Scattered, Smothered & Covered

by Chip Kasper

Scattered, Smothered & Covered

Is it going to rain today?  This seems like a simple enough question, but if this question is posed to a professional weather forecaster based in the Florida Keys, the response likely will range from a long pause to a bewildered stare.  Now, why is that?  The weather people are supposed to know what is going on, and it is a simple question.  A “yes” or “no” will do.  Well, as it turns out, the question is not at all that simple, not here in the Florida Keys, and especially, not during the rainy season.  Reasons for this complexity are associated with the unique rainy-season climate of the Florida Keys, where rain showers typically are found somewhere within about a 100-mile radius of the Keys most days from early June through late September.  Indeed, Florida Keys climate statistics reveal that about 68% of the yearly rainfall total and 91% of the yearly lightning occurs during the four months of June, July, August, and September.  However, determining exactly when and where those rain showers and thunderstorms occur, hour to hour, and mile by mile, is highly unpredictable.  Often, downpours will wet one island, and skip another.  Boaters, divers, or anglers on one section of the Florida Reef tract may be visited by lightning and violent squalls, while those only a few miles away enjoy abundant sunshine and rain-free skies.  Rarely do overcast skies and widespread rain and storms envelope the entire archipelago and adjacent reefs and waters.FishMonster Magazine-Sept/Oct 2016

Perhaps the first thing to understand about rain during the Florida Keys rainy season is the fact that it falls mostly from cumulus-type clouds versus stratus-type clouds.  Cumulus-type clouds are those that tend to “bubble” upward in the atmosphere from bases not far off the ground (typically, around 2000 feet elevation).  These cloud forms include both cumulus congestus and cumulonimbus, the thunderstorm cloud.  They are taller than they are wide, and the associated rain areas typically cover less than a few miles of horizontal distance.  Rain from cumulus-type clouds tends to be intense, localized, and short-lived.  Stratus-type clouds, on the other hand, are more “layered’ in appearance, and much shallower than cumulus clouds.  They often develop in expansive decks, covering thousands of square miles.  These cloud forms include altostratus and nimbostratus.  Rain from stratus-type clouds tends to be lighter, widespread, and longer lasting (think Seattle or London).  The stratus-type rain occurs most often with large-scale weather systems associated with the midlatitude jet stream.  These types of weather systems typically will remain far north of the Florida Keys during the June-September rainy season.  In the atmosphere, as a general rule, large-scale phenomena are more predictable than small-scale phenomena.  For example, the general paths of many hurricanes and major winter storms can be predicted within a reasonable degree of certainty several days in advance.  However, predicting the exact time and location of a tornado is nearly impossible. 

Another complicating factor in Florida Keys rainy-season weather forecasting is the absence of the sea breezes which modulate daily rainfall patterns over much of the Florida Peninsula.  The accompanying graph illustrates the influence that Florida Peninsular sea breezes have on the daily rainfall patterns at mainland South Florida locations versus Key West.  Note that at Fort Myers, Miami, and Fort Lauderdale, rain chances spike during the afternoon hours as sea breezes develop at the coast, and help initiate showers and thunderstorms through lifting of the hot, moist air at the surface.  Sea breezes of such intensity do not occur in the Florida Keys where surrounding waters keep the island air slightly cooler than over mainland South Florida.  In fact, the hourly relative frequencies of rain, or “rain chances” at Key West show little or no pattern during the 24-hour day, with rain about as likely to occur during the dark of night as it is during the middle of the afternoon.

Despite the “unpredictability” of rain in the Keys, the climate statistics do offer some useful insight, mainly related to the “steering” winds of the lower atmosphere (surface to about 10,000 feet elevation).  The most common steering wind regimes are when winds are blowing out of the east and southeast (not surprising given the Keys’ location at the northern fringe of the tropical trade-wind belt).  However, the “wettest” wind regime is when winds are blowing from the northeast.  In this regime, sea-breeze thunderstorms move southwestward off the Everglades toward the Keys; in addition, lines of cumulus clouds, showers, and thunderstorms can develop along or parallel to the Keys archipelago which runs east-northeast to west-southwest.  The least common and driest rainy-season wind regime is when winds are blowing out of the northwest. 

The most effective means by which forecasters have come up with to communicate the likelihood for “measurable” rain (0.01 inch) are probabilities.  The daily forecast communicates the chance of rain for a 12-hour period (6:00 a.m. to 6:00 p.m. or 6:00 p.m. to 6:00 a.m.) at any point in the forecast area.  In marine and aviation products, “coverage” terms typically have been used due to the transient nature of these users.  Examples include:  “numerous showers”, “scattered showers”, or “isolated thunderstorms”.  Here, these coverage terms are equated to ranges of probabilities (“isolated” = less than 25 percent; “scattered” = 25–54%; “numerous” = 55–79%; and categorical (e.g., “showers”) = 80% or greater).

For more immediate decision making, the “Short Term Forecast” is available, usually once per hour, offering an assessment of location, timing, and impacts of rain and storm areas based on Doppler radar interpretation.  Of course, the National Weather Service Doppler Radar images are available in near real time for any and all to see as well.  These sources are available online here:

Short Term Forecast:
http://forecast.weather.gov/product.php?site=NWS&product=NOW&issuedby=KEY

National Weather Service Key West
Doppler Radar Data:

http://radar.weather.gov/radar.php?rid=byx
Remember to check the radar and the forecast before heading out on the water, and as always, be weather-ready, and stay safe!




Chip Kasper
Chip Kasper

Author

Chip is a senior forecaster and marine program meteorologist at the NOAA/National Weather Service Forecast Office in Key West. The National Weather Service provides weather, water, and climate information for the protection of life and property on land and at sea. Email Chip at kennard.kasper@noaa.gov.



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