Strong vertical wind shear weakened Tropical Storm Hilda again on Wednesday and the Tropical Storm Watch for the Big Island of Hawaii was discontinued by the Central Pacific Hurricane Center.  At 11:00 p.m. EDT on Wednesday the center of Tropical Storm Hilda was located at latitude 17.2°N and longitude 152.4°W which put it about 245 miles (395 km) southeast of Hilo, Hawaii.  Hilda was moving toward the west at 4 m.p.h. (6 km/h).  The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1003 mb.

An upper level low north of the Hawaiian Islands is generating very strong southwesterly winds that are blowing right over the top of Tropical Storm Hilda.  Those winds are blowing thunderstorms away from the core of the circulation.  The western half and inner core of Hilda consists of bands of low clouds rotating counterclockwise around the exposed center of circulation.  Thunderstorms continue to form in some of the bands northeast of the center of circulation, but the strong upper level winds quickly blow the tops off of those storms.  Since the upper level low isn’t moving, strong vertical wind shear is expected to continue and Hilda should slowly spin down.  It is likely to weaken to tropical depression status on Thursday.

As the circulation gets shallower, Hilda will be steered by the winds in the lower atmosphere.  Those winds are blowing from the east and the will push the tropical storm or its remnants toward the west.  On its anticipated track Hilda would pass south of Hawaii as a tropical depression during the next several days.

Although strong vertical wind shear is weakening Hurricane Hilda, its proximity to Hawaii has prompted the issuance of a Tropical Storm Watch.  At 11:00a.m. EDT on Tuesday the center of Hurricane Hilda was located at latitude 17.5°N and longitude 151.3°W which put it about 290 miles (470 km) east-southeast of Hilo, Hawaii.  Hilda was moving toward the northwest at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 85 m.p.h. (140 km/h) and there were wind gusts to 100 m.p.h. (160 km/).  The minimum surface pressure was 985 mb.  The Central Pacific Hurricane Center in Honolulu has issued a Tropical Storm Watch for Hawaii County.

An upper level low north of the Hawaiian Islands is creating strong southwesterly winds that are blowing straight into Hurricane Hilda.  Although the circulation of Hilda appeared strong enough on Monday to deflect the upper level winds around it, those winds are tilting the vortex significantly to the east with height today.  The middle and upper portions of the hurricane’s circulation appear to still be attached to the lower level circulation, but they are tilted toward the east.  If the upper level winds get much stronger they could shear off the upper portion of the circulation from the lower level circulation.  The effect of the strong vertical wind shear is weakening Hurricane Hilda and that weakening is expected to continue during the next several days.  Hilda could weaken to tropical storm status within 24 hours and it could be a tropical depression in 48 to 72 hours.

A subtropical ridge northeast of Hilda and the upper low north of Hawaii are both affecting the movement of the hurricane.  The subtropical ridge is trying to push Hurricane Hilda toward the west, while the upper low is trying to push it to the northeast..  The combined effects of the ridge and the low are causing Hilda to move slowly northwestward.  If the strong upper level winds do shear the top off of Hilda, then winds in the lower atmosphere will steer the surface circulation toward the west.  Since Hilda could still be a minimal tropical storm when it approaches the Big Island of Hawaii, a Tropical Storm Watch has been issued.

Hurricane Hilda weakened on Sunday as it move closer to Hawaii.  At 11:00 p.m. EDT on Sunday the center of Hurricane Hilda was located at latitude 15.3°N and longitude 148.2°W which put it about 545 miles (880 km) east-southeast of Hilo, Hawaii.  Hilda was moving toward the 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 wind gusts to 115 m.p.h. (185 km/h).  The minimum surface pressure was 970 mb.

An upper level low north of Hawaii is generating southwesterly winds which are causing vertical wind shear near Hurricane Hilda.  The wind shear is inhibiting the upper level divergence on the western side of the circulation and the hurricane is weakening.  As Hilda moves farther northwest the wind shear will increase and the hurricane will weaken more quickly.  Hilda could weaken below hurricane strength on Tuesday.

A subtropical ridge was steering Hilda toward the west-northwest.  The hurricane is near the western end of the ridge and the upper level southwesterly winds are pushing Hilda more toward the northwest.  The northwesterly motion is likely to continue while Hilda produces tall thunderstorms and the circulation extends through the troposphere.  If Hilda weakens to the point where it no longer produces tall thunderstorms, then it will be steered more toward the west by the winds in the lower levels of the atmosphere.

Hurricane Hilda continued to move steadily westward on Saturday and when it moved west of longitude 140°W, Hilda officially crossed into the Central Pacific.  At that time the responsibility for issuing advisories on Hilda shifted from the National Hurricane Center in Miami, Florida to the Central Pacific Hurricane Center in Honolulu, Hawaii.  At 5:00 p.m. EDT on Saturday the center of Hurricane Hilda was located at latitude 13.7°N and longitude 143.5°W which put it about 870 miles (1400 km) east-southeast of Hilo, Hawaii.  Hilda was moving toward the west at 15 m.p.h. (24 km/h).  The maximum sustained wind speed was 140 m.p.h. (220 km/h) which made Hilda a Category 4 hurricane on the Saffir-Simpson Scale and qualified it as a Major Hurricane.  There were wind gusts to 165 m.p.h. (270 km/h) and the minimum surface pressure was 946 mb.

Hilda is moving over water where the Sea Surface Temperature (SST) is near 28°C.  The upper level winds are light and there is not much vertical wind shear.  Although Hilda is a small hurricane, it is generating upper level divergence in all directions for the center.  Hilda will remain in a favorable environment for another day or so and it could intensify further during that time.  When it moves farther west, it will move into an area where upper level winds are blowing from the west.  Those winds will cause vertical wind shear and weaken Hilda.  Because of the small size of the circulation Hilda could weaken faster than a larger hurricane would.

A subtropical ridge is steering Hilda toward the west and that general steering pattern is expected to continue for another 24 to 48 hours.  When Hilda encounters the upper level westerly winds, those winds will deflect it onto a more northwesterly track.  When Hilda weakens and the thunderstorms do not extend as high, then winds in the lower levels will push it more toward the west again.

After not changing much during the overnight hours, Hilda intensified rapidly into a hurricane on Friday afternoon.  At 5:00 p.m. EDT on Friday the center of Hurricane Hilda was located at latitude 12.8°N and longitude 138.3°W which put it about 1215 miles (1950 km) east-southeast of Hilo, Hawaii.  Hilda was moving toward the wet at 15 m.p.h. (24 km/h).  The maximum sustained wind speed was 80 m.p.h. (130 km/h) and there were wind gusts to 90 m.p.h. (145 km/h).  The minimum surface pressure was 984 mb.

Hurricane has a very small circulation.  Hurricane force winds extend out less than 15 miles (25 km) from the center of circulation and tropical storm force winds extend out about 70 miles (110 km) from the center.  In spite of its small size, the circulation is well organized.  A clear eye is visible on satellite images and there is a well formed eyewall around the eye.  There are not  many spiral bands because of the small size of the circulation.

Hilda is moving over water where the Sea Surface Temperature (SST) is near 29°C.  It is in an area where the upper level winds are light and there is not much vertical wind shear.  Further intensification is likely and rapid intensification is possible during the next 24 hours.  Eventually, when Hilda moves farther west it will move over cooler SSTs and encounter more vertical wind shear.  Given the small size of Hilda, it could also weaken very quickly when it moves into a more hostile environment.

A subtropical ridge is steering Hilda toward the west and that general motion is expected to continue for several more days.  When Hilda gets closer to Hawaii, southwesterly winds in the upper levels are expected to turn it more toward the northwest.

Small Tropical Storm Hilda intensified rapidly on Thursday.  At 5:00 p.m. EDT on Thursday the center of Tropical Storm Hilda was located at latitude 12.7°N and longitude 133.4°W which put it about 1590 miles (2550 km) east-southeast of Hilo, Hawaii.  Hilda was moving toward the west at 13 m.p.h. (21 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 70 m.p.h. (115 km).  The minimum surface pressure was 1001 mb.

Hilda is a very small tropical storm.  The area of tropical storm force winds only extends outward about 40 miles (65 km) from the center.  Even though the circulation is small, it is well organized and there are hints of a tiny eye trying to form in the center of Hilda.  Hilda is moving over water where the Sea Surface Temperature is near 28.5°C.  The upper level winds are very light and there is little vertical wind shear.  The thunderstorms in the core of Hilda are generating upper level divergence which is pumping out mass.  Hilda is likely to strengthen more during the next day or two because of the favorable environment.  However, small tropical cyclones can weaken just as fast as they intensify.  When Hilda moves farther west it will encounter some of the same wind shear that is affecting Tropical Storm Guillermo.  As a result, Hilda is likely to weaken when it gets closer to Hawaii.

A subtropical ridge is steering HIlda toward the west and a generally west-northwestward track is expected during the next several days.  As Hilda moves farther west it could turn more toward the northwest, much like Guillermo did when it neared Hawaii.

Tropical Storm Guillermo weakened further and the Tropical Storm Watch for portions of Hawaii was discontinued.  At 5:00 p.m. EDT on Wednesday the center of Tropical Storm Guillermo was located at latitude 21.4°N and longitude 151.7°W which put it about 250 miles (400 km) east-northeast of Hilo, Hawaii and about 400 miles (640 km) east of Honolulu.  Guillermo was moving toward the west-northwest at 9 m.p.h. (13 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 70 m.p.h. (110 km/h).  The minimum surface pressure was 1001 mb.  The Tropical Storm Watch for Hawaii County and Maui County including the islands of Maui, Molokai, Lanai and Kahoolawe has been discontinued.

Strong southwesterly winds in the upper levels are creating significant vertical wind shear over Tropical Storm Guillermo.  The wind shear has distorted the circulation and the only thunderstorms are in outer rainbands well northeast of the center of the tropical storm.  The wind shear is expected to continue and Guillermo will continue to weaken during the next several days.

Since Guillermo is no longer producing tall thunderstorms near the center of circulation, it is being steered by winds in the lower troposphere.  Those winds are expected to carry the weakening tropical storm north of the Hawaiian Islands.  Since Guillermo or its remnants is expected to move north of Hawaii, the Tropical Storm Watch was discontinued.

Guillermo weakened below hurricane intensity on Monday and the Central Pacific Hurricane Center issued a Tropical Storm Watch for some of the Hawaiian Islands.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Guillermo was located at latitude 18.1°N and longitude 147.4°W which put it about 515 miles (830 km) east of Hilo, Hawaii.  Guillermo was moving toward the northwest at 10 m.p.h. (16 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 991 mb.  A Tropical Storm Watch has been issued for Hawaii County and Maui County including the islands of Maui, Molokai, Lanai and Kahoolawe.

An upper level trough north of Guillermo is causing strong southwesterly winds over the top of the circulation.  Significant vertical wind shear is affecting the tropical storm and is weakening it.  The southwesterly winds are also inhibiting upper level divergence on the western side of the storm.  Most of the thunderstorms are occurring in rainbands east of the center of circulation.  The vertical wind shear is expected to increase later this week and Guillermo will be moving over cooler Sea Surface Temperatures.  The weakening trend is expected to continue.

A subtropical ridge is pushing Guillermo toward the west-northwest and the southwesterly winds in the upper levels are turning it more toward the northwest.  That general motion is expected to continue for the next day or two.  On its anticipated track Guillermo is expected to pass north of Hawaii.  However, if it weakens a little faster than expected and the circulation is not as tall, then it could be steered by winds lower in the atmosphere.  Those winds could steer it more toward the west which would bring it nearer to Hawaii.  That possibility prompted the issuance of a Tropical Storm Watch for some of the Hawaiian Islands.

A reconnaissance aircraft confirmed a weakening trend in Hurricane Guillermo that had been suggested by satellite imagery.  At 11:00 p.m. EDT on Sunday the center of Hurricane Guillermo was located at latitude 15.5°N and longitude 145.0°W which put it about 725 miles (1165 km) east-southeast of Hilo, Hawaii.  Guillermo was moving toward the west-northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 85 m.p.h. (145 km/h) and there were wind gusts to 100 m.p.h. (160 km/h).  The minimum surface pressure was 981 mb.

An upper level trough north of Guillermo is producing southwesterly winds over the top of the circulation.  Those winds are causing vertical wind shear and are inhibiting the upper level divergence on the western side of the circulation.  In addition Guillermo is gradually moving over cooler Sea Surface Temperatures (SSTs).  A combination of wind shear and cooler SSTs will continue to weaken the hurricane.

A subtropical ridge is steering Guillermo toward the west-northwest and that general motion is expected to continue.  On its anticipated track Guillermo would approach the Big Island of Hawaii in about 72 hours.  It will likely be a tropical storm at that time.

Hurricane Guillermo moved west of longitude 140°W on Saturday, which means it officially crossed into the Central Pacific Ocean.  The responsibility for issuing advisories on Guillermo shifted from the National Hurricane Center in Miami, Florida to the Central Pacific Hurricane Center in Honolulu, Hawaii when Guillermo moved west of 140°W.  At 11:00 p.m. EDT on Saturday the center of Hurricane Guillermo was located at latitude 14.0°N and longitude 141.0°W which put it about 1010 miles (1625 km) east-southeast Hilo, Hawaii.  Guillermo was moving toward the west-northwest at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 105 m.p.h. (165 km/h) and there were gusts to 125 m.p.h. (205 km/h).  The minimum surface pressure was 970 mb.

The structure of Hurricane Guillermo appeared to become less organized on Saturday.  There are fewer thunderstorms and the spiral bands appear to be weaker.  Guillermo is nearing water where the Sea Surface Temperature is near 27°C, and it will gradually move over even cooler water.  Westerly winds in the upper levels appear to be generating some vertical wind shear over the northern portion of the circulation.  Guillermo is still producing upper level divergence near the center of circulation, but the divergence appears to less over the western half of the hurricane.  The circulation may be pulling some drier more stable air into the western part of the circulation.  Guillermo will be moving into a less favorable environment and weakening is likely.

A subtropical ridge is steering Guillermo toward the west-northwest and that general motion is expected to continue during the next several days.  On its anticipated track Guillermo could approach the Big Island of Hawaii in about four days.  It is likely to be a tropical storm at that time.