Hurricane warnings and storm surge warnings are flying for portions of the Gulf Coast of Florida, as Tropical Storm Idalia has begun its northwards march toward the state. Residents in evacuation areas should heed evacuation orders quickly. Idalia is expected to accelerate its forward speed, so landfall and conditions will rapidly deteriorate beginning late Tuesday morning in the afternoon across the warned areas in Florida.
At 11 a.m. EDT Monday, Idalia was located in the Yucatan Channel between Mexico and western Cuba, with top sustained winds of 65 mph and a central pressure of 990 mb, headed north at 8 mph. Idalia was bringing heavy rains to Mexico’s Yucatan Peninsula and western Cuba, as seen on satellite imagery and radar loops from Mexico and Cuba.
As anticipated by forecasters, Idalia is waiting to embark on any significant strengthening. Idalia’s heavy thunderstorms were mostly limited to the southern side of its circulation, thanks to moderate northerly wind shear of 10-20 knots, which was pushing dry air into the core of the storm. Repeated attempts by Idalia to close off an eyewall by wrapping heavy thunderstorms all the way around its center have been stymied by this wind shear on Sunday evening and Monday morning. Otherwise, conditions were favorable for development, with warm ocean temperatures near 30 degrees Celsius (86°F) and a reasonably moist atmosphere (a midlevel relative humidity near 65%).
Track forecast for Idalia
An upper-level low dipping into the northeast Gulf began tugging Idalia northward overnight, and a larger, stronger upper low will join the action by Tuesday, accelerating Idalia north-northeastward to a forward speed of 15-20 mph into the Florida Gulf Coast.
Track forecasts along the west coast of Florida are notoriously challenging because tropical cyclones are typically approaching from the southwest, in other words, at an oblique angle. That means only a slight nudge in trajectory to the right or left can change the landfall location by 50 or 100 miles. Fortunately, data from the Hurricane Hunters overnight has helped narrow down the range of uncertainty, and as of Monday morning, forecast models were in general agreement on a landfall Tuesday night or Wednesday morning in the Florida Big Bend area, between Cedar Key — about 100 miles north of the Tampa Bay area — and the central Florida Panhandle. Tampa Bay was near the edge of the 11 a.m. EDT Monday forecast cone of uncertainty from the National Hurricane Center. Keep in mind that hurricanes can and sometimes do make landfall toward either side of forecast cones.
After landfall, Idalia should be carried northeastward near or along the Southeast U.S. coast on Wednesday and Thursday before it angles eastward away from the coast around Friday. It is possible that Idalia (or its remnants) could linger along the Southeast U.S. coast through the weekend (see Tweet below).
Intensity forecast for Idalia
Idalia will be traversing very warm waters until landfall, with sea surface temperatures increasing from around 30 degrees Celsius (86°F) in the western Caribbean to 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.
A fairly moist mid-level atmosphere (65-70% humidity) is expected to prevail through at least Monday night. Wind shear is predicted to gradually abate by Monday night through Tuesday morning, when we should pay particular attention to a period of faster intensification expected to commence by then. Wind shear will increase sharply by late Tuesday as Idalia is pulled into the eastern Gulf, but the storm will be moving along with that shear, which will help limit its effect. In fact, the configuration of upper-level winds is expected to provide a powerful outflow channel aloft, as seen in several recent Gulf hurricanes that intensified sharply before landfall.
The National Hurricane Center predicted at 11 a.m. EDT Monday that Idalia would become a hurricane by Monday night and steadily intensify, peaking at category 3 strength (115 mph sustained winds) near landfall around 8 a.m. EDT Wednesday. The 6Z Monday runs of the high-resolution hurricane models (HMON, HWRF, HAFS-A, HAFS-B, and COAMPS) predicted a peak intensity Tuesday night or Wednesday morning ranging from 115 to 135 mph (category 3 or 4).
After Idalia re-emerges into the Atlantic Ocean off the Southeast U.S. coast on Wednesday night, high wind shear of 25-35 knots should prevent re-intensification, and the storm is expected to angle sharply eastward, taking it quickly away from the coast.
A large swath of the coast under storm surge warnings
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 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 of 2016.
High/low tide this week
The difference between low tide and high tide at St. Petersburg, Florida, 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.
Since Idalia will be approaching the coast at an oblique angle – not far from being parallel to the coast – slight changes in the storm’s track will cause significant changes in the timing of landfall, and thus the arrival time of the peak storm surge. If Idalia follows the centerline of the 11 a.m. EDT Monday National Hurricane Center forecast, it will make landfall near Cedar Key around 9 a.m. EDT Wednesday. This would be approximately mid-tide for Tampa Bay – not a worst-case scenario, but plenty high enough to deliver water levels in excess of four feet. This would break the all-time water level records for the two tide gauges with long-term periods of record in the Tampa Bay region (see the next section).
Top surge events in Tampa Bay
There are two tide gauges in the Tampa Bay area with long-term periods of record. Here are the top water levels in feet above high tide (Mean Higher High Water, MHHW):
- St. Petersburg (since 1947): 4.00 feet above MHHW, Aug. 31, 1985, Hurricane Elena
- Clearwater Beach (since 1973): 4.02 feet above MHHW, March 13, 1993, “Storm of the Century”
- Cedar Key (about 100 miles north of Tampa Bay, data since 1914): 5.98 feet above MHHW, Sept. 2, 2016, Hurricane Hermine
The 11 a.m. EDT Monday NHC surge forecast is calling for seven to 11 feet of inundation at Cedar Key and five to seven feet in Tampa Bay. Unless Idalia hits at low tide, this level of storm surge is likely to break all-time water level records at all three tide gauges.
Flooding and wind threat from Idalia
Idalia’s expansive circulation will bring heavy rains to a large part of the Florida Peninsula. Widespread one-inch to four-inch totals can be expected statewide, with a corridor of four- to 12-inch rains (and even higher amounts in isolated areas) along Idalia’s eventual track from its landfall location northeastward to the Carolinas. Substantial flooding close to Idalia’s track can be expected, and outer rainbands may also produce localized flash flooding. The central Gulf Coast of Florida, including Tampa-St. Petersburg, is in severe to extreme drought, so flooding may be relatively slow to occur there.
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 35% chance of experiencing sustained winds of 58 mph (50 knots), according to the 11 a.m. EDT Monday wind probability forecast from NHC. This forecast also called for a 38% chance of hurricane-force winds at Cedar Key, 18% chance at Tampa, 13% chance in the Panhandle at Apalachicola, and a 14% chance on the other side of the Florida Peninsula, from Jacksonville, Florida, to Kings Bay, Georgia.
The worst hurricanes to strike the northwest Florida peninsula
The northwest Florida peninsula, from the Tampa Bay area to the Big Bend, is among the most geographically vulnerable spots in the nation to a hurricane landfall, especially storm surge. Yet this stretch has experienced surprisingly few encounters with intense hurricanes.
In NOAA records going back to 1851, only four major hurricanes (category 3 or stronger) have made landfall or passed offshore within 100 miles of Cedar Key, including Tampa (see Figure 2 above):
• The 1896 Cedar Key Hurricane moved from the Yucatan Channel north-northeast and made landfall as a category 3 storm near Cedar Key with top sustained winds of 125 mph and a 10.5-foot storm surge. The hurricane took at least 70 lives in Florida and at least 37 in Georgia before racing up the U.S. East Coast as a destructive post-tropical cyclone. Overall, the storm wreaked $9.6 million in damage, an enormous amount at the time and roughly equal to $338 million in today’s dollars (plus, there is far more that could be damaged in a 2023 storm).
•. The 1921 Tampa Bay Hurricane was the last major hurricane to make landfall in the Tampa Bay region. This low-end Category 3 storm with 115 mph winds at landfall brought a storm tide of 10-11.5 feet (3-3.5 meters), causing severe damage ($10 million in 1921 dollars, or more than $100 million in today’s dollars) Arriving on Oct. 25, it was the nation’s latest major landfall in any calendar year until Hurricane Zeta struck Louisiana on Oct. 28, 2020.
• The catastrophic 1935 Labor Day Hurricane, one of only four storms on record to strike the U.S. Gulf or Atlantic coast at category 5 strength, hugged the west coast of Florida after ravaging the Keys at peak strength and taking more than 400 lives there. The hurricane passed about 30 miles west of the Tampa Bay area while still at category 3 strength, then made landfall as a category 2 north of Cedar Key near Steinhatchee with top winds of 100 mph.
• Despite its innocuous name, Hurricane Easy of 1950 was a prolonged, disastrous experience for Northwest Florida. Easy angled into the coast just south of Cedar Key as a category 3 storm, with top sustained winds of 120 mph, then made a hard right turn near the coast and moved south before heading inland near Spring Hill about 12 hours later. About half of the buildings in Cedar Key were severely damaged or destroyed, and the storm’s slow movement led to enormous multi-day rainfall totals, including 45.2 inches at Yankeetown, Florida, 38.7 inches of which fell in 24 hours (still the 24-hour rainfall record for any tropical cyclone in Florida).
Two other notable storms occurred just before the 1851 start date for NOAA’s hurricane database. The Cedar Key Hurricane of 1842 struck on Oct. 5 just north of Cedar Key, most likely at category 3 strength. The storm arrived on a worst-case trajectory for inundation, delivering a 27-foot storm surge, destroying many buildings on Cedar Key (then known as Depot Key) and prompting the U.S. Army to abandon a detention camp there. The Great Gale of 1848, the most violent hurricane in Tampa’s history, roared ashore on Sept. 25 as a Category 3 or 4 hurricane with 115-135 mph winds. A 15-foot storm surge (4.6 meters) was observed in what is now downtown Tampa, and the peninsula where St. Petersburg lies in Pinellas County was inundated, making St. Petersburg an island. A large portion of what few human structures were then in the area were destroyed.
Of note: When the 1921 hurricane hit Tampa Bay, there were 160,000 residents in the 4-county region, most of whom lived in communities on high ground. Today there are more than 3.2 million residents in the fast-growing region, with about half of the population living at an elevation of less than 10 feet. An August 2015 report by Karen Clark & Company, Most Vulnerable US Cities to Storm Surge Flooding, cited Tampa/St. Petersburg as the most vulnerable metropolitan area in the U.S. to storm surge damage. Their 1-in-100 year storm — a strong Category 4 hurricane with 150 mph winds — could be expected to cause $175 billion in damage just from the storm surge.
Hurricane Franklin rapidly intensifies into the first major hurricane of 2023
Hurricane Franklin put on a very impressive burst of rapid intensification, its winds increasing by 70 mph in the 24 hours ending at 11 a.m. EDT Monday and is now a large and powerful category 4 hurricane with 145 mph winds. Franklin was located midway between the Bahama Islands and Bermuda, and the only populated area at risk of direct impacts from Franklin is Bermuda. NHC gave the island a 47% chance of seeing tropical storm-force winds by Thursday morning. Franklin is predicted to make a run at category 5 strength, peaking as a top-end category 4 storm with 155 mph winds on Monday night.
The only other area in the Atlantic of note is a tropical wave that could slowly develop as it moves west-northwest after it moves off the coast of Africa on Tuesday. In their 8 a.m. EDT Monday Tropical Weather Outlook, NHC gave this system two-day and seven-day odds of development of 0% and 50%, respectively.
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