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.

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THEWEATHERMOGIL:: Snowfall gradients

Yesterday’s winter-spring storm produced lots of severe weather on its eastern flank, but it also generated a band of heavy snow to its west. Just to the east of Chicago, IL, the infamous “rain-snow” line helped to create a rather large snowfall gradient. That gradient (0.44 inches of snow per mile, across a 34-mile distance) gave the National Weather Service (NWS) office in Chicago an opportunity to play up such gradients (Fig. 1). Bravo!

NATL001-ORD-snowfall-gradient

Yet, much larger gradients came into play for the Jan. 26-28, 2015 snowstorm in the mid-Atlantic and Northeast U.S. I refer readers of this blog site to an article I wrote after that event – THEWEATHERMOGIL:: Looking back at an almost perfect winter storm forecast (1/26-1/28/15). Gradients in that storm were some two to three times larger than the one near Chicago yesterday (Fig. 2).

NATL005MAsnowfallGradient150126to150127

What’s important is that people realize that weather-related gradients (e.g., snow, rain, temperature, wind, cloudiness) can be large and affect the viability of current, highly localized forecasts. They can also play a role in how weather is treated in legal and insurance matters.

© 2016 H. Michael Mogil

Originally posted 2/25/16