High Winds Take Aim on Eastern Seaboard for Second Day

Severe straight-line thunderstorm winds are possible on Tuesday across parts of the Northeast U.S., as residents of the Washington, D.C., area pick up the pieces from a derecho-type windstorm that rattled nerves—and a presidential motorcade—on Monday night. By late Tuesday, flash flooding may pose a new threat to the mid-Atlantic.

In its Tuesday morning outlook issued around 9 am EDT, the NOAA/NWS Storm Prediction Center called for an enhanced risk of severe weather from northeast Pennsylvania through most of southern New England, including the greater New York City area. Most of the enhanced-risk area includes the chance of wind gusts topping 75 mph. SPC warned of the potential for a “well-organized swath of damaging wind” by Tuesday afternoon. The severe storms and strong winds could extend closer to the Atlantic coast than is often the case. Large hail and tornadoes are also possible, especially before the storms congeal into a wind-producing complex. Update: At 12:30 pm EDT, SPC upgraded a portion of the enhanced-risk area to a moderate risk, the second highest of SPC's threat levels.

Further south, the tail end of the storm complex is likely to stall along an east-west axis from West Virginia across northern Maryland to the Delmarva and southern New Jersey. A flash flood watch covered most of this area, where “training” thunderstorm cells could lead to rainfall totals of 1-3” in a short amount of time. Parts of the area have already seen more than 2” of rain in the past week, priming the soil for potential flash floods.

Severe storms have also plagued the Plains and Midwest, although weak upper-level winds have kept the activity fairly disorganized across Tornado Alley. Only a handful of tornadoes have been reported in the last week, none of them intense. On Monday, two brief, weak tornadoes occurred in eastern Florida. Several other twisters were observed from a meandering, rotating supercell southeast of Wichita, Kansas (see embedded tweet below). Another storm that plowed southeast from the Denver area produced widespread hail.

Tornado near Geuda Springs right now!! #kswx pic.twitter.com/HPmVNtn5jD

— Adrian Leyser (@TheNimbus) May 14, 2018

Figure 1. Ariana McLaughlin, of Denver, jumps over a torrent of water to clean hail off her sports utility vehicle after a storm packing heavy rains and hail hit Monday, May 14, 2018, in southeast Denver. The high-powered storm swept over the area and dumped large amounts of marble-sized hail and heavy rain, snarling traffic in densely-populated parts of the metropolitan area. Image credit: AP Photo/David Zalubowski.

Supercell activity may pick up across the central High Plains on Thursday and Friday, but no major tornado outbreak is anticipated, as upper winds across the Plains will remain in a weak summerlike mode for the foreseeable future.

It was a derecho—but nothing like 2012’s

Monday night’s D.C. windstorm formed on the eastern end of a “ring of fire” that generated severe thunderstorms along an arc from west Texas to the Chicago area to the East Coast. As it whipped through the Washington area between around 5 and 7 pm EDT, a thunderstorm complex generated downburst winds in the 50-70 mph range. Thousands of patrons at Dulles International Airport were herded into an underground tunnel after the NWS issued a tornado warning based on radar indications (no tornado was reported, though). The core of the strongest winds passed just south of central D.C., which helped limit the overall damage. Capital Weather Gang reported widespread reports of downed trees in the Reston, Virginia, area.

The strong winds fanning out ahead of the storm helped produce a spectacular shelf cloud at the storm’s leading edge (see embedded tweet below).

Figure 2. Infrared image of the storm complex as it moved into the Washington, D.C., area at 6 pm EDT Monday, May 14, 2018. See this blog post for an animated IR loop. Image credit: CIMSS/SSEC/University of Wisconsin-Madison.

Immaculate shelf cloud over Reston. Take cover immediately! #vaxa pic.twitter.com/wAejCN8OZM

— Dave Dildine (@DildineWTOP) May 14, 2018

The term derecho (“straight ahead” in Spanish) gained prominence after a massive thunderstorm-driven burst of wind swept from the Midwest across the D.C. area on June 29-30, 2012, producing an estimated $2.9 billion in damage and taking 22 lives. Although Monday night’s winds were much weaker than those in 2012, Jon Erdman (weather.com) found that the windstorm met the most common working definition of derecho, which was put forth in a study by researchers Walker Ashley and Thomas Mote in 2005. Their paper is available in open access at the the Bulletin of the American Meteorological Society.

Here are three of the five ways in which the Monday storm made the grade, as explained by Erdman and Brian Donegan at weather.com:

•  Length:  The swath of high winds/wind damage was about 430 mi (692 km) long, more than meeting the 400-km criterion put forth by Ashley and Mote (and even meeting a more stringent minimum of 650 km proposed by NWS scientists in a 2016 paper).

•  Continuous nature:  There were no time gaps of more than 2.5 hours or spatial gaps of more than 2° latitude or longitude between wind reports.

•  Origin of wind swath:  All of the wind and damage reports came from the same overall storm complex, which included a few severe storms ahead of the main squall line in western Virginia.

“We would categorize this as a low-end derecho,” said Erdman and Donegan.

The chance of a tropical cyclone in the eastern Gulf is virtually gone

The NOAA/NWS National Hurricane Center put the odds of tropical development in the eastern Gulf at a mere 10% as of 8 am EDT Tuesday morning, down from a peak in the 40-60% range. Satellite loops show that the mid-level circulation had pulled away from the broad area of surface low pressure in the eastern Gulf, which makes it virtually impossible for a tropical or subtropical cyclone to organize (especially given the absence of sea-surface temperatures above the 26°C or 79°F threshold for tropical development).

The main impact of the upper- and lower-level circulations near the Southeast U.S. will be to continue funneling moisture northward along the Atlantic coast toward New England, fueling wet, stormy conditions. Most areas east of the Appalachians up to New York can expect 3-5” of rain over the next week, and some spots could see amounts in the 7-10” range.

Figure 3. Precipitation forecast for the seven-day period from 12Z (8 am EDT) Tuesday, May 15, through Tuesday, May 22, 2018. Image credit: NOAA/NWS/WPC.

The GFS model continues to project development in the western Gulf more than a week from now, but this remains a highly dubious prospect. As Jeff Masters discussed in our last post, the GFS is well known for spurious tropical development in the western Caribbean this time of year, especially toward the outer end of the GFS forecast period (roughly 1-2 weeks out). Over the last few days, the GFS has been consistent in predicting a western Caribbean spinup—but it’s also consistently kept the window of such development in the extended range beyond 180 hours, a classic sign of the usual bias.

The 2018 Eastern Pacific hurricane season has begun

Today (May 15) marks the official start of the Eastern Pacific hurricane season, with the Atlantic to follow on June 1. These starting points have seemed a bit more arbitrary than usual in the last few years, as we’ve seen a spate of named storms getting a jump start on the season. In fact, 2018 is the first year since 2014 that we’ve made it from January 1 to May 15 without a named storm in either the Atlantic or Northeast Pacific. Last year saw pre-season storms in both basins: on April 20, Tropical Storm Arlene formed in the remote northwest Atlantic, and on May 10, Adrian became the East Pacific’s earliest tropical storm on record

Figure 4. Infrared METEOSAT image of a slowly organizing disturbance east of Somalia at 1330Z (9:30 am EDT) Tuesday, May 15, 2018. Image credit: RAMMB-CIRA/CSU.

A tropical cyclone may develop in the Gulf of Aden

An area of disturbed weather several hundred miles east of Somalia on Tuesday morning could become a tropical depression or tropical storm over the next several days as it moves northwest toward the Gulf of Aden. This disturbance is traversing very warm sea-surface temperatures of 30-32°C (86-90°F), which is about 1-2°C above the seasonal average, and wind shear is predicted by the HWRF model to remain light (below 10 knots) for at least the next 36 hours. The 06Z Tuesday run of the HWRF predicts that the disturbance could become a weak tropical storm entering the Gulf of Aden and then making a cyclonic loop toward the northern Somali coast later this week. The very warm waters and light shear suggest that more robust development is quite possible.

Tropical cyclones are most common in the Arabian Sea in spring and autumn, during the transition periods between the strong southwest flow of the summer monsoon and the strong northeast flow that predominates in winter. There are signs that pre-monsoon (springtime) Arabian Sea cyclones have gotten stronger in the last 20 years, as we discussed in a 2015 post on Chapala.

On average, the Arabian Peninsula is affected by a tropical cyclone every year or two; the 2010s have seen at least nine so far. Only six tropical cyclones are known to have hit Somalia since accurate satellite measurements began in 1966: Tropical Cyclone ARB02 of 2016, Tropical Cyclone ARB01 of 2013 (the nation’s deadliest cyclone on record, with at least 162 fatalities), Tropical Cyclone Murjan of 2012, Tropical Cyclone ARB02 of 1994, Tropical Cyclone 12A of 1994, and Tropical Cyclone 4B of 1984. Notably, all six of these were post-monsoon (autumn) cyclones rather than springtime systems, and all of them struck the east coast of Somalia rather than its north coast.

The region’s strongest tropical cyclones on record both occurred in November 2015. Cyclone Chapala came ashore on the south end of the Arabian peninsula on November 4 as the first hurricane-strength cyclone ever recorded in Yemen. Just four days later, Cyclone Megh passed just north of Socotra Island east of Somalia with Category 3 winds of 110 mph, destroying hundreds of homes and killing at least 18 people. Megh grazed the north tip of Somalia before making landfall in Yemen as a weak low.

Thanks to Jon Erdman at weather.com for derecho background and analysis in today’s post.

Figure 5. A MODIS satellite view of Chapala as it approached the Gulf of Aden on Tuesday, November 2, 2015. Image credit: NASA EarthData.