Uncategorized, weather

Tropical system affecting Florida (H. Michael Mogil, CCM, CBM, NWA-DS*)

While ex-hurricane Franklin dissipated quickly over Mexico yesterday, a much weaker tropical system began affecting Florida (Fig. 1). On Wednesday, the system was located over the Bahamas and all its storminess remained well east of Florida. In fact, thanks to the dynamics of these “tropical waves,” places west of the wave (in the Northern Hemisphere) are typically afforded sunny skies.

That was the dichotomy across Florida yesterday. The wave was located across southeast Florida late in the morning. As a result, southeast Florida was under a dense cloud canopy with numerous showers and thunderstorms (Fig. 2); at the same time, southwest Florida was experiencing scattered cumulus clouds and some high-altitude cirrus clouds, blow off from the storms to the east (Fig. 3).

East of the wave, southerly wind flow at low levels was convergent (winds blowing together). This forced air to rise, and rising air often yields clouds and precipitation. West of the wave, low-level northeasterly winds were divergent, yielding sinking air and clearer skies.

As of mid-morning yesterday (Aug. 10, 2017), Fort Lauderdale / Hollywood Airport (FLL) had logged record-breaking rainfall (at least 3.36” versus the old record of 3.35” set in 2003; records for FLL date back to 1912). To highlight the variability of rainfall, nearby Fort Lauderdale Executive Airport (FXE), just 9 miles away, received a scant 0.69” during roughly the same time period.

The “tropical wave” headed westward yesterday and brought some heavy thunderstorms to southwest Florida. One thunderstorm moved from north to south across Lee, Hendry, and Collier Counties, bringing  locally rainfall amounts, of an inch to more, to the area. The storm had a very pronounced outflow boundary, with a multi-layer “shelf cloud” marking its arrival (Fig. 4).

Today, with the tropical wave remaining over south Florida, more widespread shower and thunderstorm activity can be expected. Again, rainfall amounts may be locally heavy, with ponding of water possible in usually vulnerable low-lying areas and on roadways.

© 2017 H. Michael Mogil

Originally posted 8/11/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.

climate, geography, weather

A Tropical Storm Season Update – Aug. 1, 2017 (H. Michael Mogil, CCM, CBM, NWA-DS*)

So far, the Eastern Pacific (EPAC) Basin (the North Pacific Ocean east of 140 degrees West longitude) 2017 hurricane season is “on fire.” For July alone, Accumulated Cyclone Energy (ACE), which measures the combined strength and duration of tropical storms and hurricanes, ran at 2.5 times the recent monthly average. This made July 2017 the fifth most active July on record. On the other hand, the Atlantic Basin remained “on vacation.”

This isn’t unusual for the two basins, based on a long-term (50-year) seasonal study by Matt Bolton (student at St. Leo University, St. Leo, FL and intern at How The Weatherworks) and myself. The authors also found that inter-connectivities among the ocean basins worldwide tended to keep global tropical cyclone numbers fairly constant from year to year.

According to NOAA’s National Hurricane Center (NHC), the July 2017 EPAC basin saw five named storms, with two reaching major hurricane status and three having sustained winds reaching triple digits. Fernanda’s sustained winds peaked at 145 miles per hour.

Based on a 30-year (1981-2010) NHC climatology, three to four named storms typically form in the basin in July, with two becoming hurricanes and one of those reaching major hurricane intensity. This offers further testimony that July was an active time in the Eastern Pacific.

For the North Atlantic, Caribbean Sea, and the Gulf of Mexico (considered the North Atlantic Basin), two named storms and one unnamed tropical depression formed in July 2017. Based on a 30-year climatology (1981-2010), one named storm typically forms in the basin in July, with a hurricane forming once every other year. ACE has been low, noted NHC, because all storms have been, “…relatively weak and short lived.” In fact, none of the five named Atlantic storms have had sustained winds above 60 miles per hour.

Fig. 1 summarizes activity in the two basins for the current hurricane season through July 31, 2017.

An NHC-compiled climatology for the period 1966 to 2009 indicated that the mid-point of tropical season in both the Atlantic and Eastern Pacific basins occurs during the late August to early September time period. Recent activity does not a hurricane season make. Hence, the current activity in either basin may or may not be representative of what the entire season will bring.

© 2017 H. Michael Mogil

Originally posted 8/6/17

* The National Weather Association Digital Seal (NWA-DS) is awarded to individuals who pass stringent meteorological testing and evaluation of written weather content. H. Michael Mogil was awarded the second such seal and is a strong advocate for its use by weather bloggers.