Invest 98L spinning up; outlook for remainder of hurricane season
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 spin. An ASCAT pass from 8:08 pm EDT last night showed 98L was close to closing off a 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 depression, but most of them do show some weak development. NHC gave the disturbance a 60% chance of developing into a tropical depression by Wednesday in their 8 am Tropical Weather Outlook. Given the recent increase in spin on visible satellite images and favorable environment for development, I'd bump these odds up to 70%. 98L is currently moving little, but is expected to begin a westward motion at 10 mph today. This motion would take 98L into the Lesser Antilles Islands by Friday or Saturday. The northern Lesser Antilles would be most likely to see the core of the storm, as has been the case for all of this year's disturbances. However, a more southerly path across Barbados, as predicted by the GFS model, cannot be ruled out. Once 98L does reach the Lesser Antilles, all of the models indicate the storm will see a sharp increase in vertical wind shear 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.
Atlantic hurricane outlook for the rest of September
Ocean temperatures are starting to decline in the North Atlantic, though remain much above average in the Main Development Region (MDR) for hurricanes, from the coast of Africa to the coast of Central America, between 10°N and 20°N latitude. The latest departure of sea surface temperature (SST) from average plot (Figure 2) shows a large area of ocean temperatures near 1°C above average. The water temperatures were 0.8°C above average in this region during August, which is the 4th highest such reading on record. These warm waters will allow for an above-average chance of African tropical waves developing through early October. By early October, the African Monsoon typically begins to wane, spawning fewer tropical waves that tend to be weaker, and we should stop seeing development of newly-emerged tropical waves off the coast of Africa.
Figure 2. Departure of sea surface temperature (SST) from average for September 19, 2011. Ocean temperatures were about 1°C above average over much of the Main Development Region (MDR) for hurricanes, from the coast of Africa to the coast of Central America, between 10°N and 20°N latitude. In the Pacific off the coast of South America, we can see the tell-tale signature of a La Niña event, with cooler than average waters along the Equator. Also note the cooler than average waters between Bermuda and Puerto Rico, due to the passage of Hurricane Maria and Hurricane Katia. Image credit: NOAA/NESDIS.
Wind shear has been near average over the tropical Atlantic this hurricane season, and is currently at its climatological low point, which occurs in mid-September. The latest 2-week run of the GFS model shows wind shear will remain at the sort of typical low levels we usually see this time of year. With ocean temperatures at near-record warm levels, this combination would tend to favor formation of at least two tropical storms between now and the beginning of October. One inhibiting factor, though, may be the continued presence of dry, stable air over the tropical Atlantic. Hurricanes like to have an unstable atmosphere, with moist, warm air near the surface, and cold, dryer air aloft. This situation helps the updrafts in the storm grow stronger. This year, we've had unusually stable air (Figure 3.) This has really put the brakes on intensification of most of the tropical storms that have formed. The current ratio of 14 named storms but only 3 hurricanes is unprecedented in the historical record, going back to 1851. Usually, just over half of all Atlantic tropical storms intensify into hurricanes. One other factor to consider, the 30-60 day pattern of increased thunderstorm activity known as the Madden-Jullian Oscillation (MJO), looks like it will have little influence over the coming week. The MJO has been weak all month, and is predicted to stay weak for the remainder of this week.
Figure 3. Vertical instability, as measured by the difference in temperature near the surface to the bottom of the stratosphere. The atmosphere in the Caribbean (left) and tropical Atlantic between Africa and the Lesser Antilles Islands (right) has been much more stable than average this year (average is the thick black line). Image credit: NOAA/CIRA.
Forecast of the rest of hurricane season
We are past the half-way point of the Atlantic hurricane season, which typically peaks on September 10. On average, about 60% of the activity has occurred by this point in the season. Since we've already had 14 named storms and 3 hurricanes, at the current rate, we would expect to see another 8 or 9 named storms, with 1 or 2 of them reaching hurricane strength. It's pretty tough to maintain the sort of activity levels we've seen so far this year, so I am forecasting we'll see 7 more named storms during the remainder of this season, taking us all the way to "W" in the alphabet. With the unusually stable air over the Atlantic showing no signs of abating, I predict that we'll see just 2 of these storms reach hurricane strength. As far as steering currents go, the latest 2-week forecast from the GFS model doesn't show any significant changes to the jet stream pattern we've seen all summer. There will continue to be a parade of troughs of low pressure moving off the U.S. East Coast that will tend to curve any storms northwards and then northeastwards out to sea, once they penetrate north of the Lesser Antilles Islands. This pattern favors strikes on North Carolina and New England, and discourages strikes on Texas. I doubt Texas will see a tropical storm this year given this steering pattern, and considering that Texas' tropical cyclone season tends to peak in late August and early September. It is quite unusual for Texas to have a tropical storm or hurricane this late in the season, so they will probably have to look elsewhere for drought-busting rains.
Jeff Masters
Reader Comments
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WHXX01 KWBC 201229
CHGHUR
TROPICAL CYCLONE GUIDANCE MESSAGE
NWS NATIONAL HURRICANE CENTER MIAMI FL
1229 UTC TUE SEP 20 2011
DISCLAIMER...NUMERICAL MODELS ARE SUBJECT TO LARGE ERRORS.
PLEASE REFER TO NHC OFFICIAL FORECASTS FOR TROPICAL CYCLONE
AND SUBTROPICAL CYCLONE INFORMATION.
ATLANTIC OBJECTIVE AIDS FOR
DISTURBANCE INVEST (AL982011) 20110920 1200 UTC
...00 HRS... ...12 HRS... ...24 HRS. .. ...36 HRS...
110920 1200 110921 0000 110921 1200 110922 0000
LAT LON LAT LON LAT LON LAT LON
BAMS 11.5N 37.5W 12.4N 40.5W 12.9N 44.2W 13.1N 48.4W
BAMD 11.5N 37.5W 12.1N 39.4W 12.8N 41.5W 13.6N 43.7W
BAMM 11.5N 37.5W 12.1N 39.6W 12.7N 42.2W 13.2N 44.9W
LBAR 11.5N 37.5W 12.1N 39.0W 13.0N 41.0W 14.2N 43.5W
SHIP 25KTS 29KTS 34KTS 39KTS
DSHP 25KTS 29KTS 34KTS 39KTS
...48 HRS... ...72 HRS... ...96 HRS. .. ..120 HRS...
110922 1200 110923 1200 110924 1200 110925 1200
LAT LON LAT LON LAT LON LAT LON
BAMS 12.7N 52.9W 10.8N 60.9W 9.7N 67.0W 10.4N 70.6W
BAMD 14.4N 45.9W 16.4N 49.6W 18.5N 52.8W 19.6N 54.8W
BAMM 13.6N 47.6W 14.8N 52.2W 16.9N 56.6W 18.7N 60.5W
LBAR 15.3N 46.2W 17.9N 51.7W 21.4N 56.2W 23.7N 58.3W
SHIP 42KTS 45KTS 47KTS 45KTS
DSHP 42KTS 45KTS 47KTS 45KTS
...INITIAL CONDITIONS...
LATCUR = 11.5N LONCUR = 37.5W DIRCUR = 280DEG SPDCUR = 4KT
LATM12 = 11.3N LONM12 = 36.7W DIRM12 = 316DEG SPDM12 = 4KT
LATM24 = 11.1N LONM24 = 37.2W
WNDCUR = 25KT RMAXWD = 50NM WNDM12 = 25KT
CENPRS = 1008MB OUTPRS = 1012MB OUTRAD = 150NM SDEPTH = M
RD34NE = 0NM RD34SE = 0NM RD34SW = 0NM RD34NW = 0NM
$$
NNNN
To me it still looks a little ragged to be called a tropical storm. It needs a burst of convection near the center 1st. But this is certainly at least a TD now.
note now the gfdl has 98l as hurricane at 15n 72 west will it get just below dr then turn n or ne toward fla possibly a fay track with the trofs coming down . this one would . could be all about timimg and fla should keep one eye on this one .
2011092006
11.3 322.3
11.3 319.2
100
11.3 322.3
201000
1109201000
1
WTNT21 KNGU 201000
SUBJ/TROPICAL CYCLONE FORMATION ALERT//
RMKS/1. FORMATION OF A TROPICAL CYCLONE IS POSSIBLE WITHIN 100NM
EITHER SIDE OF A LINE FROM 11.3N 37.7W TO 11.3N 40.8W OVER THE
NEXT 24HOURS. AVAILABLE DATA DOES NOT JUSTIFY ISSUANCE OF
A NUMBERED TROPICAL CYCLONE WARNING AT THIS TIME. WINDS ASSOCIATED
WITH THIS DISTURBANCE ARE ESTIMATED TO BE AROUND 25KTS.
SATELLITE IMAGERY AT 0815Z INDICATES THAT A CIRCULATION CENTER
IS LOCATED NEAR 11.3N 037.7W MOVING WESTWARD AT 5 TO 10 KTS.
2. A 1008MB LOW IS EMBEDDED ALONG THE MONSOON TROUGH
AXIS NEAR 11N37W, OR APPROXIMATELY 1300NM EAST OF WINWARD ISLANDS,
AND HAS BECOME BETTER DEFINED IN THE LAST 6 HOURS.
DVORAK SATELLITE ESTIMATE AS OF 20/0545Z IS CURRENTLY T1.5/1.5.
WIND SHEAR VALUES OF 10 TO 20 KTS ARE MARGINAL FOR SLOW DEVELOPMENT
OF THIS SYSTEM AS IT TRACKS WESTWARD. SEA SURFACE TEMPERATURES
OF 84 TO 88 DEGREES FAHRENHEIT EXIST IN THE REGION.
3. THIS ALERT WILL BE REISSUED, UPGRADED TO A WARNING, CANCELLED OR
ALLOWED TO EXPIRE BY 211000Z.//9811091700 99N 369W 15
9811091706 99N 364W 15
9811091712 100N 358W 20
9811091718 104N 353W 20
9811091800 110N 350W 20
9811091806 117N 350W 25
9811091812 120N 357W 25
9811091818 119N 362W 25
9811091900 117N 367W 25
9811091906 117N 372W 25
9811091912 111N 372W 25
9811091918 108N 364W 25
9811092000 113N 372W 25
9811092006 113N 377W 25
sometime tomorrow. But its forward speed and dry air ahead will not allow for much intensification, therefore possibly keeping it on the more southerly track into carib. We will see what happens. JMO
Glad we have something to watch into the weekend.
Link
Seems like it wants some attention.
CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
0930 AM EDT TUE 20 SEPTEMBER 2011
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 21/1100Z TO 22/1100Z SEPTEMBER 2011
TCPOD NUMBER.....11-112
I. ATLANTIC REQUIREMENTS
1. NEGATIVE RECONNAISANCE REQUIREMENTS.
2. SUCCEEDING DAY OUTLOOK: BEGIN 6-HRLY FIXES AT
23/1200Z ON A SYSTEM NEAR 13.5N 54.0W.
Well fire season has started with a bang. as of today, there is 47 bush fires burning in NSW.
2 are burning the Blue mountains west of Sydney. there is also a bush fire burning in the SW suburbs of Sydney that closed the main highway to Canberra and Melbourne. Today's weather conditions were very ugly and ripe for fire ignition. Sustained wind were 50km/h(31mph) gusting up to 80-100km/h(50-62mph). A almost completed sports Stadium in Woolongong south of Sydney has sustained damage and is on the verge of collapse.
This is what everyone in NSW was dreading, Last year there was only a few bush fires due to the inclement weather, and flooding. This has caused the undergrowth to become a cinder pot waiting to explode.
I will post more info when it comes to hand.
How is Texas and Florida going with there bush fires?
LOOKING AHEAD...A BROAD AREA OF LOW PRESSURE WILL CONTINUE TO
DEVELOP FARTHER TO THE EAST OF THE WINDWARD ISLANDS. SHOWERS AND
THUNDERSTORMS ARE GRADUALLY INCREASING...AND ENVIRONMENTAL
CONDITIONS APPEAR FAVORABLE FOR A TROPICAL DEPRESSION TO FORM DURING
THE NEXT COUPLE OF DAYS. THIS SYSTEM HAS A HIGH CHANCE OF BECOMING A
TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS IT MOVES SLOWLY
WESTWARD. HOWEVER...AT THIS TIME THE GFS AND THE ECMWF COMPUTER
MODELS MOVE THIS SYSTEM ACROSS THE LOCAL ISLANDS ONLY AS AN OPEN
WAVE LATE SUNDAY AND MONDAY. STAY TUNED.
What might the blue/purple off TX/LA, be that we don't see? Be nice if a cut-off something span up, ran south and strengthened, then drifted back to S TX.
Viewing: 801 - 832
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