Tropical Depression Dorian

[phil882]

New GFS coming in much stronger in the 2-4 day range.

 

Edit: There appears to be less wind shear, as well as overall weaker mean flow resulting in a slower forward speed. 

 

One of the things that will have to be watched over the next 24-48 hours is how Dorian evolves as it traverses sub 26C SSTs. If Dorian can maintain the moist unstable inflow and continue to grow its cirrus canopy, I think that bodes well for the health of the system long term. At present, Dorian is vulnerable right now because it has only a small cirrus canopy that will be no match to large TUTT complex located further west. However, a larger cirrus canopy (and hence larger upper-level anticyclone) has a much better chance at combating this unfavorable flow.

 

Part of the reason why global models often have such a hard time predicting forecast intensity of TCs is because of convective schemes that don't very realistically simulate how the atmosphere works. Models in the past tended to have over-active convective schemes (in the tropics) that would constantly try to build a huge upper level anticyclone over a TC. Most of us know that inner core processes are critical to fully understand intensity fluctuations in TCs, but another critical process that sometimes gets overlooked is how TCs fight off vertical wind shear with their own convective plumes.

 

Noting that convective and stratiform profiles tend to have maximum diabatic heating in the mid-levels, +PV generation takes place below, while -PV generation takes place above, which is why after a major convective burst you also see enhanced anticyclonic outflow. Upper-level troughs (and TUTT cells) are regions where there is a +PV anomaly at the tropopause which can often extend some vertical extent below based on the rossby penetration depth. One way to weaken these +PV anomalies is to introduce a bunch of convection in its immediate vicinity which will produce -PV near the tropopause and start to destroy the upper-level cyclone. This is the process that allows extratropical cyclones to undergo tropical transition and replace an upper level cyclone with an anticyclone. However, it can also work for sheared TCs as well, where the large extent of convection simply overwhelms the TUTT cell and destroys it. In addition, convection can split shortwave troughs into different ULLs that begin to act as outflow channels around a growing TC. Thats similar to what happened with Michael last year.

 

So is this whats going on with the 12z GFS? Well take a look for yourself.

 

 12z GFS 120 hour forecast

 

 

Vs.

 

06z GFS 126 hour forecast:

 

 

00z GFS 132 hour forecast:

 

[bluewave]

The WCOSS GFS is weaker than the current OP GFS.

 

WCOSS

 

 

OP

 

[NJwinter23]

Dr. No is in the houuuuse

[GaWx]

Dr. No is in the houuuuse

 

 Fwiw, the good Doc (12Z) gets Dorian to move WNW to the Bahamas on 7/31 but only as an open wave since it degenerates Dorian into just that on Friday 7/26.

 

 Edit: It then gets it to skirt SE FL on 8/1 as a very weak low moving NW and then NNW to N recurving near or just offshore the FL E coast on 8/2.

[PSUBlizzicane2007]

Interesting side note: When did the NHC change the text color for tropical storms on the homepage from purple to green? This is the first time I've noticed it.

[AutoPenalti]

All I can say is if the models keep upping the strength in the long run Florida needs to monitor this for next week

Nah, bet its just going to be one of those fishcanes, or a cruiser if models keep in check.

We are overdue for a big one though, so chances aren't really out of the cards.

[Superstorm93]

Dorian has an excellent structure, it just lacks deep convection. Though this is to be expected when a system is traversing 25-26 Celsius water temperatures. 

 

 

18z roundup 

 

[GaWx]

Nah, bet its just going to be one of those fishcanes, or a cruiser if models keep in check.

We are overdue for a big one though, so chances aren't really out of the cards.

 

 

 I'd call the climo based chance of a US hit from Dorian a respectable near 1 in 4, which is a bit higher than the hit rate for later CV formations (such as in SEP, which is only ~1 in 6) due to a higher recurvature rate E of the US in SEP vs. now.

 

Exact dates of FORMATION (TD+) for the 47 1851-2012 CV storms* that later hit the U.S:

 

7/5, 7/11, 7/15, 7/31, 8/3, 8/3, 8/5, 8/7, 8/7, 8/15, 8/15, 8/16, 8/16, 8/17, 8/17, 8/18, 8/19, 8/20, 8/20, 8/21,8/23,  8/23, 8/25, 8/25, 8/27, 8/28, 8/28, 8/29, 8/29, 9/1, 9/2, 9/3, 9/4, 9/6, 9/6, 9/7, 9/8, 9/10, 9/10, 9/10, 9/10, 9/11, 9/15, 9/16, 9/21, 9/21, 9/25

 

 *My def. of CV storm: tropical storm/hurricane that first became at least a TD E of 50W and S of 20N

 

 The four July formation CV storm US hits on record are as follows:

 

- Claudette of 7/15/1979 (TX/LA border as a TS)

- Allen of 7/31/1980 (far S TX as a cat 3)

- Bertha 7/5/1996 (NC as a cat 2)

- Emily of 7/11/2005 (not a direct hit since the center hit N MX but there were sustained TS force winds in and near Brownville, TX)

 

 All four of these formed at 12.5 N or further south vs. Dorian's 13.9 N. So, if Dorian were to hit the US, it would be the furthest north formation of a July CV storm to hit the US on record.

[phil882]

 Fwiw, the good Doc (12Z) gets Dorian to move WNW to the Bahamas on 7/31 but only as an open wave since it degenerates Dorian into just that on Friday 7/26.

 

 Edit: It then gets it to skirt SE FL on 8/1 as a very weak low moving NW and then NNW to N recurving near or just offshore the FL E coast on 8/2.

 

The key here is that the ECMWF in the short term was stronger and further south than the 00z run. However, despite the stronger and slower short-term forecast, the low and mid-level centers decouple in the 24-72 hour period as the low-level flow accelerates. However, the ECMWF has had a poor track record with Dorian so far. 

[Ed Lizard]

Question

Everyone knows CMC suffers from many spurious cyclone developments.  Once a cyclone has developed, how does it rate, compared to other models, on track (not intensity) forecasts?  Any chance of a SW Florida affect?

[It's Always Sunny]

FWIW the 12Z runs of the Euro and GEM have a tropical wave (which is actually Dorian) just north of Hispaniola at 12Z on the 30th. GFS has Dorian further east more towards Puerto Rico but the fact that the Euro and GEM have picked up on something similar around this same exact time period should warrant some interest, especially given how far out this is. GFS is obviously more progressive with the development but imho I can see the stars aligning within the next couple of model runs given this stays course.

[Ed Lizard]

My glass half full/always seeing silver linings optimism at work here, but maybe, just maybe, the somewhat flat early August height fields depicted on the FIM9 might allow Dorian (which is depicts with 70 knot winds at 850 mb) to continue WNW for a while after Hour 168, or towards the Southeast United States.  If it can stay below 28 or 29º until its gets to about 78ºW or so,  Still only 5/32nds optimistic, because of Euro, GEFS big spray, and near miss on op GFS, but pushing 3/16ths if trends continue.

 

[Derecho!]

Question

Everyone knows CMC suffers from many spurious cyclone developments.  Once a cyclone has developed, how does it rate, compared to other models, on track (not intensity) forecasts?  Any chance of a SW Florida affect?

 

It was the worst global model for 120 hr track forecasts in the Atlantic in 2013, and the only worse models at 120 hours were stuff like LBAR, BAMD, CLIPER, etc.  It's now in a class of its own as the worst global model for tracks with the retirement of NOGAPS.   Any success it has relative to other models is essentially dumb luck. It's outlier weenie-bait for track as well as cyclogenesis.

 

You'll see a few die-hard defenders claim it has some sort of unique TC ability during gibbous moons in odd-numbered years during months that end in "R" for TCs with female names.

 

I actually am not certain why it exists, unless it has a unique ability to forecast temps in Saskatoon or something.

[Floydbuster]

000

WTNT44 KNHC 242045

TCDAT4

TROPICAL STORM DORIAN DISCUSSION NUMBER 3

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013

500 PM EDT WED JUL 24 2013

CONVENTIONAL AND MICROWAVE SATELLITE IMAGERY INDICATE THAT THE

CONVECTIVE STRUCTURE OF DORIAN HAS CHANGED LITTLE SINCE THE

PREVIOUS ADVISORY. HOWEVER...UPPER-LEVEL ANTICYCLONIC OUTFLOW HAS

IMPROVED IN ALL QUADRANTS AS THE VERTICAL SHEAR HAS DIMINISHED

SIGNIFICANTLY. THE INITIAL INTENSITY OF 45 KT IS BASED ON A

SATELLITE INTENSITY ESTIMATE OF T3.0/45 KT FROM TAFB.

THE INITIAL MOTION ESTIMATE IS 285/17 KT. DORIAN IS EXPECTED TO

REMAIN SOUTH OF AND MOVE WEST-NORTHWESTWARD TO WESTWARD IN TANDEM

WITH A STRONG SUBTROPICAL RIDGE LOCATED WELL TO THE NORTH. THE NHC

MODEL GUIDANCE CONTINUES TO REMAIN TIGHTLY PACKED AND IN EXCELLENT

AGREEMENT ON THIS SCENARIO. THE ONLY DIFFERENCES IN THE MODELS ARE

THE FORWARD SPEED...WHICH IS DUE TO THE STRENGTH AND VERTICAL DEPTH

OF DORIAN. THE GFS...UKMET...AND CANADIAN MODELS MAINTAIN A ROBUST

AND MORE VERTICALLY DEEP CYCLONE...WHICH GETS STEERED MORE SLOWLY

BY THE DEEPER MEAN FLOW. IN CONTRAST...THE HWRF AND ECMWF MODELS

WEAKEN DORIAN SIGNIFICANTLY AS THE CYCLONE PASSES OVER COOLER WATER

DURING THE NEXT 48 HOURS...WHICH RESULTS IN A SHALLOWER AND WEAKER

VORTEX BEING STEERED MORE RAPIDLY WESTWARD BY THE STRONGER EASTERLY

TRADE WIND FLOW. GIVEN THAT DORIAN WILL BE MOVING OVER COOLER WATER

AT A BRISK 18 KT FOR THE NEXT 24-36 HOURS...MY FEELING IS THAT THE

CYCLONE WILL NOT WEAKEN AS MUCH AS THE SOME OF THE MODELS ARE

FORECASTING...AND AS A RESULT...THE OFFICIAL TRACK FORECAST LEANS

TOWARD THE SLOWER GFS...UKMET...AND CANADIAN MODEL SOLUTIONS.

DORIAN IS BEGINNING TO MOVE OVER SUB-26C SSTS...AND THE CYCLONE

SHOULD REMAIN OVER COOLER WATERS NEAR 25C FOR THE NEXT 12 HOURS

OR SO...WHICH COULD INDUCE SOME SLIGHT WEAKENING. HOWEVER...BY 36-48

HOURS...DORIAN WILL BE MOVING BACK OVER SSTS WARMER THAN 26C AND IN

A VERY LOW VERTICAL SHEAR ENVIRONMENT...WHICH SHOULD ALLOW FOR SOME

RE-STRENGTHENING TO OCCUR. THE LATEST RUNS OF THE GFS AND ECMWF

ARE SHOWING A LESS HOSTILE UPPER-AIR PATTERN NEAR THE END OF THE

FORECAST PERIOD...WITH MORE RIDGING AND LESS INFLUENCE FROM AN

UPPER-LEVEL TROUGH OVER THE BAHAMAS. IN CONSIDERATION OF THAT...

THE FORECAST INTENSITIES AT DAYS 4-5 HAVE BEEN INCREASED RELATIVE

TO THE PREVIOUS ADVISORY.

FORECAST POSITIONS AND MAX WINDS

INIT 24/2100Z 14.6N 31.4W 45 KT 50 MPH

12H 25/0600Z 15.3N 34.0W 45 KT 50 MPH

24H 25/1800Z 16.1N 37.5W 40 KT 45 MPH

36H 26/0600Z 16.8N 41.0W 40 KT 45 MPH

48H 26/1800Z 17.5N 44.7W 45 KT 50 MPH

72H 27/1800Z 18.7N 51.8W 45 KT 50 MPH

96H 28/1800Z 19.6N 58.9W 50 KT 60 MPH

120H 29/1800Z 20.0N 64.0W 50 KT 60 MPH

$$

FORECASTER STEWART

[Floydbuster]

It should be noted that while it is early in the year for a Cape Verde system, it's not extremely early for a Cape Verde system. August is a week away.

[HurricaneJosh]

Phil, what's with the pressure in hPa?  

[phil882]

Phil, what's with the pressure in hPa?  

 

Haha oops... fixed that, although it wouldn't have changed the numerical value either way .

[It's Always Sunny]

It was the worst global model for 120 hr track forecasts in the Atlantic in 2013, and the only worse models at 120 hours were stuff like LBAR, BAMD, CLIPER, etc.  It's now in a class of its own as the worst global model for tracks with the retirement of NOGAPS.   Any success it has relative to other models is essentially dumb luck. It's outlier weenie-bait for track as well as cyclogenesis.

 

You'll see a few die-hard defenders claim it has some sort of unique TC ability during gibbous moons in odd-numbered years during months that end in "R" for TCs with female names.

 

I actually am not certain why it exists, unless it has a unique ability to forecast temps in Saskatoon or something.

Lol. I've never relied heavily on it either, especially with short range stuff. I look at it more for enjoyment to see if maybe, maybe, it picks up on something far out to give me a glimmer of hope that a certain meteorological event could happen, like Dorian! Like I said before it does pick up on a tropical wave right around the time Dorian is supposed to roll through the Caribbean so I like looking at in in that sense to help myself determine if something could actually be legit or not. Again, still early but that's my take.

[phil882]

It should be noted that while it is early in the year for a Cape Verde system, it's not extremely early for a Cape Verde system. August is a week away.

 

Dorian is the 2nd furthest east TC to develop into a tropical storm behind Bertha (2008) in July. 

[Ed Lizard]

Dorian is the 2nd furthest east TC to develop into a tropical storm behind Bertha (2008) in July. 

 

Just distant memories of John Hope and TWC, but Bertha 1996 looked like a TD coming off Africa. 

[Superstorm93]

While the windspeeds are probably 10-15 knots too generous, WindSat still caught a pretty tight circulation and a hefty southerly flow from the ITCZ. Might be the saving grace for this one as it moves over the cool SST's and stable UL environment over the next 24 hours. 

 

We'll see how Dorian looks tomorrow since it's once again popping convection over the LLC

 

 

[HM]

Dorian is the 2nd furthest east TC to develop into a tropical storm behind Bertha (2008) in July.

It must be that "below normal" Atlantic Stability that kept it from being #1

[phil882]

Just distant memories of John Hope and TWC, but Bertha 1996 looked like a TD coming off Africa. 

 

Bertha (1996) became a TS at 11N / 39W... Dorian became a TS at 14.3N / 29.9W

 

 

It must be that "below normal" Atlantic Stability that kept it from being #1

 

Hahaha, I sometimes wonder how useful it is to have an area average stability graphic, because the vertical circulation associated with TCs (and all tropical convection for that matter) will produce distinct modes for stability with subsidence increasing stability in a larger area around the convection than in the small corridor closer to the convection.

 

In any event, the lack of SAL is quite startling right now... this is an unusual pattern for late July in the Atlantic basin, and this could be the factor that keeps Dorian from falling apart over marginal SSTs over the Central Atlantic.

 

[bluewave]

FWIW, the 18z WCOSS GFS set to go operational next week has a similar evolution as the Euro.

 

http://magpara.ncep.noaa.gov/model-guidance-model-parameter.php?group=Model%20Guidance&model=gfs&area=atlantic&cycle=20130724%2018%20UTC&param=10m_wnd_precip&fourpan=no&imageSize=

[Superstorm93]

Looking good

 

21:45

 

 

19:45

 

[Floydbuster]

Phil, I don't recall that little amount of SAL with any Cape Verde system in the last two or three years, especially not this early. I do recall a huge amount of the Saharan Air Layer in July 2005 however, with dust even coming into the skies over Central Florida.

The SAL also contributed to Hurricane Ivan's abrupt weaking from a Cat 4 to a Cat 2 prior to hitting the Windward Islands.

[It's Always Sunny]

FWIW, the 18z WCOSS GFS set to go operational next week has a similar evolution as the Euro.

 

http://magpara.ncep.noaa.gov/model-guidance-model-parameter.php?group=Model%20Guidance&model=gfs&area=atlantic&cycle=20130724%2018%20UTC&param=10m_wnd_precip&fourpan=no&imageSize=

That looks like the regular GFS to me unless I am missing something.

[bluewave]

That looks like the regular GFS to me unless I am missing something.

 

This is the old link below. The new link should be operational next week.

 

 

old site

http://mag.ncep.noaa.gov/

 

 

 

new

 

http://www.nws.noaa.gov/os/notification/pns13wcosseval.htm

 

http://magpara.ncep.noaa.gov/model-guidance-model-area.php

 

 

[andyhb]

Phil, I don't recall that little amount of SAL with any Cape Verde system in the last two or three years, especially not this early. I do recall a huge amount of the Saharan Air Layer in July 2005 however, with dust even coming into the skies over Central Florida.

The SAL also contributed to Hurricane Ivan's abrupt weaking from a Cat 4 to a Cat 2 prior to hitting the Windward Islands.

 

Ivan was a Category 3 when it hit Grenada.

[wxmeddler]

FWIW, the 18z WCOSS GFS set to go operational next week has a similar evolution as the Euro.

 

http://magpara.ncep.noaa.gov/model-guidance-model-parameter.php?group=Model%20Guidance&model=gfs&area=atlantic&cycle=20130724%2018%20UTC&param=10m_wnd_precip&fourpan=no&imageSize=

 

FWIW, the WCOSS GFS is the same as the Old "CSS" GFS. No changes have been made to the actual "code" except for a very small number of things they had to fix because the new WCOSS super computer didn't support those processes. Apparently, it runs 4 min faster on the WCOSS though. Maybe dtk can shed some more light on why it looks slightly different.