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Hurricane preparations should be rushed to completion along the coast of Florida as intensifying Hurricane Idalia steams northward. People in evacuation areas need to heed evacuation orders quickly, since Idalia has accelerated its forward speed and conditions will rapidly deteriorate across the warned areas in Florida today.

As anticipated by forecasters, Idalia waited until Tuesday morning to close off an eyewall by wrapping heavy thunderstorms all the way around its center, attaining hurricane status at 5 a.m. EDT Tuesday. At 11 a.m. EDT Tuesday, Idalia was located about 270 miles south-southwest of Tampa, Florida, with top sustained winds of 85 mph and a central pressure of 976 mb. Idalia had accelerated from its leisurely northward pace of 8 mph on Monday night, and was now moving north at 14 mph.

Satellite images at midday Tuesday showed that Idalia was growing increasingly well-organized, with more intense heavy thunderstorm activity and an expanding region of low-level spiral bands. Radar loops from Key West showed that heavy rains from Idalia were affecting much of western Cuba and the southwest coast of Florida, plus the Lower Florida Keys.

Track forecast for Idalia

An upper-level low to the north will pull Idalia north to north-northeastward until landfall on the Florida Gulf Coast in the Big Bend area occurs Wednesday morning. There is still some spread in the models, with the GFS and its ensemble members predicting a more westerly track, and the European model and its ensembles taking Idalia more to the east. There is not a lot of difference in the predicted forward speed of the hurricane, with the top models all agreeing that landfall will occur between 9 a.m. and noon EDT Wednesday.

Notably, although the northeast Florida Gulf Coast has seen many hurricanes, there is no record of any major hurricane (category 3 or stronger) having made landfall in Apalachee Bay or most of the Big Bend – between Apalachicola and Cedar Key – in the NOAA historical database extending back to 1851.

After landfall, Idalia should be carried northeastward just inland along the Southeast U.S. coast on Wednesday before it emerges over the Atlantic on Thursday, off the coast of the Carolinas. It is possible that Idalia (or its remnants) could linger several hundred miles offshore of the Southeast U.S. coast through the weekend. The long-term fate of the storm next week is unclear, with the European model predicting Idalia could bother Bermuda by next Tuesday, then head back toward the U.S., while the GFS model predicts a weakened Idalia could loop back across Florida and into the Gulf of Mexico. (Such loops are not unprececented; one occurred with Hurricane Ivan in 2004.)

Two maps showing track forecasts for Idalia.
Figure 1. Track forecasts out to five days for Idalia from the 06Z Tuesday run of the European ensemble model (left) and GFS ensemble model (right). Individual forecasts of the 51 Euro and 31 GFS ensemble members are the lines color-coded by the wind speed in knots they predict for Idalia; red colors correspond to a category 1 hurricane. The time in hours from the model initialization time is in gray text. Long-range models such as the European and GFS models are less reliable guides to storm intensity than higher-resolution, shorter-range models such as HWRF, HMON, and HAFS. (Image credit: weathernerds.org)

Intensity forecast for Idalia

Idalia will have nearly ideal conditions for intensification until landfall, as it traverses waters of 31 degrees Celsius (88°F) in the eastern Gulf of Mexico. These warm waters extend to great depths, providing high oceanic heat content and reducing the chance that Idalia’s slow movement will cause major upwelling of cooler water.

Idalia will have a moist midlevel atmosphere (70% humidity) through landfall, and wind shear is predicted to be light to moderate (5-15 knots). Wind shear will increase sharply near the time of landfall, but this will likely be too late to weaken the hurricane. In fact, the configuration of upper-level winds in the final few hours before landfall is expected to provide a powerful outflow channel aloft, allowing Idalia to continue intensifying until it moves inland over Florida.

Read: How to prepare for a hurricane

The National Hurricane Center predicted at 11 a.m. EDT Tuesday that Idalia would peak just before landfall as a high-end category 3 hurricane with 125 mph sustained winds. Landfall was predicted around 9 a.m. EDT Wednesday. The 6Z Tuesday runs of four of the high-resolution hurricane models (HMON, HWRF, HAFS-A, and HAFS-B) predicted a peak intensity Tuesday night or Wednesday morning ranging from 125 to 140 mph (category 3 or 4).

After Idalia reemerges into the Atlantic Ocean off the Southeast U.S. coast on Thursday, high wind shear of 25-35 knots should prevent re-intensification.

A large swath of the coast under storm surge warnings

Like a giant stepping into a bathtub, Idalia’s emergence into the Gulf of Mexico has already raised water levels by 0.3-1.2 feet along a huge span of the coast, from Texas to Florida (see the NOAA Tides and Currents Idalia page). At Key West, Idalia caused minor flooding during the Tuesday morning high tide cycle, with a water level 0.94 feet above normal high tide. Minor coastal flooding also occurred Tuesday morning at Southwest Pass, Louisiana, with a water level 1.15 feet above the normal high tide.

Idalia will pose a significant storm-surge threat over most of the Florida Gulf Coast, mainly near and to the right of where the center makes landfall. Idalia has a large circulation, and a larger hurricane will deliver a higher storm surge. A dangerous storm surge is predicted from the eastern Florida Panhandle to southwestern Florida – plus a portion of the Georgia coast, on the other side of the Florida Peninsula. The National Hurricane Center warned that catastrophic impacts are expected somewhere between Aucilla River and Yankeetown, Florida, from storm surge inundation of 10 to 15 feet above ground level overtopped by destructive waves. This includes Cedar Key (population 700).

The Florida Gulf Coast has a wide continental shelf with shallow waters, providing an ideal situation for high storm surges to build up. Furthermore, because of the unique water depths in the region, hurricanes tracking from south to north off the west coast of Florida can create a “shelf wave” of higher surge that can propagate northwards along the coast, inundating the coast over 100 miles to the east of where the hurricane tracks. Even a Category 1 hurricane can deliver a storm surge over seven feet high to the Big Bend area – as occurred during Hurricane Hermine in 2016.

Peak storm surge for Tampa Bay likely to occur near mid-tide

The difference between low tide and high tide at St. Petersburg, Florida, located in Tampa Bay, is about 3.2 feet, so the precise timing of the storm surge with respect to the tide matters a great deal. Low tide is at 8:43 p.m. EDT Tuesday; high tide is at 1:54 p.m. EDT Wednesday. Unfortunately, this month’s full moon occurs Wednesday night, so the tides that day are among the highest of the month.

If Idalia follows the centerline of the 11 a.m. EDT Tuesday, National Hurricane Center forecast, the hurricane will make landfall in the Big Bend area around 9 a.m. EDT Wednesday, making its closest pass by Tampa Bay several hours earlier. During Category 1 Hurricane Hermine of 2016 (which also made landfall in the Big Bend area), the peak storm surge in Tampa Bay occurred about an hour before the hurricane made landfall (Figure 2). If this same behavior holds true for Idalia, Tampa Bay can expect its highest storm surge sometime between 7-10 a.m. EDT Wednesday. This would be approximately mid-tide for Tampa Bay – not a worst-case scenario. But given that the 11 a.m. EDT Tuesday NHC surge forecast is calling for four to seven feet of inundation above normally dry ground in Tampa Bay, if the surge hits at high tide, a surge of this magnitude would break the all-time water level records for the two tide gauges with long-term periods of record in the Tampa Bay region (back to 1947 and 1973), as discussed in yesterday’s post. It is known that the last major hurricane to strike the Tampa area, the category 3 1921 hurricane, brought higher water levels.

Hurricane Hermine storm surge (2016)
Figure 2. Regional radar of Hurricane Hermine at 11 p.m. EDT September 1, 2016, with time traces of the storm surge at tide gages in Apalachicola, Cedar Key, and Tampa, Florida. The storm surge (green line) had peaked in Tampa at this time, but was still rising at Apalachicola and Cedar Key. Surge images were taken from the NOAA Tides and Currents page for Hermine. (Image credit: Weather Underground, an IBM company)

Flooding and wind threat from Idalia

Idalia’s expansive circulation will bring heavy rains and strong winds to a huge area of Florida and the Southeast Coast. Strong winds extend out a considerable distance to the east of Idalia, and a 50% chance or greater of tropical-storm-force winds is predicted along a long swath of the Florida coast, from southwest Florida (near Fort Myers) to the central Panhandle (near Apalachicola). Wind damage will be highest where the eyewall comes ashore and along a considerable distance inland. Gainesville (population 140,000), located about 50 miles inland from the coast, was being given about a 14% chance of experiencing hurricane-force winds, according to the 11 a.m. EDT Tuesday wind probability forecast from NHC. This forecast also called for a 46% chance of hurricane-force winds at Cedar Key, 3% chance at Tampa, 13% chance in the Panhandle at Apalachicola, and a 10% chance on the other side of the Florida Peninsula, at Jacksonville. Even tropical storm-force winds can be enough to bring down trees and power lines, especially in rain-saturated soils, so residents should be ready for potential disruption over a broad area.

Hurricane Franklin peaks as a Cat 4 with 150 mph winds

Hurricane Franklin peaked Monday night midway between the Bahama Islands and Bermuda as a powerful Category 4 hurricane with 150 mph winds and a central pressure of 926 mb, becoming the most powerful Atlantic hurricane ever observed so far north in the open Atlantic (not including the Gulf of Mexico).

At 11 a.m. EDT Tuesday, Franklin was located about 345 miles west-southwest of Bermuda, with top winds of 130 mph (Cat 4) and a central pressure of 941 mb. NHC gave Bermuda a 45% chance of seeing tropical storm-force winds by Thursday morning. Franklin is not predicted to threaten any land areas besides Bermuda, becoming post-tropical on Saturday over the central Atlantic.

Tropical Depression 11 could become a short-lived tropical storm

A disturbance in the central subtropical Atlantic well east of Franklin was upgraded to Tropical Depression 11 at 11 a.m. EDT Tuesday. TD 11 has had a fairly persistent core of strong showers and thunderstorms since Monday night, with outflow evident at upper levels. Sea surface temperatures are above average, around 29 degrees Celsius (84°F), and TD 11 will have weak to moderate wind shear of 5 to 15 knots through Tuesday night, with a fairly moist atmosphere on Tuesday (mid-level relative humidity of 60-65%). Wind shear will increase by late Wednesday, driving drier air into TD 11. Only a few of the GFS and European model ensemble members develop TD 11, but the shorter-range intensity models HWRF, HAFS-A, and HAFS-B agree that TD 11 could become a weak tropical storm from late Tuesday into Wednesday, as now predicted by the National Hurricane Center, as it drifts northward far from any land areas. The next name on the Atlantic list is Jose.

New African tropical wave

A tropical wave could slowly develop as it moves west-northwest after moving off the coast of Africa on Tuesday morning. In their 8 a.m. EDT Tuesday Tropical Weather Outlook, NHC gave this system two-day and seven-day odds of development of 20% and 50%, respectively.

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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...