ATL: TROPICAL DEPRESSION DANNY (05L)

[Aric Dunn]

Derek,why there are different positions from SSD dvorak and Best Track?

different agencies right..

[KWT]

Wow the Ships really do ramp this system up quite decently, has it getting upto 90kts, or a solid category-2 which would make for a pretty powerful landfalling system if it came to that.

Also starts at 40kts, so would jump right into TS status.

[KWT]

Have to admit that is quite a big difference there Cycloneye, but yeah its just different agencies that are progging where they think any center is. Its probably a mute point though because there probably isn't yet a true surface center to track.

[HURAKAN]

Erika on the SE-side of the graph? Interesting

[KWT]

Once again GFS based models are furthest east, even though the 18z has come somewhat closer to the coast on this run and does eventually come very close to the NE The other dynamic models actually in pretty good agreement it appears.

[otowntiger]

Its probably a mute point though because there probably isn't yet a true surface center to track.
Have to admit that is quite a big difference there Cycloneye, but yeah its just different agencies that are progging where they think any center is. Its probably a mute point though because there probably isn't yet a true surface center to track.

Bingo!

[Dean4Storms]

Have to admit that is quite a big difference there Cycloneye, but yeah its just different agencies that are progging where they think any center is. Its probably a mute point though because there probably isn't yet a true surface center to track.

It called playing darts! LOL

[Dean4Storms]

I think it forms further westward more in line with the CMC but maybe a tad east of it. I agree with Derek here that this is a hurricane by Friday if not by late Thursday. I think the pressure gradient helps it in obtaining hurricane strength in the NE Quad!

[KWT]

Yep weather darts!

By the way latest SHIPS start at 40kts so it will be upgraded straight to TS if a center is found it appears unless those winds die down, also SHIPS takes this upto 90kts.

[Aric Dunn]

Once again GFS based models are furthest east, even though the 18z has come somewhat closer to the coast on this run and does eventually come very close to the NE The other dynamic models actually in pretty good agreement it appears.

if you all have not noticed they are all on crack..

each run the track or the initial motion does not change... just the initial position is farther west.. which says they have no idea whats going on initially and leads to the simple conclusion they dont know whats going to happen in the future.. the Euro and ukmet are the only to maintaining the correct initial motions with the cmc close behind ..

[KWT]

Yeah I agree Aric though until an actual strong center of circulation forms we can't rule out any formation away from where we think it is which would bring the GFS back into play.

However for now if the center forms roughly where we think then the models you mention would have the best idea.

[Aric Dunn]

Yeah I agree Aric though until an actual strong center of circulation forms we can't rule out any formation away from where we think it is which would bring the GFS back into play.

However for now if the center forms roughly where we think then the models you mention would have the best idea.

no not exactly.. cause it still has the wrong initial motion.. so even if it was farther east a w to wnw motion would be likely not NW borderline nnw like the GFS and its based models..

[clfenwi]

Yeah, as everyone's saying, if there's a difference between the SAB estimate and best track, one can infer that TAFB had a conflicting opinion. Depending on the situation, either TAFB's position was favored or a compromise was struck. Given the large uncertainty inherent in this situation, one would suspect the latter in this case.
Even when you know it's a virtual dart toss, you have to give the tropical models a starting point, alas.

[Dean4Storms]

Might this slowing of events allow 92L or possibly Danny to get closer to the coast before a turn northward?

From Mobile NWS afternoon Discussion.....

SOME CONTINUITY
ARRIVES IN THE GUIDANCE ADVERTISING A SHORTWAVE SYSTEM DIVING INTO
THE UPPER TROUGH OVER THE MISS RIVER VALLEY...DISORGANIZING THE UPPER
LOW OVER THE LOWER MISS RIVER AND KICKING IT EAST. THURSDAY THROUGH
THURSDAY NIGHT.

THERE CONTINUES TO BE A PUSHING BACK OF THESE EVENTS...RESULTING IN
A SLOWER RETURN OF PRECIP TO THE FA. CURRENT TIME OF EVENTS IS ABOUT
12HRS PLUS SLOWER THAN YESTERDAY`S GUIDANCE WAS ADVERTISING. HAVE
AGAIN WENT WITH A BLENDED APPROACH WITH THE FORECAST IN THE SHORT
TERM...TO HOPEFULLY COME OUT WITH A BETTER TIMING OF THE EXPECTED
COMING PRECIP.

[btangy]

The upper level flow remains very complex with an upper level low nearly co-located with the 92L and a sharp upper level trough to the N. I remain skeptical that the global models are handling the evolution of these features with much fidelity.

At this time, the upper level low is making it hard for 92L to consolidate an area of thunderstorms to form tight low level center. Convection is quickly firing, decaying, and forming large cold pools at the surface. An example of which can be seen to the SE of the wave axis as a thin arcing cloud line with a line of storms firing behind it like you would see in the Midwest with a fast moving squall line along a gust front. This is very typical of sheared, conditionally unstable environments.

The wave axis of 92L does seem to be moving forward a bit faster than the upper level low and models are emphatic on placing 92L in a very favorable area of upper level difluence/divergence in 24-48 hours. Provided this happens, quick development may occur around this time frame, but location is key. Instead, if 92L keeps doing the tango with the upper level low, then development will be slow as it'll be more of a subtropical flavor with the cold core aloft slowly eroding away.

There's still much uncertainty as to where the initial LLC will ultimately form which will make all the difference in the future track. It is quite possible one could wake up tomorrow morning and the convection will be in a totally different location and we can all restart guessing and straining our eyes as to where the clouds appear to be closing off a circulation. IMO, I say this thing needs at least another 24 hours to marinate.

['CaneFreak]

The upper level flow remains very complex with an upper level low nearly co-located with the 92L and a sharp upper level trough to the N. I remain skeptical that the global models are handling the evolution of these features with much fidelity.

At this time, the upper level low is making it hard for 92L to consolidate an area of thunderstorms to form tight low level center. Convection is quickly firing, decaying, and forming large cold pools at the surface. An example of which can be seen to the SE of the wave axis as a thin arcing cloud line with a line of storms firing behind it like you would see in the Midwest with a fast moving squall line along a gust front. This is very typical of sheared, conditionally unstable environments.

The wave axis of 92L does seem to be moving forward a bit faster than the upper level low and models are emphatic on placing 92L in a very favorable area of upper level difluence/divergence in 24-48 hours. Provided this happens, quick development may occur around this time frame, but location is key. Instead, if 92L keeps doing the tango with the upper level low, then development will be slow as it'll be more of a subtropical flavor with the cold core aloft slowly eroding away.

There's still much uncertainty as to where the initial LLC will ultimately form which will make all the difference in the future track. It is quite possible one could wake up tomorrow morning and the convection will be in a totally different location and we can all restart guessing and straining our eyes as to where the clouds appear to be closing off a circulation. IMO, I say this thing needs at least another 24 hours to marinate.

I agree totally...24 hours and we'll be good

[KWT]

Quite possibly Dean, I wonder whether thats whats causing the GFS to slowly push the system a little further west as each cycle comes through.

[Derek Ortt]

Derek,why there are different positions from SSD dvorak and Best Track?

SSD=22.5N 67.1W

Best Track=23.3N-66.7W

probably TAFB has a different position

[Windtalker1]

Great analysis (as always) this evening from Dr. Jeff Masters....

The tropical wave (92L) a few hundred miles north of Puerto Rico is generating a large area of surface winds of 50 - 60 mph, according to the latest information from the Hurricane Hunters. Top winds seen so far at their flight level of 1,000 feet were 69 mph, which would make 92L a strong tropical storm if it had a surface circulation.
However, the aircraft has not found a surface circulation, and the satellite appearance shows virtually no change in the amount, intensity, or organization of the storm's thunderstorm activity.

Wind shear has dropped to the moderate range, 15 - 20 knots this afternoon, but the upper low 92L is moving underneath is dumping cold, dry air into the region. Dry air continues to get ingested into 92L's thunderstorms, creating strong downdrafts that are robbing 92L of heat and moisture. These downdrafts are creating surface arc clouds that spread out from where the downdraft hits the ocean surface. NHC continues to give 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday afternoon.

As 92L moves underneath the center of the upper low on Wednesday morning, the upper low is expected to weaken, and wind shear is expected to decline to the low range, 5 - 10 knots. However, the upper-level low will continue to dump dry, cold air into 92L through Thursday afternoon, slowing down development. By Thursday night, when 92L should be several hundred miles off the coast of northern Florida, the upper-level low may be weak enough and far enough away that 92L will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development.

However, this favorable environment will not last long, since a strong trough of low pressure will be approaching the U.S. East Coast on Friday. This trough will bring high wind shear of 20 - 30 knots by Friday night. This trough should be strong enough to turn 92L to the north. The models disagree substantially on how close 92L will be to the coast at that time. One camp of models, including the NOGAPS, Canadian, UKMET, and ECMWF models, predict 92L will pass very close to the Outer Banks of North Carolina on Friday night or Saturday morning. The GFS, GFDL, and HWRF models keep 92L several hundred miles out to sea. Both sets of models bring 92L north-northeastwards on Saturday, with a track over Massachusetts or Nova Scotia. The intensity forecast for 92L is problematic, since it's eventual strength depends upon how quickly it manages to become a tropical depression. Given that 92L will find itself in a favorable environment for strengthening for about 36 hours this week, and marginal for the remainder of the week, I give the system these odds:

10% chance of never getting a name.
20% chance of becoming a weak tropical storm (40 - 50 mph winds).
40% chance of becoming a strong tropical storm (55 - 70 mph winds).
30% chance of attaining hurricane strength.

Elsewhere in the Atlantic
The ECMWF and UKMET models predict the development of a tropical wave coming off the coast of Africa late this week. The GFS model no longer shows this.

I'll have an update Wednesday morning.

Jeff Masters

Although I respect Jeff Masters, I have to wonder why our local Weather here in TN will not see this cold front pass through until Sometime Saturday. It could be Monday before this front makes it to the east coast:

NATIONAL WEATHER SERVICE NASHVILLE TN
329 PM CDT TUE AUG 25 2009

.DISCUSSION...
CU FORMATION AGAIN AS A WEAK UPPER LOW REMAINS POSITIONED ACROSS MS.
UPPER RIDGING STRETCHES FROM TX THROUGH MO AND IN. THUS...CU
DEVELOPMENT LESS EXTENSIVE ACROSS THAT AREA. A FEW ISOL SHOWERS
ATTEMPTING TO DEVELOP ACROSS THE NORTHERN PLATEAU BUT AIRMASS IS
JUST TOO DRY TO SUPPORT EVEN 5-10 PERCENT COVERAGE.

UPPER LOW WILL STRENGTHEN SOME AS UPPER RIDGING WEAKENS. AS A SFC
TROUGH GAINS STRENGTH OVER THE LOWER MS VALLEY...A SOUTHERLY
COMPONENT TO THE SFC WIND FLOW PATTERN....THOUGH WEAK...WILL RETURN
TOWARD THE END OF THE WEAK. AS HUMIDITY LEVELS CLIMB...PRECIP
CHANCES IN THE FORM OF SHOWERS AND TSTMS WILL CONTINUE IN THE FCST
FOR FRIDAY AND SATURDAY.

FOR THE NEAR TERM TEMPS...WILL GO ABOVE THE MAV GUIDANCE FOR
TONIGHT. DEW POINTS HAVE INCREASED ABOUT 5 DEGREES COMPARED TO
YESTERDAY AT THIS TIME. WILL THEREFORE LEAN TOWARD THE WARMER MET
SCENARIO. OTW...HIGH TEMPS TO REACH NEAR THE 90 DEGREE LEVEL FOR THE
NEXT SEVERAL AFTERNOONS.

IN THE EXT FCST...COLD FRONT TO MOVE THROUGH ON SATURDAY. GOOD
THERMAL TROUGHING BEHIND THE FRONT WITH 850 MB TEMPS DROPPING TO
ABOUT 12C FOR EARLY NEXT WEEK. TROUGH IS RATHER BROAD WITH A GOOD
PUSH OF DRY AIR BEHIND THE FROPA. SO...SHOULD SEE MORE SUNSHINE IN
THE POST FRONTAL SECTOR AS COMPARED WITH THE LAST FROPA.

FOR TEMPS...LOOK FOR BELOW NORMAL LEVELS SUNDAY THROUGH TUESDAY

[Derek Ortt]

Ortt's first call

hurricane by Friday

Now that's what I call making a bold forecast! Props for putting your neck out Derek

Probably going to get even busier for you with the model support of the African system and the subtropical ridge building in...

not overly so. Only have resources to model one storm at a time. The forecasting can be handled by another forecaster