Tropical Storm Leslie - Discussion, Images, Forecasts

[turtlehurricane]

Ekman upwelling doing it's thing, this buoy is 100 km NNW of the center, so waters are already probably much cooler towards the center where cyclonic wind stress is the strongest. Probably one of the explanations for the lackluster convection.

http://www.ndbc.noaa.gov/plot_met.php?station=41049&meas=wtmp&uom=E&time_diff=0&time_label=GMT

[phil882]

Ekman upwelling doing it's thing, this buoy is 100 km NNW of the center, so waters are already probably much cooler towards the center where cyclonic wind stress is the strongest. Probably one of the explanations for the lackluster convection.

http://www.ndbc.noaa...&time_label=GMT

Agreed generally although oceanic heat content is also generally above normal in this region right now (pre-Leslie of course).

[turtlehurricane]

Agreed generally although oceanic heat content is also generally above normal in this region right now (pre-Leslie of course).

Looks like an average sea level anomaly of 15 cm throughout most of the basin, jives with the subtropical high being stronger than normal and the concurrent drought. When that + MJO comes back around there's gonna be alot of fuel to work with. As for Leslie, it could spell trouble for Bermuda since it will encounter untapped warm waters as it accelerates north towards the island... I expect intensification Friday and Saturday when motion starts. Also can't discount that when Leslie begins to accelerate it will be an optimal time for the trough to aid intensification as the outflow couples with the jet stream. Things could get hairy very quickly even if this remains a borderline cane until Friday.

[andyhb]

Looks like an average sea level anomaly of 15 cm throughout most of the basin, jives with the subtropical high being stronger than normal and the concurrent drought. When that + MJO comes back around there's gonna be alot of fuel to work with. As for Leslie, it could spell trouble for Bermuda since it will encounter untapped warm waters as it accelerates north towards the island... I expect intensification Friday and Saturday when motion starts. Also can't discount that when Leslie begins to accelerate it will be an optimal time for the trough to aid intensification as the outflow couples with the jet stream. Things could get hairy very quickly even if this remains a borderline cane until Friday.

I'd say that Leslie has a good chance to be the first MH of the season (if Kirk wasn't already), as you mentioned that acceleration would aid in creating a healthy upper level environment for the storm to work with.

[wxmx]

Not only the bulk of the correction was on the western tracking models like the Euro, but even it's looking a bit more east than what the GFS showed a couple days ago. The GFS looks like once more it will mop with the competition.

[am19psu]

Not only the bulk of the correction was on the western tracking models like the Euro, but even it's looking a bit more east than what the GFS showed a couple days ago. The GFS looks like once more it will mop with the competition.

Remember when the Euro used to be good at forecasting tropical cyclones?

[phil882]

Not only the bulk of the correction was on the western tracking models like the Euro, but even it's looking a bit more east than what the GFS showed a couple days ago. The GFS looks like once more it will mop with the competition.

Ehh its more like a Tie... the GFS was well east of Bermuda in its early forecasting for Leslie and WAY too fast. Speed of a TC is just as important as the track.

Leslie continues to look like crap. Finally the NHC is acknowledging that its probably due to SST decrease over the core of the storm. Both the HWRF and GDFL (which are pretty much only useful for their initial conditions over Leslie) show a nice patch of sub 26 degree C water nearly over the core. Since they are dynamically coupled, they are the best models we have for forecasting SST cool down due to upwelling of a storm circulation. Below is the analysis HWRF depiction of Leslie. Unfortunately, the HWRF and GFDL aren't the best models when it comes to interpreting these SST changes over the core of the system, and they both are still forecasting rapid intensification. This might happen were Leslie to start moving away rapidly from the coolest SSTs, but you have to remember how large Leslie circulation is currently. Thus, upwelling is occurring well ahead of the main core, which is why we weren't able to get a well organized inner core to form thus far. I think its under-appreciated how much upwelling affects the structure of these larger TCs. We have seen plenty of examples of major hurricanes that can rapidly intensify in 27-28 degree C waters (Ophelia a great example last year). However, these larger TCs already modify their SST environment long before the inner core of the cyclone can move over those warm waters, limiting the heat fluxes the storm can take in from the inner core of the circulation. This might be one reason (inertia aside) why its hard for a large wind field to contract effectively, and why its rare to see a TC shrink in size rather than vice versa.

[CoastalWx]

Remember when the Euro used to be good at forecasting tropical cyclones?

Euro is a good model, but I trust the GFS more lately when it comes to TC..at least int he ATL basin. Euro does tend to be a little overzealous with troughing in the east so that might be one reason it tried bringing it west.

[am19psu]

post

I don't disagree with your SST assessment, but it's pretty obvious looking at the MW pattern that Leslie is still dealing with westerly shear, too.

[am19psu]

Euro is a good model, but I trust the GFS more lately when it comes to TC..at least int he ATL basin. Euro does tend to be a little overzealous with troughing in the east so that might be one reason it tried bringing it west.

I'm mostly joking, but it's definitely been owned by the GFS for the last two systems.

[CoastalWx]

I'm mostly joking, but it's definitely been owned by the GFS for the last two systems.

Oh I know..lol...but I agree. It hasn't been stellar.

[phil882]

I don't disagree with your SST assessment, but it's pretty obvious looking at the MW pattern that Leslie is still dealing with westerly shear, too.

True... but this could be more of a function of a TC having an inability to produce deep convection completely around its circulation. As we saw with Michael, deep convection modulates the atmosphere through diabatic heating in the mid-levels, producing anticyclonic flow aloft due to mass flux adjustments due to the temperature perturbation. Without a nice CDO, the storm struggles to fight off the synoptic scale westerly flow. Since the global models don't have a good handle on the SST pattern under the core, they are going to overdo the convective adjustment of the synoptic flow.

[wxmx]

Remember when the Euro used to be good at forecasting tropical cyclones?

Bring back the good ol' times please...when the Niños were known to have little cyclone activity, and the Euro was king...I guess we'll have to adjust to the new era .

Ehh its more like a Tie... the GFS was well east of Bermuda in its early forecasting for Leslie and WAY too fast. Speed of a TC is just as important as the track.

Will probably have to wait for objective verifications, since the mind can play tricks on you (general you, more like me ). Two days ago the GFS was slower than the Euro and way east. That has verified better so far, at least. I remember that well because Riptide's blew away my analysis with his "GFS will bust!"

[phil882]

Will probably have to wait for objective verifications, since the mind can play tricks on you (general you, more like me ). Two days ago the GFS was slower than the Euro and way east. That has verified better so far, at least. I remember that well because Riptide's blew away my analysis with his "GFS will bust!"

I generally agree, it seems like the GFS is handling the 48-72 hour forecast period better... I'm just referring to when Leslie first developed into a TC... the GFS had it all the way up way east of Bermuda in the 180-144 hour time frame. That forecast is not verifying. Its hard to do model verification when it always changes each run

Kyle Griffin's Dprog/Dt map shows this blatant NE bias of the GFS in the longer time fields. It does seem to key into the correct solution within 144 hours, although there were still some runs a little too far to the NE.

http://www.atmos.alb...patl_dprog.html

[OSUmetstud]

True... but this could be more of a function of a TC having an inability to produce deep convection completely around its circulation. As we saw with Michael, deep convection modulates the atmosphere through diabatic heating in the mid-levels, producing anticyclonic flow aloft due to mass flux adjustments due to the temperature perturbation. Without a nice CDO, the storm struggles to fight off the synoptic scale westerly flow. Since the global models don't have a good handle on the SST pattern under the core, they are going to overdo the convective adjustment of the synoptic flow.

I don't remember the stastical models or the nhc having much intensification in the current time range...the shear was always supposed to drop off during the day today through the night and especially tomorrow. I think shear has decreased today...it will just take some time to develop a more coherent inner core with some upwelled ssts and the ambient dry air.

and yeah gfs ftw on this one...leslie is going east of Bermuda.

[Ed Lizard]

WooT! Intermediate advisories!

WATCHES AND WARNINGS

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

CHANGES WITH THIS ADVISORY...

THE WEATHER SERVICE OF BERMUDA HAS ISSUED A TROPICAL STORM WATCH FOR

BERMUDA.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WATCH IS IN EFFECT FOR...

* BERMUDA

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE

POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 2 TO 3

DAYS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR

PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.

[OSUmetstud]

outflow has expanded significantly to the west of Leslie now, indicative of the decrease in shear today. UW-CIMSS has about 10 kt over the center now. Latest WSAT Microwave was still lackluster but I wonder if it could begin to get it's act together tonight.

[phil882]

outflow has expanded significantly to the west of Leslie now, indicative of the decrease in shear today. UW-CIMSS has about 10 kt over the center now. Latest WSAT Microwave was still lackluster but I wonder if it could begin to get it's act together tonight.

Until it moves away form the cold waters it has upwelled, its unlikely. I'd say another 24 hours or so until the storms forward motion increases above 5 mph before we really see any substancial development. The alternative is that it never really moves fast enough to get under warm enough waters to sustain anything more than a moderate cat 1. The SSTs drop off gradually north of Bermuda.

[OSUmetstud]

Until it moves away form the cold waters it has upwelled, its unlikely. I'd say another 24 hours or so until the storms forward motion increases above 5 mph before we really see any substancial development. The alternative is that it never really moves fast enough to get under warm enough waters to sustain anything more than a moderate cat 1. The SSTs drop off gradually north of Bermuda.

I think you're overstating the importance of upwelling a bit. The major problems with intensification have always been dry air and westerly shear imo.

SSTs are 28C up to 38N (after it gets away from the upwelling)

[OSUmetstud]

Leslie looks like **** lol

[Ed Lizard]

NOAA 42 tweeting from Leslie. (I follow...)

ETA

114900 2645N 06213W 6947 03039 9836 +138 +114 055014 017 003 001 00

114930 2643N 06212W 6948 03039 9831 +141 +111 042007 009 000 001 03

115000 2642N 06210W 6947 03040 9829 +143 +109 354002 003 005 001 00

115030 2640N 06209W 6946 03038 9833 +137 +114 231003 006 016 001 00

115100 2639N 06207W 6947 03039 9838 +131 +118 221012 014 022 001 00

Does the WP-3D fly at 700 mb for any scientific reason, or just because the planes are almost 40 years old?,

[gkrangers]

NOAA 42 tweeting from Leslie. (I follow...)

ETA

114900 2645N 06213W 6947 03039 9836 +138 +114 055014 017 003 001 00

114930 2643N 06212W 6948 03039 9831 +141 +111 042007 009 000 001 03

115000 2642N 06210W 6947 03040 9829 +143 +109 354002 003 005 001 00

115030 2640N 06209W 6946 03038 9833 +137 +114 231003 006 016 001 00

115100 2639N 06207W 6947 03039 9838 +131 +118 221012 014 022 001 00

Does the WP-3D fly at 700 mb for any scientific reason, or just because the planes are almost 40 years old?,

700mb (About 10,000 feet) is a standard recon altitude for hurricanes if I'm not mistaken.

[Ed Lizard]

700mb (About 10,000 feet) is a standard recon altitude for hurricanes if I'm not mistaken.

I might be thinking of HDOBs for invests...

Leslie looks awful, BTW

[phil882]

I think you're overstating the importance of upwelling a bit. The major problems with intensification have always been dry air and westerly shear imo.

SSTs are 28C up to 38N (after it gets away from the upwelling)

LESLIE HAS BEEN NEARLY STATIONARY TOO LONG AND THE OCEAN BENEATH HAS

COOLED SIGNIFICANTLY. IN FACT...ONE OF THE AXBT SONDES LAUNCHED FROM

THE NOAA PLANE CURRENTLY IN LESLIE MEASURED A SEA SURFACE

TEMPERATURE OF 24.5 DEGREES CELSIUS. CONSEQUENTLY...THE CONVECTION

HAS DIMINISHED AND THE CLOUD PATTERN HAS BECOME LESS ORGANIZED.

[Amped]

Appears to be forming a large eye LMAO.

And it still looks better than Irene.

[Amped]

Subtropical @ 11pm anyone?

[HurricaneJosh]

Another illustration of the current state of intensity forecasting. The science just isn't there.

[phil882]

Another illustration of the current state of intensity forecasting. The science just isn't there.

The key player in this track evolution is not the impeding trough in the long range, but actually the mid level ridge in the 48-96 hour period. This again is the period where the GFS has had difficulty forecasting enough mid level ridging. We saw this with Isaac in the absence of an upper level trough and we are seeing it again with Leslie. While I still believe Leslie has a very low chance of affecting the US other than high surf... The threat for the Canadian maritimes and Bermuda has increased substancially.

I do think that both the GFS and ECMWF are over forecasting intensity. Upwelling has got to be occurring in such a low OHC environment over the next 72 hours and that's something none of the global models will have a good handle on.

Haha I was typing on my phone last night, so the autocorrect was working well it seems.

Notice that Leslie doesn't seem to be in any state to intensify despite the fact the shear is decreasing. The system has done a number to its SST field, and as long as Leslie continues to be nearly stationary, its unlikely the system will intensify as robustly as most of the global models are indicating.

The GFS and ECMWF to my knowledge are not dynamically coupled with the ocean. From my understanding the "ocean" as seen by these models are merely a Reynolds weekly average that is not updated over the model integration. Thus, things like upwelling for vertical mixing of the ocean have little representation in the model fields nor how they might translate into the TC dynamics.

Models like the GFDL and HWRF, however, are, and should provide a better gauge to storm intensity in regards to upwelling ect.

dtk can provide a much better (and perhaps more correct) answer than myself.

Ehh its more like a Tie... the GFS was well east of Bermuda in its early forecasting for Leslie and WAY too fast. Speed of a TC is just as important as the track.

Leslie continues to look like crap. Finally the NHC is acknowledging that its probably due to SST decrease over the core of the storm. Both the HWRF and GDFL (which are pretty much only useful for their initial conditions over Leslie) show a nice patch of sub 26 degree C water nearly over the core. Since they are dynamically coupled, they are the best models we have for forecasting SST cool down due to upwelling of a storm circulation. Below is the analysis HWRF depiction of Leslie. Unfortunately, the HWRF and GFDL aren't the best models when it comes to interpreting these SST changes over the core of the system, and they both are still forecasting rapid intensification. This might happen were Leslie to start moving away rapidly from the coolest SSTs, but you have to remember how large Leslie circulation is currently. Thus, upwelling is occurring well ahead of the main core, which is why we weren't able to get a well organized inner core to form thus far. I think its under-appreciated how much upwelling affects the structure of these larger TCs. We have seen plenty of examples of major hurricanes that can rapidly intensify in 27-28 degree C waters (Ophelia a great example last year). However, these larger TCs already modify their SST environment long before the inner core of the cyclone can move over those warm waters, limiting the heat fluxes the storm can take in from the inner core of the circulation. This might be one reason (inertia aside) why its hard for a large wind field to contract effectively, and why its rare to see a TC shrink in size rather than vice versa.

True... but this could be more of a function of a TC having an inability to produce deep convection completely around its circulation. As we saw with Michael, deep convection modulates the atmosphere through diabatic heating in the mid-levels, producing anticyclonic flow aloft due to mass flux adjustments due to the temperature perturbation. Without a nice CDO, the storm struggles to fight off the synoptic scale westerly flow. Since the global models don't have a good handle on the SST pattern under the core, they are going to overdo the convective adjustment of the synoptic flow.

Until it moves away form the cold waters it has upwelled, its unlikely. I'd say another 24 hours or so until the storms forward motion increases above 5 mph before we really see any substancial development. The alternative is that it never really moves fast enough to get under warm enough waters to sustain anything more than a moderate cat 1. The SSTs drop off gradually north of Bermuda.

I sorta disagree. The writing was on the wall that both the dynamical/statistical models were not going to handle the intensity of Leslie correctly due to the SST differences from reality. The most perplexing thing to me is why the GFDL and HWRF continued to rapidly intensify Leslie even after it was over sub 26 degree waters. That just is a large red flag that these models have a poor handle on heat flux dynamics in TCs, which is troubling considering their higher resolution. I think the science was there to correctly forecast the general intensity of Leslie. The modeling was not.

[HurricaneJosh]

I sorta disagree. The writing was on the wall that both the dynamical/statistical models were not going to handle the intensity of Leslie correctly due to the SST differences from reality. The most perplexing thing to me is why the GFDL and HWRF continued to rapidly intensify Leslie even after it was over sub 26 degree waters. That just is a large red flag that these models have a poor handle on heat flux dynamics in TCs, which is troubling considering their higher resolution. I think the science was there to correctly forecast the general intensity of Leslie. The modeling was not.

If the science was there, why was this forecast to strengthen and consolidate, and why is it doing the opposite? What am I missing here?

[phil882]

If the science was there, why was this forecast to strengthen and consolidate, and why is it doing the opposite? What am I missing here?

The NHC forecast is almost constantly tied to a dynamical/statistical model mean. Sometimes some of the forecasters like to get more agressive/conservative, but they don't like to stray too far away from the model consensus. Most of the time this is the smart course of action. This particular case, however, is where the models would not have a good handle on the intensity since none of the global dynamical guidance can forecast upwelling since they are not dynamically coupled with the ocean. This is something the mesoscale models (GFDL,HWRF) should have a handle on, but apparently not. Anyone that has taken a basic class in tropical meteorology can deduce that 24.5 degree sea surface temperatures are not favorable for intensification of any pure tropical cyclone, let alone a hurricane. The models however did have a good handle on the extremely slow motion, and on the overall size of the circulation. Slow moving storm + large storm is a winning combination for substancial upwelling. That is why one could make, with reasonable accuracy, that the global models wouldn't handle the intensity of Leslie well.

Of course it would take a much more careful analysis of Leslie to deduce this scientifically, but I think there is evidence here that its the models, not the current science of TC dynamics, that have failed in the forecast of Leslie.