Erick to Bring Heavy Rains, High Surf to Hawaii; Watching 96L in the Atlantic

A flash flood watch was posted for Hawaii’s Big Island as a weakening Hurricane Erick is on track to pass just to its south on Friday, bringing high surf and heavy rain. Another system—Tropical Storm Flossie—could make its way even closer to Hawaii by early next week. Meanwhile, a strong tropical wave in the central Atlantic dubbed 96L looks increasingly likely to develop this weekend as it approaches the northern Lesser Antilles.

The #GOESWest satellite captured this view of hurricanes #Erick and #Flossie on July 31, 2019. The storms are expected to bring heavy rainfall to Hawaii and dangerous surf conditions. Follow them via our #hurricane tracker: https://t.co/rt6evQ7nmg pic.twitter.com/SMQ5butJkC

— NOAA Satellites (@NOAASatellites) July 31, 2019

Hurricane Erick, which peaked as a Category 4 storm on Tuesday with top sustained winds of 130 mph, is in the midst of a well-predicted steady weakening. As of 5 pm EDT Wednesday, Erick’s top winds were down to 115 mph, near the bottom of the Category 3 range. The once-distinct eye within Erick’s shield of showers and thunderstorms (convection) eroded on Wednesday, as relatively dry air (mid-level relative humidity of 50-55%) began to infiltrate the storm.

As Erick continues moving just north of due west, it will enter a zone of steadily increasing wind shear that should degrade the storm further, in spite of warm sea-surface temperatures (SSTs). The Central Pacific Hurricane Center predicts that Erick will be a tropical storm by Friday morning, as it passes about 200 miles south of Hawaii’s Big Island, and a tropical depression by Monday. A tropical storm warning is in effect for the waters south and west of the Big Island, with a hurricane warning for all of Hawaii’s surrounding offshore waters, mainly for high surf. Wave heights could reach 7 to 14 feet from Thursday night into Friday. Flash flooding is most likely on east- and south-facing slopes of the Big Island, which will be hit by upslope winds circulating around Erick.

Figure 1. Probability of tropical-storm-force winds from Hurricane Erick in the five days starting at 8 am HST (2 pm EDT) Wednesday, July 31, 2019. Odds of tropical-storm-force winds are less than 20% for Hawaii, except for parts of the south coast of the Big Island. Some higher-elevation points on the Big Island could see stronger winds. Image credit: NOAA/NWS.

Up-and-down Flossie on a steady track toward Hawaii

After reaching hurricane strength on Tuesday afternoon, Flossie was downgraded to a tropical storm at 5 pm EDT Wednesday, a victim of persistent wind shear that eroded convection on Flossie’s northwest side. The storm resembled a giant convective band late Wednesday, arcing in comma-shaped fashion from its east to south sides. The most intense convection was limited to a small area near Flossie’s center.

Forecast models indicate that wind shear should decrease to around 10-15 knots from Thursday into Friday, which may give Flossie a second lease on life. Unusually warm SSTs of around 27°C (81°F)—about 1°C above average—would also support a potential rebound if Flossie’s current struggles do not completely undermine its structure. The 12Z Wednesday run of the HWRF high-resolution model brings Flossie back to hurricane strength by Friday and suggests Flossie could reach Category 2 strength over the weekend. The National Hurricane Center’s forecast is more measured, bringing Flossie to Category 1 strength from Friday through Sunday.

Figure 2. WU depiction of NHC forecast track for Flossie as of 21Z (5 pm EDT) Wednesday, July 31, 2019.

The track forecast for Flossie from both the GFS and European models has been remarkably consistent for days, still bringing the storm very close to Hawaii around Monday. The 12Z Wednesday ensemble data from the GFS and European models indicate that Flossie could move over or very near the eastern island chain, but it could just as easily arc to the right shortly before reaching Hawaii. We’ll have to see if such a turn becomes the favored model solution, but for now an unusual Hawaii landfall remains a distinct possibility. Increased wind shear will be taking its toll on Flossie by late in the weekend, despite continued warm water, so we can expect it to be a tropical storm or perhaps a weakening Category 1 hurricane as it approaches Hawaii by Monday. Hawaii could wind up on the weaker left-hand side of Flossie should the storm angle to the right.

As with Erick, heavy rains and high surf are the main threats to expect from Flossie, although it’s too soon to iron out details.

Figure 3. Infrared image of Invest 96L at 2355Z (7:55 pm EDT) Wednesday, July 31, 2019. Image credit: tropicaltidbits.com.

The Antilles will need to watch 96L

A strong tropical wave traversing the central Atlantic was designed Invest 96L on Wednesday evening by NHC. We’ll be hearing about 96L for several days to come, as the system makes its way into the western part of the Main Development Region, the deep tropics that extend from the Caribbean to Africa.

Although the strongest convection was far west of 96L's center on Wednesday night, the disturbance had “good bones”, with a large field of moisture and cloudiness and a broad, open circulation focused around 8-10°N and 31-33°W. The ASCAT advanced scatterometer detected surface winds of up to 25-30 knots at the heart of 96L. Over the next several days, 96L will be heading west-northwest toward an area of warmer SSTs where several upper-level features may align to support it.

The upper-pattern for the Central Atlantic wave as it approaches the Antilles is worth watching. A couple of upper lows to the north could ventilate the storm on the north side if things line up, but they could also bring shear (mid-level RH isn't high either). Tricky setup. pic.twitter.com/UZpTAMUVF4

— Andy Hazelton (@AndyHazelton) July 31, 2019

GFS ensemble runs from Wednesday morning were near-unanimous in developing 96L into at least a depression by the time it approaches the northern Leeward Islands on Monday. About 20-30% of the European ensemble runs supported development in that period. In its 8 pm EDT Wednesday tropical weather outlook, NHC ended up in between, raising the odds of development in the 3- to 5-day period to 70%.

There is a good bit of ensemble support for 96L beyond the five-day period, but increasing spread over time (and with subsequent model runs) in where the system might go from that point onward.

Meanwhile, odds are low that the system formerly known as 95L will develop as it moves into the vicinity of The Bahamas. NHC gives the system a less-than-10% chance of becoming at least a tropical depression between Friday and Monday as it translates northward well off the Southeast U.S. coast.