A Year of Tropical Cyclone Precipitation Records (Imagery Credit: Meteorologist Dakota Smith)10/4/2018 
DISCUSSION: Over the past year and counting, there is no debate that a good portion of both the contiguous United States as well as Hawaii (even as disconnected from the United States mainland as it is), has not had any shortage of tropical cyclone activity. On that note, it is important to acknowledge the fact that with a good portion of this tropical cyclone activity also came with the shattering of state rainfall records in North Carolina, South Carolina, Texas, and Hawaii. During the past 12 months, the world watched as Hurricane Harvey impacted southeast Texas, Hurricane Lane impacted parts of southern and central Hawaii, as well as Hurricane Florence which severely impacted North Carolina and South Carolina.
It goes without saying that with four states breaking tropical cyclone event rainfall records within the time-span of 12 months there are plenty of legitimate questions which are raised throughout the community. First and foremost, whether future climate change will make such events more and more common with time. The simple answer to this question is the fact that someone cannot pin any given event on various impacts of current and future climate change. Therefore, to attribute that some given percentage of a tropical cyclone’s rainfall is directly connected to climate change would be a premature statement since any one event may have be impacted in a subtle manner from warmer-than-average sea-surface temperatures and other factors such as favorable mid/upper-level jet stream dynamical support. Thus, there are manners by which a tropical cyclone can be positively influenced with respect to an increasing intensity feedback loop but there should never be an assumption for there to be a direct connection between tropical cyclones rainfall records and climate change. To learn more about other flooding events and/or flooding topics from around the world, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz
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Historic Flooding Event from Hurricane Florence That Seems Never Ending. (Image Credit: USA Today)9/14/2018  DISCUSSION: There is no debate that the biggest ticket issue during both the pre-landfall, landfall, and post-landfall phases of Hurricane Florence, then Tropical Storm Florence, and now Tropical Depression Florence was the persistent flooding event which unfolded. It is worth noting that many towns and cities across North Carolina and South Carolina got so much rain, that they broke all-time 24-hour rainfall records as well as rainfall records from a given tropical cyclone event. Thus, it goes without saying that the flooding impacts from Hurricane Florence across this part of the Mid-Atlantic region of the United States will go down as one of the worst flooding events from a tropical cyclone landfall in recorded history for the United States.
The worst part about this ongoing flooding situation is precisely the fact that it is still going on right now with more rainfall still coming down heavily across a good portion of North Carolina, Virginia, West Virginia, and beyond right at this very moment. Hence, this flooding still far from over with more rainfall on the way. Attached above is a brief video briefing which details a good portion of why this ongoing flooding event from what is now Tropical Depression Florence is indeed so historic in the context of recorded history with respect to both general flooding events as well as tropical cyclone-based flooding events. To learn more about other high-impact flooding events from around the world, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz  Water begins to cover the Skater’s Cabin walkway to Mendenhall Lake Thursday morning July 19, after reports that Suicide Basin to the side of Mendenhall Glacier had released. Mendenhall Glacier in the background (Sharon Sullivan) The jökulhlaup has struck again. Jökulhlaup (“yo-kel-yawp”) comes from the Icelandic word meaning “glacial run”. This unique annual flooding event in Southeast Alaska results from a glacial dam outburst from Suicide Basin, located about 2 miles up from the eastern side of the Mendenhall Glacier. The basin is like a bowl with an opening on the bottom. The exit is normally sealed with ice, allowing rainwater and glacial melt to collect during the spring and summer months. Once the weight of the water becomes too great for the icy seal, it is lets loose and sends a surge of water through the drainage system below the glacier into the Mendenhall Lake. Floods can occur all at once or have multiple releases. The first jökulhlaup occurred in 2011, with 2016 being the largest.  (NWS Juneau) An abrupt change in the basin’s water level near the end of June 2018 indicated that Suicide Basin was beginning to drain. Upon further examination, a calving event with a chunk of ice 1/5 of a mile wide disrupted the cycle by splashing into the basin and displacing the sensor, similar to dropping an ice cube into a glass of water. The calving event seemed to strengthen the dam holding the water in place, but a flood watch was issued as a precaution. By July 17, the water level in Suicide Basin had dropped overnight, but there was no response from Mendenhall Lake. Waters began responding by rising accordingly for the lake and the Mendenhall River on Wednesday, July 18 as glacial water flowed overtop the wall of ice surrounding the Mendenhall Glacier. By 10 am, a flood watch was in effect for the Mendenhall Lake area, with an expected 12 foot crest by Thursday evening and the potential for record flooding. At 8 am on July 19th, a flood warning was issued by NWS Juneau when the current stage level reached 8.99 ft. The flood stage for Mendenhall Lake is 9 feet.  Major flood stage of Mendenhall Lake occurs at 14 ft. The highest recorded water levels on the lake occurred in 2016, when the jökulhlaup caused the lake to reach heights of 11.99 feet (Alaska- Pacific River Forecast Center) The crest occurred at 4:30 pm at 10.92 feet, tying for the 3rd highest lake level (11.99 feet in 2016), which was lower than the crest that was predicted. Significant flooding was reported along view drive and water inundated the Mendenhall Campground. According to the NWS Hydrologic Prediction Center, water levels begin dropping at a rate of about 3.5 inches per hour. The Mendenhall Campground, which had been evacuated the previous day was back open as of 9 am Friday. As of 11 am Friday, people were once again setting up their tents and enjoying the sunshine. It is difficult to predict when the jökulhlaup will happen during the summer and if significant flooding will occur, but it is important that people are aware of the threat and prepare accordingly. In any flooding event, residents are advised to keep an emergency bag with important documents, dry foods, tools, and medication to take on the go. If you see water on the roadways, do not proceed to drive through it, as water may be deeper/ faster than you estimate. And, in the case of a jökulhlaup event, glacial waters can reach below freezing so hypothermia may become a threat. The response to the flooding went generally well due to a collaborative effort between the National Weather Service office in Juneau, the River Forecast Center, the Forest Service to evacuate the campgrounds, USGS river gauges, and the Department of Transportation’s thorough inspections on important bridges and roadways every year. Dynamics of a glacier in a changing climate, unknowns in the basin water levels, and small sample sizes may lead to varying crest heights and timing, but significant progress has been made since 2011 to better understand, forecast, and monitor this phenomena. (Follow-up: On August 8th, the Mendenhall Lake and River were once again under Flash Flood Warnings as a significant heavy rain event over the northern panhandle and some additional contributions from Suicide Basin increased waters past flood stage). To learn more about other flooding events, please click here! ©2018 Meteorologist Sharon Sullivan  “The Flooded Stairway to Nowhere” (Sharon Sullivan)
 In the last two weeks, Japan has been inundated with rainfall totaling over 70 inches in areas over the extreme southwestern part of the country. This has been cited as one of worst flooding disasters to ever occur there. The flooding rains have lead to numerous landslides as well as rivers overflowing their banks which has caused immense damage to cities and villages. The death toll has risen to 200 and continues to rise by the day, with another 54 people that remain unaccounted for. The Hiroshima and Okayama Prefecture areas took the brunt of the storms, with rainfall rates exceeding three inches per hour at times. Japan’s Shikoku Island was also hit hard. The Associated Press reported that 10.4 inches of rain accumulated in its Kochi Prefecture in just three hours, with more than 70 inches of rain totaling at the end of the storm system. The World Meteorological Organization reports that the total precipitation at many of the observation sites reached two to upwards of four times the annual mean which also happens to be the monthly precipitation for the month of July in southwestern Japan.
Japan is no stranger to natural disasters, with the catastrophic tsunami in 2011 that caused the meltdown at the Fukushima Daiichi Nuclear Power Plant. The scenes of the flooded landscapes with rooftops just barely visible this past week eerily resembles the aftermath of the 2011 tsunami. In the case of this storm, the torrential rainfall can be blamed on a combination of events occurring. Tropical moisture was pulled north, colliding with the remnants of Typhoon Prapiroon as well as colliding with a front stalled over Japan. While uncommon to occur, the atmospheric conditions were just right in this case, creating multiple storms behind the parent thunderstorms. This process had the storms lining up and inundating the same region over and over, causing the massive landslides and flooding to occur instantaneously. While natural disasters are bound to happen throughout the world even without climate change existing, the continuous acknowledgment as well as educating others on this matter will really make a difference on tackling these problems in the world. For more about flooding and other applied meteorology topics, please click here! ©2018 Weather Forecaster Michael Ames If you’re like me and enjoyed the past few days of low humidity with plentiful sunshine in the northern Mid-Atlantic, you will not like this 2-week outlook. The Climate Prediction Center has issued their 6-10 (below) and 8-14 day precipitation probability outlook through the end of July into the beginning of August. It’s anything but sunshine and low humidity, but let’s dive into the setup and why the forecast calls for above normal precipitation along the East Coast.  The shortwave that is diving into the Ohio Valley shown below which was the lifting mechanism for the severe weather outbreak across Kentucky this afternoon and evening (July 20th) will be one of the keys to this upcoming wet forecast along the East Coast. This animation from GOES-16 Water Vapor shows this interaction between the shortwave moving into the Ohio Valley igniting the severe storms in Kentucky/Indiana, (satellite animation credited to College of DuPage).  Looking at the 500 mb pattern this weekend (July 21-22) an almost wintertime storm pattern develops. There is a substantial cut-off low pressure across the Ohio Valley, the same system that produced the severe weather mentioned above. The two ridges, one to the west across the High Plains region as well as the Bermuda high stretching and strengthening into portions of eastern Canada, keep will keep this pattern locked in.  The position of the cut-off low sits around the Ohio Valley will allow for a low pressure to develop and rotate around the counterclockwise circulation and up the East Coast. Shown below is an animation from the GFS depicting the coastal low’s location and intensity with the precipitation overlaid, courtesy of Tropical Tidbits. Notice the upper level low’s influence on the track of the coastal low in this animation.  As the shortwave drifts to the south while the high pressure continues to stay strong to the east, a stationary front develops and sits over the area Monday morning through mid-week (WPC photo below #1). Also notice the increase in moisture distinguished by the dew points surging back into the 70+ degree mark as Gulf of Mexico and Atlantic moisture is driven up the coast into early next week (photo below #2). With all the moisture in the area, CAPE values near 1000 J/kg in some places, the stationary front as the lifting mechanism along with upper level support from a jet streak to the north and northwest (jet stream photo below #3), periods of rain and thunderstorms will develop throughout much of next week. 
Just to give a perspective on how much rainfall is possible, WPC paints a healthy amount of rain, up to 5 inches in areas from the interior Northeast down through portions of the Mid-Atlantic.  A wet and soggy weekend into next week is on tap for many along the East Coast especially from the Mid-Atlantic to the Northeast. Always be sure to look out for flooding and know the safety and precautions that come with it (click here to review flooding safety protocols from the NWS).
Stay tuned to GWCC and click here to view other potential flooding events! ©2018 Meteorologist Joe DeLizio Understanding What a 1,000-year Flood Means: The Ellicott City Tale (credit: NOAA NWS Baltimore, MD)6/16/2018  DISCUSSION: On Sunday, May 27th, 2018 Ellicott City in Maryland experienced a devastating flood that tore through the heart of the town. Cars were washed away by what looked like a raging river as water rushed down main street. The water rose fast and high in some locations above the first story of houses. When all was said and done, an estimated eight inches of rain fell over the city in less than 12 hours, with the majority of the rain falling over a three-hour time span. Analysis of the flood determined that Ellicott City was hit with a 1,000-year flood. However, this was not the first 1,000-year flood to hit the city. On July 30, 2016, Ellicott City experienced a similar flooding event, with approximately six inches of rain falling over a two-hour period. Like the most recent event, the 2016 flood left the city in ruins.
But how is it possible to have two 1,000-year floods less than two-years apart from each other? The answer is in what a 1,000-year flood actually means. The term itself is misleading, suggesting that such a flood should happen only once every 1000 years. However, 1,000-year flood represents a hydrological probability defining the likelihood or rarity of a flood occurring. A 1,000-year refers to a 1 in 1000 chance or a 0.1% chance of the flooding event occurring any given year. Similarly, a 100-year flood has a 1 in 100 chance or a 1% chance of occurring each year while a 10-year flood has a 1 in 10 chance or a 10% chance of occurring each year. The U.S. Geological Survey, also known as USGS, refer to this probability as the one percent Annual Exceedance Probability, or the AEP. Along rivers, gauges measure the height of the water and the quality of the waters’ flow across America. The information from the gauges is then statistically analyzed for any given location and the flood probabilities are determined. However, other factors play a role in how likely a flooding event is, which can not necessarily be included in the statistical analysis. For example, the accuracy of the incoming data and changes in how the land is used in any given year. These, combined with generally unpredictable changes in yearly weather patterns, lead to rare events, like a 1,000-year flood, happening in the same location less than two-years apart. Information provided by the USGS and the NWS in Baltimore (LWX) Image from the Hydrometeorological Design Studies Center ftp://hdsc.nws.noaa.gov/pub/hdsc/data/aep/201805_Ellicott_City/AEP_Ellicott_City_May2018.jpg To learn more about other flooding-related stories and topics from around the world, be sure to click on the following link: https://www.globalweatherclimatecenter.com/flooding! ©2018 Meteorologist Sarah Trojniak   Subtropical Storm Alberto indirectly brought torrential downpours and thunderstorms to the Mid-Atlantic Region. While Alberto did not go straight over the region, a flow of moisture was moving northeast coming from Alberto. The northeast flow moved towards a stationary front that was sitting over the Tennessee Valley and the Mid-Atlantic region. These were the ingredients for the destructive flooding that occurred in Maryland, Virginia and North Carolina.
Ellicott City in Maryland was under water on Sunday, May 27, 2018 when a nearby river overflowed its banks and into the streets of local homes and businesses. Many people needed to be rescued from buildings and vehicles as the force of the water pushed vehicles down the streets. One person lost his life when he was attempting to help a person in need. The above image is from Syria, Virginia where a woman and a young girl were swept away in the flooding. The young girl was quickly found; however, the woman has yet to be found as of the time this article was posted. The image on the top used to be a bridge where people could walk, but the strength of the flooded waters pushed the bridge off and you can see it in the image on the bottom. On Route 33, in Central Virginia, a landslide covered the highway prompting several road closures and detours. If we continue south, North Carolina had its share of landslides and flooding. One landslide compromised the integrity of the Lake Tahoma Dam in North Carolina prompting evacuations. Some areas have received over 6 inches of rain and the threat of rain continues. Many roads have been closed throughout the Mid-Atlantic due to debris and flooding. Always remember; if you see high water, turn around, don’t drown. It only takes 6 inches of water to stall a vehicle while a foot of water will float many vehicles. Once the water level reaches two feet, your vehicle will likely be carried away. Flooded waters are typically muddy so any debris lurking in the water may not be visible. Alberto is no longer a threat, but rain will persist through the weekend. With already saturated soil, any additional rainfall will produce more flooding. After the first weekend of June dry weather will make its way back into the area. Stay tuned for more updates on the flooding situation in the Mid-Atlantic by clicking www.globalweatherclimatecenter.com/flooding ⓒ 2018 Meteorologist Brandie Cantrell  DISCUSSION: This past weekend, Ellicott City, Maryland was devastated by what was referred to as a 1000-year flood. In recent years, there have been multiple storms that have produced 100-year, 500-year, and even 1000-year floods. But what do these terms actually mean and how can there be multiple of these historic floods within the given timespan?
Contrary to the name, a 1000-year flood does not necessarily mean this type of flooding will occur exactly once every 1000 years. Weather does not occur on a timed pattern so it is not possible to say that a certain severity of flooding will only occur exactly every 1000 years. What is actually meant by the name is that there is a 1 in 1000 chance of a storm producing flooding of that severity occurring within a year. It is not unheard of to have multiple historic floods within a relatively short time period. In 2017, flooding from Hurricane Harvey was reported as the third 500-year flood to affect Houston, Texas in three years. There was some confusion from the public as to how it was possible to have multiple 500-year floods less than 500 years apart from each other within one area. Since the naming of the category of these floods is based on probability, we can’t exactly say for certain there will or won’t be more than one flood of that magnitude within the 100/500/1000 etc. year timeframe. When you flip a coin, there is a 1 in 2 chance of the coin landing on heads. Say you flip the coin twice. The only possible outcomes are the coin landing on heads twice, the coin landing on tails twice, or the coin landing on each once. Two of these scenarios do not represent this 1 in 2 chance probability. That is because the probability does not mean that for every 2 coins one of them must be heads. More accurately what this probability represents is that for a single coin toss, there is a 1 in 2 chance that the coin will land on heads. When the coin is tossed again, there is once again a 1 in 2 chance that the coin will land on heads since each coin toss is treated as an individual event. Since the 100/500/1000 year floods are based on the probability that type of flooding will occur within a given year, each year would be treated as an individual event. This means that for each year, there is a 1 in 100 chance that a 100-year flood will occur. The occurrence of that type of flooding in one year does not affect the probability of it happening again the next year, which is why it is entirely possible to have more than one flood of the same probability within that given time frame. Ultimately, weather does not occur like clockwork. We cannot predict that a severe flood will occur exactly every 1000-years. We can, however, deduce that there is a 1 in 1000 chance that particular type of flood can occur in any given year. While the terminology may seem confusing, it is important to remember it refers to the probability of a flood, not an exact timeline of occurrence. To learn more about high-impact flooding and flooding-related stories from around the world, be sure to click here! ©2018 Meteorologist Stephanie Edwards Heavy Rainfall Causes Flash Flooding in Ellicott City, Maryland (Credit: NWS DC/Baltimore, NWS)5/27/2018
Discussion: Heavy rainfall from deep convective storms fell this afternoon in Ellicott City, Maryland. Earlier this afternoon, severe weather moved through the area producing torrential downfalls. Radar estimates and actual reports of seven inches or more fell in just a few short hours. The rainfall that fell this afternoon caused the Patapsco River, to overflow their banks and reach above major flood stage. That water flowed downstream into Ellicott City, flooding the historic Main Street. As of tonight the heavy rain has moved out and the flood waters have receded.
This is not the first time Ellicott City has seen flooding like this. The geography of Ellicott City is unique in which it has streams upstream that converge to flow into the Patapsco River, making it susceptible to flooding and or flash flooding events. Two years ago, strong storms brought heavy rain that fell in a short amount of time producing flooding that destroyed businesses, homes and cars. That flash flood back in 2016 had two fatalities. A state of Emergency has been declared for the state of Maryland, as it starts to clean up and recover from another historic flood. Now the focus turns to recovery for this city once again. With the flash flood waters receding, there is still a danger being outside. Some streets and buildings are not structurally sound which can collapse, certain roadways may be partially washed away or still covered in standing water making it dangerous to drive. The National Weather Service advises people to stay informed and to avoid standing waters due to not knowing what is underneath them whether it be the condition of the road or the toxins and chemicals in the water. In the image above are more after the flood safety tips from the National Weather Service. For more information on flash floods, how to stay safe after a flash flood and other flooding stories be sure to click here! © 2018 Meteorologist Shannon Scully Identifying Warm, Moist Flow for the Ongoing Severe Weather Threat. (credit: UW-Madison CIMSS MIMIC)5/1/2018 
DISCUSSION: As the Central United States continues to be under a larger-scale severe weather threat overnight tonight and into the day on Wednesday and Thursday, there is also a substantial threat for flooding and even potential flash flooding. The reason for this is due to the fact that the moisture track which is ending up northward into the southern and central Mississippi River Valley is tracking out of the Gulf of Mexico. This can be seen in the CIMSS-MIMIC Total Precipitable Water imagery courtesy of the University of Wisconsin animated imagery attached above. Whenever there are situations characterized by warm, moist flow emerging out of the Gulf of Mexico, this greatly favors heavier rainfall events due to the fact there is also substantial persistence to these moisture plumes.
The core of this particular moisture plume can be identified via the yellow to orange shaded colors emerging from the Gulf of Mexico. This yellow to orange coloring represents the core of the deepest moisture within this particular Gulf of Mexico moisture plume which was coming ashore during the overnight hours from Monday night (04/30/2018) to Tuesday morning (05/01/2018). Thus, per the track of this moisture plume via the animated CIMSS-MIMIC imagery attached above, the core of this moisture plume was ideally oriented for heavy rainfall across the southern to central Mississippi River valley. Hence, it is always critical to respect the potential for heavy rainfall events coincident with severe weather events during the Spring to Summer time-frame across the Central United States. To learn more about other high-impact flooding or flooding-related events from around the world, be sure to click here! |
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