Rainy Daze

by Chip Kasper

Rainy Daze

Rain is a rather peculiar weather phenomenon, and one that often garners a great deal of attention from both the weather wise and the otherwise. If you have lived in or traveled to various corners of North America, you, perhaps, may have realized that a “California rain” differs from a “Florida rain”, and that a “Seattle rain” often is distinct from a “New England rain”. Think about all of the terms we have for various kinds of rain, from drizzles to downpours. One can observe “intermittent” rain or “steady” rain, and “vertical” rain versus “horizontal” rain. Rain can be mixed with sleet, snow, hail, or even salt. Up north, folks talk about “freezing” rain. In the Florida Keys, we have spring rains and sunshowers, and summer rain from cumulus towers. On some days, rain may take the form of a fine mist, whereas on others, it may fall in a “cloudburst”.

Rain is necessary, but too much of a good thing can be a bad thing on the water, from both a comfort and safety perspective. The details matter (rainfall rate, intensity, duration, presence of lightning or wind) when considering the impacts on a voyage (planning, navigation, vessel maneuvering, passenger safety and comfort, etc.). However, these specific attributes of “rain” are not only difficult and often impossible to predict, but also are not explicitly forecast by your local weather people. In fact, even when asked the simple question, “Is it going to rain?”, the meteorologist often directs their perplexed gaze toward the sky--a “rainy daze”, perhaps. The answer often is, “It depends”. Since the mid-1960s, the National Weather Service has been preparing “probability of precipitation” (POP) forecasts. At the Florida Keys National Weather Service, our POP is defined as the probability of at least 0.01 inch of rainfall at a point in a particular forecast zone during a 12-hour period (6:00 a.m. to 6:00 p.m. or 6:00 p.m. to 6:00 a.m.). The POP (chance of rain) is just a number, and says nothing about many of the characteristics of rainfall that matter. In addition, the POP gives you no information about the prospects for flooding rain or wind-driven rain, and it will not tell you if it is going to rain once, twice, or thrice during the day or night.

Meteorology is a science that always has evolved with technology, and there are some very promising applications of weather prediction modeling and telecommunications technology in the works. These advances may one day soon improve our forecasts of rain and its impacts. In the interim, we do offer a few ways beyond the POP for you to obtain additional information about the prospect for a rainy day. The “Area Forecast Discussion” is a prognostic narrative written by the forecaster on duty, which gives one a glimpse of the forecast challenge of the day, the scientific uncertainty and associated confidence level in a forecast, and even some additional details regarding actual impacts. With rain, that may be information on rainfall intensity and associated reductions in horizontal visibility. Visibility is important for watercraft, aircraft, and land-based vehicles alike. The Area Forecast Discussion also may provide additional details on expected rain rates and potential for street flooding, or whether multiple episodes of rainfall are expected at a particular location during a given time frame. Another helpful and freely available product is the “Short Term Forecast” or “Nowcast”. This product is a very effective addendum to a radar animation, focusing on the next one to three hours, and providing information pertaining to the location, movement, and impacts from rain showers, squalls, and thunderstorms. It helps answer the question, “Will this affect me, and, if so, how?”.

You may access both the Florida Keys “Area Forecast Discussion” and “Short Term Forecast” at the following websites:

Area Forecast Discussion:https://forecast-v3.weather.gov/products/locations/KEY/AFD/1

Short Term Forecast:https://forecast-v3.weather.gov/products/locations/KEY/NOW/1

Bookmark these links, and place them next to your Doppler radar link/app, and you will be on your way to going “beyond the POP”. Finally, you may access the Short Term Forecast and all available marine weather observations, forecasts, warnings, and advisories via the NOAA All Hazards Weather Radio broadcasts on marine VHF Channels 2 (lower/middle Keys), 5 (middle/upper Keys), and 4 (North Key Largo/Ocean Reef).

Please be marine weather-ready, and stay safe in 2018!




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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