Gulf of Mexico's Hermine Finally Gets its Name; Hurricane Madeline Lashing Hawaii

Tropical Depression Nine in the Gulf of Mexico finally got its act together enough to deserve a name, the NOAA Hurricane Hunters discovered on Wednesday afternoon. They found top sustained winds of 45 mph in Tropical Storm Hermine, ending a week-long drama that left us all wondering if someone had cast a “hold” spell on the storm. However, the aircraft found that the storm’s central pressure remained a fairly high 1004 mb, and Hermine has a lot of organizing to do before it can become a hurricane. Late Wednesday afternoon, the strong winds from Hermine were already creating storm surge heights over 1’ along the entire Gulf Coast from New Orleans, Louisiana to Naples, Florida. The maximum surge was just over 2’ at Cedar Key, Florida on Wednesday afternoon. Satellite images on Wednesday afternoon showed a much more organized storm, with heavy thunderstorms building near the storm’s center and some significant low-level spiral bands forming. Wind shear continued to be a moderate 10 - 15 knots on Wednesday afternoon, but water vapor satellite imagery still showed plenty of dry air to the storm’s north and west; the combination of these factors is likely slowing down the intensification process. Sea surface temperatures (SSTs) near Hermine’s center remained favorable for development, near 30.5°C (87°F).


Figure 1. MODIS visible satellite image of Hermine on Wednesday afternoon, August 31, 2016. Note the large area of intense thunderstorms to the northeast of the storm, over the west coast of Florida. This chunk of moisture and its associated spin are from a non-tropical low pressure system, and appear poised to get entrained into the circulation of Hermine. When this happens, Hermine will expand considerably in size, and may dump heavier rains than are currently forecast. Image credit: NASA.

Intensity forecast: Hermine may become a Category 1 hurricane
The SHIPS model on Wednesday afternoon predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through landfall on Thursday evening. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models--the HWRF, DSHIPS and LGEM models--were in good agreement with their latest runs available Wednesday afternoon, with landfall intensities for Hermine ranging from 75 - 80 mph—Category 1 hurricane strength. NHC is going with a forecast of a high-end 70 mph tropical storm at landfall. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast. On Wednesday afternoon, NHC increased their maximum storm surge forecast to 4 - 6’ above ground along a stretch of the Florida coast to the right of where the center is expected to make landfall.

Track forecast for Hermine: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest Wednesday morning runs of our top models are in solid agreement that Hermine will make landfall along the Florida Big Bend coast north of Tampa on Thursday evening. In their 5 pm EDT Wednesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from Hermine along the Gulf Coast of Florida were 76%, 74%, and 64%, respectively, for Apalachicola, St. Marks and Panama City, Florida.

After landfall, Hermine will begin transitioning to a powerful extratropical storm as it sweeps northeastward along the coast, and begin deriving energy from atmospheric dynamics, rather than from the heat energy of the ocean. This extra energy source should allow Hermine to maintain tropical storm intensity as it traverses the Southeast U.S. In their 5 pm EDT Wednesday Wind Probability Forecast, NHC gave odds of at least 25% for tropical storm-force winds affecting the entire U.S. coast from northern Florida to Delaware, including Washington D.C.


Figure 2. Radar estimated rainfall between August 29 - and 6:08 pm EDT August 31, 2016 from the Tampa radar. Swaths of 2 - 4” of rain (yellow colors) were common over Florida, with 4 - 6” near Tampa.

Extremely rich moisture available to Hermine
Near record-warm ocean temperatures in the Gulf of Mexico are evaporating near-record amounts of water vapor into the atmosphere for Hermine to feed off of. At 8 am EDT Wednesday, the upper-air balloon sounding at Tampa, Florida measured 2.5” of total precipitable water (TPW)—the amount of water that would result if one condensed all the water vapor in a column above and precipitated it out. This value ranked in the upper 1% of all TPW measurements taken at the site since 1948. According to the National Weather Service, Tampa’s all-time greatest precipitable water sounding was 2.85” on September 6, 2004, when Hurricane Frances was crossing Florida (though SPC lists one higher value around 3.1”, year unknown). TPW values close to that record level were analyzed by satellite over the Gulf of Mexico on Wednesday afternoon, so it is possible Tampa may challenge its TPW record in the next day or two (thanks go to Sheldon Kusselson for some of these links).

Very heavy rains have already occurred in many locations across Florida with Hermine. A more concentrated band of 2” - 6” rains, with local totals perhaps exceeding 10”, can be expected across far northern Florida and far southeast Georgia as Hermine moves ashore. Beyond that point, the divergent model guidance keeps us guessing, but there is a good chance of very heavy rain along the Southeast coast, perhaps extending into central parts of the Carolinas. If Hermine approaches the mid-Atlantic coast and slows as much as some model runs are predicting, very heavy rains are possible in the Delmarva area (Delaware, eastern Maryland, and eastern Virginia). This area could extend north over the holiday weekend depending on Hermine’s eventual track.

Figure 3. Rainfall totals from Hurricane Agnes, which slogged through the eastern United States in late June 1972.Image credit: NOAA/NWS Weather Prediction Center.
 

In June 1972, Hurricane Agnes (only briefly a hurricane) took a largely-inland track from the Florida Panhandle into the mid-Atlantic, then moved back offshore before a second landfall near New York City. As it interacted with a strong midlatitude trough, Agnes produced massive flooding, at least 128 deaths, and $3 billion in damage (1972 dollars), making it the most expensive U.S. hurricane on record up to that point. The point is not that Hermine would mirror Agnes precisely (the upper-level forecasts are far from identical, and Agnes was much larger than Hermine), but that even a minimal hurricane or tropical storm that spends much of its life inland can cause huge amounts of rain if it moves very slowly or interacts with other weather features. We will need to be ready for a potentially significant rain/flood event if Hermine decides to slow significantly and hang out near the U.S. East Coast.

A weakening Madeline edges toward Hawaii
Hawaii’s Big Island is catching a break as Hurricane Madeline—a Category 4 storm at its peak— has been losing much of its power en route to the island. Located about 95 miles southeast of Hilo at 5 pm EDT (10 am HST) Wednesday, Madeline has weakened dramatically over the last 24 hours, with top sustained winds now down to minimal hurricane strength, 75 mph. That trend is expected to continue until Madeline is safely past Hawaii, although some restrengthening is possible as Madeline approaches Johnston Atoll. Northeast winds were gusting to 30-35 mph just after 11 am HST at Discovery Harbor, near the south tip of the Big Island. At an airport near Waimea (elevation 2600 feet), storm force winds of 43 mph, gusting to 52 mph, were reported.


Figure 4. Rainbands arc around the center of Madeline, well southeast of the Big Island, at 1716Z (5:16 pm EDT and 11:16 am HST) Wednesday, August 31, 2016.

Madeline is moving just south of due west at 13 mph, a trajectory that should take it south of the Big Island but within 100 miles of it. A tropical storm warning remains in effect for the islands of Hawaii, Maui, Molokai, Lanai and Kahoolawe. Even the weakened version of Madeline could deliver quite a punch to Hawaii, especially the Big Island. Surf as high as 25 feet can be expected along the Big Island’s east- and south-facing coastlines. Torrential rains could lead to flooding and mudslides in some locations as Madeline’s winds slam against the Big Island’s mountainous terrain. Rainfall amounts could exceed 15” along east-facing slopes.

The best place to find frequently updated local statements on Madeline’s expected impact is at a dedicated website maintained by the National Weather Service office in Honolulu. See our Wednesday morning post for more background on Hawaii’s hurricane history and what the future may have in store.


Figure 5. Enhanced infrared image of Hurricane Madeline (left) and Hurricane Lester (right) at 2013Z (4:13 pm EDT and 10:13 am HST) Wednesday, August 31, 2016. Image credit: NASA/MSFC Earth Science Office.

Category 4 Lester still a threat to Hawaii
About 1000 miles east of Hilo, Hurricane Lester continues to blast the open ocean with top sustained winds of 130 mph as of the 5 pm EDT advisory from NHC. (Advisories on Lester will be issued by CPHC starting at 11 pm EDT, as the hurricane moves west of 140°W into that agency’s area of responsibility.). Computer models agree that Lester’s westward path will start bending toward the west-northwest by Thursday, with the hurricane gradually weakening as it encounters greater wind shear and waters churned up by Madeline. The NHC outlook has Lester as a Category 1 hurricane on Saturday, as it approaches the longitude of the Big Island.


Figure 6. The official NHC forecast track of Hurricane Lester as of 2100Z (5:00 pm EDT) Wednesday.

Models agree that Lester should parallel the Hawaiian island chain as it moves west-northwest, but they disagree on how close it may come to the state. The 12Z Wednesday runs of the GFS and UKMET models bring Lester very near Oahu and Kauai on Sunday. The 12Z ECMWF keeps the storm about 100-200 miles north of the islands, as do the HWRF and GFDL models. Lester will most likely be a strong tropical storm or a Category 1 hurricane by this point. One positive factor is that a track north of the islands would put them on the weaker left-hand (south) side of Lester, reducing the potential impact. Huge surf is a safe bet.

It’s still uncertain exactly how close Lester’s path will be to Hawaii, especially Oahu, so residents throughout the state need to keep tabs on this powerful storm. On Wednesday afternoon, the NCAR/NSF Gulfstream-V jet was gathering data on the environment around Lester. The data will feed into the next round of computer model guidance (00Z Thursday), hopefully giving us more clarity on Lester’s weekend track.


Figure 7. The NCAR/NSF Gulfstream-V is assisting in high-altitude hurricane monitoring acoss the Atlantic and Pacific through mid-October while NOAA’s Gulfstream-IV undergoes unscheduled maintenance. Image credit: UCAR, photo by Carlye Calvin.

Links
Regional Hawaii radar
Mauna Kea weather (elevation 13,796’)
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu


Figure 8. MODIS visible satellite image of Invest 92L on Wednesday morning, August 31, 2016. The tropical wave was embedded in a large area of African dust to its west and north. Image credit: NASA.

92L off the coast of Africa embedded in dry air
A large tropical wave with plenty of spin that emerged from the coast of Africa on Monday was designated Invest 92L by NHC, but NHC is no longer issuing their suite of model forecasts for the system, due to the system’s lack of potential for development. The wave was just west of the Cabo Verde Islands on Wednesday, and was embedded in a major area of dust and dry air from the Sahara Desert. This dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic. The latest 12Z Wednesday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models—had one of the three, the UKMET, showing development of the system over the next five days. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through or just north of the Lesser Antilles Islands on Sunday. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC reduced their 2-day and 5-day development odds to 0% and 30%, respectively.

We’ll be back with a new post late Thursday morning.

Bob Henson and Jeff Masters