climate, weather, weather safety/preparedness

Fiona fades; Gaston grows; but there is another tropical system (99L) on Florida’s horizon… (H. Michael Mogil, CCM, CBM, DMS)

Fiona (the sixth Atlantic tropical cyclone of the 2016 season) recently passed. In her wake, Gaston briefly reached hurricane strength far out in the Atlantic. Another tropical system (99L) near the Bahamas is still showing signs of intensifying this weekend as it approaches the Florida Straits. And, more African waves seem ready to exit the west African coast and join this Atlantic train of tropical weather systems. All this as the peak of hurricane season (Sept. 10) looms.

For obvious reasons, the potential system nearing the Bahamas brings the greatest concern. During the past two days, National Hurricane Center (NHC) forecasters have begun to downplay chances for tropical storm or tropical depression formation (although, anytime a system like this is near the U.S., it warrants close scrutiny).

However, the bigger threat seems to be heavy rainfall and rip currents. The persistent strong easterly wind flow (and the expected elevated wind speeds as the system approaches later this weekend) will allow for rip currents to develop along east-facing beaches across much of the so-called Sunshine State. I say, “so-called,” because extensive cloud cover and widespread precipitation areas (filled with heavy showers and thunderstorms) will make folks wonder if the sun even exists. The Naples area forecast, for example, keeps mostly cloudy skies as the main sky condition until next Thursday. If the sun does peak out of thick, multi-layered clouds, it will likely be for very short periods, at best.

Fig001-seven-day-qpf-thru-160902NOAA’s Weather Prediction Center (WPC) has issued its latest quantitative precipitation forecast (QPF) for the seven-day period ending next Friday. This places much of Florida in a five to seven inch rainfall zone (Fig. 1). As always, local amounts can often be double the expected general rainfall numbers.

Fig002-pct-annual-rainfall-FL-thru-160825Much of the southern half of Florida is already reporting above average yearly rainfall (Fig. 2). Some places are as much as fifty percent or more above average. Fort Myers, for example, is already nearly 12 inches above its yearly average rainfall of 36 inches through Aug. 25. Additional rainfall will only add to water management issues across the southern part of the state, including increased agricultural and fertilizer runoff (which contribute to the development of algal blooms in near-shore Gulf waters).

Since sand is the dominant ground material in south Florida’s (not clay soil like Louisiana), any localized heavy rainfall will not likely generate massive flooding. However, normally poor drainage areas, places with storm sewer gratings filled with leave sand twigs and even some urban areas can expect ponding of water on roadways and in low-lying areas. Coastal urban flooding will be exaggerated if heavy rain falls during the time of high tides.

The best advice for now is to keep up-to-date on the tropical activity near Florida, and be very cautious of any heavy rainfall and localized ponding water, especially while driving.

© 2016 H. Michael Mogil

Originally posted 8/26/16

weather

Significant, but localized, tornado outbreak hits Indiana and Ohio… (H. Michael Mogil, CCM, CBM, DMS)

Fig001-unfiltered-storm-reports-map-160824On the afternoon and early evening of Aug. 24, 2016, a significant, albeit localized, tornado outbreak occurred across Indiana and Ohio. By the time the situation calmed down (meteorologically-speaking), 42 tornado reports (Fig. 1) had been logged at the Storm Prediction Center (SPC) web site. During the overnight hours, ABC-7 (Chicago) reported that Indiana State Police (ISP) had tallied between 15 to 20 (mostly minor) injuries from the storms. The ABC report quoted Captain David Bursten (ISP) as stating, “We were very, very fortunate, and you have to credit the advance warnings and people heeding those warnings.”

Fig002-storm-reports-160824The focus here, however, is on the number of tornadoes that occurred. A close examination of the SPC reports listing suggests that the actual number of reported twisters will eventually be much lower.

Fig003-filtered-storm-reports-map-160824First, within the storm report listing, it is easy to see multiple reports of the same tornado (Fig. 2). This happens in, “the heat of battle.” During that time, SPC lists all reports received because screening reports for consistency and redundancy is a low priority task.

Later on, the reports undergo an initial filtering. In this case, Fig. 3 shows that the report number had already dropped to 35 (almost a 20 percent reduction).

Fig004-two-tornado-tracks-map-160824Next, a closer examination of the region affected (Fig. 4) shows two main tornado tracks running along a southwest to northeast axis. These tracks likely mark the path of the parent meso-cyclone that spawned the tornadoes. Hence, some of the reports may be multiple sightings of a long-tracked tornado or they may be multiple touchdowns from the same parent super-cell thunderstorm.

To be fair, some reports may not have been received in real time and other tornado tracks may be uncovered later today, as National Weather Service (NWS) teams conduct damage surveys and interview local residents. During these field investigations, the meteorologists will be able to ascertain the specific number of tornadoes, and each twister’s width, length and strength.

Regardless of the actual number of twisters, there’s little doubt that this was a significant summer-time outbreak.

© 2016 H. Michael Mogil

Originally posted 8/25/16

climate, weather

The next hurricane…

Fig001-tropical-storm-day-of-year-climatologySeptember 10 marks the average day of peak tropical cyclone activity in the Atlantic Ocean basin (Fig. 1). As we approach that day, the atmosphere is delivering. Early this Monday morning (Aug. 22, 2016), there were three tropical systems at play in the Atlantic – Tropical Depression Fiona and two tropical convective areas (Fig. 2). All of these systems started their lives as tropical waves coming off the west coast of Africa. Note that there is another thunderstorm cluster getting ready to exit Africa. So, all eyes across the eastern half of the U.S. are turning to the east.

Fig002-Atlantic-satellite-160822-1030amEDTThe media is showing spaghetti plots and folks in Florida are starting to talk about these weather systems. Fortunately, the bread, water and toilet paper are not yet flying off supermarket shelves.

Successive runs of various computer models are showing an ever-evolving set of potential atmospheric solutions. The other day, one of the tropical systems was expected to pass over southwest Florida enroute to Louisiana. The model run six hours later had the storm in the Atlantic completely missing Florida. The solution set continues to grow. Several things are important to note:

(1)  Official National Hurricane Center (NHC) center position forecast errors (based on 2011-2015 data for the Atlantic basin) are about 272 miles at day five (120-hour forecast). We are currently at day 7 or 8 before the storm might affect the U.S. If forecasts were made this far ahead, errors would be expected to be even greater.

(2)  Longer-term forecast errors in storm strength are about 15 knots (17 miles per hour), roughly equivalent to one storm strength category.

(3)  Nearby weather systems (and sometimes systems far removed from the tropical cyclone) can affect the cyclone’s formation, development and movement.

(4)  Computer models are based on past computer model outputs and updated observations. Missing or incorrect data, model boundary conditions, and even the thermodynamics and physics built into the model can all generate errors or incorrect outcomes.

So, this far ahead, it just pays to watch the tropics and know what thenext named storm might be called. Keep up-to-date on any storms that have or may form and monitor official forecast tracks. As any storm gets closer to the U.S. (if one does), NHC forecasts and discussions (plus information here at GWCC, and on local and national media) will help guide our readers to risks and any needed safety actions.

© 2016 H. Michael Mogil

Originally posted 8/22/16

weather, weather safety/preparedness

Louisiana under a flooding emergency (H. Michael Mogil, CCM, CBM, DMS)

Early this Saturday morning, many parts of southern and central Louisiana were under a flooding emergency. Widespread 10 to 20 inch rainfall totals (Fig. 1) Fig001-LA-32-hr-rainfall-thru-160813-amCDThave led to flooding and flash flooding with some places under evacuation orders and others dealing with water reaching and entering homes and businesses. The National Weather Service (NWS) has been passing on information from emergency management officials and also adding some of its own strong wording. One NWS office even urged listeners not to, “drive cars through flooded areas. Remember, two feet of rushing water can carry away most vehicles including pickups. Turn around and don`t drown!”

The culprit behind this flooding situation is a slowly moving low-pressure system, albeit with a weak meteorological wind circulation (Fig. 2). The low formed just south of Florida’s “Big Bend,” northwest of Tampa, last weekend and brought some heavy rainfall to places in Florida; however, most of its more active convection remained over Gulf of Mexico waters. Coupled with sea level pressure readings remaining near average global values (29.92 inches of mercury), the low stood little chance of developing into a tropical storm.Fig002-GOES-EIR-160812-1815Z

Then, during the past few days, as the low drifted slowly to the west, the precipitation region keyed on Louisiana and remained there. As seen in Fig. 1, rainfall totals have been extraordinary.

NWS precipitation forecasters are not painting a rosy picture for Louisiana. For the next few days, parts of the state could see general rains approaching 10 inches, with a predicted weekly total of more than 13 inches (Fig. 3). Locally, amounts could easily be double these, given the high atmospheric moisture values in the region and the system’s slow movement. Fig003-7-day-QPF-thru-160820-amCDT

Widespread flooding will continue, even if rainfall tapers off, because rains already fallen have entered upstream watersheds and will be making their way downstream toward the Gulf of Mexico. Flooding can even develop in a downstream under clear to partly skies.

The low-pressure system will continue to plod slowly westward and some of the system’s moisture is expected to interact with a frontal system to the north of Louisiana. Hence, parts of the Ohio and Middle Mississippi River Valleys (and even parts of east Texas) could see excessive rainfall during the upcoming week.

© 2016 H. Michael Mogil

Originally posted 8/13/16