98L organizing; September temperatures in the U.S. return to normal

By: Dr. Jeff Masters , 1:29 PM GMT on September 20, 2011

A tropical wave midway between Africa and the Lesser Antilles (Invest 98L) continues to look well-organized on satellite imagery, with a modest amount of heavy thunderstorm activity and excellent spin. An ASCAT pass from 7:47 pm EDT last night showed that 98L had a moderately well-defined surface circulation. Wind shear as diagnosed by the SHIPS model is light, less than 10 knots, and is predicted to stay light to moderate through Friday. Ocean temperatures are 28 - 28.5°C, well above the threshold typically needed for a tropical storm to spin up. Water vapor satellite images show 98L is embedded in a moist environment, but there is dry air to the system's northwest. However, given the light wind shear, this dry air may not pose a hindrance to development at this time. An analysis of upper level winds from the University of Wisconsin CIMMS group shows a pattern favorable for development, with an outflow channel open to both the north and south available to ventilate the storm and allow 98L to efficiently lift plenty of moisture to high levels.

Figure 1. Morning satellite image of 98L.

The models are not very aggressive about developing 98L into a tropical storm, but most of them do show some development. NHC gave the disturbance a 70% chance of developing into a tropical depression by Thursday in their 8 am Tropical Weather Outlook. 98L's westward motion of 5 - 10 mph should bring the storm into the Lesser Antilles Islands by Saturday, though the models are not in strong agreement about the forward speed of the storm. The GFDL model brings 98L into the islands on Friday, while the NOGAPS model keeps the storm east of the islands through Tuesday. If 98L takes a more west-northwesterly path through northern Lesser Antilles, which has been the preferred track for tropical systems this year, the disturbance should encounter high wind shear in excess of 20 knots due to strong upper-level winds out of the west. This shear should make it difficult for 98L to intensify as it moves though the islands. However, if 98L takes a more southerly path across Barbados, as predicted by the GFDL model, the storm will miss seeing the high shear area that lies over the northern islands, and the storm would have more opportunity to strengthen. The most likely scenario I see at this point is for 98L to be a weak tropical storm on Saturday as it moves through the Lesser Antilles--but there is more than the usual amount of uncertainty in both the track and intensity forecast.

September temperatures return to normal over the U.S.
The summer of 2011 was the second hottest in U.S. history, but September of 2011 is so far shaping up to be an average one for temperature. A series of cold fronts and cold-cored low pressure systems have moved southwards out of Canada this month, bringing typical amounts of cool air to the country. If you want to select dates for the start and end of the U.S. heat wave of 2011, the dates to pick would be May 20 - September 4. During the period May 20 - September 4, 2011, the number of daily record high temperatures at the 515 major airports in the U.S. exceeded the number of daily low temperature records every day but one. That's an astonishing 107 out of 108 days! Only July 15 had more record daily lows than highs during that 108-day period. I doubt one could find a similar stretch of days anytime in U.S. weather history where such a lopsided ratio of high temperature to low temperature records existed. For the 3-month summer period of June, July, and August, 2703 daily high temperature records were set, compared to 300 daily low temperature records--a ratio of 9-to-1. Not surprisingly, the summer of 2011 wound up as the hottest summer in 75 years in the U.S., and was only 0.1°F cooler than the all-time record hottest summer, during the Dust Bowl year of 1936. But so far this September, the ratio of high temperature records to low temperature records has been close to 1-to-1. There were 283 daily high temperature records set during the first sixteen days of September, and 246 low temperature records. Eight of the first sixteen days of September have seen the lows outnumber the highs, and eight have seen the highs outnumber the lows. The latest 2-week forecast from the GFS model shows a continuation of pretty normal weather over the U.S. for the rest of the month, and September temperatures will end up close to average.

Jeff Masters

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Old Invest 99L is tiny and its about to get out of some of the dry air and move into a moisture environment. Gotta see if it hangs together long enough to survive that long tho. Still a tight little spinner tho.

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Speaking of lopsided: for June, 2,731 daily high temperature records were broken (not tied), compared to just 263 daily low temperature records, for a ratio of 10.38:1. July was a bit more balanced, at 1,618 to 260, or 6.22:1. The truly lopsided month, however, was August, when 3,168 daily high temperature records were broken, against just 144 daily low temperature records, an astounding ratio of exactly 22:1. For the three summer months, the tally was 7,517 to 667, or 11.26:1, and for the entire year (January 1 through yesterday), that is 14,819 to 5,376, or 2.75:1. (For September, the balance stands at just 1,067 to 932, or 1.14-to-1.) (http://www.ncdc.noaa.gov/extremes/records/)

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which model solution do u buy for 98L

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unusual weather in many aspects...
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We have started getting some much needed rain here in NE Fl. Not flooding just gentle rain with ALOT of lightshows
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Curious that the weather is mimicing the events of the 30's, coincident with the financial storm. Can history repeat itself?
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Roke is an INCREDIBLE looking storm...
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Japan Meteorological Agency
Tropical Cyclone Advisory #85
21:50 PM JST September 20 2011

SUBJECT: Category Four Typhoon In Sea South Of Japan

At 12:00 PM UTC, Typhoon Roke (940 hPa) located at 30.3N 133.6E has 10 minute sustained winds of 95 knots with gusts of 135 knots. The cyclone is reported as moving northeast at 15 knots

Dvorak Intensity: T6.0

Storm Force Winds
80 NM from the center

Gale Force Winds
280 NM from the center in southeast quadrant
200 NM from the center in northwest quadrant

Forecast and Intensity

24 HRS: 36.5N 139.3E - 65 knots (CAT 3/Strong Typhoon)
48 HRS: 46.7N 147.1E - Extratropical

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Thanks doc
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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