91L near tropical depression status

By: Dr. Jeff Masters , 2:08 PM GMT on August 02, 2010

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A tropical wave near 12N 41W, midway between Africa and the Lesser Antilles Islands, is very close to being a tropical depression. NHC labeled this system Invest 91L yesterday, and is giving it a 90% chance of developing into a tropical depression by 8am Wednesday. Wind shear is a low 5 - 10 knots, sea surface temperatures are at record highs, and the dust and dry air of the Saharan Air Layer (SAL) are far enough to the north of 91L to potentially allow further development. The main negative for development appears to be the storm's small size, which makes it vulnerable to modest increases in wind shear or dry air entrainment. A Windsat pass from 5am EDT this morning did not show a closed circulation. Satellite imagery shows that the intensity and areal extent of 91L's heavy thunderstorms is very limited, but that a closed surface circulation may be close to forming. Low-level spiral bands and upper-level outflow are not apparent yet.


Figure 1. Morning satellite image of Invest 91L.

Forecast for 91L
There is modest model support for 91L developing into a tropical depression. Three out of six of our reliable models for forecasting tropical cyclone genesis predict 91L will develop into a tropical depression by Tuesday or Wednesday. A west-northwest motion at 10 -15 mph is predicted, which should carry 91L a few hundred miles northeast of the northernmost Lesser Antilles Islands. However, it is possible that 91L would track over the northernmost Lesser Antilles Islands, as predicted by the Canadian model. Squalls from the outer rainbands of 91L may affect islands such as Antigua and Barbuda as early as Wednesday afternoon. As 91L makes its closest approach to the Lesser Antilles on Wednesday, the storm will begin to encounter strong upper-level westerly winds associated with the counter-clockwise flow of air around an upper-level low pressure system centered between Bermuda and Puerto Rico. The latest SHIPS model forecast predicts that these winds will cause wind shear to rise to the moderate level, 10 - 20 knots, by Tuesday night, and to the high level, 20 - 30 knots, by Wednesday. There is also a great deal of dry air associated with the upper level low that may cause problems for 91L. The high wind shear and dry air should weaken and possibly destroy 91L late this week.

A trough of low pressure is expected to move off the U.S. East Coast on Friday, and this trough may be strong enough to recurve 91L far enough to the northwest that the storm will threaten Bermuda. The HWRF and ECMWF models predict 91L could pass very near to Bermuda on Saturday. It is uncertain at this time if the trough will be strong enough to recurve 91L all the way out to sea early next week, as predicted by the GFS model, or leave 91L behind to potentially move westward again into the U.S. East Coast, as predicted by the Canadian model. The amount of wind shear that might be present early next week is also highly uncertain.

Cyprus records its hottest temperature in history yesterday
The island of Cyprus recorded its hottest temperature in its history on August 1, 2010 when the mercury hit 46.6°C (115.9°F) at Lefconica. The old record for Cyprus was 44.4°C (111.9°F) at Lefkosia in August 1956. An older record of 46.6°C from July 1888 was reported from Nicosia, but is of questionable reliability.

The year 2010 is now tied with 2007 as the year with the most national extreme heat records--fifteen. There has been one country that has recorded its coldest temperature on record in 2010; see my post last week for a list of the 2010 records. My source for extreme weather records is the excellent book Extreme Weather by Chris Burt. His new updates (not yet published) remove a number of old disputed records. Keep in mind that the matter of determining extreme records is very difficult, and it is often a judgment call as to whether an old record is reliable or not. For example, one of 2007's fifteen extreme hottest temperature records is for the U.S.--the 129°F recorded at Death Valley that year. Most weather record books list 1913 as the year the hottest temperature in the U.S. occurred, when Greenland Ranch in Death Valley hit 134°F. However, as explained in a recent Weatherwise article, that record is questionable, since it occurred during a sandstorm when hot sand may have wedged against the thermometer, artificially inflating the temperature. Mr. Burt's list of 225 countries with extreme heat records includes islands that are not independent countries, such as Puerto Rico and Greenland. Seventy four extreme hottest temperature records have been set in the past ten years (33% of all countries.) For comparison, 14 countries set extreme coldest temperature records over the past ten years (6% of all countries). I thank Mr. Burt and weather record researchers Maximiliano Herrera and Howard Rainford for their assistance identifying this year's new extreme temperature records.

Next update
I'll have an update later today if 91L develops into a TD.

Jeff Masters

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Quoting Orcasystems:


It would be nice if we were getting some of this moisture heading towards the Islands. We have not had rain for over a month, They are finally calling for a bit of rain here this week :)


Is this typical of you guys in a La-Nina year? It seems here in the mid-west U.S. we become the deep south and they become something like Hades. C'mon December!
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I got a feeling that TD4 will move much more further west than what is forecasted and I also think in due time the forecast track will also change further south
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Wow. 1 hour after the NHC classifies TD 4, people are RIPping it and calling it a fish.
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Note there are two things keeping TD4/Colin more north and east after the islands.
The Low and associated trough into the Bahamas off the NC coast.
The upper level Trough which will shear it and/or push it north between PR and Bermuda.
If it continues it's development and northward track which it should because..
Warm water Low shear
It's got it's convection in front of it to moisten and remove dust from it's environment.
The stronger part of the MA/BA rdige is north of 20N
I expect it to take the GFS track and then would see landfall north of SC or re-curve out to see..
The forward motion is picking up on sat loops and should pick up even more making re-curving out to sea most likely..

On the other hand I am done with hurricane Prep here on ST Thomas.. everythign si put awayy taken down tied up and secured and I'll be taking the boat to Culebra tommorow to tie up in the mangroves just in case....
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So I see we have a TD4 now. Looks like we are in the clear. Kinda surprised though, thought this was going to be the year of everything going more west, not fishies...
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TD 4 looks like its getting ripped apart!
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We do not have 92L.
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Quoting ShenValleyFlyFish:
Dry as 30's Oklahoma here. Fool farm workers burned down a 120+ yr. barn here last weekend. Missed the show as was staying with daughter recovering from surgery. Only thing the fire companies (5 local volunteer stations responded)could do was back off and wet down adjacent buildings and hot spots in fields. Wonder they didn't get more stuff burning. Whatever possessed them to start a brush pile burning is beyond me. Road sides are flaring up from tossed cigarettes. Too bad ignorance isn't a painful condition.


It is, but only for the rest of us typically
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Quoting RitaEvac:
If I remember, Ike at this stage of the game was a fish storm


Ike was never a fish

Link
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Member Since: July 15, 2006 Posts: 178 Comments: 56141
This site is like my August - November bible, living on the east coast of Florida. I appreciate the level of knowledge that is shared here and how it helps me to be prepared for what is coming. I live in an evacuation area, so I monitor this site religiously during this time of the year.
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346. IKE
Quoting Irth:
The area of dry air to the west of TD4 is already having a effect on the western area of convection (inside red oval). It's starting to dry out the air ahead of the system. At this rate, I think that TD4 will definitely be starved for fuel in the next few days. Even if it recurves, it will just get pulled in to the other area of dry air spinning to the north of the Antilles. There is nothing but bone dry air ahead of this storm.

Sorry for the small size, I have no idea how to get the full size image from flickr...



My posts are not posting. Here's a closer view....

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344. 900MB
TD4 is impressive for a little bugger.
My initial thought is that the substantial amount of dry air and decent shear coming up could make it go poof! The blob behind it, that could be scary if it develops, and TD4 moistens the environment.
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Quoting Patrap:
I think I saw a Pigeon burst spontaneously into Flame Mid Flight.



Isn't that a recipe for squab?
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Quoting ho77yw00d:


Jaws theme song after that statement lol
any chance of you changing you profile pic to some sort of weather image so i can get some work done.... LOL
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In two hours we may have another invest 92L. Note the firepower of the ITCZ as a better moistened environment and energy from the strongest axis remains in the vicinity of 32W. In addition, there are numerous outflow boundaries from TD4 that are going to initiate convection with 32West.
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Quoting Irth:
Sorry for the small size, I have no idea how to get the full size image from flickr


That's OK. If we want the graphics we can go pull them up ourselves. Thanks.
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Quoting KEEPEROFTHEGATE:
there is a lot doing this




Too Funny!
People can be crazy
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I am agreeing whith Stormw blogg the models will shft left and we will have a whole diffrent scenario after lunch when the next models come out . Also look on the IR Loop this system is going almos due west maybe w-wnw
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Quoting ho77yw00d:


Jaws theme song after that statement lol


lol, easy now
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AOI/XX/XL
MARK
9.8N/30.3W
Member Since: July 15, 2006 Posts: 178 Comments: 56141
333. Irth
The area of dry air to the west of TD4 is already having a effect on the western area of convection (inside red oval). It's starting to dry out the air ahead of the system. At this rate, I think that TD4 will definitely be starved for fuel in the next few days. Even if it recurves, it will just get pulled in to the other area of dry air spinning to the north of the Antilles. There is nothing but bone dry air ahead of this storm.

Sorry for the small size, I have no idea how to get the full size image from flickr...

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Besides TD4 I think within 7 to 10 days we will have 2 more storms to follow.
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Quoting RitaEvac:
If I remember, Ike at this stage of the game was a fish storm


Jaws theme song after that statement lol
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330. IKE
Quoting KEEPEROFTHEGATE:
there is a lot doing this


I agree with what you said the other day...there's just something missing this season, so far, in the ATL.
Member Since: June 9, 2005 Posts: 23 Comments: 37860
Quoting Orcasystems:


It would be nice if we were getting some of this moisture heading towards the Islands. We have not had rain for over a month, They are finally calling for a bit of rain here this week :)
Dry as 30's Oklahoma here. Fool farm workers burned down a 120+ yr. barn here last weekend. Missed the show as was staying with daughter recovering from surgery. Only thing the fire companies (5 local volunteer stations responded)could do was back off and wet down adjacent buildings and hot spots in fields. Wonder they didn't get more stuff burning. Whatever possessed them to start a brush pile burning is beyond me. Road sides are flaring up from tossed cigarettes. Too bad ignorance isn't a painful condition.
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Quoting sammywammybamy:


My Track.

Good or Bad?
Everything I am reading your models look good. This Storm will work around the Huge High Pressure in the Atlantic, question is will that High Pressure move back to the West???
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The blog is always more active trying to decide/fight about when something is going to develop.

Its usually very quiet after they decide where its going.
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If I remember, Ike at this stage of the game was a fish storm
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Quoting Orcasystems:


I see me! haha
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Quoting howarjo1943:
Note I said most not all...This system could end up impacting the U.S. but if it moves north of the islands this is much less likely.
I also dont get where some folks are saying the flow is westerly. TD4 is moving WNW and the flow as seen on water vapor imagery is also WNW at the upper levels. Also note that cyclones tend to move a little to the right of the mean flow. Take CIMSS maps with large sodium chloride grains.
Take a look at the steering maps--low, mid, and upper levels--all almost due west. That is what the core of the storm will respond to, not what the water vapor flow likes like.
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It will be interesting to see what plays out with this storm's eventual track. Dr. Masters and NHC put alot of faith in the current run of the models and believe this will be a fish storm (if it isn't destroyed by shear) while Storm W. and Rob L. (from Crown Weather) believe this will NOT go out to sea and will remain on a more WNW course. Based on everything I have read and the supporting data I would go with the latter, a more WNW storm that may take a track similar to Bonnie. Time will tell.
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Quoting sammywammybamy:


My Track.

Good or Bad?


You're right in line with my track :)
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Quoting Patrap:
I think I saw a Pigeon burst spontaneously into Flame Mid Flight.



lol
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Quoting IKE:


Notice how the blog posts have slowed down since the NHC advisory?
there is a lot doing this
Member Since: July 15, 2006 Posts: 178 Comments: 56141
317. IKE
Dry air doing a number on TD4....



Member Since: June 9, 2005 Posts: 23 Comments: 37860
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The ITCZ is actually just below 10N so one could argue that the northern portion of the AOI is detached.

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Quoting jeebsa:
Jeanne the notorious loop. What caused that loop anyway?


A ridge building back in.
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119 down here in Navarre on the Beach. Not even the hot part of the day.
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I bet that Colin will be even worse than Bonnie!
Link
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311. xcool
PRELIMINARY EXTENDED FORECAST DISCUSSION

--------------------------------------------------------------------------------



PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
855 AM EDT MON AUG 02 2010

VALID 12Z FRI AUG 06 2010 - 12Z MON AUG 09 2010

AS HAS BEEN THE STORY FOR WEEKS...TROUGHING IS EXPECTED TO REMAIN
ACROSS THE NORTHWEST AND NORTHEAST WITH A WARM CORE RIDGE
SANDWICHED IN BETWEEN ACROSS THE SOUTHERN PLAINS. THE ONLY ASPECT
OF THE FORECAST WHICH HAS CHANGED HAS BEEN THE STRENGTH OF THESE
FEATURES...WHICH HAS BEEN INCREASING AS THE SUN ANGLE SLOWLY
LOWERS WITH TIME. THE 06Z GFS/00Z UKMET PROGRESS TROUGHING OUT OF
THE NORTHWEST...WHICH WAS NOT SUPPORTED BY THE REMAINDER OF THE
DETERMINISTIC OR ENSEMBLE MEANS...SO THEIR SOLUTIONS WERE NOT
USED. THE GEFS/ECMWF ENSEMBLE MEANS...00Z CANADIAN...AND 00Z
ECMWF WERE ALL CONSIDERED USABLE WITHIN THE WESTERLIES.
CONFIDENCE IN THIS PATTERN ACROSS THE NORTHERN AND CENTRAL UNITED
STATES IS NEAR TO ABOVE AVERAGE.

ANOTHER WRINKLE IN THE FORECAST IS THE TROPICAL DISTURBANCE NEAR
12N 41W THIS MORNING...WHICH THE DETERMINISTIC GUIDANCE FLING NEAR
OR OFFSHORE THE EAST COAST OF NORTH AMERICA. NONE OF THE
DETERMINISTIC GUIDANCE HAS SHOWN MUCH CONTINUITY WITH THIS SYSTEM
FROM DAY TO DAY. THE ENSEMBLE GUIDANCE WITH THIS FEATURE HAS ALSO
BEEN QUESTIONABLE WITHIN ANY OF ITS 00Z AND 12Z RUNS OVER THE PAST
SEVERAL DAYS...THOUGH THE MULTI-DAY WAVERING IS AROUND A TRACK
WHICH MOVES JUST NORTH OF THE GREATER ANTILLES UP THE BAHAMAS AND
OFFSHORE THE FLORIDA COAST. HOWEVER...THERE ARE A FEW ENSEMBLE
MEMBERS WHICH SUPPORT THE ECMWFS ATTEMPT TO RECURVE THIS SYSTEM
OFFSHORE ATLANTIC CANADA IN A WEEK...BUT JUST AS MANY WHICH MOVE
THROUGH THE FLORIDA STRAITS AND KEYS. THE TROUGH IN THE EAST IS
NOT EXPECTED TO BE STRONG ENOUGH TO CAUSE FULL RECURVATURE...AND
THE COL IN THE FLOW PATTERN SHOULD REMAIN TO ITS NORTH...SO
FLORIDA AND THE GULF COAST ARE NOT IN THE CLEAR. THE WEAKNESS
ALOFT TO ITS NORTH SHOULD SLOW ITS FORWARD MOTION AS IT NEARS
FLORIDA AT THE END OF THE PERIOD. DUE TO THIS TRACK EXPECTATION
AND CONTINUITY FROM THE 16Z CALL ON SUNDAY...THE 00Z CANADIAN WAS
USED AS PART OF THE PREFERENCE. THE 00Z ECMWF HAD TO BE DITCHED
FOR THE 00Z ECMWF ENSEMBLE MEAN AS PART OF THE PREFERENCE LATE IN
THE PERIOD DUE TO ITS QUESTIONABLE HANDLING OF THIS TROPICAL
DISTURBANCE. NEEDLESS TO SAY...CONFIDENCE IS LOWER THAN AVERAGE
IN THIS ASPECT OF THE FORECAST...AND CHANGES MAY OCCUR DURING THE
16Z COORDINATION CALL WITH NHC TODAY.

ROTH


hpc
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Note I said most not all...This system could end up impacting the U.S. but if it moves north of the islands this is much less likely.
I also dont get where some folks are saying the flow is westerly. TD4 is moving WNW and the flow as seen on water vapor imagery is also WNW at the upper levels. Also note that cyclones tend to move a little to the right of the mean flow. Take CIMSS maps with large sodium chloride grains.
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Quoting divdog:
caught up in itcz .. must separate from the itcz and with altitude maybe some spin then you may be on to something


Not for an invest. 04L is just to the left around 12N.
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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