Waiting on Alberto: Tropical Storm in Gulf Still on the Table

Subtropical Storm Alberto began moving into the southeast Gulf of Mexico on Saturday, and computer models agree it has a good chance to transition into a tropical storm as it heads toward the central Gulf Coast. As of 11 AM EDT Saturday, the poorly defined center of Alberto was in the Yucatan Channel about 20 miles west of the western tip of Cuba. Top sustained winds were 40 mph. Alberto was moving north at about 10 mph, and that general trajectory is expected to continue until Alberto reaches U.S. shores on Monday or early Tuesday.

Update (11 am EDT Sunday): Tropical storm warnings are in effect from the Mississippi/Alabama border east and south to Bonita Beach, Florida.

Subtropical storms are typically driven by large-scale upper-level processes more than by warm water (see our subtropical storm tutorial for more information on the difference between a tropical and subtropical storm), and Alberto fills the bill. An upper-level trough to the west of Alberto has fed energy into the system, but it has also kept Alberto’s broad center under persistent southwesterly wind shear. As a result, virtually all of the showers and thunderstorms are on the east side of Alberto’s gyre-like low-level circulation, a common occurrence with subtropical storms that’s especially evident in this case (see image at top). Alberto’s former low-level center was left behind on Friday as the bulk of the storm moved north.

On Saturday morning, hurricane hunters found a new low-level circulation attempting to form about 75 miles north-northwest of the western tip of Cuba. Thunderstorms were bubbling near that new center, and the central pressure was down to at least 998 mb, so it is likely that Alberto will be officially relocated northward on Saturday afternoon.

And just like that, it already looks like #Alberto has a center reformation in progress just N of #Cuba! Nice burst of lightning with the new vortex which suggests significant updrafts stretching & amplifying llvl vorticity.

Animation w/ lightning overlay from @CODMeteorology pic.twitter.com/tnWFbeRLqM

— Philippe Papin (@pppapin) May 26, 2018

Forecast for Alberto

Conditions will be getting more favorable for Alberto to strengthen as the holiday weekend unfolds. The 12Z Saturday run of the SHIPS statistical model shows that wind shear will drop from an unfavorable 20-25 knots on Saturday to only around 10 knots from Sunday through Monday. Sea surface temperatures (SSTs) along Alberto’s projected path through the eastern Gulf are around 27-28°C (81-82°F), which is just warm enough to support tropical development. As wind shear relaxes, there will be more opportunity for a low-level center to consolidate and become vertically stacked beneath the mid-level low, which would herald Alberto’s likely transition into a tropical storm.

The amount of deep oceanic heat content is fairly low along most of Alberto’s path, so Alberto is not expected to strengthen rapidly, especially given its current disorganized state.

Our best track models—the European, UKMET, GFS and HWRF models (see the Jeff Masters post last Friday on model performance in 2017)—agree that Alberto’s center will carry out a slight cyclonic bend along its northward trek, as the low-level center translates around the broad circulation. By late Sunday, Alberto is expected to be within 200 miles of the Gulf Coast, with a good chance of evolving into a tropical storm. At this point, its forward motion will likely be slowing as steering currents weaken, and it may take until late Monday for Alberto to make landfall. The 0Z run of the ECMWF model and the 0Z, 6Z, and 12Z runs of the GFS model agree on placing Alberto’s center near the western end of the Florida panhandle by Monday afternoon. The 0Z and 6Z HWRF model runs were somewhat faster, putting Alberto near the coast on Monday morning.

The Friday night runs of our top intensity models—the HWRF, DSHIPS, COAMPS-TC, LGEM and HMON models—generally pulled back on Alberto’s anticipated strength in the northeast Gulf. Only HMON had Alberto approaching the coast as a minimal hurricane. Most runs from 0Z and 6Z had Alberto making landfall as a low- to mid-range tropical storm. However, we will have to watch closely for changes in the model runs at 18Z and 0Z now that Alberto appears to be forming a new and somewhat stronger low-level center to the north of where models placed it before.

Alberto’s slow motion will extend the length of time that tropical storm conditions prevail near and east of its center. A long period of high seas can be expected. 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, and flooding is possible across multiple tidal cycles if Alberto slows as expected. Update (11 am EDT Sunday): A storm surge watch is in effect a large swath of the northeast Gulf Coast, from the Alabama/Florida border to Crystal River, Florida.

Figure 1.  Five-day rainfall totals predicted by NOAA for the period from 8 am EDT Saturday, May 26, through Thursday, May 31, 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 slows or stalls near the Gulf Coast.

A large mass of dry air in the western Gulf will be wrapping around Alberto’s south and east sides on Sunday and Monday (see Levi Cowan’s video discussion from Friday night), which complicates the rainfall forecast somewhat. It appears Alberto may be able to wall off a moist center and maintain its intensity on Monday, but a relative dry slot east of the center may divide Alberto’s heaviest rains into two swaths, one near the center and the other extending from the Florida peninsula along the coast to North Carolina. The focus of the first swath would most likely progress across and near Alabama from south to north on Monday and Tuesday, while the second swath will work its way from South Florida on Sunday to the eastern Carolinas on Monday and Tuesday. Even further north, moisture channeled by Alberto will encounter a weak front, which could lead to a band of 1-3” rains over the water-weary mid-Atlantic.

Rainfall totals by midweek could reach 10” or more near the coast of Alabama and far western Florida, with 2”-7” totals widespread across the entire Southeast.

Figure 2.  A dry slot is clearly evident in the GFS 24-hour forecast for Subtropical Storm Alberto, valid at 8 am EDT Sunday, May 26, 2018. The tan colors southeast of Alberto’s center in the Gulf of Mexico denote relative humidities below 50% in the layer from about two to four miles above the surface, which are predicted to drop below 50%.

Climatology of named Atlantic storms in May

According to Dr. Phil Klotzbach (CSU), since 1950, there have been 14 years with named storms in May—an average of one May storm every five years. Those 14 years had near-average activity for the entire season, so a May storm is not necessarily a harbinger of an active hurricane season to come. He also reports that the earliest date the U.S. has seen a hurricane landfall since record-keeping began in 1851 is June 9, 1966, when Category 2 Hurricane Alma hit Florida. Wunderground member Mark Cole reported this interesting stat: 2018 is the fourth season in a row for the Atlantic to see a named storm before the official June 1 start of hurricane season. That has happened only one other time in recorded history, back in 1951 – 1954:

YEAR     DATE     NAME

1951       Jan 4     Tropical Storm One
1952       Feb 3     Tropical Storm One
1953       May 25   Tropical Storm Alice
1954       May 28   Tropical Storm One
2015       May 8     Tropical Storm Ana
2016       Jan 12    Hurricane Alex
2017       Apr 19    Tropical Storm Arlene
2018       May 25   Subtropical Storm Alberto

Dr. Jeff Masters co-wrote this post. If there is no significant change to Alberto, our next post will be Sunday.