DISCUSSION: There is no question that year in and year out, tropical cyclones can often confound even the most educated of scientists and people located all over the globe. More specifically, as tropical cyclones begin to form and develop within a region of relatively adverse conditions, the overall cloud structure and corresponding rain-band structure can often be misleading. That is, misleading in terms of how the evolving structure of a developing tropical cyclone often has a very minimal correlation to the potential total impacts from a given tropical cyclone.
This inherent level of psychological confusion with developing tropical cyclones is that a more organized and a more intense tropical cyclone will more likely than not have more destructive impacts to any regions which lie in its path. However, the imperative point of clarification which needs to be made here is the fact that a disorganized storm from a satellite and/or radar-based perspective is no less dangerous than a well-organized storm. This is due to the reality that even with a disorganized system, there are still major threats for both wind and flooding damage since an unbalanced storm (i.e., with respect to moisture and wind intensity distribution) can still delivered tremendous amounts of rainfall (i.e., rainfall totals in excess of 10 to 20 inches within 24 hours) and locally strong winds as well. A perfect example of this narrative could be found with the quick but short-lived development of Tropical Storm Imelda in the vicinity of southeast Texas earlier this year.
In the case of Tropical Storm Imelda, an invigorated tropical disturbance quickly blossomed into a tropical depression and then was upgraded to a tropical storm just hours before the center of Imelda’s still-developing circulation came ashore in southeast Texas. Despite the storm being weak and disorganized overall, this storm went on to dump copious amounts of rainfall in and around coastal and semi-coastal portions of southeast Texas. Thus, this just goes to show that regardless of a tropical cyclone’s overall structural breakdown and its given intensity, all tropical cyclones MUST ALWAYS be given do respect since any single storm can be a storm to be reckoned with for hours or even days to come once a system closes in on a given coastal region. Attached above is a graphic courtesy of The Weather Channel which approximately reflects the overall distribution of the rainfall which resulted from Tropical Storm Imelda across southeast Texas.
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©2019 Meteorologist Jordan Rabinowitz
This year’s hurricane season has been nothing short of impressive thus far and there are still two months left. Thankfully, most of the hurricanes that have formed this year have had minimal impact to the United States as opposed to similarly active years in the past. However, that doesn’t mean that the hurricanes that have hit the U.S. have been a cake walk. So far this season there have been 11 named storms and 2 major hurricanes. At one point, there were six named storms going on at once in the Atlantic and Pacific basins, but is this more active than usual? The answer is yes, but only for this time of year. In the Atlantic between September 14-23, there have only been four named storms occurring at once as the record; this year that record was broken when there were 5 named storms during this span. However, is this the most active season ever? Not really.
The most active Atlantic hurricane season was 2005 with 28 named storms, and 15 of those became major hurricanes. In fact, that season was so active that the National Hurricane Center ran out of names through the alphabet and had to start using names from the Greek alphabet. This year has nothing on 2005, however, it is rather active compared to the last few hurricane seasons. The National Hurricane Center normally issues pre-season predictions based on sea-surface temperature trends and whether we are in an El-Nino or El-Nina phase, the two different phases of the Southern Oscillation Index which affect the trade winds and sea surface temperatures. This year, they predicted between 8-11 named storms and 2-4 major hurricanes, which is fairly accurate with what has been going on so far. However, it is only September and it doesn’t appear that the Atlantic or Pacific Basins will be calming down anytime soon. Hurricane Lorenzo has broken many records in the Atlantic this past week, one including the strongest hurricane to have been observed that far north and east in the Atlantic. It is currently a Category 1 and will be affecting the Azores Islands in the coming days.
Shear-wise, the Atlantic is beginning to look unfavorable as autumn sets in. Wind shear is the variation of wind velocity along a given direction perpendicular to overall wind direction. Favorable wind shear in the mid-to-upper levels of the atmosphere for hurricanes is 15 knots and below, while less favorable wind shear is above 20 knots. The reasoning behind this is that hurricanes get torn apart by strong upper-level shear and it can cause them to not become organized. Also, the trough-ridge pattern is starting to set up quite nice, regardless of the steep ridge that has been stationed over the Southeast U.S. during the last couple of weeks. The trough-ridge pattern is set up by the jet stream. The jet stream moves further north in the summertime and farther south in the fall and winter season due to the seasonal change. When the jet stream moves further north, the tropical jet will also move further north in its place, which brings much hotter and more humid conditions over the southern U.S. and Mexico. The troughs that are formed by the jet stream are normally lined by a cold front ahead of them, with an area of high pressure behind them. Ridges are areas that are preceded by a warm front, with an area of low pressure in them. Hurricanes tend to want to avoid areas of high pressure because of the dry air associated with the pressure, so with an active frontal pattern typically means that devastating landfalls are harder to come by. However, sea-surface temperatures are very steamy with temperatures ranging from the upper-70s to mid-80s. These are very favorable conditions to support hurricane development, along with many disturbances still coming off the west coast of Africa. Hurricanes need warm, steamy temperatures for fuel. Any sea-surface temperatures (temperature of the ocean from the surface down to around 20 meters, but can be as deep as 700 meters below the surface) that are below 70 degrees are typically unfavorable for hurricanes.
It is something to monitor over the next couple months as the Atlantic still has an abundance of energy. Fortunately, with the frontal systems beginning to start up for the autumn and winter months, the chances for hurricane landfall in the southern U.S. is low. The U.S. East Coast could still potentially face landfalls, but the overall chances decrease.
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©2019 Meteorologist Ashley Lennard