97L growing more organized, will bring heavy rain to the Lesser Antilles
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
Reader Comments
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That is not what I heard, but I could have misinterpreted. Doubtful, though.
After all, why does westerly shear often destroy westward-moving tropical waves, and yet they can survive easterly shear?
That Valid Time is a little confusing...That is the time that the forecast is for...matter of fact that image doesn't even show what time the model was run.
It won't develop until 75W, when shear becomes more favorable.
Doesn't westerly shear come from the west? Easterly shear comes from the east? Like a west wind is from the west and an east wind is from the east.
So a storm going west would run straight into westerly shear.
20 knots from the west at 200mb and 10 knots from the east at 850mb equals 30 knots of shear.
20 knots from the east at 200mb and 10 knots also from the east equals 10 knots of shear.
Wind Shear Tutorial-J. Masters
Unless I am doing my math wrong.... its 0218Z
So the OO would have been over 2 hours ago
Yeah, that was my point. 97L is running into westerly shear.
Models take a while to run that's why there's lag. The early cycle models like the bams are probably already out.
ROFLMAO
Correct Orca...But most of the global models do not finish processing data and producing output until 6-9 hrs later.
So SJ is right. The storm is not going with the shear. It's traveling against the shear thus it's worse.
Not all are new
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.
Well, that was what I was trying to say, I guess I just got my facts slightly mixed up. Thanks. >_>
A vigorous tonight? o_O
Might want to edit that. :P
It's all good.
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)
lol I already, you did not give me a chance :p
Any radar maps please
How to use your board and still ride the water when there is no surf.
LINK
It'd be funny if it weren't true.
Why is every thing about south florida?
Miamicaster.
your examinations are incorrect lol or at least premature anyway
good to see you folks again.
XD
Early Model 00Z runs
Nah, not NOLA. This year, it's Miami.
yep, but can you tell from reading all the posts? lol
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.
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.
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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