

MetStorm, equipped with hourly and daily rain gauges, dual-pol QPE, and satellite data, estimated a maximum of 23.64″ fell near Oak Ridge, LA. This amount of rain in this period of time equates to over a 1000-year average recurrence interval, meaning a storm of this magnitude has less than a 0.1% chance of occurring in any given year. An excerpt from the NWS storm summary number 13 published on 3/11 describes the synoptic pattern that produced this extreme weather:

“…AT 800 AM CST…AN ANOMALOUSLY LARGE AND DEEP UPPER-LEVEL LOW
LOCATED OVER SOUTH TEXAS WAS SLOWLY MOVING NORTHWARD AS
SIGNIFICANT MOISTURE AND UNSTABLE AIR STREAMED NORTHWARD FROM THE
GULF OF MEXICO INTO THE WESTERN GULF COAST REGION. AN AREA OF LOW
PRESSURE…WITH A CENTRAL PRESSURE OF 1011 MB…29.85 INCHES…WAS
LOCATED ABOUT 90 MILES WEST-SOUTHWEST OF CORPUS CHRISTI…TEXAS.
AN OCCLUSION EXTENDED FROM THIS LOW ACROSS SOUTHEAST TEXAS WITH A
STATIONARY FRONT DRAPED ACROSS THE LOWER MISSISSIPPI VALLEY AND
INTO THE TENNESSEE VALLEY. THERE WAS ALSO A COLD FRONT MOVING
THROUGH THE CENTRAL GULF OF MEXICO. NATIONAL WEATHER SERVICE
RADARS AND SURFACE OBSERVATIONS INDICATED THAT THE HEAVIEST
RAINFALL AND STRONG THUNDERSTORMS WERE FALLING ACROSS SOUTHEASTERN
LOUISIANA…SOUTHERN MISSISSIPPI…AND SOUTHWEST ALABAMA. MODERATE
TO HEAVY RAINFALL WAS ALSO FALLING ACROSS NORTHERN
MISSISSIPPI…NORTHEASTERN LOUISIANA…AND SOUTHERN ARKANSAS.
LIGHT TO MODERATE RAIN WAS REPORTED IN THE VICINITY OF THE
OCCLUSION IN WEST CENTRAL TEXAS…”

For more information or to request data from this storm, please contact us at media@metstat.com, or through our contacts page at here.

-MetStat Team



Hurricane Patricia remnants cause extreme rains in Texas and Louisiana

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

Hurricane Patricia was a force to be reckoned with not only in its record breaking intensity by central pressure, but also by its impressive rainfall in the Gulf coast states. If the minimum pressure recorded by NOAA’s Hurricane Hunter aircraft of 879 hPa verifies, it will beat the Hurricane Wilma for the most intense tropical cyclone by central pressure in the Western Hemisphere. Another astonishing fact about Hurricane Patricia was its impressively quick transition from tropical storm to category 5 hurricane in 24-hours. Winds recorded on October 22nd at 3UTC (9pm MDT) were at 65mph, which classified it as a tropical storm. One day later on October 23rd at 3UTC (9pm MDT) winds had reached 160mph, classifying it as a category 5 Hurricane. In the same 24-hour period the central pressure had decreased from 994 hPa to 924 hPa, making an impressive 70 hPa drop in pressure.

As the system passed over the Sierra Madre mountains in Mexico, it weakened rapidly and dropped below the wind requirements for a tropical depression before it arrived on the east coast of Mexico. As the remnants continued east-northeast into the Gulf of Mexico, the warm moist air was advected over the Gulf coast states. This moisture fueled the large rainfall totals experienced in Texas and Louisiana over the weekend. In a preliminary MetStorm analysis from October 22nd at 12Z (6am MDT) through October 26th at 12Z (6am MDT) the maximum estimated precipitation was 22.5″ over the location south-southeast of Dallas, Texas. Other precipitation estimates for this storm near North Houston were around 10″ and around Baton Rouge were a little over 9″. The bulk of the precipitation, as shown by the mass curve plot below, at the storm center near south-southeast Dallas occurred in a short period of time, with showers trailing over the next day and a half.

P_allsites_20151023410001_googlemap

mass_curve_31.9962-96.4838_20151023410001_Zone1

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




 

1000-year Rains in South Carolina, October 2015

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

Here at MetStat, we are continuously monitoring the situation as Hurricane Joaquin continues on its path northward. As the impacts from the precipitation of this event unfold we will be regularly updating our blog, facebook, and twitter pages with the latest information available, so check back often. Our Quantitative Precipitation Forecast (QPF) product, initialized from the 15Z (11 EDT) Weather Research and Forecasting (WRF) model run, shows significant precipitation forecasted to impact South Carolina in a 24-hour period ending at 3Z (23 EDT) this Sunday, October 4th. Shown below following the QPF map, is the Extreme Precipitation Index map, which depicts the rarity of the forecasted 24-hour precipitation ending at 3Z (23 EDT) October 4th. If the current forecast verifies, this event will be extremely rare, with a return period at over 1000-years (in other words, this event has less than a 0.1% chance of occurring in any given year). A similar QPF map of ours was featured in Dr. Jeff Masters blog about this storm. Dr. Masters post has great commentary on the meteorology and mechanisms involved in the set up and forecast of Joaquin. For additional information, visit his blog post here.

ppt_qpfconus_1500_f03600_024h

ari_qpfconus_1500_f03600_024h

If you want to monitor the rarity of the latest 6- and 24-hour precipitation in real time, there are also live Extreme Precipitation Index analysis maps available here.

As precipitation intensifies in the Carolinas and along the east coast we will be initiating MetStorm runs, in near-real time, to keep a pulse on the amount and rarity of precipitation falling, so make sure to check back often.

Update 10/9/2015

MetStat, Inc. is proud to share the most comprehensive rainfall and frequency analysis of the October 1-5, 2015 storm that caused catastrophic flooding in South Carolina. As forecasted, an “atmospheric river” of deep tropical moisture emanating west from Hurricane Joaquin interacted with a frontal boundary and a strong upper-level low parked over the Southeast. This produced a NW-to-SE band of extremely heavy rain that stretched across much of central South Carolina and caused 1000+ year rains. MetStat’s analysis was conducted using MetStorm™, a new, state-of-the-science hydrometeorological tool for characterizing storm precipitation.  This analysis utilized dual-polarization radar mosaics from Weather Decision Technologies, satellite estimated rainfall from NOAA, 845 quality-controlled rain gauge measurements (hourly and daily) from Synoptic Data Corp and NOAA, and MetStorm’s innovative algorithms to produce high-resolution, 5-minute rainfall grids/maps.  Below is a preliminary 96-hour rainfall map and corresponding Average Recurrence Interval (ARI) map. The maximum measured 96-hour rainfall was 26.88 inches at CoCoRaHS gauge MOUNT PLEASANT 6.4 NE, SC, while the highest derived rainfall was 30.68 inches along the central coast of South Carolina, just northeast of Charleston. The 96-hour ARI based on the MetStorm™ results and official precipitation frequency data from NOAA, exceeded 1,000-years (0.1% chance of occurring in any given year) across large areas of South Carolina.

Please email us at info@metstat.com or through our contacts page at http://metstat.com/contact-us/ for more information and other analytics (e.g. Depth-Area-Duration (DAD) plots/tables) available for this storm analysis.

-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.

ussatsfc2015091503

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.

AZtopo

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.

NWSwarnings

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 and Temporal Distribution of the Northwestern Iowa storm of August 16-17, 2015

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

In mid-August, a cold front moving through the Midwest stalled and transitioned to a stationary front, causing significant rainfall and subsequent flooding to occur. The cold front can be seen in the 00 Z (7pm CDT) Weather Prediction Center’s (WPC) satellite overlayed surface analysis. Another prevalent feature in the Midwest at this time were squall lines centered parallel and to the east of the cold front. The lines of slow moving thunderstorms were the driving factor for this event, as they dropped heavy rain over the same areas for an extended period of time.
ussatsfc2015081700

The National Weather Service’s (NWS) local storm report, shown below, depicts all reports received and all watches/warnings issued with a radar overlay of the rainband that caused this event. Local NWS offices issued several flash flood and flood warnings in the area and received dozens of local storm reports of heavy rain and a few reports of floods spanning from the 16th through the 17th. One report had just under 6″ of rain fall in under a 5 hour period from 6pm CDT through 10:30pm on the 16th.

NWSchat

 

The 48-hour maximum precipitation for this event, based on the MetStorm™ analysis, was 8.56″. There were Two major areas of heavy precipitation, one located north-northwest of Fort Dodge, IA and the other located at, and to the northeast of Sioux City, IA.

P_allsites_201529_googlemap

For this event, an ARI for a 6-hour duration yielded a maximum of ~151 years. In frequency terms this means a 6-hour maximum precipitation of this magnitude in this area has a 0.6% chance of occurring in any given year.

ARI_metstorm201529_6hr_max_ppt

Looking at the temporal distribution using the mass curve plot at the center of the storm, there were two distinct periods of intense precipitation. The first occurred at about 2UTC on the 17th (9pm CDT on the 16th) and the second at about 16UTC on the 17th (11am CDT on the 17th).

mass_curve_43.0938-95.4737_201529_Zone1

Overall, this storm which dropped a maximum of 8.56″ of rain in two major pulses in a 48-hour period was determined to be a rare event for northwestern Iowa with a ~151-year ARI. The resulting flood warnings and reports exemplify the consequences of a storm of this magnitude occurring in 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 our blog today!

-MetStat Team



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.
Surface Analysis 20150721

Sounding_2015072112Streamlines_2015072112

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.

 

Flood_warning_20150721

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