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Post Tropical Cyclone Debby


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Isn't it the other way around?

SFMR has to be supported by the flight level winds

Not quite... before SFMR, recon primarily used flight level winds that were then reduced down by some fraction to obtain the surface winds. However, SFMR actually measures the surface winds, and probably represents a more accurate reading (outside of areas with high rainfall rates) than flight level winds alone can obtain, even after reduced down to their equivalent sfc wind speed. As the NHC gets more confortable using SFMR look for it to be used more and more frequently as the primary wind source, although oftentimes you still see a blend of the two data methods being used.

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BULLETIN

TROPICAL STORM DEBBY ADVISORY NUMBER 1

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042012

400 PM CDT SAT JUN 23 2012

...TROPICAL STORM DEBBY FORMS IN THE CENTRAL GULF OF MEXICO...

...TROPICAL STORM WARNING ISSUED FOR A PORTION OF THE LOUISIANA

COAST...

SUMMARY OF 400 PM CDT...2100 UTC...INFORMATION

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

LOCATION...26.2N 87.6W

ABOUT 220 MI...355 KM SSE OF THE MOUTH OF THE MISSISSIPPI RIVER

MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H

PRESENT MOVEMENT...N OR 360 DEGREES AT 6 MPH...9 KM/H

MINIMUM CENTRAL PRESSURE...1001 MB...29.56 INCHES

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Tropical Storm DEBBY Public Advisory

Home Public Adv Fcst/Adv Discussion Wind Probs Archive

000

WTNT34 KNHC 232051

TCPAT4

BULLETIN

TROPICAL STORM DEBBY ADVISORY NUMBER 1

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042012

400 PM CDT SAT JUN 23 2012

...TROPICAL STORM DEBBY FORMS IN THE CENTRAL GULF OF MEXICO...

...TROPICAL STORM WARNING ISSUED FOR A PORTION OF THE LOUISIANA

COAST...

SUMMARY OF 400 PM CDT...2100 UTC...INFORMATION

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

LOCATION...26.2N 87.6W

ABOUT 220 MI...355 KM SSE OF THE MOUTH OF THE MISSISSIPPI RIVER

MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H

PRESENT MOVEMENT...N OR 360 DEGREES AT 6 MPH...9 KM/H

MINIMUM CENTRAL PRESSURE...1001 MB...29.56 INCHES

WATCHES AND WARNINGS

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

CHANGES WITH THIS ADVISORY...

A TROPICAL STORM WARNING HAS BEEN ISSUED FROM THE MOUTH OF THE PEARL

RIVER WESTWARD TO MORGAN CITY LOUISIANA...NOT INCLUDING THE CITY OF

NEW ORLEANS OR LAKE PONTCHARTRAIN.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...

* THE COAST OF LOUISIANA FROM THE MOUTH OF THE PEARL RIVER WESTWARD

TO MORGAN CITY...NOT INCLUDING THE CITY OF NEW ORLEANS OR LAKE

PONTCHARTRAIN

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE

EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...INCLUDING POSSIBLE

INLAND WATCHES AND WARNINGS...PLEASE MONITOR PRODUCTS ISSUED BY

YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST OFFICE.

DISCUSSION AND 48-HOUR OUTLOOK

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

AIRCRAFT RECONNAISSANCE AND BUOY DATA INDICATE THAT THE AREA OF LOW

PRESSURE IN THE GULF OF MEXICO HAS BECOME TROPICAL STORM DEBBY.

AT 400 PM CDT...2100 UTC...THE CENTER OF TROPICAL STORM DEBBY WAS

LOCATED NEAR LATITUDE 26.2 NORTH...LONGITUDE 87.6 WEST. DEBBY IS

MOVING TOWARD THE NORTH NEAR 6 MPH...9 KM/H. A SLOW NORTHWARD

MOTION IS EXPECTED TONIGHT...FOLLOWED BY A WESTWARD TURN ON SUNDAY.

ON THE FORECAST TRACK...THE CENTER OF DEBBY WILL BE MOVING OVER THE

NORTHERN GULF OF MEXICO DURING THE NEXT FEW DAYS.

MAXIMUM SUSTAINED WINDS ARE NEAR 50 MPH...85 KM/H...WITH HIGHER

GUSTS. THESE WINDS ARE OCCURRING WELL EAST OF THE CENTER OF

CIRCULATION. SOME STRENGTHENING IS FORECAST DURING THE NEXT 48

HOURS.

TROPICAL-STORM-FORCE WINDS EXTEND OUTWARD UP TO 175 MILES...280 KM

TO THE EAST OF THE CENTER.

MINIMUM CENTRAL PRESSURE ESTIMATED FROM RECONNAISSANCE DATA IS 1001

MB...29.56 INCHES.

HAZARDS AFFECTING LAND

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

WIND...TROPICAL STORM CONDITIONS ARE EXPECTED TO FIRST REACH THE

COAST WITHIN THE WARNING AREA BY SUNDAY NIGHT...MAKING OUTSIDE

PREPARATIONS DIFFICULT OR DANGEROUS.

STORM SURGE...THE COMBINATION OF A STORM SURGE AND THE TIDE WILL

CAUSE NORMALLY DRY AREAS NEAR THE COAST TO BE FLOODED BY

RISING WATERS. THE WATER COULD REACH THE FOLLOWING DEPTHS ABOVE

GROUND IF THE PEAK SURGE OCCURS AT THE TIME OF HIGH TIDE...

MISSISSIPPI AND SOUTHEASTERN LOUISIANA...1 TO 3 FT

THE DEEPEST WATER WILL OCCUR ALONG THE IMMEDIATE COAST IN AREAS OF

ONSHORE FLOW. SURGE-RELATED FLOODING DEPENDS ON THE RELATIVE

TIMING OF THE SURGE AND THE TIDAL CYCLE...AND CAN VARY GREATLY OVER

SHORT DISTANCES. FOR INFORMATION SPECIFIC TO YOUR AREA...PLEASE

SEE PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE OFFICE.

RAINFALL...DEBBY IS EXPECTED TO PRODUCE RAIN ACCUMULATIONS OF 3 TO 6

INCHES ALONG THE GULF COAST FROM SOUTHERN LOUISIANA TO THE FLORIDA

PANHANDLE...WITH POSSIBLE ISOLATED MAXIMUM AMOUNTS OF 10 INCHES.

NEXT ADVISORY

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

NEXT INTERMEDIATE ADVISORY...700 PM CDT.

NEXT COMPLETE ADVISORY...1000 PM CDT.

$$

FORECASTER BERG/AVILA

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Not quite... before SFMR, recon primarily used flight level winds that were then reduced down by some fraction to obtain the surface winds. However, SFMR actually measures the surface winds, and probably represents a more accurate reading (outside of areas with high rainfall rates) than flight level winds alone can obtain, even after reduced down to their equivalent sfc wind speed. As the NHC gets more confortable using SFMR look for it to be used more and more frequently as the primary wind source, although oftentimes you still see a blend of the two data methods being used.

In this particular case, I don't think the SFMR and the flight level winds are inconsistent with each other, since the flight level is so low and the reduction factor is probably something like 75-80%. Plus it is under heavy convection as well, which makes the SFMR less reliable, but also means the reduction factor probably isn't super low.

There have been cases in the past where the dropsondes have recorded substantially higher surface winds (i.e. more than 5 knots) than SFMR. Really, there's potential error with any method you use.

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Looks like the NHC is taking the westward route... I am glad they also mentioned this in their discussion, you can pretty much discount the SHIPS and LGEM until the GFS comes around to the westward track, or the NHC tracks comes around to the eastward track ;)

000

WTNT44 KNHC 232052

TCDAT4

TROPICAL STORM DEBBY DISCUSSION NUMBER 1

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042012

400 PM CDT SAT JUN 23 2012

AIRCRAFT RECONNAISSANCE...SATELLITE...AND BUOY DATA INDICATE THAT

THE CIRCULATION ASSOCIATED WITH THE AREA OF LOW PRESSURE IN THE

GULF OF MEXICO HAS BECOME SUFFICIENTLY DEFINED TO CLASSIFY THIS

SYSTEM AS A TROPICAL STORM. THE INITIAL INTENSITY IS BASED ON A

HIGHEST 1000-FT FLIGHT-LEVEL WIND OF 61 KT AND SFMR WINDS NEAR 45

KT. THESE WINDS ARE ONLY OCCURRING TO THE EAST OF THE CENTER.

DEBBY IS MOVING SLOWLY NORTHWARD...OR 360 DEGREES AT 5 KT. THE

CYCLONE IS IN A WEAK STEERING ENVIRONMENT BETWEEN TWO MID-LEVEL

RIDGES LOCATED OVER THE GREATER ANTILLES AND THE SOUTHERN UNITED

STATES...AND DEBBY IS NOT EXPECTED TO MOVE MUCH DURING THE NEXT

COUPLE OF DAYS. THE TRACK GUIDANCE IS SPLIT IN TWO BRANCHES...WITH

THE GFS AND GFDL SHOWING AN EASTWARD MOTION ACROSS FLORIDA...AND

THE REST OF THE MODELS BRINGING DEBBY WESTWARD ACROSS THE NORTHERN

GULF TOWARDS TEXAS. EVEN THOUGH THE DETERMINISTIC GFS SHOWS AN

EASTWARD TRACK...MORE THAN HALF OF THE GFS ENSEMBLE MEMBERS ARE IN

AGREEMENT WITH THE WESTWARD-MOVING MODELS...MAKING THE WESTWARD

SCENARIO SEEM MORE LIKELY. IT IS IMPORTANT TO NOTE TOO THAT THE

ECMWF MODEL SHOWS A FASTER WESTWARD MOTION THAN INDICATED IN THE

NHC FORECAST...SO IT IS POSSIBLE THAT DEBBY COULD REACH THE COAST

EARLIER THAN INDICATED.

DEBBY IS CURRENTLY A SHEARED CYCLONE WITH NEARLY ALL OF THE DEEP

CONVECTION LOCATED IN A CURVED BAND TO THE EAST OF THE CENTER.

HOWEVER...AN UPPER-LEVEL LOW OVER THE NORTHWESTERN GULF OF MEXICO

IS FORECAST TO DIG SOUTHWESTWARD...WHICH WILL RESULT IN A DECREASE

OF VERTICAL SHEAR AFFECTING THE SYSTEM...AND A GREATER CHANCE OF

INTENSIFICATION OVER THE NEXT FEW DAYS. BECAUSE THE OFFICIAL TRACK

FORECAST IS SO DIFFERENT FROM THE GFS SOLUTION...THE GFS FIELDS USED

BY THE SHIPS AND LGEM MODELS ARE NOT LIKELY TO BE REPRESENTATIVE OF

THE CONDITIONS ENCOUNTERED BY DEBBY. THE NHC FORECAST THEREFORE

SHOWS MORE STRENGTHENING THAN INDICATED BY THE INTENSITY GUIDANCE.

DEBBY IS THE EARLIEST 4TH STORM IN THE HISTORICAL RECORDS...

SURPASSING HURRICANE DENNIS OF 2005...WHICH BECAME A TROPICAL STORM

ON JULY 5TH.

FORECAST POSITIONS AND MAX WINDS

INIT 23/2100Z 26.2N 87.6W 45 KT 50 MPH

12H 24/0600Z 26.9N 87.8W 45 KT 50 MPH

24H 24/1800Z 27.3N 88.1W 50 KT 60 MPH

36H 25/0600Z 27.4N 88.5W 50 KT 60 MPH

48H 25/1800Z 27.5N 89.5W 55 KT 65 MPH

72H 26/1800Z 27.5N 91.5W 55 KT 65 MPH

96H 27/1800Z 27.5N 93.5W 60 KT 70 MPH

120H 28/1800Z 27.5N 95.0W 60 KT 70 MPH

$$

FORECASTER BERG/AVILA

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In this particular case, I don't think the SFMR and the flight level winds are inconsistent with each other, since the flight level is so low and the reduction factor is probably something like 75-80%. Plus it is under heavy convection as well, which makes the SFMR less reliable, but also means the reduction factor probably isn't super low.

There have been cases in the past where the dropsondes have recorded substantially higher surface winds (i.e. more than 5 knots) than SFMR. Really, there's potential error with any method you use.

Good points... I guess just for myself I'd rather be starting off with an value that is actually a surface measurement (albiet empirically derived through sea foam) rather than obtaining a flight level measurement and then trying to extrapolate the winds based on what level you are above the surface. In the latter case you are trying to apply an average that has been estimated through other TCs that doesn't necessary apply in every TC.

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Anyone notice in the latest vis that the naked southern swirl has fallen apart and a new LL swirl is taking shape to the north and seems to be starting to pull some of the deeper convection westward.

Looks like typical mesovortice orbiting to me, there's probably 1-2 hiding in the convection besides the 2 we can see. Common to see these things get deformed and interact with each other, etc. I wouldn't focus too much on what they're doing since they're basically inconsequential to the overall storm structure, they're only a sign that this storm is very disorganized right now.

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Assuming the western track verifies closer to the ECMWF operational and ensemble runs, Debby should remain under significant shear until Monday. Afterwards, the ECMWF has the upper low cutting off and becoming smaller in scale, likely in response to the diabatic heating and outflow from Debby's convection. This should allow the upper level westerly wind component around Debby to diminish, resulting in a reduction of the shear. Therefore, I don't expect much, if any, intensification until sometime on Monday, at which point we'll probably see a reorganization of Debby's convective pattern into something that looks more symmetric. Then the intensification rate should increase on Tuesday.

I think the environmental conditions are good enough for this to become a hurricane, honestly.

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The weather is so boring up here, Tulsa led off their afternoon AFD with an "off-topic" commentary on Debby. :popcorn:

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE TULSA OK

402 PM CDT SAT JUN 23 2012

.DISCUSSION...

GOOD TIME NOT TO BE A FORECASTER ON/NEAR THE GULF

COAST. GFS TAKES TROPICAL DISTURBANCE...OR MORE ?

EAST INTO FLORIDA... ECMWF MODEL ABOUT 180 DEGREES

OPPOSED TAKES TROPICAL SYSTEM INTO TEXAS. CANADIAN

MODEL SPLITS THE DIFFERENCE AND TRYS A RUN AT THE

LOWER MISSISSIPPI VALLEY. SHOULD BE A CLEAR WINNER

AND LOSER ON THIS ONE. WILL PLACE BETS ON ECMWF.

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Assuming the western track verifies closer to the ECMWF operational and ensemble runs, Debby should remain under significant shear until Monday. Afterwards, the ECMWF has the upper low cutting off and becoming smaller in scale, likely in response to the diabatic heating and outflow from Debby's convection. This should allow the upper level westerly wind component around Debby to diminish, resulting in a reduction of the shear. Therefore, I don't expect much, if any, intensification until sometime on Monday, at which point we'll probably see a reorganization of Debby's convective pattern into something that looks more symmetric. Then the intensification rate should increase on Tuesday.

I think the environmental conditions are good enough for this to become a hurricane, honestly.

Euro shows a storm with great structure within a day or two of landfall in Texas. Pretty much fits your description very well.

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Looks like typical mesovortice orbiting to me, there's probably 1-2 hiding in the convection besides the 2 we can see. Common to see these things get deformed and interact with each other, etc. I wouldn't focus too much on what they're doing since they're basically inconsequential to the overall storm structure, they're only a sign that this storm is very disorganized right now.

I disagree with the following statement. Mesovorticies are important to the structure of a TC because the dominant mesovorticies can influence the motion and intensity of a tropical cyclone. A good example is Claudette in 2003. The system had multiple mesovoritices that were rotating around a mean center. When each mesovortex interacted with the convection, it often resulted in Claudette taking a jog to the right of the mean track, with a corresponding drop in the pressure. Some excerpts from the NHC discussions reveal this.

CLAUDETTE HAS TAKEN A LITTLE

JOG TO THE NORTH TODAY...PERHAPS IN RESPONSE TO THE PERSISTENT DEEP

CONVECTION THAT DEVELOPED NEAR THE CENTER IN THE NORTHEAST QUADRANT

THIS MORNING.

ONCE AGAIN THE WIND CENTER OF CLAUDETTE HAS BEEN PULLED INTO THE

DEEP CONVECTION...BUT THIS TIME THERE WAS A RESPONSE IN THE

PRESSURE FIELD. THE MOST RECENT RECONNAISSANCE FIX WAS 996

MB...DOWN ABOUT 11 MB SINCE YESTERDAY. FLIGHT LEVEL WINDS ARE NEAR

60 KT AND THE SURFACE WINDS ARE ESTIMATED TO BE ABOUT 50 KT. IT

REMAINS TO BE SEEN WHETHER THIS IS A TRANSITORY PRESSURE FALL.

ALTHOUGH THE RECON CENTER FIX WAS A SOLID ONE...THE FEATURE THEY

ARE TRACKING STILL APPEARS TO BE ROTATING AROUND WITHIN A BROADER

CIRCULATION.

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I disagree with the following statement. Mesovorticies are important to the structure of a TC because the dominant mesovorticies can influence the motion and intensity of a tropical cyclone. A good example is Claudette in 2003. The system had multiple mesovoritices that were rotating around a mean center. When each mesovortex interacted with the convection, it often resulted in Claudette taking a jog to the right of the mean track, with a corresponding drop in the pressure. Some excerpts from the NHC discussion reveal this.

Sounds like typical downshear center reformation to me, which I'm sure we'll see with this storm. You could say that the mesovortices enhance this process when they re-enter the convection and move towards the best updrafts, which would supercharge the mesovortex and make it the deepest pressure minima in the storm (and thus move the storm center towards the convection). However, the driving process is environmental wind shear which is displacing convection from the center imo, not the mesovortices.

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