97L growing more organized, will bring heavy rain to the Lesser Antilles

By: Dr. Jeff Masters , 3:55 PM GMT on July 19, 2009

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The tropical wave near 12N 52W, about 600 miles east of the Lesser Antilles Islands (97 L), has grown more organized this morning as it tracks west at about 20 mph. This wave is surrounded by an area of very dry air from the Sahara Desert, but 97L has been able to steadily moisten a large region of the atmosphere over the past day, insulating itself from the dry air. This moistening process has been aided by sea surface temperatures (SSTs) that have steadily increased from 26.5°C to 27.5°C, plus the presence of only 10 knots of wind shear. The system now has a small area of intense thunderstorms near its center, with some rotation of the cloud pattern evident at mid-levels of the atmosphere. An upper-level outflow channel has opened to the north, and there is evidence that surface spiral bands are beginning to form. This morning's QuikSCAT pass missed 97L, so we don't know what is happening at the surface.

Wind shear is a modest 10 knots over the disturbance, and is forecast to remain in the moderate 10 - 15 knot range through Monday morning. This should allow further development to occur today, and 97L could be approaching tropical depression strength on Monday as it moves through the central Lesser Antilles Islands. Monday night, shear is expected to rise to 20 - 30 knots, thanks to the presence of a trough of low pressure at upper levels of the atmosphere over the eastern Caribbean. Since 97L is a relatively small system, it is very vulnerable to wind shear. This shear may be able to drive some of the dry air west of 97L deep into its interior, significantly disrupting the disturbance. Shear will remain high along 97L's path through Thursday, when the storm should be in the western Caribbean near Cuba. If there is anything left of 97L by then, some development is possible. The National Hurricane Center gave 97L a low (less than 30% chance) of developing into a tropical depression in the next 48 hours in their 8am Tropical Weather Outlook. However, I'd say the odds are now in the medium range (30 - 50%).

None of the computer models are forecasting tropical storm development over the next seven days.


Figure 1. Current satellite image of African wave 97L.

I'll have an update Monday morning. As 97L approaches the islands, you may want to follow local observations there using our wundermap for 97L.

Jeff Masters

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1815. Vero1
MARINE WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE MIAMI FL
226 PM EDT SUN JUL 19 2009

... GLOBAL MODELS CONTINUE TO INDICATE THAT THE
NORTHERN EXTENT OF A TROPICAL WAVE NEAR 70W MOVING WEST TOWARD
THE TURKS AND CAICOS ISLANDS WILL SPLIT OFF AND MOVE NW THROUGH
THE LENGTH OF THE BAHAMAS...DELIVERING INCREASED SHOWERS AND
TSTMS. A PAIR OF TROPICAL WAVES BEHIND THIS WILL MIGRATE ACROSS
THE CARIBBEAN THROUGH MIDWEEK..MAINTAINING 20 TO 25 KT EASTERLY
WINDS MAINLY S OF 23N TO INCLUDE THE SOUTHERN BAHAMAS AND TURKS
AND CAICOS. THE PERSISTENT STRONG EAST FLOW WILL ALLOW SEAS TO
BUILD TO 10 FT IN E SWELL OVER MUCH OF THE AREA BETWEEN THE
LEEWARDS AND THE SOUTHERN BAHAMAS THROUGH MIDWEEK. DECAYING SE
SWELL MIGRATES NW TO THE EAST OF THE BAHAMAS THU AND FRI.
THE RIDGE AXIS SHIFTS SOUTHWARD THU AND FRI...WITH WINDS AND SEAS
DIMINISHING.
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yeah shear is currently 23.6 mph.
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Also its southern convection is in 10kt.So north part is likely 15kt.
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.....more rain for the caribbean. It's help to clean the Saharian dust from our sky....
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SJ it may not be pretty in about an hour or two lol, that shear is making a beeline for the core of the system
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Wrf weakens shear gradually to 0 to 10kt.
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Expect fluctatuons in shea rof 5 to 15kt overnight.Gradually shear will weaken next few days over caribean as trough weakens/moves northwest
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Quoting STORMMASTERG:
Its 10kt shear.


no SJ is right 20 knots going right over the system as we speak
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When will the shear leave?Because I don't think there will be nothing tomorrow if conditions right now persist for 97L.
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Quoting sporteguy03:


yep, but can you tell from reading all the posts? lol
Exactly.
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1805. Gorty
Since it may enter the Carriebean, the shear will destroy it. I doubt anything will be left when it enters the western cariebean.

With this model: http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8shr.html

For the windhsear there, do you have to look at both levels of the atmosphere, or just one or the other?
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Quoting StormJunkie:
Looks like shear is already 20knts over the system.
Its 10kt shear.
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Quoting StormJunkie:
Looks like shear is already 20knts over the system.


It sure is and going against the shear, 97L may have a tough time surviving

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i haven't been on since yesterday, has there been a lot of miamicasting since i was gone?
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Looks like shear is already 20knts over the system.
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1800. Vero1
000
AXNT20 KNHC 192334
TWDAT

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
805 PM EDT SUN JUL 19 2009

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL
AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHERN SECTIONS
OF SOUTH AMERICA...AND ATLANTIC OCEAN TO THE AFRICAN COAST
FROM THE EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED
ON SATELLITE IMAGERY...METEOROLOGICAL ANALYSIS...WEATHER
OBSERVATIONS...AND RADAR.

BASED ON 1800 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY
THROUGH 2245 UTC.

...TROPICAL WAVES...
A TROPICAL WAVE IS ALONG 41W/42W S OF 19N MOVG W NEAR 20 KT.
THIS WAVE COINCIDES WITH A DEEP LAYER MOISTURE MAXIMUM OBSERVED
IN TOTAL PRECIPITABLE WATER IMAGERY. THIS WAVE IS JUST W OF A
DRY SAHARAN AIR LAYER. DUE TO THE PRESENCE OF THIS DRY AIR...NO
DEEP CONVECTION IS CURRENTLY ASSOCIATED WITH THE WAVE.

A TROPICAL WAVE IS ALONG 52W S OF 18N MOVG W NEAR 15 KT. THIS
WAVE COINCIDES WITH A LOW TO MIDDLE LEVEL CYCLONIC CIRCULATION
NEAR THE WAVE AXIS AT 13N. SCATTERED MODERATE CONVECTION IS FROM
12N-15N BETWEEN 51W-54W.

A HIGH AMPLITUDE TROPICAL WAVE IS ALONG 71W S OF 26N MOVG W
20-25 KT. THIS WAVE COINCIDES WITH A HIGH AMPLITUDE MOISTURE
MAXIMUM OBSERVED ON TOTAL PRECIPITABLE WATER IMAGERY. AN
INVERTED-V PATTERN OF LOW CLOUDS IS ALSO EVIDENT ON VISIBLE
SATELLITE IMAGERY. SCATTERED MODERATE/ISOLATED STRONG CONVECTION
IS FROM 16N-20N BETWEEN 69W-72W INCLUDING PORTIONS OF CUBA. THIS
WAVE MAY FRACTURE OVER THE NEXT DAY WITH THE N PORTION OF THE
WAVE FORMING A TROUGH THAT WILL CONTINUE TO THE NW...WHILE THE S
PORTION OF THE WAVE CONTINUES TO PROPAGATE WWD.

...THE ITCZ...
ITCZ AXIS IS CENTERED ALONG 12N15W 7N27W 10N40W 10N51W 9N62W.
SCATTERED MODERATE CONVECTION IS FROM 9N-13N BETWEEN 13W-17W. A
CLUSTER OF NUMEROUS MODERATE/SCATTERED STRONG CONVECTION IS FROM
5N-9N BETWEEN 26W-29W. SCATTERED MODERATE CONVECTION IS FROM
7N-14N BETWEEN 60W-64W.

...DISCUSSION...

THE GULF OF MEXICO...
A STATIONARY FRONT IS OVER THE NRN GULF OF MEXICO ALONG 31N81W
28N88W 29N96W. AN UPPER LEVEL TROUGH EXTENDING DOWN THE ERN
CONUS INTO THE GULF TO NEAR 26N90W SUPPORTS THE FRONT. SURFACE
CONVERGENCE AHEAD OF THE FRONT ALONG WITH UPPER LEVEL DIFFLUENCE
ARE SUPPORTING SCATTERED SHOWERS/THUNDERSTORMS FROM 24N-29N E OF
96W. THE REMAINDER OF THE GULF IS UNDER A BROAD SURFACE RIDGE
ANCHORED BY A 1020 MB SURFACE HIGH NEAR 26N87W BRINGING
GENERALLY FAIR WEATHER S OF 24N. A MID-LEVEL DRY SAHARAN AIR
LAYER IS ACROSS THE SRN GULF OF MEXICO SUPPORTING THE NOTED FAIR
WEATHER. LIGHT ANTICYCLONIC FLOW IS AROUND THE SURFACE RIDGE
WITH E-SE WINDS UP TO 15 KT PRESENT ACROSS THE SRN GULF...EXCEPT
FOR A SMALL PORTION OF NE WINDS UP TO 20 KT NEAR THE YUCATAN
PENINSULA COAST. EXPECT THE FRONT TO STALL TONIGHT AND BECOME
DIFFUSE TOMORROW.

THE CARIBBEAN SEA...
FAIR WEATHER IS NOTED IN THE FAR NW PORTION OF THE CARIBBEAN AS
IT IS UNDER THE INFLUENCE OF A MID-LEVEL DRY SAHARAN AIR LAYER.
HOWEVER...ATMOSPHERIC INSTABILITY IS CAUSING A FEW ISOLATED
SHOWERS TO FLARE UP OVER CUBA AND THE YUCATAN PENINSULA. AN
UPPER LEVEL RIDGE DOMINATES THE MAJORITY OF THE CARIBBEAN
CENTERED NEAR 14N75W. UPPER LEVEL DIFFLUENCE AND THE PROXIMITY
OF THE E PACIFIC ITCZ ARE SUPPORTING SCATTERED
SHOWERS/THUNDERSTORMS OVER CENTRAL AMERICA S OF 14N W OF 80W
INCLUDING PORTIONS OF NICARAGUA...COSTA RICA...AND PANAMA.
SURFACE CONVERGENCE AROUND A 1008 MB SURFACE LOW OVER COLUMBIA
NEAR 8N76W IS SUPPORTING SCATTERED SHOWER/THUNDERSTORMS S OF 10N
BETWEEN 75W-78W. UPPER LEVEL DIFFLUENCE BETWEEN THE UPPER LEVEL
RIDGE OVER THE CARIBBEAN AND AN UPPER LEVEL LOW IN THE ATLC NEAR
27N55W IS SUPPORTING SCATTERED SHOWERS/THUNDERSTORMS OVER THE
ERN CARIBBEAN N OF 14N BETWEEN 63W-69W. THIS SHOWER ACTIVITY IS
PROBABLY BEING ENHANCED BY THE TROPICAL WAVE ALONG 71W.
SCATTERED SHOWERS/THUNDERSTORMS ARE ALSO AFFECTING THE SRN
LESSER ANTILLES S OF 14N W OF 62W DUE TO UPPER LEVEL DIFFLUENCE
AND THE PROXIMITY OF THE ATLC ITCZ. EXPECT A SECOND TROPICAL
WAVE TO REACH THE ERN CARIBBEAN EARLY TUE.

THE ATLANTIC OCEAN...
SCATTERED SHOWERS/THUNDERSTORMS ARE ACROSS S FLORIDA ENTERING
THE ATLC AND APPROACHING THE BAHAMAS FROM 25N-29N W OF 79W. THIS
SHOWER ACTIVITY IS SUPPORTED BY A SMALL UPPER LEVEL LOW OVER THE
BAHAMAS NEAR 25N77W. UPPER LEVEL DIFFLUENCE BETWEEN THE UPPER
LEVEL TROUGH OVER THE ERN CONUS AND AN UPPER LEVEL RIDGE
CENTERED AROUND 39N54W IS SUPPORTING ISOLATED
SHOWERS/THUNDERSTORMS ACROSS THE SW NORTH ATLC N OF 24N BETWEEN
68W-75W. THE REMAINDER OF THE ATLC IS DOMINATED BY A BROAD RIDGE
ANCHORED BY A 1029 MB SURFACE HIGH N OF THE DISCUSSION AREA NEAR
35N46W BRINGING GENERALLY FAIR WEATHER ACROSS THE MAJORITY OF
THE CENTRAL AND ERN ATLC. HOWEVER...A LINE OF SHOWERS ARE FROM
17N58W TO 24N50W DUE TO UPPER LEVEL DIFFLUENCE BETWEEN AN UPPER
LEVEL HIGH NEAR 15N50W AND AN UPPER LEVEL LOW NEAR 27N55W.

$$
WALTON
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Quoting BurnedAfterPosting:


your examinations are incorrect lol or at least premature anyway
i know, im just putting that i out there.
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Quoting KoritheMan:


Nah, not NOLA. This year, it's Miami.


Oh trust me, they'll manage to slip in a NOLA. "It's going to pull an Andrew and skip right over FL to hit NOLA!"

I too think FL is in for a bit of a beating this season. I'm not expecting too much though.
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Quoting KoritheMan:


Nah, not NOLA. This year, it's Miami.


Yeah, 2008 was the year of NOLAcasters. Now its "Miamicasters". I don't think I've ever seen so many posts and different people saying a blob/invest was going to post a threat to South Florida like this one.
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Quoting FloridaTigers:


Miamicaster.
i pomise you im not a miamicaster im just following the new models.
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Quoting TampaMishy:
I see nothing has changed with 97L from this afternoon.


yep, but can you tell from reading all the posts? lol
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Quoting frostynugs:
"yawn"... awakening from my off-season hybernation. I see we have a little semi-swirly puff of clouds out there. *waits on "CAT5 NOLA!"*

good to see you folks again.


Nah, not NOLA. This year, it's Miami.
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Early Model 00Z runs
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I still haven't changed my opinion on the forcast of 97l.Expect 5 to 15 kt shear based on wrf and other models.Ts is possible by mid day tomorrow.
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Quoting WxLogic:
Night guys... I'll praying tonight... that 97L doesn't get pounded too much tonight so we can have a focused blog instead of talking about snakes and turtles. :)


XD
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"yawn"... awakening from my off-season hybernation. I see we have a little semi-swirly puff of clouds out there. *waits on "CAT5 NOLA!"*

good to see you folks again.
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Quoting canesrule1:
hello everyone, i am currently examining some of the stats and models of 97L and well i have concluded 2 things, South Florida is in danger and 97L is strengthening, in the past 12 hours 97L pressure has decreased to TD status. I do agree convection is going down i will probably pick up tomorrow.


your examinations are incorrect lol or at least premature anyway
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Quoting canesrule1:
hello everyone, i am currently examining some of the stats and models of 97L and well i have concluded 2 things, South Florida is in danger and 97L is strengthening, in the past 12 hours 97L pressure has decreased to TD status. I do agree convection is going down i will probably pick up tomorrow.


Miamicaster.
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Any naked swirls yet?
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Quoting canesrule1:
hello everyone, i am currently examining some of the stats and models of 97L and well i have concluded 2 things, South Florida is in danger and 97L is strengthening, in the past 12 hours 97L pressure has decreased to TD status. I do agree convection is going down i will probably pick up tomorrow.

Why is every thing about south florida?
Member Since: May 16, 2009 Posts: 1 Comments: 1231
1785. WxLogic
Night guys... I'll praying tonight... that 97L doesn't get pounded too much tonight so we can have a focused blog instead of talking about snakes and turtles. :)
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Quoting futuremet:
Watch this blog do a 180 tomorrow morning lol


It'd be funny if it weren't true.
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The weather udnerground one/deep red cores.
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hello everyone, i am currently examining some of the stats and models of 97L and well i have concluded 2 things, South Florida is in danger and 97L is strengthening, in the past 12 hours 97L pressure has decreased to TD status. I do agree convection is going down i will probably pick up tomorrow.
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A little diversion for surfmom.
How to use your board and still ride the water when there is no surf.
LINK
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Quoting largeeyes:
Looks like rain chances every day for next 7 days here in Eastern NC. Is anything still showing a chance of something forming off the coast from one of these areas of low pressure?
The AFD from Wilmington stated that an area of low pressure would ride up the coast sometime on Tuesday, but I believe that this would be from the upper levels, not at the surface. The only other than that to note would be the wave along 10N50W. The AFD again, stated that the WRF model and the DGEX model develop it well offshore of NC, but at this time it is not their favored solution.
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Watch this blog do a 180 tomorrow morning lol
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Quoting STORMMASTERG:
Tstms beginning to erupt like last night/but this night it already has a better structure so expect more than last night.


Any radar maps please
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I see nothing has changed with 97L from this afternoon.
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Quoting KoritheMan:


A vigorous tonight? o_O

Might want to edit that. :P


lol I already, you did not give me a chance :p
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Tstms beginning to erupt like last night/but this night it already has a better structure so expect more than last night.
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Quoting StormJunkie:



Correct Orca...But most of the global models do not finish processing data and producing output until 6-9 hrs later.


ROFL, not to mention.. it easy to cheat... the Blog is in Zulu time.

I know it takes a few hours to run them... when I first looked at the post.. it looked like he was using Zulu as his local time (the description match the times)
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Quoting KoritheMan:


Well, that was what I was trying to say, I guess I just got my facts slightly mixed up. Thanks. >_>


It's all good.
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Quoting futuremet:
Convection is building near the quasi-LLC. The establishment of a vigorous tonight is vital for organization.


A vigorous tonight? o_O

Might want to edit that. :P
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Quoting HurricaneKing:


So SJ is right. The storm is not going with the shear. It's traveling against the shear thus it's worse.


Well, that was what I was trying to say, I guess I just got my facts slightly mixed up. Thanks. >_>
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Convection is building near the quasi-LLC. The establishment of a vigorous CDO tonight is vital for organization.
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I found this in Wilmington, NC's area forecast discussion. Just interesting to see them include a tropical wave that far out that hasn't even developed yet. " TROPICAL WAVE NOW AROUND 10N50W IS PROGGED TO
DEVELOP BY WRF/DGEX WELL OFFSHORE BUT THIS IS CURRENTLY NOT
FAVORED.
" Im really interested to see what happens with this wave over the next week. It may not be amounting to much right now, but I think that changes as time goes by.
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Some new models out


Not all are new
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Quoting KoritheMan:


Yeah, that was my point. 97L is running into westerly shear.


So SJ is right. The storm is not going with the shear. It's traveling against the shear thus it's worse.
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Quoting Orcasystems:


Unless I am doing my math wrong.... its 0218Z
So the OO would have been over 2 hours ago


Correct Orca...But most of the global models do not finish processing data and producing output until 6-9 hrs later.
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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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