Category Archives: Eastern and Central Pacific

TCs between Mexico and Hawaii

Possible Hurricane Formation Southwest of Mexico

Guidance from recent runs of the Global Forecasting System (GFS) model indicate that a tropical cyclone could form southwest of Mexico during the next few days.  A tropical wave will move into the Eastern North Pacific Ocean south of Mexico.  The wave will move over water where the Sea Surface Temperature is between 29°C and 30°C.  Vertical wind shear caused by an upper low near the west coast of Mexico will initially inhibit development of the wave.  The GFS model is forecasting that the tropical wave will move into a region where the upper level winds are light and there will not be much vertical wind shear.  If that scenario occurs then a tropical cyclone could develop during the weekend near latitude 10°N and longitude 105°W.

If a tropical cyclone forms, a ridge of high pressure to the north of the cyclone would likely steer it in a generally west-northwesterly direction which would move it away from Mexico.  The environment would be favorable for intensification.  Rapid intensification may be possible and the system could become a major hurricane next week.

The development of this tropical wave could also be a signal that the Eastern North Pacific is getting active.  Although it has been a bit of a slow start to the hurricane season in that basin, the formation of a hurricane during the next week or so would be consistent with the long term climatology for that region.

Tropical Depression One-E Forms South of Mexico

A low level center of circulation developed near the southern edge of thunderstorms near the coast of Mexico on Monday and the National Hurricane Center designated the system as Tropical Depression One-E.  At 5:00 p.m. EDT on Monday the center of Tropical Depression One-E (TD1E) was located at latitude 14.2°N and longitude 97.0°W which put it about 185 miles (295 km) southwest of Salina Cruz, Mexico.  TD1E was moving toward the northeast at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h).  The minimum surface pressure was 1006 mb.  The government of Mexico issued a Tropical Storm Watch for the portion of the coast that extends from Puerto Escondido to Boca de Pijijiapan, Mexico.

Tropical Depression One-E is not a particularly well organized circulation.  A large upper level trough that extends from west of Mexico over the Gulf of Mexico is producing strong southwesterly winds that are blowing over the top of the depression.  The is the same upper level trough that is also causing the vertical wind shear over Tropical Storm Colin in the Gulf of Mexico.  As a result of the significant vertical wind shear, most of the strong thunderstorms are occurring north and east of the center of circulation.  The wind shear may be causing the tropical depression to be tilted toward the northeast with height.  The low level center of circulation is near the southern edge of the stronger thunderstorms.  The thunderstorms generating some upper level divergence to the east of the depression.

The environment around TD1E is only marginal for further intensification.  The depression is over water where the Sea Surface Temperature is near 31°C.  However, the vertical wind shear is forecast to continue and TD1E does not have much time to intensify before the center moves over land.  Little change in intensity is expected and only slight strengthening may occur before landfall.  TD1E is likely to weaken rapidly after landfall.  However, it will be interesting to see if the middle and upper part of the circulation make it to the Gulf of Mexico.

The upper level trough is expected to continue to steer TD1E toward the northeast.  On its anticipated track, the low level center is forecast to reach the coast of Mexico in 12 to 24 hours.  In cases like this when there is strong vertical wind shear near the coast of Mexico, the upper portion of the circulation sometimes gets separated from the lower level circulation.  In that case, the upper half of the circulation would continue northeast while the lower circulation either lingers near the coast or moves slowly inland and dissipates.

The primary risks from Tropical Depression One-E are locally heavy rain and flash flooding.

Hurricane Season Officially Starts in East Pacific

May 15 is the official start of the hurricane season for the Eastern North Pacific Ocean.  The Sea Surface Temperatures (SSTs) should be cooler than they were in 2015, since the El Niño is dissipating.  That could produce a hurricane season in which the activity is nearer to normal.  In a normal year we might expect 13-17 named storms, 7-10 hurricanes and 3-4 major hurricanes.  Of course, other environmental factors including the location and strength of atmospheric systems will also determine how active the season is.

The National Hurricane Center (NHC) is currently monitoring Invest 90E which is about 950 miles (1530 km) south-southwest of the southern tip of Baja California.  There is a well defined low level circulation in Invest 90E, but there are no deep thunderstorms.  NHC is giving Invest 90E a 20 percent chance of developing into a tropical cyclone during the next five days.

Hurricane Pali Weakens As It Nears the Equator

Hurricane Pali weakened quickly on Wednesday as it moved closer to the Equator and it is now classified as a tropical storm.  At 10:00 p.m. EST on Wednesday the center of Tropical Storm Pali was located at latitude 2.7°N and longitude 172.2°W which put it about 330 miles (530 km) east-northeast of Howland Island.  Pali was moving toward the south-southwest at 9 m.p.h. (17 km/h).  The maximum sustained wind speed was 65 m.p.h. (105 km/h) and there were wind gusts to 80 m.p.h. (130 km/h).  The minimum surface pressure was 993 mb.

As Hurricane Pali moved closer to the Equator, it moved into an area of strong vertical wind shear and weakened.  A large area of thunderstorms east-northeast of Samoa could be consolidating into a new tropical cyclone.  That area of thunderstorms is generating a lot of upper level divergence which is spreading across the Equator as strong upper level winds from the south.  Those upper level winds created strong vertical wind shear over Pali as the hurricane moved south on Wednesday.  The wind shear pushed the upper portion of Tropical Storm Pali toward the north and disrupted the vertical integrity of Pali’s circulation.  The lack of vertical integrity resulted in a significant decrease in the surface wind speed on Wednesday.

The environment around Tropical Storm Pali will be unfavorable for intensification for the next several days.  The vertical wind shear will continue and it could increase if a tropical cyclone develops east of Samoa.  Tropical Storm Pali is expected to continue to weaken on Thursday.  However, if the surface circulation remains intact for another 48 hours, the wind shear could decrease during the weekend.  The Sea Surface Temperature is near 30°C and so there is sufficient energy to support intensification, if the upper level winds decrease.  If there is still a surface circulation on Saturday, then it could begin to reorganize and get stronger.  Alternatively, if the wind shear gets stronger, Tropical Storm Pali could dissipate over the weekend.

A subtropical ridge to the northwest of Pali is steering the tropical storm toward the south.  As Pali weakens, the low level circulation will be steered more by winds closer to the surface.  Those winds are blowing from the east and Tropical Storm Pali or its remnants are expected to move toward the west during the next few days.

Pali Intensifies into Rare January Hurricane

Tropical Storm Pali intensified steadily on Monday and it has become a rare January hurricane over the Central Pacific Ocean.  At 10:00 p.m. EST on Monday the center of Hurricane Pali was located at latitude 8.1°N and longitude 171.9°W which put it about 1305 miles (2100 km) southwest of Honolulu, Hawaii.  Pali was moving toward the east-southeast at 6 m.p.h. (10 km/h).  The maximum sustained wind speed was 85 m.p.h. (135 km/h) and there were wind gusts to 100 m.p.h. (160 km/h).  The minimum surface pressure was 982 mb.

The structure of Hurricane Pali improved significantly on Monday.  A primary rainband wrapped all of the way around the center of circulation and became a well formed eyewall.  The eye has been clearly visible on satellite images from the past few hours.  Thunderstorms around the eye are generating upper level divergence in all directions which is pumping out mass.

Hurricane Pali is in an environment favorable for intensification.  An upper level ridge is generating light westerly winds over the top of Pali, but the vertical wind shear is modest.  Hurricane Pali is moving over water where the Sea Surface Temperatures (SSTs) are near 28°C.  Wind shear will be the primary factor that will determine intensify changes during the next 24 to 48 hours.  If the wind shear remains minimal, then Hurricane Pali will intensify further.  If the wind shear increases to the magnitude that existed during the weekend, then Pali will weaken.  The most likely scenario is for Pali to intensify during the next 12 to 24 hours, then maintain a steady state or slowly weaken during the middle of the week.  Rapid intensification could continue during the next few hours.

Hurricane Pali remains in an area where the steering currents are weak.  It could meander slowly toward the southeast or east for another day or two.  After that time a subtropical ridge could strengthen and start to steer Pali more toward the west.

Wind Shear Weakens Tropical Storm Pali

Upper level winds blowing from the east over the top of Pali weakened the tropical storm on Saturday.  At 10:00 p.m. EST on Saturday the center of Tropical Storm Pali was located at latitude 7.7°N and longitude 174.5°W which put it about 1450 miles (2335 km) southwest of Honolulu, Hawaii.  Pali was moving toward the west at 2 m.p.h. (3 km/h).  The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were wind gusts to 50 m.p.h. (80 km/h).  The minimum surface pressure was 1000 mb.

A narrow upper level ridge is generating easterly winds that are blowing over the top of Tropical Storm Pali.  The vertical wind shear produced by those winds caused the structure of Pali to become asymmetrical on Saturday.  Most of the stronger thunderstorms formed west of the center of circulation.  There are rainbands east of the center, but they are not as tall as the bands in the western half of the circulation.  Occasionally, when the wind shear lessens, new thunderstorms form closer to the center of circulation.  At times the wind shear also causes the circulation to tilt toward the west with height.  Tropical Storm Pali is over water where the Sea Surface Temperature is near 28°C.  So, there is sufficient energy in the upper ocean to support intensification, if the wind shear decreases.  However, if the wind shear stays at its current magnitude, slow weakening will continue.

Tropical Storm Pali is embedded in a larger trough of low pressure.  As a result the steering winds are relatively weak.  The easterly winds are slowly pushing Pali toward the west.  That general motion is expected to continue for another day or two.  On its anticipated track Tropical Storm Pali could meander over the Central Pacific well southwest of Hawaii for a few more days.

Tropical Storm Pali Forms Southwest of Hawaii

A surface circulation organized quickly on Thursday in an area of thunderstorms southwest of Hawaii and the Central Pacific Hurricane Center designated the system as Tropical Storm Pali.  At 4:00 p.m. EDT on Thursday the center of Tropical Storm Pali was located at latitude 4.7°N and longitude 171.2°W which put it about 1450 miles (2335 km) southwest of Honolulu, Hawaii.  Pali was moving toward the north-northwest at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were wind gusts to 65 m.p.h. (105 km/h).  The minimum surface pressure was 1000 mb.

Several well formed spiral bands developed within a large area of thunderstorms and the system exhibited sufficient organization to be classified as a tropical storm.  Two of the inner spiral bands appear to be wrapping around the center of circulation and the inner core of Tropical Storm Pali is organizing quickly.  A partial eyewall may be forming around the southern and western sides of the center of circulation.  Thunderstorms near the core of Pali are generating upper level divergence, which is pumping out mass.

The environment surrounding Tropical Storm Pali is favorable for further intensification.  It is moving over water where the Sea Surface Temperatures are near 29°C.  An upper level ridge east of Pali is generating southeasterly winds over the top of the tropical storm, but the vertical wind shear is not too strong.  Pali is likely to intensify further during the next 24 hours.  A period of rapid intensification may be possible, if the inner core consolidates around an eye.

A subtropical ridge east of Pali is steering the tropical storm toward the north-northwest and that general motion is expected to continue.  However, the winds steering the storm are not too strong, and it may not move much during the next several days.

Although the historical record of tropical storms over the Central Pacific is relatively short, it appears that Tropical Storm Pali may have reached tropical storm intensity at a lower latitude than any other tropical storm on record did in that region.

Tropical Depression 09C Forms Near Equator

Sufficient evidence of a surface center of circulation within a broad area of thunderstorms near the Equator prompted the Central Pacific Hurricane Center to classify the system as Tropical Depression 09C.  At 10:00 p.m. EST on Wednesday the center of Tropical Depression 09C (TD09C) was located at latitude 2.8°N and longitude 177.8°W which put it about 1850 miles (2975 km) southwest of Honolulu, Hawaii.  TD09C was moving toward the west-northwest at 8 m.p.h.  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (75 km/h).  The minimum surface pressure was 1001 mb.

The circulation of Tropical Depression 09C is not particularly well organized and there may be multiple small centers inside the larger circulation of the system.  One small center of circulation is near a cluster of thunderstorms in the northern half of the circulation and that center is currently being designated as the center of TD09C.  Another circulation center is near latitude 1.0°N and longitude 174.6°W, but there are no thunderstorms near that center.  The main area of thunderstorms is north of the official center of circulation.  There are a few partial spiral bands northeast of the center of the depression.

The environment around TD09C is only marginal for intensification.  The depression is moving over water where the Sea Surface Temperatures are near 30°C.  So, there is plenty of energy in the upper ocean to support intensification.  However, a strong upper level ridge east of TD09C is generating strong southeasterly winds over the top of the depression.  The strong vertical wind shear is contributing to the poor organization of TD09C.  The wind shear is likely to inhibit intensification during the next several days.

A subtropical ridge east of TD09C is expected to steer the depression slowly toward the west-northwest during the next few days.  The actual track of TD09C could be somewhat erratic, especially if some of the small centers of circulation dissipate and additional centers form in clusters of thunderstorms that develop.  On its anticipated track Tropical Depression 09C could approach Tarawa in about three days and it could be approaching the Marshall Islands this weekend.

The most remarkable thing about Tropical Depression 09C is that is developed so close to the Equator.  Conventional wisdom often says that tropical cyclones cannot form near the Equator because the Coriolis effect is too small.  However, although tropical cyclones are rare near the Equator, Tropical Depression 09C is another example that they can form in that region.

Sandra Weakens to a Tropical Storm

Strong vertical wind shear weakened Hurricane Sandra to a tropical storm on Friday.  At 10:00 p.m. EST on Friday the center of Tropical Storm Sandra was located at latitude 21.2°N and longitude 108.8°W which put it about 155 miles (250 km) west of Las Islas Marias and about 205 miles (330 km) southwest of Mazatlan, Mexico.  Sandra was moving toward the north at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 65 m.p.h. (105 km/h) and there were wind gusts to 80 m.p.h. (130 km/h).  The minimum surface pressure was 990 mb.  Tropical Storm Warnings are in effect for the portion of the coast from Altata to San Blas, Mexico and for the Las Islas, Marias.

A large upper level trough centered over the southwestern U.S. generated strong southwesterly winds that blew the top half of Hurricane Sandra east of the low level circulation.  The strong wind vertical wind shear will continue, but it will take another day or two for the low level circulation to spin down.  Sandra could weaken to a tropical depression on Saturday and it could be classified as a remnant low by the end of the weekend.

Since the upper portion of the circulation is detached from the lower portion, the surface circulation is being steered by the winds in the lower atmosphere.  A ridge in the lower atmosphere is steering the surface circulation toward the north and that general motion is expected to continue for the next 24 to 48 hours.  On its anticipated track the surface center of Tropical Storm Sandra will pass near the southern tip of Baja California on Saturday.  The surface center could make landfall on the west coast of Mexico on Sunday.

Hurricane Sandra Turns Toward Mexico

Hurricane Sandra moved around the western end of a subtropical ridge and turned toward Mexico on Thursday.  At 10:00 p.m. EST on Thursday the center of Hurricane Sandra was located at latitude 18.0°N and longitude 109.7°W which put it about 340 miles (545 km) south of the southern tip of Baja California and about 490 miles (790 km) south-southwest of Culiacan, Mexico.  Sandra was moving toward the north at 13 m.p.h. (20 km/h).  The maximum sustained wind speed was 120 m.p.h. (195 km/h) and there were wind gusts to 150 m.p.h. (240 km/h).  The minimum surface pressure was 956 mb.  The government of Mexico issued a Tropical Storm Watch for the coast of Baja California from Todos Santos to Los Barriles.

Hurricane Sandra appeared to develop concentric eyewalls on Thursday.  A primary rainband wrapped around the tiny pinhole eye at the center of the hurricane.  Satellite imagery suggests that the tiny inner eye still exists inside the larger outer eye.  Sandra reached Category 4 on the Saffir-Simpson Scale on Thursday morning, but the formation of concentric eyewalls weakened the hurricane back to Category 3 later in the day.  Sandra remains a well organized hurricane.  It has a well defined core surrounded by a ring of strong thunderstorms.  Spiral bands of storms are in the northeastern quadrant of the circulation.

Hurricane Sandra will move into a less favorable environment on Friday.  It will be over water where the Sea Surface Temperature is near 29°C.  However, a large upper level trough along the west coast of North America will generate strong southwesterly winds.  Those winds will cause strong vertical wind shear, which will tilt the circulation toward the northeast.  The upper level winds will also inhibit upper level divergence from the western part of Sandra.  The strong wind shear will weaken Hurricane Sandra on Friday.  Some models are predicting that the shear could be strong enough to blow the middle and upper parts of the circulation northeast of the low level circulation.  If that happens, then Hurricane Sandy could weaken very quickly,

The upper level trough will steer Hurricane Sandra toward the northeast on Friday.  Sandra will likely move faster toward the northeast by Friday night.  On its anticipated track Sandra could be near the southern tip of Baja California by Friday night and it could approach the west coast of Mexico on Saturday.  Sandra could still be a hurricane when it nears Baja California, but it is more likely to be a tropical storm.  It could be a tropical storm or depression by the time it reaches the west coast of Mexico.