Tropical weather analysis - July 5, 2014

By: KoritheMan , 7:42 AM GMT on July 05, 2014

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Arthur

Arthur remains a hurricane as of the 0600Z NHC intermediate advisory bulletin:

Wind: 75 mph, with higher gusts
Location: 42.3°N 67.6°W
Movement: NE at 31 mph
Pressure: 981 mb
Category: 1 (Saffir-Simpson Hurricane Wind Scale)

Arthur is quickly becoming an extratropical cyclone. Satellite and radar data from Portland, Maine show a sizable area of rain to the north of the center, but little apparent precipitation near the center. In addition, 0z upper air soundings from Portland along with manual analysis of water vapor imagery show a pronounced tongue of cooler and drier air becoming rapidly entrained into the circulation as a cold front approaches and overtakes the cyclone from the west. Satellite estimates are beginning to fall rather abruptly, and Arthur will likely be downgraded to a tropical storm on the forthcoming NHC 0900Z full advisory package. While Arthur is still producing an area of fairly cold convection, satellite data suggest that this convection is occurring well ahead of the center over southwestern Nova Scotia.



Figure 1. Latest infrared satellite image of Hurricane Arthur. Image credit: NOAA's Satellite Services Division (SSD).

While Arthur is probably not quite extratropical yet, it should get there within the next few hours. Although the last AMSU temperature profiles still showed a respectable warm core, the most recent of those assessments were near 2000Z, nearly nine hours ago at the time of this writing, and it is likely that the column has cooled significantly within Arthur since that time as the rapidly transforming tropical cyclones heads over cold water and encounters westerly shear. While a gradual decay of the circulation is forecast throughout the forecast period, Arthur is still expected to make landfall over Atlantic Canada with near hurricane-force winds, especially in gusts. The global models do not suggest baroclinic reintensification of Arthur as an extratropical cyclone, but instead signify a gradual elongation of the circulation and a steady decrease in storm-relative wind speeds as post-tropical cyclone Arthur rotates around a larger extratropical gyre over the north Atlantic.

Arthur is accelerating to the northeast embedded in mid-latitude southwesterly flow connected to the aforementioned cold front. Arthur or more likely its extratropical remnants are expected to make landfall along the southwestern coast of Nova Scotia near Yarmouth within the next several hours. After landfall, most of the guidance shows Arthur briefly moving into the Bay of Fundy before turning to the east-northeast as it rounds the northern periphery of the Atlantic subtropical ridge and begins to also feel the counterclockwise circulation associated with a larger extratropical cyclone over the north Atlantic. The model consensus has shifted a little to the left on the latest cycle, but my forecast remains a little to the right of that guidance, and is fairly close to the current National Hurricane Center forecast track. While a forecast point is not provided out to five days here due to the limitations of the map I use to make the track forecasts, the models show extratropical Arthur ending up near southern Greenland by that time, where it is likely to be significantly weaker.

Earlier scatterometer data indicates that Arthur has a rather expansive wind field, and recent offshore buoy data indicate that sustained tropical storm force winds are not far offshore the coast of Maine (Station M1SM1 located at 43.7°N 68.8°W). While a tropical storm warning has not been issued for that area with the expectation that the strongest winds will arrive behind the cold front, the overall impact will be the same, and interests there should follow the products issued by their local National Weather Service forecast office.

Tropical storm force sustained winds with possible embedded hurricane force gusts are likely over much of Nova Scotia before the circulation begins to wind down in about 24 hours. Residents there should anticipate the likelihood of tree and power line damage. Rainfall should be fairly minimal since Arthur will be moving quickly through the region.

The tropical storm warning for Nantucket and Cape Cod will likely be discontinued by the National Hurricane Center with the next advisory, as surface observations show westerly winds that are well below tropical storm force. A tropical storm warning remains in effect for all of Nova Scotia, and a wide swath of tropical storm force winds is expected to occur here.

Intensity forecast

Initial 07/05 0600Z 42.3°N 67.6°W 65 kt 75 mph
12 hour 07/05 1800Z 44.7°N 65.1°W 50 kt 60 mph: extratropical
24 hour 07/06 0600Z 47.8°N 60.8°W 45 kt 50 mph: extratropical
36 hour 07/06 1800Z 49.8°N 57.8°W 40 kt 45 mph: extratropical

Track forecast



Figure 2. My forecast track for Arthur.

NHC storm information


000
WTNT31 KNHC 050549
TCPAT1

BULLETIN
HURRICANE ARTHUR INTERMEDIATE ADVISORY NUMBER 17A
NWS NATIONAL HURRICANE CENTER MIAMI FL AL012014
200 AM EDT SAT JUL 05 2014

...ARTHUR RACING TOWARD NOVA SCOTIA...


SUMMARY OF 200 AM EDT...0600 UTC...INFORMATION
----------------------------------------------
LOCATION...42.3N 67.6W
ABOUT 130 MI...210 KM SW OF YARMOUTH NOVA SCOTIA
MAXIMUM SUSTAINED WINDS...75 MPH...120 KM/H
PRESENT MOVEMENT...NE OR 45 DEGREES AT 31 MPH...50 KM/H
MINIMUM CENTRAL PRESSURE...981 MB...28.97 INCHES


WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY...

NONE.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* NANTUCKET
* CAPE COD FROM PROVINCETOWN TO WOODS HOLE
* NOVA SCOTIA INCLUDING CAPE BRETON ISLAND
* PRINCE EDWARD ISLAND
* NEW BRUNSWICK FROM THE U.S./CANADA BORDER TO GRAND-ANSE

IN ADDITION...HIGH WIND WARNINGS...FOR WINDS IN EXCESS OF 40 MPH...
ARE IN EFFECT FOR PORTIONS OF EASTERN MAINE.

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. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.


DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
AT 200 AM EDT...0600 UTC...THE CENTER OF HURRICANE ARTHUR WAS
LOCATED BY AN AIR FORCE RECONNAISSANCE PLANE NEAR LATITUDE 42.3
NORTH...LONGITUDE 67.6 WEST. ARTHUR IS MOVING TOWARD THE NORTHEAST
NEAR 31 MPH...50 KM/H...AND THIS MOTION IS EXPECTED TO CONTINUE WITH
SOME DECREASE IN FORWARD SPEED DURING THE NEXT DAY OR SO. ON THE
FORECAST TRACK...THE CENTER OF ARTHUR WILL BE NEAR OR OVER WESTERN
NOVA SCOTIA IN THE NEXT FEW HOURS...AND OVER THE GULF OF ST.
LAWRENCE TONIGHT.

MAXIMUM SUSTAINED WINDS REMAIN NEAR 75 MPH...120 KM/H...WITH HIGHER
GUSTS. WEAKENING IS FORECAST DURING THE NEXT 48 HOURS...AND ARTHUR
IS EXPECTED TO BECOME A POST-TROPICAL CYCLONE LATER THIS
MORNING.

HURRICANE FORCE WINDS EXTEND OUTWARD UP TO 70 MILES...110 KM...FROM
THE CENTER...AND TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO
160 MILES...260 KM.

THE MINIMUM CENTRAL PRESSURE RECENTLY REPORTED BY AN AIR FORCE
HURRICANE HUNTER PLANE WAS 981 MB...28.97 INCHES.


HAZARDS AFFECTING LAND
----------------------
WIND...TROPICAL STORM CONDITIONS ARE SPREADING ACROSS PORTIONS THE
WARNING AREA IN NEW ENGLAND. THESE WINDS SHOULD GRADUALLY SUBSIDE
IN THE NEXT FEW HOURS. TROPICAL STORM CONDITIONS ARE
EXPECTED TO REACH THE WARNING AREA IN CANADA DURING THE NEXT
FEW HOURS AND SPREAD NORTHWARD LATER TODAY. IN ADDITION...WINDS IN
EXCESS OF 40 MPH ARE EXPECTED OVER PORTIONS OF EASTERN MAINE
TODAY...BEHIND A COLD FRONT OVERTAKING THE CIRCULATION OF ARTHUR.

STORM SURGE...COASTAL FLOODING IS POSSIBLE ALONG CAPE COD
TODAY. FOR INFORMATION SPECIFIC TO YOUR AREA...PLEASE SEE PRODUCTS
ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE OFFICE.

RAINFALL...RAINFALL ACCUMULATIONS OF 2 TO 4 INCHES ARE EXPECTED
OVER EASTERN MASSACHUSETTS...COASTAL MAINE...NOVA SCOTIA...
NEWFOUNDLAND...AND NEW BRUNSWICK...WITH ISOLATED MAXIMUM AMOUNTS OF
6 INCHES POSSIBLE OVER DOWNEAST MAINE AND INTO NEW BRUNSWICK CANADA.

SURF...SWELLS GENERATED BY ARTHUR ARE STILL AFFECTING PORTIONS OF
THE COAST OF NORTH CAROLINA...THE MID-ATLANTIC...AND NORTHEAST
UNITED STATES. SWELLS WILL BEGIN AFFECTING PORTIONS OF SOUTHEASTERN
CANADA SOON. THESE SWELLS ARE EXPECTED TO CAUSE LIFE-THREATENING
SURF AND RIP CURRENTS. FOR MORE INFORMATION...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST
OFFICE.


NEXT ADVISORY
-------------
NEXT COMPLETE ADVISORY...500 AM EDT.

$$
FORECASTER AVILA

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9. WunderAlertBot (Admin)
3:07 AM GMT on July 08, 2014
KoritheMan has created a new entry.
8. WunderAlertBot (Admin)
3:06 AM GMT on July 08, 2014
KoritheMan has created a new entry.
7. KoritheMan
8:48 AM GMT on July 05, 2014
Quoting 6. DeepSeaRising:

And the obvious answer is, NHC wouldn't say there were those surface winds if they hadn't measured them in the first place.


Yes! Exactly.

There is a small reduction factor that's only applicable to the coast if I'm not mistaken. So inland areas should still receive the strongest winds.
Member Since: March 7, 2007 Posts: 602 Comments: 21574
6. DeepSeaRising
8:29 AM GMT on July 05, 2014
And the obvious answer is, NHC wouldn't say there were those surface winds if they hadn't measured them in the first place.
Member Since: January 31, 2014 Posts: 0 Comments: 8
5. DeepSeaRising
8:24 AM GMT on July 05, 2014
Very interesting. So you'd argue that in a hurricane estimated winds do happen at the surface but maybe just in very limited areas that aren't always verifiable. So in many hurricanes, that don't do horrible damage that is widespread, there are, at least typically, small areas that do receive NHC measure surface winds. Another worthy study.
Member Since: January 31, 2014 Posts: 0 Comments: 8
4. KoritheMan
8:15 AM GMT on July 05, 2014
Quoting 3. DeepSeaRising:



Well that's a variable that changes with every hurricane. Some Cat ones produce far more severe damage and many Cat 2's have been devastating. Every storm is different and the reasons vary why. Clearly dry air, I at least believe was the main culprit with Arthur. There really is no usual or unusual wind readings or damage norms for hurricanes. They all differ and for different reasons. Would like to see the percentage of hurricanes that make landfall and do not produce operational intensity estimate. Would be a cool research. Guessing at least 60%, probably higher. Thanks Kori.


Recon was measuring 85 kt surface winds at the time of landfall. It looked like the strongest portion of the eyewall was over water at Arthur's first landfall, though, which could explain the discrepancy.

Also, landfalling hurricanes likely do produce the winds advertised in the NHC advisory packages, but probably outside of official observing stations. I've seen many instances of amateur storm observers and/or unofficial reporting stations reporting winds closer to the advisory intensity than the official stations indicate. NHC doesn't typically use those in the storm's final report due to things like calibration issues, personal biases, etc.
Member Since: March 7, 2007 Posts: 602 Comments: 21574
3. DeepSeaRising
8:03 AM GMT on July 05, 2014
Quoting 2. KoritheMan:



I don't really see how the wind reports overland were that unusual for an 85 kt Category 2. Maybe a little weaker than what we'd expect, but the majority of landfalling hurricanes do not produce the operational intensity estimate; at least in areas that can directly measure them.


Well that's a variable that changes with every hurricane. Some Cat ones produce far more severe damage and many Cat 2's have been devastating. Every storm is different and the reasons vary why. Clearly dry air, I at least believe was the main culprit with Arthur. There really is no usual or unusual wind readings or damage norms for hurricanes. They all differ and for different reasons. Would like to see the percentage of hurricanes that make landfall and do not produce operational intensity estimate. Would be a cool research. Guessing at least 60%, probably higher. Thanks Kori.
Member Since: January 31, 2014 Posts: 0 Comments: 8
2. KoritheMan
7:55 AM GMT on July 05, 2014
Quoting 1. DeepSeaRising:

Thanks Kori. Arthur's going to be a Cat 2 with dry entrainment lessons to be learned. Winds due to eye entrainment of dry air never was as strong as one would expect a Cat 2 eye to be and outer bands were full of collapsing TS compared to strong convection MCV's. Good news and a fun storm to watch and learn much from.


I don't really see how the wind reports overland were that unusual for an 85 kt Category 2. Maybe a little weaker than what we'd expect, but the majority of landfalling hurricanes do not produce the operational intensity estimate; at least in areas that can directly measure them.
Member Since: March 7, 2007 Posts: 602 Comments: 21574
1. DeepSeaRising
7:51 AM GMT on July 05, 2014
Thanks Kori. Arthur's going to be remembered as a Cat 2 with dry entrainment lessons to be learned. Winds due to eye entrainment of dry air never were never as strong as one would have expected a Cat 2 eye to be and outer bands were full of collapsing TS compared to strong convection MCV's. Good news and a fun storm to watch and learn much from.
Member Since: January 31, 2014 Posts: 0 Comments: 8

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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