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Two Tropical Depressions in Atlantic; More Record Heat in U.S., Europe

By: Bob Henson 7:27 PM GMT on September 18, 2015

Tropical activity is at a low ebb globally compared to the breakneck pace of the last month. The only hurricane-strength tropical cyclone on the planet as of Friday was Typhoon Krovanh, which peaked at Category 3 strength over the last 24 hours. At 1500 GMT (11:00 am EDT) on Friday, Krovanh was located about 80 miles northeast of the island of Iwo To, gradually recurving to the north and packing top sustained winds of 100 mph. Iwo To and its neighboring islands have seen little impact from Krovanh, as the typhoon's convection and its strongest winds are focused on its east side. Kovanh should be below typhoon strength by Monday as it sweeps well to the east of Japan's main islands.

In the Atlantic, we have two tropical depressions, both far out at sea in the central North Atlantic, as well as a disturbance closer to the southeast U.S. coast. Tropical Depression 9 has missed its chance to become a named storm. Now undergoing southwesterly shear, TD 9 is largely devoid of convection, and statistical and dynamical forecast models generally weaken it over time as it drifts to the west-northwest. The National Hurricane Center is projecting TD 9 to become a remnant low by tonight. Further to the southeast, newborn Tropical Depression 10, also heading west-northwest, has a better chance of becoming a named storm. Models are consistent in bringing TD 10 to tropical storm strength by Sunday and keeping it there for several more days. The track forecast becomes more uncertain after Monday, but the overall tendency is toward a gradual recurvature well east of the Caribbean and North America.

Figure 1. Satellite imagery of water vapor shows the extensive swath of convection associated with Invest 96L (left). Also evident are tiny Tropical Depression 9 (center) and much larger Tropical Depression 10 (right). Image credit: CIMMS/SSEC/University of Wisconsin.

About 100 miles east of Jacksonville, Florida, Invest 96L is a region of surface low pressure associated with an vast, loosely organized area of showers and thunderstorms and a weak surface boundary that extends from south Florida into the Northwest Atlantic. A flood watch is in effect over eastern Florida on Friday, with heavy thunderstorms pounding the populated strip from Miami to Palm Beach. NHC gives Invest 96L a 20% chance of development into a tropical cyclone through Monday and a 40% chance through Wednesday, as the low continues to spin off the southeast U.S. Coast. WIth an upper-level trough predicted to dig over the eastern United States next week, 96L could take on at least some hybrid or subtropical characteristics if it were to intensify. As high pressure builds to the north of 96L, the next few days may bring an extended period of high surf along parts of the U.S. East Coast, especially in the Carolinas.

Figure 2. Surface temperatures on Friday, September 18, 2015, remained far above normal across central Europe, from Italy into western Russia. Image credit: climatereanalyzer.org, University of Maine.

Extreme mid-September heat in Europe
An exceptional summer of record-breaking heat waves across Europe is hanging on with remarkable vigor as the fall equinox approaches. On Thursday, September 17, dozens of locations in at least six countries set all-time high temperature records for any date in September, according to international weather records researcher Maximiliano Herrera. This is truly an amazing accomplishment for so late in a transition month like September when average temperatures are dropping throughout the month. Among the locations that set all-time September highs on Thursday were these four stations in southeast Germany, all of which broke monthly records that had just been set on September 1 of this year (thanks to Michael Theusner, Klimahaus, for these statistics).

Gottfrieding: 34.0°C (93.2°F), old record 32.3°C
Mühldorf: 33.3°C (91.9°F), old record 32.6°C
Straubing : 32.9°C (91.2°F), old record 31.4°C
Chieming: 31.1°C (88.0°F), old record 30.6°C

Still more records fell on Friday, as dozens more stations with long periods of record set all-time September highs, according to Herrera. Sarajevo, the capital of Bosnia and Herzegovina, hit 38.0°C (100.4°F), beating its previous September record of 37.7°C. Weather records in Sarajevo extend back to 1880. Several locations came within 2°F of their all-time highs for any time of year--again, a remarkable outcome for the final week of astronomical summer! Special thanks for these data go to Maximiliano Herrera, who maintains a comprehensive list of extreme temperature records for every nation in the world on his website.

See the Jeff Masters post from Thursday for details on the many heat records set in Europe and elsewhere last month, which was the warmest August in global record-keeping that goes back to 1880.

Warmest September on record for U.S.? It's possible
Summer isn't quite ready to let go of the contiguous U.S., either, as warm weather has predominated in many areas through September. In a swath running from Colorado's Front Range to New England, a number of locations are on track for what could be their warmest September on record. These include:

Figure 3. The 8- to 14-day outlook from the NOAA Climate Prediction Center shows a high probability that most of the contiguous U.S. will have above-average temperatures during the last week of September. Image credit: NOAA/CPC

All of these places, and others around the nation, have a running start toward possibly setting their all-time monthly record for September. One big obstacle in their way is the climatological tendency for temperatures to drop as we move toward the end of the month. However, that tendency will be muted somewhat by persistent upper-level high pressure that's keeping any major pushes of Canadian or Arctic air at bay. NOAA's Weather Prediction Center is projecting high odds of above-normal temperatures across much of the nation for the 8- to 14-day period (see Figure 3). With these positive signals, I won't be surprised to see a number of U.S. cities end up with their warmest September on record. The wide geographic spread of the warmth also suggests that the contiguous U.S. as a whole may be in the midst of its warmest September in more than a century of record-keeping. The just-released autumn/winter outlook from weather.com projects more warmth for October and beyond, especially for the eastern U.S.

Have a great weekend, everyone!

Bob Henson

Hurricane Heat

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.