Published: July 28,2015
(MORE: Expert Analysis | Hurricane Central)
Whether or not this low-pressure center becomes a full-fledged tropical cyclone, the weather will remain unsettled for a while over parts of the Sunshine State.
(MORE: Florida Flooding Photos and News)
Let's break down what we know now about this still-evolving situation.
The Setup
Satellite and Surface Weather Map
A frontal boundary stalled Sunday from near Bermuda southwestward to near the northwest Bahamas, across the Florida peninsula to the eastern Gulf of Mexico.
Boundaries such as this one help focus clusters of showers and thunderstorms over the mainland in the spring, summer and fall, and the same can be true, at times, in the Gulf of Mexico as well as the subtropical Atlantic basin.
As expected, a weak area of low pressure developed at sea level along that stationary front over the weekend, and weak low pressure also formed in the upper levels of the atmosphere over that same area. The sea-level zone of low pressure has been drifting toward the west coast of Florida since then and was just north of Tampa Bay as of Tuesday morning.
The warm waters in the Gulf Stream current of the western Atlantic, as well as the northeast Gulf of Mexico itself, are adding warmth and moisture to the air, increasing instability, or the ability to generate convection (thunderstorms). This is a factor favorable for tropical cyclone development.
The stationary front itself has dissolved, since the air north of the front has become about as warm and muggy as the air south of it. This turn of events is also favorable for tropical cyclone development, as tropical cyclones cannot form in the presence of strong temperature gradients.
In theory, if clusters of showers and thundershowers can develop in the same place as the low-pressure center and persist, it could eventually develop into at least a tropical depression.
Tropical Development: Will It or Won't It?
The National Hurricane Center defines a tropical cyclone as "a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation." The phrase "tropical cyclone" is the blanket term for tropical depressions, tropical storms, and hurricanes, which are simply different categories of intensity among tropical cyclones.Arguably the most subjective part of the definition is the word "organized." While the National Hurricane Center described the low-pressure center as a "surface weakness" in its Monday morning weather discussion, thunderstorm activity has increased since then, organizing into spiral bands mainly south of the circulation center.
Senior meteorologist Stu Ostro of The Weather Channel said, "This is in the category (no pun intended) of, 'If it looks, swims and quacks like a tropical depression.' It arguably is meeting the meteorological definition of that. Regardless, the system affecting Florida with heavy rain, brisk breezes and an elevated rip current risk."
But no matter what term you use to describe the Gulf low, winds in the upper atmosphere are expected to remain hostile for development. The National Hurricane Center had been highlighting an area from the eastern Gulf of Mexico to off the Southeast coast for possible subtropical or tropical development during the next five days. They dropped that highlight early Sunday morning and have not reintroduced it.
The weak area of low pressure is expected to move northeast across Florida on Tuesday, further crimping its already-minimal chances of turning into a tropical cyclone. After that it will move away into the Atlantic Ocean, but showers and thunderstorms are expected to linger for several more days in its wake over Florida.
Rainfall Forecast
Current Flood Alerts, Radar
Even though we are unlikely to see a tropical depression or storm form, there are two significant impacts that are resulting from this pattern.
Heavy Rain and Flood Threat
While at least isolated thunderstorms are almost a daily occurrence in mid-summer in Florida, there is a swath of the state that will see repeated clusters of storms with heavy rain into the new week ahead.Some flooding is likely where rainfall persists the longest. Flash flood watches are in effect for a part of the west-central Florida coast, including the Tampa Bay area, through Tuesday evening.
More than 11 inches of rain fell near Holiday, Florida, in the 72 hours ending 10:19 p.m. EDT Sunday, resulting in some reports of flooding. A volunteer observer near Tarpon Springs, Florida, has reported 11.39 inches of rain since Friday. Many areas of Pasco and Pinellas counties have received at least 6 inches of rain since Friday.
Flooding on the Anclote River from heavy rain the last several days resulted in an evacuation of some homes in Elfers (southwest Pasco County) on Sunday.
Here is a general look at the forecast:
- Most numerous thunderstorms, flood threat: Tampa Bay Area, central Florida, Nature Coast, Daytona Beach area (FORECASTS: Orlando | Daytona Beach | Tampa/St. Pete)
- Less numerous thunderstorms: Southern third of Florida (FORECASTS: Miami | Key West)
Of course, this doesn't mean a complete washout is likely in the areas mentioned above for most numerous thunderstorms. There will be some breaks in the rain.
However, keep in mind your outdoor plans may be interrupted multiple times during the week ahead.
Incidentally, parts of Florida need the rain.
The most serious drought in the mainland U.S. is in south Florida, with "extreme" drought in parts of Miami-Dade and Palm Beach Counties, according to the latest Drought Monitor analysis.
Miami International Airport is running a 2015 rainfall deficit of nearly 12 inches. Several groundwater wells in Palm Beach and Miami-Dade Counties have plunged to the lowest 10 percent of levels on record, according to the U.S. Geological Survey.
As mentioned above, while parts of drought-stricken southeast Florida will see some rain, the heaviest rain will generally target areas of the Sunshine State not in drought.
High Surf/Rip Currents
Northeast winds may produce elevated surf along parts of the Southeast coast, from Florida's First Coast to southeast Georgia and South Carolina.Accompanying this will be a threat of rip currents. Keep this in mind particularly if you're vacationing in these areas. Rip currents can and often do occur when the weather is otherwise tranquil. Heed all warnings from lifeguards.
Onshore winds are currently contributing to a high risk of rip currents along central and southern portions of the Florida Peninsula's west coast, bordering the Gulf of Mexico.
It's Happened Already This Season
Again, we think the chance of a tropical depression or storm forming in the eastern Gulf of Mexico or off the Southeast coast is very low, but not zero.This is a relatively common early-season pattern known to generate tropical cyclones near the U.S. coast.
Despite high wind shear squashing any chance of tropical development in the Caribbean Sea, the Atlantic Basin has manufactured three named storms so far this season, all either close to, or landfalling in, the U.S.
On Mother's Day weekend, Tropical Storm Ana was the earliest East Coast landfall on record.
A little more than a month later, Tropical Storm Bill soaked an already saturated southern Plains, Ozarks and Ohio Valley.
Finally, a disturbance that originated from a Midwest thunderstorm cluster sprouted convection over the Gulf Stream and developed into Tropical Storm Claudette on July 13.
Last July, Hurricane Arthur formed in this same general area, off the southeast U.S. coast, pounding eastern North Carolina and southeast Virginia on the July 4 holiday weekend.
If you have vacation plans this week from the northern and eastern Gulf Coasts to Florida and the Southeast coast as far north as the Outer Banks of North Carolina and southeast Virginia, don't cancel them yet.
But, do check back with us at weather.com, Weather Underground, and The Weather Channel for the latest on this situation.
Meteorologist Chris Dolce contributed to this report.
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