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Brian Crow

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.

Heavy Precipitation in the Northern Sierras Leads to Oroville Dam Crisis

By | Extreme General Storm Precipitation | No Comments

A dominant story in news headlines over the past few days has been the crisis at California’s Lake Oroville dam, the tallest earthen dam in the United States. Buoyed by weeks of heavy precipitation throughout much of central and northern California, the reservoir for the first time in its 49-year history exceeded its capacity and began to overflow into the emergency spillway (Figures 1 & 2). The temporary evacuation of some 200,000 people occurred on Sunday when engineers became alarmed by the rate of erosion at the lip of the emergency spillway. This came just days after standard releases from the dam began to cause serious erosion of the main spillway, necessitating the reduction of flows while engineers inspected the damage. While the erosion of the main spillway has apparently stabilized as of this writing, the emergency spillway remains potentially unstable if needed again before proper repairs can be made.

Figure 1. An aerial view of the severely eroded main spillway of Lake Oroville on Saturday, 11 Feb 2017. Water can also be seen running over the emergency spillway above and to the left of the main spillway. Image credit: Wikimedia Commons.

Figure 2. Lake Oroville water storage levels in 2016-2017 as compared to other historically wet and dry years. Image source: CA Dept. of Water Resources.

As indicated by the water storage graph in Figure 2, water levels have spiked sharply multiple times already this water year (water years run from October to September). These spikes are the result of a series of powerful low pressure systems that have produced copious amounts of precipitation over the Lake Oroville catchment area, particularly in the mountains. Since mid-December, there have been at least four major storm systems that produced widespread storm-total liquid precipitation totals in excess of 10″ (Figure 3). Above 6,000-8,000 feet in elevation, much of this moisture remains locked up as snow and will only gradually contribute to the flow of water into Lake Oroville. However, as evidenced by the time series below, taken at a point at just 3,300′ elevation, the extremely heavy precipitation has also occurred at lower, warmer altitudes, where almost all of it becomes immediate runoff.

Figure 3. A time series of analyzed daily precipitation values at a point in the mountains upstream of Lake Oroville. Image source: PRISM Explorer.

Based on this time series, MetStat decided to analyze four recent weeklong periods of precipitation in the Oroville drainage area: 10-16 December 2016, 5-11 January, 19-25 January, and 4-10 February. Gridded precipitation data was acquired from NOAA’s Multi-sensor Precipitation Estimates (MPE) and compared to precipitation frequency grids from NOAA’s Atlas 14 Vol. 6 to produce the Average Recurrence Interval (ARI) product. The ARI shows the approximate rarity of an event expressed in terms of the average amount of time that would be expected to pass between events of identical magnitude. In other words, an ARI value of 200 years would indicate a storm that would only occur once every 200 years on average. The images below show the storm total precipitation for each event on the left and the 7-day ARI on the right.

Note that while no individual storm is incredibly rare individually, the combined effects of a 1-2 year storm (storm 1), a 10-25 year storm (storm 2), a common but heavy storm (storm 3), and another 10-25 year storm (storm 4) all in succession has led to some truly staggering rainfall totals. This is illustrated most starkly in the 45-day precipitation totals (from 28 Dec to 10 Feb) and 45-day ARI analysis, which conveys the rarity of seeing this much precipitation in the course of a month and a half (Figure 5).

Figure 5. 45-day total precipitation (left) and and 45-day average recurrence interval (right) for the Lake Oroville drainage area.

The precipitation during this period was definitely impressive: over 20″ of liquid has fallen on all of the hilly and mountainous terrain in the region, and a substantial area upwards of 60″ of liquid is analyzed in the heart of the mountains! Such huge precipitation values are consistent with average return intervals of anywhere from 10 to 250 years in the Oroville region, underscoring the infrequent nature of such a persistently wet pattern in the Northern Sierras. It is worth reiterating that much of that high-elevation precipitation remains in the form of snow, but this highlights the issue at hand: Lake Oroville will be dealing with enormous amounts of water for months to come. In addition to gradual snowmelt over the next several months, the wet pattern looks to continue for the immediate future. The Weather Prediction Center is forecasting another 4-10″ of precipitation to fall over much of the basin in the 7 days to come (Figure 6; see also the WPC website).

Figure 6. A Google Earth view of the WPC’s 7-day precipitation forecast, valid from 15-21 Feb 2017.

With a near-record snowpack building in the Northern Sierra Mountains and another 6-12 weeks of California’s wet season still to come, the crisis at the Lake Oroville dam will likely be ongoing for weeks. Officials are confident that the spillways can be reinforced enough to survive the season, but another extended stormy period could still be a burden and potential threat to water infrastructure all throughout the Sierra Nevada. This event emphasizes the importance of the hydrometeorological expertise that MetStat provides to help design, engineer, and operate safe water infrastructure.

Data Sources:
HDSC Precipitation Frequency Grids
AHPS Precipitation Analysis

Images:
Aerial view of spillways
Lake Oroville storage
PRISM precipitation time series

Coastal Washington and Oregon Deluge Sets Up Region for Wettest October on Record

By | Extreme General Storm Precipitation | No Comments

Earlier this month (October 2016), parts of the coastal Pacific Northwest faced nearly a week of heavy rains and high winds as a series of storms crashed ashore. An intense Pacific jet stream, amplified by interaction with the remnants of Typhoon Songda, carried copious amounts of moisture into the coastal mountains of Washington and Oregon, with the most intense rainfall occurring October 13th-17th. Forecasts of widespread hurricane-force wind gusts failed to materialize, but localized wind gusts were nonetheless impressive: peak wind gusts were 70 mph near Honey Lake, CA, 89 mph near Incline Village, NV, 94 mph near Megler, WA, and 103 mph near Oceanside, OR. In addition, two rare tornadoes managed to touch down in the coastal Oregon cities of Manzanita and Oceanside, with substantial structure and tree damage reported in Manzanita.

manzanita_tornado_track

Noteworthy though the winds may have been, the widespread heavy rainfall was the most impactful aspect of these storms. Our preliminary MetStorm analysis of regional rainfall shows just how widespread the heavy rainfall was, particularly in mountainous areas. Many high-elevation locations recorded in excess of 6″ of rain during this 4-day period, with our analysis indicating a local maximum of up to 20″ in the Olympic Mountains of northwestern Washington.

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Even in lower-lying areas between mountain ranges, including such cities as Portland, OR and Seattle, WA, rainfall was substantial enough to push several areas towards their wettest Octobers on record. The rainy season is usually just ramping up through October in the Northwest, so several-inch lowland rainfall events in October are not particularly common, although they wouldn’t be out of place later in the season. The majority of the area received rainfall consistent with what one would expect to occur at least once a year. However, isolated pockets in the mountains and in the Puget Sound region saw much rarer heavy rains, consistent with recurrence intervals in the 10- to 1000-year range. This is particularly impressive when considering that the heaviest rains in these areas generally tend to occur between November and April.

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MetStorm’s mass curve analysis illustrates how this multi-day event unfolded. The point of heaviest rainfall in our analysis is isolated, and two fields from that location are plotted below: hourly rainfall values throughout the event (shaded), and cumulative rainfall since the beginning of the event. Several distinct low pressure systems, including one associated with the remnants of what had been Super Typhoon Songda in the west Pacific Ocean several days earlier, brought distinct waves of rainfall to the region. While hourly values never exceeded 1″, the persistence of moderate to heavy rain over such a lengthy period added up–to over 20″ in this case.

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Flooding fortunately proved to be minimal with this event, but many areas in the Northwest are now dealing with very saturated ground heading into what forecasters expect could be a wetter-than-average winter. If rainfall events of this magnitude continue to occur this winter, more severe flooding and mudslides are certainly a possibility.

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