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Tropical Depression Don


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I was about to add that. There is a nice 25°F DP Depression at 850 mb.

That might explain why we saw such a lack of convection last night... dry air was very close to the circulation yesterday afternoon and it appears it got ingested last night at some point. We'll see how the convection recovers today, but it might be a fight for this little guy if those dry values stick around. I think the saving grace is that the shear vector is not from the west and that's why we should still see some intensification. That said, I might need to modify my forecast later if Don doesn't start becoming significantly better organized.

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what does that mean? Thanks

Ideally, tropical cyclones should be saturated at all levels to the tropopause. By saturated I mean that that Temperature and Dew Point are the same at every level. When the temperature and dew point are not the same we call the difference (Temperature - Dew Point) the "dew point depression". The Bigger the dew point depression is at a certain level the more dry air the cyclone is ingesting at that level at that point. Dry Air can be a major factor in staving off convection and if there is enough killing at all together. Though this is hard to detect since satellites can't penetrate the higher cirrus outflow field (minus microwave imagers). Thats why we look at microwave images so much, to get a sense of the dry air. But even that doesn't tell at what level that dry is at. That's where the Hurricane Hunters and NOAA G-IV come in.

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Ideally, tropical cyclones should be saturated at all levels to the tropopause. By saturated I mean that that Temperature and Dew Point are the same at every level. When the temperature and dew point are not the same we call the difference (Temperature - Dew Point) the "dew point depression". The Bigger the dew point depression is at a certain level the more dry air the cyclone is ingesting at that level at that point. Dry Air can be a major factor in staving off convection and if there is enough killing at all together. Though this is hard to detect since satellites can't penetrate the higher cirrus outflow field (minus microwave imagers). Thats why we look at microwave images so much, to get a sense of the dry air. But even that doesn't tell at what level that dry is at. That's where the Hurricane Hunters and NOAA G-IV come in.

Thank you. Yeah, I had never come across dew point depression before. I thought that's what it was, but since I'm in a science forum, there's no reason to stay in the dark.
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Aircraft now approaching Don from the North, and still an impressive lack of moisture.

About that huge dewpoint depression. Natural half glass full optimist in me is hoping -70ºC topped convection is helping transfer moisture from the Gulf and low level environment into Don. Recon doesn't suggest must good happened overnight, pressure wise.

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About that huge dewpoint depression. Natural half glass full optimist in me is hoping -70ºC topped convection is helping transfer moisture from the Gulf and low level environment into Don. Recon doesn't suggest must good happened overnight, pressure wise.

how were the dewpoint depressions to the south and east of the system?

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Extrap 999 mb, and it appears that's even with missing the center since winds were 30 kt where that measurement was taken. With the NHC 10 kt = 1 mb rule of thumb that suggests a minimum pressure of 996 mb. Vigorous convection like we're seeing right now can do some amazing things.

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Product: Air Force Vortex Message (URNT12 KNHC)

Transmitted: 28th day of the month at 14:22Z

Corrected: This observation corrected a previous observation.

Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 303)

Storm Number & Year: 04L in 2011

Storm Name: Don (flight in the North Atlantic basin)

Mission Number: 2 seeall.png

Observation Number: 07

A. Time of Center Fix: 28th day of the month at 13:50:10Z

B. Center Fix Coordinates: 23°59'N 89°55'W (23.9833N 89.9167W) viewmap.png

B. Center Fix Location: 414 miles (667 km) to the S (179°) from New Orleans, LA, USA.

C. Minimum Height at Standard Level: 1,442m (4,731ft) at 850mb

D. Estimated (by SFMR or visually) Maximum Surface Wind: 29kts (~ 33.4mph)

E. Location of the Estimated Maximum Surface Wind: 2 nautical miles (2 statute miles) to the SSW (201°) of center fix

F. Maximum Flight Level Wind Inbound: From 81° at 32kts (From the E at ~ 36.8mph)

G. Location of Maximum Flight Level Wind Inbound: 2 nautical miles (2 statute miles) to the NNW (346°) of center fix

H. Minimum Sea Level Pressure: 1001mb (29.56 inHg) - Extrapolated

I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 18°C (64°F) at a pressure alt. of 1,523m (4,997ft)

J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 26°C (79°F) at a pressure alt. of 1,515m (4,970ft)

K. Dewpoint Temp (collected at same location as temp inside eye): 8°C (46°F)

K. Sea Surface Temp (collected at same location as temp inside eye): Not Available

L. Eye Character: Not Available

M. Eye Shape: Not Available

N. Fix Determined By: Penetration, Radar, Wind, Pressure and Temperature

N. Fix Levels (sfc and flt lvl centers are within 5nm of each other): Surface and 850mb

O. Navigation Fix Accuracy: 0.02 nautical miles

O. Meteorological Accuracy: 1.5 nautical miles

Remarks Section - Remarks That Were Decoded...

Maximum Flight Level Wind: 52kts (~ 59.8mph) in the east quadrant at 11:57:10Z

Maximum Flight Level Wind Outbound: 38kts (~ 43.7mph) in the southeast quadrant at 14:15:50Z

Sea Level Pressure Extrapolation From: 850mb

Remarks Section - Additional Remarks...

EYE SONDE MEASURED 1004MB WITH 17 KTS SFC WIND

FEEDERBAND CROSSING SE OUTBOUND LEG 70 NM SE OF CTR

COR FOR MAX OUTBOUND FL WIND IN CONVECTIVE BAND

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Could someone please explain what are t# numbers and extrapolated pressure and what they are used for? Thanks

Extrapolated pressure is simpily the surface estimated pressure from aircraft. This uses a general equation (hipsometric for math buffs) that is reasonably accurate but extrapolated down from the elevation of the aircraft. This is generally considered less accurate however than direct observations from dropsondes.

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TS Warnings hoisted from Port Mansfield to San Luis Pass Texas...Don is a bit stronger as well...

BULLETIN

TROPICAL STORM DON ADVISORY NUMBER 4

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042011

1000 AM CDT THU JUL 28 2011

...HURRICANE HUNTER AIRCRAFT FINDS THAT DON IS A LITTLE STRONGER...

SUMMARY OF 1000 AM CDT...1500 UTC...INFORMATION

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

LOCATION...24.1N 90.1W

ABOUT 520 MI...840 KM ESE OF CORPUS CHRISTI TEXAS

ABOUT 480 MI...770 KM ESE OF BROWNSVILLE TEXAS

MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H

PRESENT MOVEMENT...NW OR 305 DEGREES AT 14 MPH...22 KM/H

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

WATCHES AND WARNINGS

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

CHANGES WITH THIS ADVISORY...

A TROPICAL STORM WARNING IS NOW IN EFFECT FOR THE TEXAS COAST FROM

PORT MANSFIELD TO SAN LUIS PASS.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...

* TEXAS COAST FROM PORT MANSFIELD TO SAN LUIS PASS

A TROPICAL STORM WATCH IS IN EFFECT FOR...

* TEXAS COAST SOUTH OF PORT MANSFIELD TO THE MOUTH OF THE RIO GRANDE

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE

EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE

POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED

STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE

MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE

FORECAST OFFICE.

DISCUSSION AND 48-HOUR OUTLOOK

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

AT 1000 AM CDT...1500 UTC...THE CENTER OF TROPICAL STORM DON WAS

LOCATED NEAR LATITUDE 24.1 NORTH...LONGITUDE 90.1 WEST. DON IS

MOVING TOWARD THE NORTHWEST NEAR 14 MPH...22 KM/H. A GENERAL MOTION

TOWARD THE NORTHWEST OR WEST-NORTHWEST IS EXPECTED THROUGH SATURDAY.

ON THIS TRACK...THE CENTER OF DON SHOULD CROSS THE CENTRAL GULF OF

MEXICO TODAY...APPROACH THE TEXAS COAST ON FRIDAY....AND REACH THE

TEXAS COAST FRIDAY NIGHT OR SATURDAY.

MAXIMUM SUSTAINED WINDS ARE NEAR 45 MPH...75 KM/H...WITH HIGHER

GUSTS. GRADUAL STRENGTHENING IS FORECAST DURING THE NEXT 36 HOURS.

TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 60 MILES...95 KM

FROM THE CENTER. A MEXICAN AUTOMATED STATION AT ISLA PEREZ IN THE

GULF OF MEXICO RECENTLY REPORTED A WIND GUST OF 38 MPH...61 KM/H.

THE MINIMUM CENTRAL PRESSURE EXTRAPOLATED BY THE HURRICANE HUNTER

AIRCRAFT IS 1001 MB...29.56 INCHES.

HAZARDS AFFECTING LAND

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

WIND...TROPICAL STORM CONDITIONS ARE EXPECTED WITHIN THE WARNING

AREA BY LATE FRIDAY OR FRIDAY NIGHT. TROPICAL STORM CONDITIONS ARE

POSSIBLE IN THE WATCH AREA BY LATE FRIDAY OR FRIDAY NIGHT.

STORM SURGE...A STORM SURGE WILL RAISE WATER LEVELS BY AS MUCH AS

1 TO 2 FEET ABOVE GROUND LEVEL...MAINLY ALONG THE IMMEDIATE COAST

NEAR AND TO THE NORTHEAST OF WHERE THE CENTER MAKES LANDFALL. NEAR

THE COAST...THE SURGE WILL BE ACCOMPANIED BY LARGE AND DAMAGING

WAVES.

RAINFALL...DON IS EXPECTED TO PRODUCE TOTAL RAIN ACCUMULATIONS OF

3 TO 5 INCHES FROM THE CENTRAL TEXAS COAST WESTWARD INTO SOUTH

CENTRAL TEXAS...WITH POSSIBLE ISOLATED MAXIMUM AMOUNTS OF 7 INCHES.

NEXT ADVISORY

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

NEXT INTERMEDIATE ADVISORY...100 PM CDT.

NEXT COMPLETE ADVISORY...400 PM CDT.

$$

FORECASTER BEVEN

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