Alberto Taking on Tropical Characteristics: Warnings along NE Gulf Coast

Subtropical Storm Alberto was in transition on Sunday, taking on more characteristics of a full-fledged tropical storm as it moved north at a brisk 13 mph across the eastern Gulf of Mexico. At 2 pm EDT, Alberto was located about 135 miles west of Tampa, Florida, and about 185 miles south-southeast of Apalachicola. Top sustained winds increased on Sunday morning to 50 mph, according to the NOAA/NWS National Hurricane Center (NHC).

Update (5 pm EDT Sunday): Landfall is now expected by NHC in the western Florida Panhandle around midday Monday. Tropical storm warnings are in effect from the Mississippi/Alabama border to the Anclote River, Florida.

In a flight at midday Sunday through Alberto, Air Force hurricane hunters found top flight-level winds of close to 50 knots (57 mph) and a minimum central pressure of 994 mb, suggesting that the storm was only gradually strengthening.  Alberto’s transformation includes a more robust stacking of lower- and upper-level centers, leading to the warm core characteristic of full-fledged tropical cyclones. On the larger scale, Alberto remains quite asymmetric, with a comma-shaped cloud field evident on satellite imagery (see above) that’s more reminiscent of a midlatitude winter storm than a tropical storm. Dry air wrapping into Alberto’s east side was accentuating the asymmetry. Still, the inner core of Alberto is expected to take on enough tropical characteristics for the storm to be reclassified as a tropical storm by late Sunday or early Monday.

Figure 1. WU’s regional depiction of NEXRAD radar at 2:10 pm EDT Sunday, May 27, 2018, showed the compact center of Alberto in the eastern Gulf, with a broad band of heavy rains located well to the north and east of the center.

Alberto heading to the coast more quickly

As Alberto evolved across the southeast Gulf late Saturday, its low-level center redeveloped considerably to the north of the old center, which hastened the storm’s northward progress. Alberto’s steady northward motion on Sunday means the storm will be reaching the Florida panhandle sooner than we thought a day ago.

The operational runs of our best track models—the European, UKMET, GFS and HWRF models (see the Jeff Masters post last Friday on model performance in 2017)—are in general agreement on a faster pace for Alberto’s approach to U.S. shores, but they are divided into two camps on the specifics.

—Faster and further east: The 0Z Sunday run of the European model called for Alberto to reach the Florida coast late Sunday night or early Monday in Apalachee Bay south of Tallahassee. The 0Z, 6Z, and 12Z runs of the HWRF model paint a similar picture, with landfall near the west end of Apalachee Bay late Sunday night.

—A bit slower and a bit further west: The 0Z, 6Z, and 12Z runs of the GFS model have consistently called for a landfall between Pensacola and Panama City on Monday afternoon. The 12Z run of the UKMET model is very similar to the GFS solution.

Figure 2. The 12Z Sunday run of the HWRF model placed Alberto’s center at 3Z Monday (11 pm EDT Sunday) just south of the Florida Panhandle, with top sustained winds of 50-55 mph. Predicted winds above tropical storm strength (34 knots) are shown here in green. Please consult the official NHC forecast for details on expected landfall strength, location, and impacts. Image credit: tropicaltidbits.com.

Given the peppy pace of Alberto thus far, and the standout performance of the Euro and HWRF models in recent years, I’m leaning toward their track solutions. However, the exact landfall location with Alberto will not make a huge difference in terms of impacts. Now that it’s moving more quickly, Alberto will have less time to potentially intensify. Guidance from our top intensity models is near-unanimous that Alberto will reach the Florida coast short of hurricane strength, although it is likely to be classified as a tropical storm before it makes landfall. The official NHC forecast as of 11 am EDT Sunday split the difference among the two model camps, bringing Alberto onto the coast early Monday morning near Panama City with top sustained winds of 60 mph.

Update (5 pm EDT Sunday): The NHC has shifted its forecast toward the slower UKMET and GFS solutions, calling for a landfall midday Monday between Pensacola and Panama City. A storm surge watch remains in effect from Navarre to Crystal River, Florida. Up to 4 feet of inundation is possible in locations where Alberto’s storm surge happens to coincide with high tide. The approach of Tuesday’s full moon will add to high-tide levels.

Tornadoes are possible in Florida on Sunday

Much of Florida will be located in a favorable zone for severe thunderstorms on the east side of Alberto’s circulation. Southeasterly surface winds below southerly winds aloft will lead to vertical wind shear supportive of rotating storms and possible short-lived tornadoes. Much of Florida is in a slight risk for severe weather, according to the Sunday-morning outlook from the NOAA/NWS Storm Prediction Center.

Figure 3. Five-day rainfall totals predicted by NOAA for the period from 8 am EDT Sunday, May 27, through Friday, June 1, 2018. Image credit: NOAA/NWS/WPC.

Heavy rain the main threat

The main impact from Alberto’s approach will be heavy rains across most of the Southeast U.S. over the next few days. Dry conditions over recent weeks have left much of the Southeast abnormally dry, which means the rains will have a beneficial aspect. However, flooding may become a significant threat, especially if Alberto’s forward motion slows as expected after it moves inland. 

A dry slot wrapping around the east side of Alberto’s core circulation on Sunday night and Monday will divide heavy rains into two main swaths, one near the center of the storm and the other extending from the Florida peninsula along the coast to North Carolina. The focus of the first swath will progress northward across Alabama and into western Tennessee from Monday to Tuesday. Totals of 3-5” will be widespread, and local amounts of 7-10” can be expected in southern Alabama and far western Florida.

The second swath of heavy rain will work its way from South Florida on Sunday to the eastern Carolinas on Monday and Tuesday. Already, CoCoRaHS reports for the 24 hours ending at 7 am EDT Sunday show a number of 2”-4” rainfall totals across southeast Florida. Totals may exceed 5” across southeast Florida and over the eastern Carolinas.

Heavy rain may also affect parts of the southern Appalachians, where it’s been an exceptionally wet May. Asheville, NC, picked up 10.72” for the month through Saturday, which is already the most for any May in records back to 1869. Asheville’s all-time monthly record of 13.71” (September 2004) could be reached before Alberto is done.

We'll have another update on Alberto late Sunday night.