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Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.Which Intensity Model Should You Trust for Tropical Storm Arthur?
The Atlantic's first storm of 2014 is here, as Tropical Storm Arthur was named at 11 am EDT Tuesday by NHC. Arthur's formation date of July 1 comes a week before the typical July 8 appearance of the Atlantic's first named storm, but is the latest the first named storm of the season has appeared since 2009, when Tropical Storm Ana formed on August 12.
Figure 1. Melbourne, Florida radar image of Tropical Storm Arthur at 2:20 pm EDT July 1, 2014.
Arthur was drifting northwest at 5 mph towards the east coast of Central Florida early Tuesday afternoon. Long-range radar out of Melbourne, Florida on Tuesday afternoon showed that bands of heavy rain from Arthur were affecting the Northwest Bahamas, and the top sustained winds observed in the Bahamas as of 2 pm EDT Tuesday were 36 mph, gusting to 41 mph at Settlement Point in the Northwest Bahama Islands at 8 am EDT. Top winds Tuesday morning and early afternoon at the buoy 23 miles east of Cape Canaveral, Florida were 22 mph, gusting to 26 mph, with a significant wave height of 4.3'. Satellite loops showed heavy thunderstorms were limited to the south side of Arthur's center of circulation early Tuesday afternoon, and were slowly increasing in intensity and areal coverage, with new spiral bands forming. The clockwise circulation of an upper level high pressure over Florida was bringing northerly winds over Arthur at high altitude, and these winds were creating moderate wind shear of 10 - 15 knots. Water vapor satellite loops showed dry air to the north of Arthur, and the northerly winds were driving this dry air into the storm, interfering with development. However, Arthur has moistened its environment considerably since Monday, and the storm is beginning to wall off the dry air to the north.
Figure 2. Water vapor satellite image of Arthur taken at 1:15 pm EDT Monday July 1, 2014. A large area of dry air (black colors) was to the north of the system, and was interfering with development. Image credit: NOAA/NESDIS.
Forecast for Arthur
The models are in good agreement that a trough of low pressure passing to the north of Arthur will turn the storm northwards by Tuesday night and northeastwards by Wednesday. On this track, the Outer Banks of North Carolina will be the land areas at greatest risk of a direct hit, though the cone of uncertainty encompasses the entire coast of North Carolina. Given the degree of model unanimity, the cone of uncertainty is likely thinner than presented. The latest 12Z Tuesday runs of our top two track models, the GFS and European (ECMWF) models, both show Arthur passing very close to Cape Hatteras, North Carolina between 8 am - 2 pm EDT Friday, July 4.
Which Intensity Model Should You Trust?
According to the 2013 National Hurricane Center Forecast Verification Report, issued in April 2014, the official NHC intensity forecasts in 2013 for Atlantic storms were their best ever. However, this success is likely because of the lack of any rapidly intensifying major hurricanes in 2013, which are typically the source of the largest forecast errors. NHC 72-hour intensity forecasts errors in 2013 averaged about 10 mph, and were biased too high at most forecast times. As shown in Figure 3, over the past three years, the official NHC intensity forecast has generally outperformed their four main intensity models. These four models were the dynamical HWRF and GFDL models, which subdivide the atmosphere into a 3-D grid around the storm and solve the atmospheric equations of fluid flow at each point on the grid, and the statistics-based LGEM and DSHP models (DSHP is the SHIPS model with inland decay of a storm factored in.) The top-performing global dynamical models for hurricane track, the GFS and European (ECMWF) models, are typically not considered by NHC forecasters when making intensity forecasts. The GFS model has done a decent job at making intensity forecasts over the past three years, but the European model has made poor intensity forecasts. In 2013 and for the period 2011 - 2013, the HWRF model was the best-performing intensity model for forecasts of 48 hours or less. The LGEM statistical model was the best one at longer-term intensity forecasts of 3 - 5 days.
Figure 3. Skill of computer model intensity forecasts of Atlantic named storms in 2011 - 2013, compared to a "no skill" model called "Decay-SHIFOR5" that uses just climatology and persistence to make a hurricane intensity forecast (persistence means that a storm will tend to maintain its current behavior.) NHC=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ECMWF=European Center for Medium Range Weather Forecasting model; LGEM=Logistic Growth Equation Model; DSHP=Statistical Hurricane Intensity Prediction Scheme with inland Decay. Image credit: 2013 National Hurricane Center Forecast Verification Report.
The official NHC 72-hour intensity forecast issued at 11 am EDT Tuesday makes Arthur a Category 1 hurricane with 80 mph winds on Friday morning at 8 am EDT, when the storm is expected to make its closest pass to the North Carolina coast. The two best models we have for making 72-hour intensity forecasts, the LGEM and DSHP models, predicted in their 8 am EDT runs that at 8 am EDT Friday Arthur would be a tropical storm with 70 mph winds or a Category 1 hurricane with 95 mph winds, respectively. NHC typically relies on consensus models that average together several of the intensity models, so it is not a surprise that the official forecast of an 80 mph storm at 72 hours is close to the result gotten by averaging together the two top intensity models for making 72-hour forecasts, the LGEM and DSHP models.
Sources of Model Data
You can view 7-day ECMWF and 16-day GFS forecasts on wunderground's wundermap with the model layer turned on.
FSU's experimental hurricane forecast page (CMC, ECMWF, GFDL, GFS, HWRF, and NAVGEM models)
NOAA's HFIP model comparison page (GFS, ECMWF, FIM, FIM9, UKMET, and CMC models.)
Experimental HFIP models
Links
Webcam on the buoy 23 miles east of Cape Canaveral, FL
I'll have a new post on Wednesday morning at the latest.
Jeff Masters
Figure 1. Melbourne, Florida radar image of Tropical Storm Arthur at 2:20 pm EDT July 1, 2014.
Arthur was drifting northwest at 5 mph towards the east coast of Central Florida early Tuesday afternoon. Long-range radar out of Melbourne, Florida on Tuesday afternoon showed that bands of heavy rain from Arthur were affecting the Northwest Bahamas, and the top sustained winds observed in the Bahamas as of 2 pm EDT Tuesday were 36 mph, gusting to 41 mph at Settlement Point in the Northwest Bahama Islands at 8 am EDT. Top winds Tuesday morning and early afternoon at the buoy 23 miles east of Cape Canaveral, Florida were 22 mph, gusting to 26 mph, with a significant wave height of 4.3'. Satellite loops showed heavy thunderstorms were limited to the south side of Arthur's center of circulation early Tuesday afternoon, and were slowly increasing in intensity and areal coverage, with new spiral bands forming. The clockwise circulation of an upper level high pressure over Florida was bringing northerly winds over Arthur at high altitude, and these winds were creating moderate wind shear of 10 - 15 knots. Water vapor satellite loops showed dry air to the north of Arthur, and the northerly winds were driving this dry air into the storm, interfering with development. However, Arthur has moistened its environment considerably since Monday, and the storm is beginning to wall off the dry air to the north.
Figure 2. Water vapor satellite image of Arthur taken at 1:15 pm EDT Monday July 1, 2014. A large area of dry air (black colors) was to the north of the system, and was interfering with development. Image credit: NOAA/NESDIS.
Forecast for Arthur
The models are in good agreement that a trough of low pressure passing to the north of Arthur will turn the storm northwards by Tuesday night and northeastwards by Wednesday. On this track, the Outer Banks of North Carolina will be the land areas at greatest risk of a direct hit, though the cone of uncertainty encompasses the entire coast of North Carolina. Given the degree of model unanimity, the cone of uncertainty is likely thinner than presented. The latest 12Z Tuesday runs of our top two track models, the GFS and European (ECMWF) models, both show Arthur passing very close to Cape Hatteras, North Carolina between 8 am - 2 pm EDT Friday, July 4.
Which Intensity Model Should You Trust?
According to the 2013 National Hurricane Center Forecast Verification Report, issued in April 2014, the official NHC intensity forecasts in 2013 for Atlantic storms were their best ever. However, this success is likely because of the lack of any rapidly intensifying major hurricanes in 2013, which are typically the source of the largest forecast errors. NHC 72-hour intensity forecasts errors in 2013 averaged about 10 mph, and were biased too high at most forecast times. As shown in Figure 3, over the past three years, the official NHC intensity forecast has generally outperformed their four main intensity models. These four models were the dynamical HWRF and GFDL models, which subdivide the atmosphere into a 3-D grid around the storm and solve the atmospheric equations of fluid flow at each point on the grid, and the statistics-based LGEM and DSHP models (DSHP is the SHIPS model with inland decay of a storm factored in.) The top-performing global dynamical models for hurricane track, the GFS and European (ECMWF) models, are typically not considered by NHC forecasters when making intensity forecasts. The GFS model has done a decent job at making intensity forecasts over the past three years, but the European model has made poor intensity forecasts. In 2013 and for the period 2011 - 2013, the HWRF model was the best-performing intensity model for forecasts of 48 hours or less. The LGEM statistical model was the best one at longer-term intensity forecasts of 3 - 5 days.
Figure 3. Skill of computer model intensity forecasts of Atlantic named storms in 2011 - 2013, compared to a "no skill" model called "Decay-SHIFOR5" that uses just climatology and persistence to make a hurricane intensity forecast (persistence means that a storm will tend to maintain its current behavior.) NHC=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ECMWF=European Center for Medium Range Weather Forecasting model; LGEM=Logistic Growth Equation Model; DSHP=Statistical Hurricane Intensity Prediction Scheme with inland Decay. Image credit: 2013 National Hurricane Center Forecast Verification Report.
The official NHC 72-hour intensity forecast issued at 11 am EDT Tuesday makes Arthur a Category 1 hurricane with 80 mph winds on Friday morning at 8 am EDT, when the storm is expected to make its closest pass to the North Carolina coast. The two best models we have for making 72-hour intensity forecasts, the LGEM and DSHP models, predicted in their 8 am EDT runs that at 8 am EDT Friday Arthur would be a tropical storm with 70 mph winds or a Category 1 hurricane with 95 mph winds, respectively. NHC typically relies on consensus models that average together several of the intensity models, so it is not a surprise that the official forecast of an 80 mph storm at 72 hours is close to the result gotten by averaging together the two top intensity models for making 72-hour forecasts, the LGEM and DSHP models.
Sources of Model Data
You can view 7-day ECMWF and 16-day GFS forecasts on wunderground's wundermap with the model layer turned on.
FSU's experimental hurricane forecast page (CMC, ECMWF, GFDL, GFS, HWRF, and NAVGEM models)
NOAA's HFIP model comparison page (GFS, ECMWF, FIM, FIM9, UKMET, and CMC models.)
Experimental HFIP models
Links
Webcam on the buoy 23 miles east of Cape Canaveral, FL
I'll have a new post on Wednesday morning at the latest.
Jeff Masters
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.