Tropical Depression Don

[turtlehurricane]

One heck of a convective tower right now, and the low-level circulation isn't that displaced anymore so I think it's tapping into it. We've seen pretty good intensification before with sheared tropical storms like this, albeit for a short duration sometimes.

[wxmx]

From flight level, the center is well west of last recon fix (even a tad south of due west).... a jog or vortex tilt? Rain contaminated, but several SFMR 60+kts obs.

[wxmeddler]

One heck of a convective tower right now, and the low-level circulation isn't that displaced anymore so I think it's tapping into it. We've seen pretty good intensification before with sheared tropical storms like this, albeit for a short duration sometimes.

Well, were going to see what's going on down there in a few minutes. Recon is entering in at 8 kft.

Edit.. 72 mph SMFR unflagged?

[kab2791]

RECON SFMR estimates of 66kts highly contaminated by heavy rainfall rates in the hot tower area.

[dan88]

Mid level center appears to be a good ways to the south. Recon is flying at 750mb, and found the windshift down about 24.25N, in a burst of exceptionally heavy precip (50 mm/hr). The last advisory places the surface center at 24.9N

NOAA2 0304A DON                HDOB 12 20110728
214430 2438N 09057W 7508 02561 0064 +150 +130 126018 019 036 000 03
214500 2437N 09059W 7508 02561 0062 +150 +138 126017 017 036 001 00
214530 2435N 09100W 7506 02561 0057 +153 +139 132017 018 037 000 00
214600 2434N 09101W 7508 02558 0057 +152 +137 127018 019 038 000 00
214630 2432N 09103W 7507 02556 0052 +155 +139 124017 018 038 000 00
214700 2431N 09104W 7507 02556 0050 +157 +135 123018 020 037 000 00
214730 2429N 09106W 7505 02556 0048 +159 +127 126021 021 038 000 00
214800 2427N 09107W 7506 02555 0046 +159 +132 126022 022 039 000 00
214830 2426N 09108W 7505 02555 0047 +155 +135 142020 022 039 001 00
214900 2424N 09110W 7515 02543 0050 +152 +139 145022 023 038 000 00
214930 2423N 09111W 7520 02536 0048 +154 +136 142025 027 038 000 00
215000 2422N 09113W 7531 02526 0049 +155 +136 142026 027 038 000 00
215030 2420N 09115W 7519 02537 0048 +153 +141 153025 025 039 001 00
215100 2419N 09116W 7511 02544 0046 +152 +140 158029 031 042 003 00
215130 2418N 09118W 7512 02543 0049 +147 +152 154025 025 048 016 00
215200 2417N 09120W 7513 02539 //// +145 //// 158017 024 054 024 21
215230 2415N 09121W 7512 02539 //// +143 //// 154011 012 058 032 21
215300 2414N 09123W 7517 02533 //// +141 //// 211004 006 063 037 21
215330 2413N 09124W 7501 02549 //// +140 //// 271011 012 066 046 25
215400 2412N 09126W 7499 02548 //// +134 //// 285009 019 063 050 25

[HKY_WX]

Shear looks to have greatly reduced and it's much more symmetrical this afternoon. Could see it crank up pretty good overnight.

[wxmeddler]

Need a dropsonde to get a hold of whats going on here.

[turtlehurricane]

Mid level center appears to be a good ways to the south. Recon is flying at 750mb, and found the windshift down about 24.25N, in a burst of exceptionally heavy precip (50 mm/hr)

NOAA2 0304A DON                HDOB 12 20110728
214430 2438N 09057W 7508 02561 0064 +150 +130 126018 019 036 000 03
214500 2437N 09059W 7508 02561 0062 +150 +138 126017 017 036 001 00
214530 2435N 09100W 7506 02561 0057 +153 +139 132017 018 037 000 00
214600 2434N 09101W 7508 02558 0057 +152 +137 127018 019 038 000 00
214630 2432N 09103W 7507 02556 0052 +155 +139 124017 018 038 000 00
214700 2431N 09104W 7507 02556 0050 +157 +135 123018 020 037 000 00
214730 2429N 09106W 7505 02556 0048 +159 +127 126021 021 038 000 00
214800 2427N 09107W 7506 02555 0046 +159 +132 126022 022 039 000 00
214830 2426N 09108W 7505 02555 0047 +155 +135 142020 022 039 001 00
214900 2424N 09110W 7515 02543 0050 +152 +139 145022 023 038 000 00
214930 2423N 09111W 7520 02536 0048 +154 +136 142025 027 038 000 00
215000 2422N 09113W 7531 02526 0049 +155 +136 142026 027 038 000 00
215030 2420N 09115W 7519 02537 0048 +153 +141 153025 025 039 001 00
215100 2419N 09116W 7511 02544 0046 +152 +140 158029 031 042 003 00
215130 2418N 09118W 7512 02543 0049 +147 +152 154025 025 048 016 00
215200 2417N 09120W 7513 02539 //// +145 //// 158017 024 054 024 21
215230 2415N 09121W 7512 02539 //// +143 //// 154011 012 058 032 21
215300 2414N 09123W 7517 02533 //// +141 //// 211004 006 063 037 21
215330 2413N 09124W 7501 02549 //// +140 //// 271011 012 066 046 25
215400 2412N 09126W 7499 02548 //// +134 //// 285009 019 063 050 25

This is some awesome convective burst data. SFMR is clearly too high, maybe 45-50 kt in reality based on the trend in the previous data. Still stronger than before though.

[wxmx]

...and extent of <-80C cloud tops seems to be on the increase.

[wxmeddler]

...and extent of <-80C cloud tops seems to be on the increase.

Would explain the 2 in/hr rain rates..

[dan88]

Peak rainfall rates in next set of data hit 71 mm/hr, which upon conversion is nearing 3 inches an hour

[Floydbuster]

No doubt some of those winds look suspect but clearly Don has at the very least strengthened a little bit. I'd venture to guess winds are around 50-60 mph by 11pm tonight, and tonight will be critical in deciding if Don ends up landfalling as an ill defined tropical storm or a strengthening hurricane.

[CUmet]

I wouldn't necessarily take the intense convection as a sign that this is getting better organized. It might be, or it could just be getting ready to dump another round of cool, dry air into the boundary layer as it's been doing over the past 12 hours.

Now that we have recon fixing a center that's pretty far south and almost colocated with the convective burst, I think we can definitely take this as a positive sign now.

[turtlehurricane]

They're following the 750 hPa pressure surface and dove 70 m from the outside to the center, so about a 5-7 hPa drop from outside to the center.

[HurricaneJosh]

These data are heartwarmin'. :wub:

[Srain]

These data are heartwarmin'. :wub:

Was there ever any doubt Don would not let us down...

[wxmx]

Was there ever any doubt Don would not let us down...

Yeah, Adam never liked it ...

[Floydbuster]

Don is the main feature but I suspect we will have invest 91L in the next couple of days in the Central Atlantic. The CMC has been persistent before this wave even showed up that this would become a hurricane for the Greater Antilles. Not sure about that, but definitely something to watch.

[phil882]

I'm quite a bit more pessimistic about Don's future than I was yesterday. The SHIPS now has pretty substantial northerly to northeasterly shear developing over the next day, with magnitudes of around 15 kt. A lot of people have said that the shear is advecting dry air into the core, but this isn't necessarily the case here or in many other instances for that matter. The shear tilts the vortex such that anomalous storm-relative inflow in the low-levels occurs on the downshear half, and anomalous storm-relative outflow occurs on the upshear half. Taking into account the advection by the TC circulation itself, the most effective source-region for dry air tends to be downshear-right of the center. For more on this idea, see Fig. 6 of this paper, which shows backward trajectories in a modeled TC under easterly shear: Riemer and Montgomery (2010)

This isn't always applicable to real TCs, but it seems consistent with what we're seeing with Don. I've decomposed the recon flight-level winds into storm-relative tangential and radial components, and there's definitely 5-10 m/s inflow in the western and northwestern quads, and outflow on the eastern and northeastern quads.

This would be consistent with the above paper, and considering that MIMIC-TPW shows a nice blob of dry air to the west of Don, it would suggest that this air is being injected into the core and likely being brought down into the boundary layer by the persistent convective downdrafts in the south quad. The outflow boundaries shooting out of the east quad is evidence of this.

I'm not sure how Don is going to deal with this going forward. TC interaction with shear is a complicated thing and very difficult to forecast. I'd go with an intensity somewhere between 50-70 kt at landfall, which is certainly more conservative than the 60-85 I had yesterday.

I didn't have a chance to reply earlier, but this is a very interesting read and great post... I'll certainly have to reassess how I look at the advection of dry air in tropical cyclones.

[dan88]

Really need to see a drop, but with the 750mb low location, it's likely that the surface center is no longer at the very northern edge of the convection. the 24.25N found here at 750mb is close to the center of the convective ball. the 5mp advisory had the surface low located at 24.9N, along the very northern edge of the convection. Going to need a drop to see how tilted the system is now.

[LakeEffectKing]

Well the fact that it changed so dramatically over the course of a short period makes me think it's useless. I'm not that interested in defining the conversation but I worry about slippery slopes. IMO any posting of a map with no discussion is "eh" tho of course a red-tagger who is "respected" in these threads gets some leeway. It's sort of like the rapid fire posts of location and speed with no thought behind it.. wasted space and time reading. If he wants to keep posting them I'd suggest a separate thread devoted to them so people can come and go as they please and not be bombarded by something with what may be no meaning.

I agree with this Ian, and am often temped to not put out a map when I know I don't have "enough" time to explain an entire methodology...With Don, I have sprinkled in some of my thoughts regarding the modeling, "not so bad" shear, and some of the reasons for my thoughts on gradual intensification wrt Don being small and env. conditions seemingly to me, to at least allow for modest intesification. But due to work time constraints, (and family) I probably haven't invested as much time in a true disscussion as I have done in the past.

The warm eddy that Don with transverse (during DMAX) tonight was always the wild card for me to possibly pull the trigger on a stronger system, however as noted before, the conditions are (and were progged back 2 and 1/2 days ago) to be marginal at best, wrt shear, and dry air issues....thus my thinking has been that tonight will essentially be interesting, but lead to very little chance of robust intensification.

The historical model depictions of Don have been quite interesting, and of smaller value, IMO, to a forecaster than one would expect with a larger system for obvious gridding reasons.

And to cover one final point, one I never had though of before, being that any forecasts I've done in the past have always been in the spirit of competative "fun", I however agree with another post wrt the disclaimer idea and believe I will implement such a statement on future calls.

[phil882]

Now that we have recon fixing a center that's pretty far south and almost colocated with the convective burst, I think we can definitely take this as a positive sign now.

Agreed. I imagine there is still some vortex tilting with height... but 750mb is more of a low level reflection than a mid-level cyclone. What is interesting is that it seems like the inner core is rebuilding, because there is a distinct double wind maxima on the NE quad, with the stronger FL winds still confined in the outer maxima.

[TUweathermanDD]

I agree with this Ian, and am often temped to not put out a map when I know I don't have "enough" time to explain an entire methodology...With Don, I have sprinkled in some of my thoughts regarding the modeling, "not so bad" shear, and some of the reasons for my thoughts on gradual intensification wrt Don being small and env. conditions seemingly to me, to at least allow for modest intesification. But due to work time constraints, (and family) I probably haven't invested as much time in a true disscussion as I have done in the past.

The warm eddy that Don with transverse (during DMAX) tonight was always the wild card for me to possibly pull the trigger on a stronger system, however as noted before, the conditions are (and were progged back 2 and 1/2 days ago) to be marginal at best, wrt shear, and dry air issues....thus my thinking has been that tonight will essentially be interesting, but lead to very little chance of robust intensification.

The historical model depictions of Don have been quite interesting, and of smaller value, IMO, to a forecaster than one would expect with a larger system for obvious gridding reasons.

And to cover one final point, one I never had though of before, being that any forecasts I've done in the past have always been in the spirit of competative "fun", I however agree with another post wrt the disclaimer idea and believe I will implement such a statement on future calls.

Well said LEK, and tonight could get you a bit closer to verifying those initial intensity estimates!

[CUmet]

Agreed. I imagine there is still some vortex tilting with height... but 750mb is more of a low level reflection than a mid-level cyclone. What is interesting is that it seems like the inner core is rebuilding, because there is a distinct double wind maxima on the NE quad, with the stronger FL winds still confined in the outer maxima.

That's what I thought at first, but this flight is only 1 km above the previous one, so the offset really shouldn't be that much. I really do think the low-level center is further south relative to the convection than it was before.

[wxmx]

That's what I thought at first, but this flight is only 1 km above the previous one, so the offset really shouldn't be that much. I really do think the low-level center is further south relative to the convection than it was before.

Yep, it's almost time to up the upper end of the intensity range... probably back to the original one... but I will wait for more data.

[Srain]

Sweet...

[Ed Lizard]

And to cover one final point, one I never had though of before, being that any forecasts I've done in the past have always been in the spirit of competative "fun", I however agree with another post wrt the disclaimer idea and believe I will implement such a statement on future calls.

I don't think anyone will ever sue Randy or Ian if one of the LEK specials doesn't work out perfectly.

[dan88]

Going to have two planes in the system for a while, Air Force recon just took off as well, which will presumably be doing it's mission at a much lower height then the 750mb NOAA is. Should help a great deal in figuring out how tilted Don is this evening.

[Floydbuster]

[wxmx]