The center of Tropical Storm Blanca is passing west of Cabo San Lucas and it is weakening as it moves over cooler Sea Surface Temperatures.  At 11:00 p.m. EDT on Sunday the center of Tropical Storm Blanca was located at latitude 22.3°N and longitude 111.2°W which put it about 95 miles (150 km) west-southwest of Cabo San Lucas, Mexico and about 185 miles (300 km) south-southeast of Cabo San Lazaro.  Blanca was moving toward the north-northwest at 10 mp.h. (17 km/h).  The maximum sustained wind speed was 65 m.p.h. (100 km/h) and there were gusts to 75 m.p.h. (120 km/h).  The minimum surface pressure was 988 mb.

Blanca will continue to weaken as it moves toward the southwestern part of Baja California.  However, it could still produce locally heavy rainfall and a potential for flooding exists.  It is possible that the upper portion of the circulation will detach from the lower portion of the circulation.  If that happens, then the upper part of the circulation will be pulled northward toward the southwestern U.S., while the lower portions moves slowly toward Baja California.  The upper portion could bring some additional moisture to the southwestern U.S. during the early part of next week.

The government of Mexico issued a Tropical Storm Watch for the southern portion of Baja California.  At 5:00 p.m. EDT on Friday the center of Hurricane Blanca was located at latitude 15.3°N and longitude 107.5°W which put it about 550 miles (880 km) south-southeast of Cabo San Lucas, Mexico and about 330 miles (535 km) southwest of Manzanillo, Mexico.  Blanca was moving toward the northwest at 9 m.p.h. (15 km/h).  The maximum sustained wind speed was 90 m.p.h. (150 km/h) and there were gusts to 115 m.p.h. (185 km/h).  The minimum surface pressure was 976 mb.  The Hurricane Intensity Index (HII) was 13.9, the Hurricane Size Index (HSI) was 17.7 and the Hurricane Wind Intensity Size Index (HWISI) was 31.6.  A reconnaissance aircraft investigated Blanca on Friday afternoon and so there is high confidence in the data about its strength and position.  The government of Mexico issued a Tropical Storm Watch for the portion of the coast from La Paz to Santa Fe including Cabo San Lucas.

Satellite imagery and data from the reconnaissance plane indicate that the structure of Blanca is more well organized.  A ring of thunderstorms surrounds a circular eye with a diameter of about 60 miles (65 km).  However, the better organization has not produced an increase in the wind speed as yet.  However, the aircraft did find winds to 105 m.p.h. (170 km/h) in the northwestern portion of the eyewall.  If downdrafts in thunderstorms in the eyewall bring those winds to the surface, then there could be some intensification.

Blanca is moving over water where the Sea Surface Temperature (SST) is about 29°C and there is sufficient energy in the upper ocean to support some increase in wind speed.  As the hurricane moves farther north it will move over cooler SSTs and when it gets near latitude 20°N in about 36 hours, it will start to move over much cooler water.  So, there is a potential for some intensification on Saturday, but Blanca will start to weaken as it approaches the southern tip of Baja California on Sunday.

A ridge northeast of Blanca and a trough west of Baja California are combining to steer Blanca toward the northwest.  As it moves farther north, it is likely to turn a little more toward the north.  Blanca is likely to approach the southern tip of Baja California on Sunday which is the reason for the Tropical Storm Watch.

A combination of upwelling of cooler water and a possible eyewall replacement weakened Hurricane Blanca on Thursday.  At 11:00 p.m. EDT on Thursday the center of Hurricane Blanca was located at latitude 13.7°N and longitude 105.7°W which put it about 690 (1115 km) miles south-southeast of Cabo San Lucas, Mexico and about 380 (610 km) miles south-southwest of Manzanillo, Mexico.  Blanca was moving toward the north-northwest at 8 m.p.h. (13 km/h).  The maximum sustained wind speed was 100 m.p.h. (160 km/h) and there were gusts to 120 m.p.h. (195 km/h).  The minimum surface pressure was 975 mb.  The Hurricane Intensity Index (HII) was 16.5, the Hurricane Size Index (HSI) was 16.1 and the Hurricane Wind Intensity Size Index was 32.6.

The structure of Hurricane Blanca underwent a significant transformation on Thursday.  After being nearly stationary for approximately three days, the winds of Blanca stirred up the water in the upper ocean and mixed cooler water to the surface.  The cooler water reduced the energy available to drive the circulation of Blanca.  In addition, Blanca may have undergone an eyewall replacement cycle on Wednesday night and Thursday morning.  When a rainband wraps around an existing eyewall, it produces dissipation of the original eyewall.  The result is often a larger hurricane with a bigger eye, which is the appearance of Blanca on satellite imagery.  The combination of cooler water and a possible eyewall replacement changed the structure of the hurricane from a small intense circulation to a weaker and larger circulation.  However, Blanca is still a 100 m.p.h. (160 km/h).  Blanca is moving again and it will start to move away from the region of cooler water.  The upper level winds are relatively light and the environment is favorable for some intensification during the next day or two.  Since Blanca is larger and less well organized, it will respond more slowly to the favorable environment.  As the hurricane nears latitude 20°N, it will start to encounter cooler water and weaken will be likely.

A ridge to the northeast of Blanca and a trough west of Baja California are combining to steer it toward the north-northwest.  That steering pattern is likely to continue for the next several days.  On its anticipated track Blanca would approach the southern portion of Baja California in about three days.  A reconnaissance aircraft is scheduled to investigate the hurricane on Friday.

Hurricane Blanca intensified very rapidly on Wednesday into a Category 4 hurricane on the Saffir-Simpson Scale.  At 5:00 p.m. EDT on Wednesday the center of Hurricane Blanca was located at latitude 12.3°N and longitude 104.7°W which put it about 810 miles (1300 km) south-southeast of Cabo San Lucas, Mexico and about 465 miles (745 km) south of Manzanillo, Mexico.  Blanca drifted slowly southward during the day, but it is essentially stationary.  The maximum sustained wind speed was 140 m.p.h. (220 km/h) and there were gusts to 165 m.p.h. (270 km/h).  The minimum surface pressure was 945 mb.  Blanca had a Hurricane Intensity Index (HII) of 28.3, a Hurricane Size Index (HSI) of 8.6 and a Hurricane Wind Intensity Size Index (HWISI) of 36.9.  Blanca is almost as strong and slightly larger than Hurricane Charley was when it hit southwest Florida in 2004.  At that time Charley had a HII of 29.9, a HSI of 7.7 and a HWISI of 37.6.

Blanca intensified from 75 m.p.h. (120 km/h) to 140 m.p.h. (220 km/h) during the past 24 hours which is a very rapid rate of intensification.  However, it is nowhere near the record for rapid intensification over the Eastern North Pacific.  In 1997 the maximum sustained wind speed in Hurricane Linda increased from 75 m.p.h. (120 km/h) to 175 m.p.h. (290 km/h) in a 24 hour period.  Blanca’s relatively small size means that there is less mass to accelerate in order to increase the wind speed.  It has efficiently extracted energy from the upper ocean and the wind speed in the small inner core increased rapidly today.  It is over water where the Sea Surface Temperature (SST) is near 30°C.  If Blanca were to remain stationary long enough, its winds would mix some cooler water to the surface.  Some subsidence and drier air is approaching the northwestern periphery of Blanca, but that air appears to have not had an effect on the core of the hurricane today.  Upper level winds are light right over the center of Blanca and there is little vertical wind shear over its inner core.  As long as Blanca can extract sufficient energy from the upper ocean, the potential for further intensification exists.  Much of the guidance suggests that Blanca could intensify for another 12 to 24 hours.  However, eyewall replacement cycles can occur in strong hurricanes and cause fluctuations in intensity.  Changes in intensity in small hurricanes can occur rapidly as we have seen today.  Eventually, when Blanca starts to move northward, it will encounter cooler SSTs and more wind shear which will cause it to weaken.

A ridge over Baja California is preventing Blanca from moving toward the north.  The hurricane is in an area where the steering currents are weak, which resulted in a slow southward drift today.  The ridge is forecast to shift to the east during the next several days and it will start to steer Blanca toward the northwest.  On its anticipated track Blanca could approach the southern tip of Baja California late in the weekend.  Because of a possible threat to Baja California a reconnaissance plane has been scheduled to investigate Blanca on Friday.  Blanca will weaken before it approaches that area, but it may still posed a significant risk.

The structure of Tropical Storm Blanca continued to improve on Tuesday and it intensified into a hurricane.  At 5:00 p.m. EDT on Tuesday the center of Hurricane Blanca was located at latitude 13.1°N and longitude 104.6°W which put it about 375 miles (605 km) south-southwest of Zihuatanejo, Mexico and about 410 miles (655 km) south of Manzanillo, Mexico.  Blanca was stationary.  The maximum sustained wind speed was 75 m.p.h. (120 km/h) and there were gusts to 90 m.p.h. (145 km/h).  The minimum surface pressure was 960 mb.  The Hurricane Intensity Index (HII) was 10.4, the Hurricane Size Index (HSI) was 5.8 and the Hurricane Wind Intensity Size Index (HWISI) was 16.2.  Since Blanca and Andres both became hurricanes this year, June 2 becomes the earliest date on record by which time two hurricanes formed over the Eastern North Pacific Ocean.  However, it should be noted that the most reliable records only go back to 1971.

Hurricane Blanca is sitting over water where the Sea Surface Temperature (SST) is near 30°C.  It is efficiently extracting energy from the upper ocean and that energy is fueling its intensification.  Spiral bands of thunderstorms are well organized and the strongest storms are near the center of circulation.  The thermodynamic environment is very favorable for further intensification and rapid intensification is possible.  There are a several of factors that could slow the rate of intensification at times.  Upper level outflow from Hurricane Andres, which is about a thousand miles west of Blanca, appears to be generating some vertical wind shear on the northern and western sides of Blanca.  Thunderstorms in Blanca are producing upper level divergence, but some of the divergence may be reduced in the northwestern quadrant of the hurricane.  Andres is slowly spinning down and the wind shear is likely to lessen during the next several days.  A second potential inhibiting factor is the fact that Blanca is not moving much.  The warm water underneath Blanca is relatively deep, but if it sits in one location long enough, its winds will eventually begin  to mix some cooler water to the surface.  For the time being Blanca has sufficient energy in the upper ocean to support further intensification.  Once Blanca starts moving, then it will be over warm SSTs until it approaches latitude 20°N.  Finally, if Blanca gets as strong as some guidance suggest it could, then eyewall replacement cycles also could produce fluctuations in intensity.  As I mentioned above, Blanca is likely to intensify further and it could intensify rapidly at times.

A ridge in the middle levels centered near Baja California is blocking the northward progress of Blanca and the hurricane is in an area where the steering currents are weak.  The ridge is forecast to move slowly eastward toward a position over northern Mexico and Texas.  As the ridge moves eastward it will initially steer Blanca toward the northwest.  In several days the ridge will cause Blanca to turn more toward the north.  On its anticipated track, Blanca could approach the southern tip of Baja California in a few days.  It is too early to know how strong Blanca might be at that time or how much of a risk it might pose to that area.