Stay safe out there today! Severe storms likely and in progress.

Nebraska is in the ice box still…but big storms, including tornadoes and significant straight line wind damage possible in parts of Indiana and Kentucky today.

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Warm, moist air, and lots of vertical wind shear. Good set up for some nasty severe weather.

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Prominent severe weather threat is between the polar and subtropical jets (northerly and southerly branches, respectively). In between, lots of mass is being removed allowing for storms to explode from below, intensify rapidly and maintain themselves in the highly sheared environment (winds rapidly increasingly with height).

There is a lot of speed shear, but not as much change in direction with height, which favors lines of storms. So the threat for damaging winds is quite significant, hence the “moderate risk” (4 out of 5 on the overall scale by the Storm Prediction Center) for a 45% chance of damaging wind reports within 25 miles of a point within the given region, and a 10% or higher chance of winds over 74 mph (hurricane-force). However, a significant tornado risk also exists for storms which do isolated themselves earlier this afternoon. So remain vigilant if you are in these areas and tell anyone you know to remain ready to take cover this afternoon and evening!

My part of Nebraska?

Unseasonably cold. Just straight up…cold. Starting off spring as if it were February with temperatures 25-30 degrees F below normal for highs and 15-20 degrees below normal for lows. Will have some recovery later this week, but near normal conditions (+-5 degrees F) don’t appear consistently likely until early next week. Thursday may give us a one day break with mid-50s (normal is around 60 F). So far our warmest day this year was March 3rd (73 F). The Great Plains have been part of the very wild weather pattern impacting much of the mid to upper mid-latitudes this year thanks to a highly oscillating jet stream with periods of very cold and very warm conditions relative to local norms. Much of Europe has gone through the same with very cold Arctic air mass spells, while the parts of the East Coast had record heat in February, followed by multiple cold and heavy snow periods from damaging nor’easters. All while the Arctic roasted in heat waves in this winter (relative to their norms) has significant heat and moisture moved northward, hitting sea ice hard. Here in my locality, we’ve had the roller-coaster ride of going from a a high of 4 to a high of 56 in ten days (January), a high of 22 to a high of 65 in seven days (February; and actually the high was 58 two days before that high of 22…ha!) to our mid-May days in early March (low-70s). Now after the last 5 days of March in the more seasonable 50s to low-60s, we’re spending the first three days of April barely above freezing. Winter was wild and Spring is starting off confusing weather even by spring standards. At least it’s not record breaking cold, it is unusually cold regardless though. Looking forward to the actual warmth of spring again.

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Discussion of Final Analysis of 2017 Hurricane Harvey

The 2017 North Atlantic Hurricane season was a devastating one in terms of loss of life as well as property damage for the United States and the Caribbean. The National Hurricane Center released its post-season report on Harvey which caused great destruction to parts of Southeast Texas and Southwest Louisiana. What follows is a brief summary and discussion of Harvey based on info from that report as well as other sources related to Harvey’s impacts. The full report is linked at the end of this post in the references.

Meteorological Discussion

What became Harvey was originally a tropical disturbance which came off the West Coast of Africa on August 12th. It is common during August and September for land-based thunderstorm complexes known as mesoscale convective systems to move westward off the African coast near or south of the Cape Verde (also known as the Cabo Verde) Islands and later develop into long-lived tropical cyclones. Harvey was a classic “cape-verde” type storm as it would later develop into a tropical depression with a well-defined center on August 16th.

The depression intensified into a storm and given its name 12 hrs after initial development. It peaked over the open Atlantic at 40 knots (~45 mph), moving over the islands of Barbados and St. Vincent on August 18th. However, increasing vertical wind shear (increasing winds with height tilting and blowing the thunderstorms away from the low pressure center) over the central Caribbean Sea lead to Harvey’s dissipation to a remnant low later that day.

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Harvey moving over Barbados and St. Vincent on August 18, 2017.

The remnant circulation moved over the Yucatan Peninsula on Aug 22nd and redeveloped into a tropical depression over Bay of Campeche on August 23rd, 150 n mi west of Progreso, Yucatan, Mexico.

The initially poor organization of the reformed Harvey transitioned to a period of rapid intensification late on the 23rd as deep convection began to concentrate near the center. This was aided by an environment of light shear, very warm sea surface temperatures and high mid-level moisture. Intensification would continue until landfall on the 26th. Harvey reached Category 3 midday on the 25th and intensified into a Category 4 as it made its landfalls on the Texas coast early August 26th (the evening of the 25th local time). The initial landfall was on San Jose Island, TX as a Category 4 with maximum sustained winds of 130 mph (115 knots) with a second landfall on mainland Texas in northeast Copano Bay as a Category 3 with maximum sustained winds of 120 mph (105 knots). Wind damage was extreme and devastating in Aransas, Nueces, Refugio and the eastern part of San Patricio Counties. 15,000 homes were destroyed and 25,000 homes damaged. The City of Rockport was hit the hardest as the Category 3+ wind field moved into that area causing both extensive wind and surge impacts. The highest surge observed in Harvey was generally in the range of 9-11 ft.

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Hurricane Harvey approaching landfall on the Texas Coast the evening of August 25, 2017 (local time).
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Hurricane Harvey making landfall in Texas as seen by radar. Note the “lumpy”, wavy undulations within the eye (such as near Rockport and north of Port Aransas in this image). These are mesovortices where winds may have been locally stronger within the inner eye wall of the hurricane.

Harvey meandered in light steering currents, “stuck” between a mid-tropospheric high pressure system over the Four Corners states and another mid-troposphere high over the Gulf of Mexico. Torrential rains fell over Houston Metro and the Golden Triangle near a stationary front which formed on the north and east side of Harvey.

 

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The rainfall of Harvey was truly incredible. A storm total of 60.58 inches was confirmed Nederland, TX; 60.54 inches in Groves, TX. Much of the heaviest precipitation fell in the first 72 hrs of the event. Previous continental US record for a tropical cyclone is 48 inches in Medina, TX (1978). The extreme nature of Harvey was displayed in that 18 values over that continental record of 48 inches reported across southeastern TX, with 36-48 inches recorded across the Houston metro area. However, Multi-Sensor Precipitation Estimates (MPE), which includes radar-derived rainfall intensity estimates suggests 65-70 inches where few observations were available or observations failed early in the event. Maximum rainfall measured in Louisiana was 23.71 inches in Vinton, LA, with MPE suggesting a more representative 40 inches as Southeast Southwest LA obs were sparse.

By Jordan Tessler for Capitol Weather Gang.

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The large-scale or synoptic set up for the Harvey exceptional rainfall event is not particularly unique. Heavy rain bands formed along a modest frontal boundary situated initially near Houston, then the Golden Triangle region in Southeast TX (Beaumont, Port Arthur, Orange, TX area). Enhanced convergence and convective lift with warm cloud droplet precipitation processes allowed for enhanced rainfall rates in abundant thunderstorms. The combination of extremely high rainfall rates of up to 5-7 inches per hour and the stationary nature of the near coastal frontal boundary and Harvey itself contributed to the extreme total accumulation and massive flooding.

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Training rain bands moving over the Houston Metro area the morning of August 27, 2017.
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Very heavy rainfall in the Golden Triangle region of east TX the early morning of August 30, 2017.

NOAA analysis determined that areas of Southeast TX experience a flood with an annual probability of <0.1% (equivalent to a >1000 year flood event). I believe this is one of the most important parts of the National Hurricane Center report, so I’ll quote it:

While established records of this nature are not kept, given the exceptional exceedance probabilities, it is unlikely the United States has ever seen such a sizable area of excessive tropical cyclone rainfall totals as it did from Harvey.

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Mesoscale Precipitation Discussion by the National Weather Service – Weather Prediction Center on August 27, 2017. Historic, devastating flooding underway in the Houston Metro Area at the time.

In addition to storm surge, wind and flooding rains, Harvey produced 57 tornadoes (many in the Houston Metro area) and killed 68 people directly with an additional 35 indirect deaths. All direct deaths were in Texas and it was the deadliest tropical cyclone for Texas since 1919. All but three direct deaths were caused by freshwater flooding.

According to NOAA, preliminary damage analysis suggests estimated damages of $125 billion, making Harvey the second-costliest hurricane on record in the North Atlantic basin, only behind Hurricane Katrina, when adjusted for inflation.


Connection to Anthropogenic (human-caused) Climate Change

During and immediately following the events of Hurricane Harvey, there was intense controversy over even discussing climate change as it related to the extreme events related to Hurricane Harvey. Even mentioning climate change in reference to an individual extreme weather event. A lot of opinions were thrown about, but the science of climate change has evolved dramatically in the past 10 years and climate researchers have a much better understanding of many of the connections between climate variables and the statistics of weather which make up the recent past and current climate. From this, attribution studies can be conducted to determine a likelihood of connection to the changing climate regime. A attribution study was done by World Weather Attribution (#2 below) and the probabilistic statistical analysis determined that the record rainfall from Harvey was approximately a) 3 times more likely and b) 15% more intense in terms of rainfall rate because of climate change. One location witnessed a return period for extreme rainfall of 9000 years with a high degree of statistical confidence. The impacts were consistent with what would be expected with 1 degree C+ of global warming since the late 19th century (the world has thought to have begun warming because of humanity since the mid 18th century). I did an extensive post previously during this most recent hurricane season on the climate change connection with includes references to numerous recent peer reviewed papers HERE.

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References

#1 – Harvey Report (National Hurricane Center, 2018)

#2 – Oldenborogh et al. 2017

See my previous posts in this blog on Hurricane Harvey from last August HERE.

–Meteorologist Nick Humphrey

Strong Winter Storm Impacting portions of Plains and Midwest Early Week

A strong winter storm is pushing across the Great Plains tonight. North of the low will experience widespread blizzard conditions and heavy snow. South of the low milder conditions with rain.

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Infrared image, but with surface frontal analysis added, valid at 6 pm CST.

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Parts of far southern Nebraska, into Iowa have a tight gradient between little to no snow vs. heavy snow with high winds. An example…my location in Lincoln, NE where the National Weather Service is calling for 3-4 inches for the day Monday and just added the county under a blizzard warning after 24 hrs ago thinking the area would only receive up to 1 inch with gusty winds and much better travel conditions.

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A difference less than 24 hrs makes. Lincoln now expects up to half a foot of snow (see below) and places expecting 7-8 inches are now expecting 12-18 inches!

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The whole system has been trending southeastward in the models and in reality and so the official forecast has been trended slightly higher and significantly so in places of central and northeastern Nebraska which may get up to a foot with isolated amounts up to 18 inches! The bigger story are winds which may gusts 35-50+ mph across much of the north-central Plains during the night and during the day Monday. This will induce the blizzard conditions, with very low visibilities.

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Conditions will improve across Nebraska and South Dakota by tomorrow evening as the storm shifts northeastward, continuing to impact northern Iowa and southern Minnesota, as well as Wisconsin and upper Michigan with locally heavy snow and gusty winds.

If you’re in an area under blizzard or winter storm warnings, stay off the roads during the worst of the conditions unless absolutely necessary as the roads will be treacherous and visibility poor, particularly outside major cities, where snow can blow around easily. If you have to travel, drive slowly and with care.

Update on Hurricane Maria (6:30 am EDT)

#Maria is making landfall in eastern Puerto Rico as a Category 4 Hurricane with maximum sustained winds of 155 mph. Gusts of 160+ are likely in progress over northeast coast of PR. Catastrophic weather conditions will continue to spread across the island over the next couple of hours. 

Terrible situation for the 3.5 million people hunkering down this morning. 

PR Radar failed just before 6 am EDT. More inbound winds of 155 mph or higher as eyewall moved over radar site (which is in elevated terrain and winds likely much stronger).

Catastrophic Hurricane Irma heading for Leeward Islands. Florida must get ready.

Category 5 Hurricane Irma bearing down on the Leeward Islands at 8:45 am CDT Tuesday.

If you’re in South Florida or the Florida Keys…you need to know what you’ll to once watches and warnings are issued BEFORE they are issued. I do have enough confidence in the path of Hurricane Irma to state that Florida is in a very serious situation based on forecast data and so folks need to 1) track the forecast 2) know your plan, especially if you live in coastal evacuation zones 3) have emergency supplies ready. This storm already has the potential to be devastating for many people all week as it buzzsaws its way through the northern Leeward Islands, close approach to US and British Virgin Islands, then the US territory of Puerto Rico with 3.5 million people, the Dominican Republic, then portions of the Bahamas, before a close passage of the coast of Cuba. And it could do this as a large Category 4-5 hurricane. 
Most likely time of arrival of tropical storm force winds overlay 7 am CDT probability of those winds during 5 day forecast period. Note South Florida is most likely to see winds arrive by Saturday morning and they could arrive overnight Fri-Sat, with the eye not expected near Florida until Sat night-Sun morning.

Get ready, be ready to act if/when watches/warnings are issued and keep track of the latest forecast. Also, hope the best for our friends facing Irma right now.

I’ll updates on Irma throughout the week and you can follow me on Facebook and Twitter.

EDIT AT 10:20 am CDT: Winds in Hurricane Irma increased to 180 mph, making it the most powerful hurricane (by wind) ever observed in the North Atlantic outside the Gulf of Mexico and Caribbean Sea.

–Meteorologist Nick Humphrey

My God…

Estimated 5-day total rainfall from Hurricane Harvey. In most tropical events, the light blue on this color scheme (12-16 inches) would be highly significant. And for many of subregions on this map, the bulk of rainfall fell in just 3 days. Again that would be significant alone. Here a wide swath of land was doused in in up 1 1/3 ft of rain from San Marcos, TX south to the coast and east of Lake Charles, LA. Within that rainfall totals turn into something nightmares are made of. Again much fell in subregions within 3 days. Beaumont, TX received nearly 2 feet of rain in 24 hrs yesterday where catastrophic flooding is in progress.

By Jordan Tessler for Capitol Weather Gang.

Flooding in Port Arthur this morning as witnessed by @MayorofPA on Twitter.
Flooding in Port Arthur as seen by @OliverTerry on Twitter.

–Meteorologist Nick Humphrey 

After Days of Agony…

The continuous rainfall in Houston…is over.



Unfortunately, Harvey remains a 45 mph tropical storm. Although its center has moved east, ending rain in Houston, the torrential rain is spreading into far SE Texas with increasing concerns in Southwest and southern Louisiana as well.

Heavy rain cause by thunderstorm activity north Harvey’s center to circulation. Far southeast TX into Southwest LA have been hard hit today with flash flooding.

Harvey is expected to make landfall on the Southwest LA Coast Wed morning. No further strengthening is expected as the system is too disorganized and in a high vertical wind shear environment to support rapid intensification (thankfully). It will move across western LA Wed-early Thurs, being heavy rain to much of the state and spreading into Arkansas.

New Record-

Today, Harvey (preliminarily) set the Continential US record for highest storm total rainfall for a tropical cyclone at 51.88 inches at Cedar Bayou in the Houston area. The previous record was Hurricane Amelia in 1978 with 48 inches (also in TX). Almost all of this rain fall in the 72 hr period with Saturday night-Sunday morning being the absolute worst with 15- 25 inches of rain in 12-15 hrs, initially the catastrophic urban flood event in Houston. One automated rain gauge which captured rainfall per min saw an equivalent rainfall rate ~12 inches/hr very briefly late Saturday night!


And additional 15-25 inches fell Sunday-Monday worsening the situation and conditions for those waiting for rescue miserable.

Storm totals ending Tuesday morning.

As of this post, 31 people are reported to have died as a result of Harvey in the US…one in Rockport who died in a house fire at the height of the storm; and 30 in the Houston area, most flood-related.

See photos of the disaster and human response by tve New York Times: HERE

Photo of flooded Houston street Sunday taken by a relative who lives there. She is safe.

(Title image is courtesy of Alyssa Schukar of the New York Times).

–Meteorologist Nick Humphrey