Tropical Depression Dorian

[wxmeddler]

Regardless of whatever pin the tail on the east coast the GFS is showing, I'm quite supprised how much the GFS slows Dorian down post 7 days. Very little steering in the Bahamas region being shown at that time frame.

[Floydbuster]

Regardless of whatever pin the tail on the east coast the GFS is showing, I'm quite supprised how much the GFS slows Dorian down post 7 days. Very little steering in the Bahamas region being shown at that time frame.

 

I just watched the loop several times and it also shows some significant tightening of the system between 65W and 75W.

[yoda]

Looks like 00z GFS is going to be a SE coast hit by the looks/pos of Dorian at 192 unless there is a hard recurve starting right there

 

Of course, we could always then buy that from 192-240 Dorian sits around off SE coast and meanders... prob due to poor steering currents

 

Meanders enough that looks like it eventually gets picked up and booted NE by hr 300 barely grazing SE US at all... interesting run... before smashing NF way out in la la land

[greensunrise]

And retrogrades into Nova Scotia.

[TropicalAnalystwx13]

 

[It's Always Sunny]

Meanders enough that looks like it eventually gets picked up and booted NE by hr 300 barely grazing SE US at all... interesting run... before smashing NF way out in la la land

...and New England for that matter.  . Not reading too much into it because it's still so early, but yet so hard not to get excited about it.

[CUmet]

We'll have to see that the latest microwave passes show... it could be anywhere from a minimal hurricane with a microwave eye to a partially exposed circulation on the edge of the cirrus canopy. Really no way to know unless we get a good overnight TRMM pass or SSMI. The lower resolution microwave imagers don't help a bunch. 

 

I agree it's tough to tell given the lack of data, although to my subjective eyes the convection looks like it's sticking close to the vortex center.  It doesn't look like a convective burst forced by shear to me.

I do think a minimal hurricane is within the realm of possibility over the next couple days.  Once we get into Saturday and beyond, Dorian may not have such a nice area of moisture to work with as it becomes detached from the ITCZ moisture (also seen in the decreasing 700-500 RH in the SHIPS), and the accelerated storm motion and the potential mid-level shear that tends to happen with it.  This is a very difficult system to get a handle on intensity-wise, given the factors above and Dorian's small size making it particularly vulnerable to environmental influences.

[phil882]

Regardless of whatever pin the tail on the east coast the GFS is showing, I'm quite supprised how much the GFS slows Dorian down post 7 days. Very little steering in the Bahamas region being shown at that time frame.

 

This seems to happen a lot when the storm moves over the mid-level ridge axis, where the storm gets stuck between the trade winds to the south of the mid-level ridge, and the westerlies on the northern flank of the mid-level ridge. If there isn't an amplified trough that aids ridge building downstream, the ridge does little to guide the storm into the westerlies. Instead the TC gets stuck on a COL region between two similar latitude mid-level ridges

 

Lets just try to get through the first 96 hours though, where the GFS still shows significant shear as the storm reaches the longitude of the lesser Antilles. If the storm isn't significantly larger than it is currently, its still more than enough for dissipation. The GFS is banking on a significantly larger system by the time Dorian reaches 65W, with a huge cirrus shield. 

 

[metalicwx366]

00z WCOSS GFS actually has it as a TC instead of an open wave as it moves into the Bahamas. Much stronger run than 18z.

http://magpara.ncep.noaa.gov/Imageanis.php

[phil882]

I agree it's tough to tell given the lack of data, although to my subjective eyes the convection looks like it's sticking close to the vortex center.  It doesn't look like a convective burst forced by shear to me.

I do think a minimal hurricane is within the realm of possibility over the next couple days.  Once we get into Saturday and beyond, Dorian may not have such a nice area of moisture to work with as it becomes detached from the ITCZ moisture (also seen in the decreasing 700-500 RH in the SHIPS), and the accelerated storm motion and the potential mid-level shear that tends to happen with it.  This is a very difficult system to get a handle on intensity-wise, given the factors above and Dorian's small size making it particularly vulnerable to environmental influences.

 

TRMM just missed it... but it does look like the center is embedded in the convection. I agree with your thoughts, the 200 hPa flow will probably have a hard time keeping up with the stronger low and mid-level flow. Small storms like Dorian can be significantly disrupted by such a pattern (Chantal had the same problem) whereas larger storms, while also affected often can survive and recover (Danielle, Earl 2010) 

 

[thewxmann]

Lollllzzzz the GFS.

 

Question: given that Dorian is heading towards drier air, wouldn't that tend to keep storm size small? I remember we've had lengthy discussion about TC size correlation to environmental mid-lvl RH.

[Floydbuster]

Lollllzzzz the GFS.

 

Question: given that Dorian is heading towards drier air, wouldn't that tend to keep storm size small? I remember we've had lengthy discussion about TC size correlation to environmental mid-lvl RH.

 

Most likely, however this thing has quite a nice moisture envelope with it. Of course, we'll have to see how it does after it disconnects completely from the ITCZ.

[HurricaneJosh]

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

 

 

Re: Dorian... I'm having trouble getting really excited just yet.  The issue is its small size.  I microcanes, but when I think of the great Cape Verde cyclones that made it all the way across the ocean from Africa to North America, I think mostly of large systems that weren't so delicate.  I'm having trouble imagining this dainty little thing making it across.

 

Granted, deep-tropical cyclones like Dean 2007 and Janet 1955 were small systems that traveled good distances... but they were at very low latitudes.

 

I'm not saying it never happens, just saying I'm having trouble thinking of examples.  Andrew 1992 was a CV system, and it wasn't exactly a micro, but it was on the small side.  That's one, maybe.

[Floydbuster]

 

Re: Dorian... I'm having trouble getting really excited just yet.  The issue is its small size.  I microcanes, but when I think of the great Cape Verde cyclones that made it all the way across the ocean from Africa to North America, I think mostly of large systems that weren't so delicate.  I'm having trouble imagining this dainty little thing making it across.

 

Granted, deep-tropical cyclones like Dean 2007 and Janet 1955 were small systems that traveled good distances... but they were at very low latitudes.

 

I'm not saying it never happens, just saying I'm having trouble thinking of examples.  Andrew 1992 was a CV system, and it wasn't exactly a micro, but it was on the small side.  That's one, maybe.

 

About time. Where you been? I think that's why the NHC is so cautious. If Dorian survives as well as it has been another 24-36 hrs, then you'll see more bullishness.

[phil882]

Lollllzzzz the GFS.

 

Question: given that Dorian is heading towards drier air, wouldn't that tend to keep storm size small? I remember we've had lengthy discussion about TC size correlation to environmental mid-lvl RH.

 

Yes generally that's the case. Which is why there are some questions as to why the GFS has a very large cirrus shield associated with Dorian while the storm is traversing between 50-60W. 

 

 

 

Re: Dorian... I'm having trouble getting really excited just yet.  The issue is its small size.  I microcanes, but when I think of the great Cape Verde cyclones that made it all the way across the ocean from Africa to North America, I think mostly of large systems that weren't so delicate.  I'm having trouble imagining this dainty little thing making it across.

 

Granted, deep-tropical cyclones like Dean 2007 and Janet 1955 were small systems that traveled good distances... but they were at very low latitudes.

 

I'm not saying it never happens, just saying I'm having trouble thinking of examples.  Andrew 1992 was a CV system, and it wasn't exactly a micro, but it was on the small side.  That's one, maybe.

 

The best two example I can think of (and note these didn't make US landfall, but traveled over the vast majority of the Atlantic) are Alberto (2000) and Danielle (1998). Both were midget TCs at some point and both experienced bouts of upper level wind shear, but ultimately survived by growing in spatial scale, and forcing the upper level trough to shift southwestward out of the TCs way. Danielle is probably the better analog since it tracked WNW at a rapid pace and approached the Bahamas before recurving. 

[AutoPenalti]

000

WTNT44 KNHC 250837

TCDAT4

TROPICAL STORM DORIAN DISCUSSION NUMBER 5

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013

500 AM AST THU JUL 25 2013

DORIAN HAS BECOME A LITTLE BETTER ORGANIZED THIS MORNING...WITH

STRONGER CONVECTION OCCURRING NEAR THE CENTER. A RECENT AMSU

OVERPASS SHOWED A MID-LEVEL EYE ASSOCIATED WITH THIS CONVECTION...

ALTHOUGH IT WAS UNCLEAR IF THIS FEATURE WAS DIRECTLY OVER THE

LOW-LEVEL CENTER. SATELLITE INTENSITY ESTIMATES ARE 55 KT FROM SAB

AND 45 KT FROM TAFB...AND A RECENT AMSU-BASED INTENSITY ESTIMATE

FROM CIMSS WAS 48 KT. BASED ON THIS THE INITIAL INTENSITY IS

INCREASED TO 50 KT.

THE INITIAL MOTION IS 290/15. DORIAN IS ON THE SOUTH SIDE OF A

DEEP-LAYER RIDGE THAT COVERS THE CENTRAL AND EASTERN ATLANTIC.

THIS FEATURE SHOULD STEER THE CYCLONE QUICKLY WEST-NORTHWESTWARD TO

WESTWARD FOR THE NEXT 72 HOURS OR SO. AFTER THAT...DORIAN SHOULD

APPROACH A WEAKNESS IN THE RIDGE...WHICH SHOULD LEAD TO A

WEST-NORTHWESTWARD MOTION AT A DECREASED FORWARD SPEED. THE

GUIDANCE ENVELOPE HAS SHIFTED A BIT TO THE NORTH SINCE THE PREVIOUS

ADVISORY. BASED ON THIS...THE NEW FORECAST TRACK WILL BE NUDGED A

LITTLE TO THE NORTH...BUT IT LIES SOUTH OF THE CENTER OF THE

GUIDANCE ENVELOPE AND THE MODEL CONSENSUS.

THE INTENSITY FORECAST IS COMPLEX AND OF LOW CONFIDENCE. DORIAN

SHOULD MOVE OVER WARMER SEA SURFACE TEMPERATURES DURING THE NEXT

2-3 DAYS...BUT DURING THIS TIME IT ALSO MAY ENCOUNTER SOUTHWESTERLY

VERTICAL WIND SHEAR AND DRIER AIR AT THE MID-LEVELS. IN

ADDITION...THE UPPER-LEVEL WIND FORECASTS FROM THE DYNAMICAL MODELS

BECOME QUITE DIVERGENT BETWEEN 72-120 HOURS...WITH THE

POSSIBILITIES RANGING FROM CONDITIONS FAVORABLE FOR INTENSIFICATION

TO SHEAR STRONG ENOUGH TO CAUSE DORIAN TO WEAKEN. OVERALL...THE

INTENSITY GUIDANCE FORECASTS LITTLE CHANGE IN STRENGTH DURING THE

NEXT FIVE DAYS...AND THUS THE NEW FORECAST IS THE SAME AS THE

PREVIOUS FORECAST. THE NEW FORECAST IS IN BEST AGREEMENT WITH THE

SHIPS MODEL AND IS STRONGER THAN THE INTENSITY CONSENSUS.

FORECAST POSITIONS AND MAX WINDS

INIT 25/0900Z 15.6N 34.5W 50 KT 60 MPH

12H 25/1800Z 16.3N 37.0W 50 KT 60 MPH

24H 26/0600Z 17.0N 40.5W 50 KT 60 MPH

36H 26/1800Z 17.8N 44.4W 50 KT 60 MPH

48H 27/0600Z 18.3N 48.3W 50 KT 60 MPH

72H 28/0600Z 19.5N 55.5W 55 KT 65 MPH

96H 29/0600Z 20.5N 62.0W 55 KT 65 MPH

120H 30/0600Z 21.5N 67.0W 55 KT 65 MPH

$$

FORECASTER BEVEN

[jlauderdal]

I'll be preparing in Ft.Lauderdale just incase.

This is getting exciting by the day.

Im sure channel 7 will be at your doorstep any minute filming your preps, anything this far out is fodder for their newscasts..i used to warm up my generator in advance of a storm and they always seem to miss us so i stopped that practice years ago

[wxmeddler]

-75°C tops and nice outfow on all quads.

[NaoPos]

Fwiw, 6z GFS keeps it on the weak side, north of Antilles, through the Bahamas, scooting between Cuba/Florida and up Florida's gulf coast fairly weak.

[bluewave]

The strength of the West African Monsoon this month helped Dorian develop so far east in the 

Atlantic for July. But the drier air further west is the reason the Euro shows weakening 

over the next few days.

 

[Ed Lizard]

I'm a fan of the 6Z at truncation, aimed for PBI or MLB.

 

A boy can dream.  A certain je' ne sais quoi about the 0Z 15 km FIM at 1 week as well.

 

 

 

 

 

 

[dtk]

Yes generally that's the case. Which is why there are some questions as to why the GFS has a very large cirrus shield associated with Dorian while the storm is traversing between 50-60W.

It's a ~27km global model relying heavily on parameterizations for cumulus/convective processes with things like diffusion to help control noise and increase stability.  Of all of the aspects of a tropical cyclone, storm size is the one I would expect such a model to be the worst at.  In fact, I wonder if there is almost some sort of resonance phenomena (i.e. the model likely generates storm sizes that are a function of strength [i.e. constant gradient]).  There must be some interplay between the spectral truncation, momentum diffusion, cumulus parameterization (redistribution of heating),  and so on....to generate "preferred" storm sizes.

[HM]

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.

 

 

That's a very good point about the instability graphics. If conditions are more favorable for convection over Africa and we have a pretty organized easterly heading into "below normal instability," it is quite possible it will not matter with proper organization ahead of time. Sometimes above normal instability is associated with +PV anomalies in the upper troposphere over very warm SST, allowing the equivalent potential temp to become unstable. A lot of different circumstances possible with each anomaly.

 

With deep westerlies descending into the troposphere right now and cooling ENSO, we are once again seeing an enhanced monsoonal circulation occurring (the timing is pretty ridiculous). There is anomalous poleward propagating anticyclones from eastern Asia (while anticyclonic breaking is suppose to happen here in the summer, it is certainly stronger/more poleward than usual) and the Hadley Cell is larger than normal. This happened to some extent during the spring when the last +AAM cycle occurred. Once the +AAM flux into the subtropics is over, it will be interesting to see how the -AAM comes on (if at all). In late May, we barely were neutral during it. We'll get to see what kind of ASO we are in for once this propagation happens and if the Hadley Cell will be like the last few years or not. Clearly, the negative dipoles across the IO are making their presence known in the various metrics; so, it will be interesting to see it in action when the subsidence state advances across this region.

[HurricaneFrances04]

Definitely some dry air. Center is becoming exposed.

 

[IsentropicLift]

Definitely some dry air. Center is becoming exposed.

 

 

Just looks like perhaps a little bit of easterly shear. In any event their is a big burst of new convection now on the western side.

 

[AutoPenalti]

ZCZC MIATCDAT4 ALL

TTAA00 KNHC DDHHMM

TROPICAL STORM DORIAN DISCUSSION NUMBER 6

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042013

1100 AM AST THU JUL 25 2013

CONVENTIONAL AND MICROWAVE SATELLITE IMAGERY INDICATE THAT THERE HAS

BEEN NO SIGNIFICANT CHANGE IN THE OVERALL CONVECTIVE PATTERN OR

STRUCTURE OF DORIAN SINCE THE PREVIOUS ADVISORY. THE COMPACT

TROPICAL CYCLONE HAS MAINTAINED A SMALL BUT SYMMETRIC OUTFLOW

PATTERN...ALONG WITH GOOD LOW-LEVEL EQUATORIAL INFLOW. SATELLITE

INTENSITY ESTIMATES ARE 55 KT FROM TAFB AND AND 45 KT FROM TAFB...

SO THE INITIAL INTENSITY IS BEING MAINTAINED AT 50 KT. THE INITIAL

POSITION IS A LITTLE SOUTH OF THE CONVENTIONAL SATELLITE FIX

POSITIONS BASED ON MICROWAVE IMAGERY OVER THE PAST 6 HOURS.

THE INITIAL MOTION ESTIMATE IS 285/15. THERE IS ESSENTIALLY NO

CHANGE TO PREVIOUS FORECAST TRACK OR REASONING. THE NHC MODEL

GUIDANCE CONTINUES TO BE IN EXCELLENT AGREEMENT ON DORIAN MOVING IN

A GENERAL WEST-NORTHWESTWARD DIRECTION THROUGHOUT THE FORECAST

PERIOD. THE MAIN DIFFERENCE BETWEEN THE MODELS CONTINUES TO BE THE

FORWARD SPEED OF DORIAN...WITH THE ECMWF AND HWRF BEING THE FASTEST

MODELS WITH A WEAKER CYCLONE...WHILE THE GFS AND GFS-ENSEMBLE MEAN

MODELS ARE THE SLOWEST DUE TO THEIR FORECASTING A STRONGER VORTEX.

BASED ON THE BETTER PERFORMANCE OF THE GFS MODEL INTENSITY

FORECASTS OVER THE PAST FEW DAYS...THE OFFICIAL FORECAST LEANS

TOWARD THE LATTER SOLUTION AND SHOWS A GRADUAL DECREASE IN FORWARD

SPEED TO BEGIN BY 48-72 HOURS AS DORIAN APPROACHES A WEAKNESS IN

THE SUBTROPICAL RIDGE. THE NEW TRACK FORECAST IS JUST AN EXTENSION

OF THE PREVIOUS ADVISORY TRACK...AND IS VERY CLOSE TO BUT SLOWER

THAN THE CONSENSUS MODEL TVCN.

DORIAN APPEARS TO HAVE SURVIVED PASSAGE OVER THE REGION OF COOLEST

SSTS OVERNIGHT AND IS NOW MOVING OVER AND TOWARD WARMER WATERS.

ALTHOUGH THE AFOREMENTIONED EQUATORIAL INFLOW OF WARM UNSTABLE AIR

LIKELY HELPED OFFSET THE COOLING BENEATH DORIAN...THAT LIFELINE IS

FORECAST BY ALL OF THE MODELS TO BE CUT OFF DURING THE NEXT 24

HOURS...WHICH WILL RESULT IN THE COMPACT CYCLONE MOVING THROUGH A

REGION OF DECREASING MID-LEVEL MOISTURE AND MORE STABLE AIR...

PRODUCING SOME SLIGHT WEAKENING FOR THE NEXT 24 HOURS OR SO. AFTER

THAT...THE VERTICAL SHEAR IS EXPECTED TO REMAIN AT OR BELOW 10-15

KT...WHICH WOULD FAVOR AT LEAST SOME GRADUAL INTENSIFICATION IF THE

INNER CORE CONVECTION CAN REMAIN INTACT. THE OFFICIAL INTENSITY

FORECAST LIES BETWEEN THE SHIPS AND LGEM SOLUTIONS...AND ASSUMES

DORIAN WILL SURVIVE ITS TREK THROUGH UNFAVORABLE THERMODYNAMIC

CONDITIONS DURING THE NEXT 2 DAYS.

FORECAST POSITIONS AND MAX WINDS

INIT 25/1500Z 16.0N 35.9W 50 KT 60 MPH

12H 26/0000Z 16.6N 38.5W 45 KT 50 MPH

24H 26/1200Z 17.4N 42.3W 45 KT 50 MPH

36H 27/0000Z 18.0N 46.2W 50 KT 60 MPH

48H 27/1200Z 18.7N 49.9W 50 KT 60 MPH

72H 28/1200Z 19.8N 57.0W 55 KT 65 MPH

96H 29/1200Z 20.7N 63.0W 60 KT 70 MPH

120H 30/1200Z 21.5N 67.5W 60 KT 70 MPH

$$

FORECASTER STEWART

NNNN

[phil882]

I would argue there has been quite a significant change to the overall structure. Dorian looks like its about to go on life support. 

[Marion_NC_WX]

I would argue there has been quite a significant change to the overall structure. Dorian looks like its about to go on life support. 

 

You got to consider, a forecaster can't go off a 2-3-4 hour convective trend when writing a forecast discussion, he/she has to go off a trend over a 6-12 hour timeframe...I'm sure the recent exposure of the LLC and entrained dry air will be mentioned if it continues.

[AvantHiatus]

You got to consider, a forecaster can't go off a 2-3-4 hour convective trend when writing a forecast discussion, he/she has to go off a trend over a 6-12 hour timeframe...I'm sure the recent exposure of the LLC and entrained dry air will be mentioned if it continues.

Good call from NHC forecasting slight weakening in the next 12 hours.

[HurricaneFrances04]

12Z GFS using the new super computer takes Dorian into Hispaniola.