By David Cannon and Jeremy Davis – Weather Routing Inc.

When one looks at the area between the Central American coast and the west coast of the United States, a fairly obvious observation can be made: this is a fairly large area to consider. With that in mind, changes in the weather can always be expected when traveling throughout this area even during the late spring through early autumn. There’s a lot to keep in mind weather-wise when one makes travel plans throughout this part of the world.

Panama Canal to the Mexican Riviera:

Tropical cyclone season in the Eastern Pacific begins on 15 May, with tropical cyclone development in May occurring (on average) about every other year. However, the frequency of cyclones increases during June and much of July, with the peak period for development occurring during late July/August. Tropical cyclone frequency diminishes significantly during September/October with development becoming quite rare in November, as tropical season nears its conclusion (tropical season concludes on 30 November).

Development of tropical cyclones is found within the so-called “Inter-Tropical Convergence Zone” (ITCZ). Simply put, the ITCZ is the convergence area between northerly/easterly winds to the north and easterly/southerly winds to the south. The location and coverage of the ITCZ shows a north/south “oscillation” over the course of the year, typically reaching its northernmost coverage zone in July, as tropical cyclone frequency nears its peak. Generally speaking, tropical cyclones will form in the far Eastern Pacific (east of 110W toward the Central American coast, and in waters between approximately 06N and 15N).

Once development occurs, the primary track for tropical systems will be toward the west or west-northwest, into the open Pacific. Cooler waters (typically below 80 degrees F/26.5 degrees C) west of approximately 110W will allow for weakening of tropical cyclones. An alternate track for cyclones will be one that takes systems on a track more toward the northwest to north, toward the Mexican coast. This is more likely to occur later in the season, near or past the peak development period (August and beyond), as larger troughs of low pressure in the upper levels of the atmosphere “pick up” and re-curve cyclones toward this track (see figure 1).

Without the presence of tropical cyclones, the weather tends to be rather benign (for the most part) along the Central American coast and across the Gulf of Tehuantepec. General wind directions in that area are northeast to southeast to south and much lighter. Only periodic bouts of higher north to northeast to east winds occur as high pressure builds westward into the Western Caribbean. These higher winds are most common near the Gulfs of Nicoya, Dulce, Papagayo, and Fonseca, as well as near and gaps in coastal ranges and along the south side of the Peninsula de Azuero. In these areas, wind speeds generally reach as high as force 5-6, with northeast to east combined seas in offshore waters as high as 6-7 feet.

Further north along the Mexican Riviera, west to northwest winds are most common, speeds generally no more than force 3-4. Along immediate coastal areas (within approximately 2 nautical miles from shore), these winds will tend to be highest during the late morning and afternoon hours and in cloud/rain-free areas, due to daytime sea breezes. Also, offshore land breezes (generally force 3 or less) will occur at night along these immediate coastal areas. Combined seas region-wide will tend to be west to northwest in direction, generally no more than 5-6 feet.

 East Pacific tropics

Sea of Cortez, Gulf of California and Baja Peninsula:

Later in the spring and during the summer months, cold fronts are less common, meaning Northerly wind surges become more of a rarity, almost never occurring during the summer. However, as with areas further to the south and east, the tropics become more of a concern during this period. These concerns are exacerbated later in the tropical season (late August through November), as the possibility of a “recurvature” in tropical cyclone tracks toward the Mexican coast, the Southern Baja, and the Sea of Cortez becomes greater. This recurvature occurs as increasing south to southwest winds aloft from large troughs of low pressure steer tropical cyclones toward a northwest to north track.

Recurving tropical cyclones will tend to weaken due to the stronger winds in the upper levels of the atmosphere, although the warmer waters within the Sea of Cortez and Gulf of California can actually allow cyclones to sustain themselves longer or allow for a slower weakening of systems under a less hostile environment aloft.

Otherwise, the thermal trough tends to be stronger during this period, extending well into and through the Gulf of California and the Baja Peninsula. Interaction between the thermal trough and high pressure further west will bring generally northwest to north winds to the region, higher when the axis of the trough is further east, though generally no more than Beaufort force 5-6. Along or near the trough axis, the winds tend to be very light, no more than force 3, and one might even find lighter southeast to south winds, generally no more than force 3-4 (see figure 2). Any southeast to southerlies will tend to occur on the east side of the trough axis, mainly occurring in the far eastern Sea of Cortez and extreme Eastern Gulf of California.

Combined seas will tend to be larger along the west side of the Baja, where greater trough/ridge interaction is more likely to occur, and where the fetch is greater. Periods of stronger northwest to north winds will induce seas as high as 8-10 feet across this region. However, further east and closer to the thermal trough axis, seas will tend to be lower, generally northwest to north, only as high as 5-6 feet.

Mexico Summer Pattern

U.S. West Coast:

The weather settles into a somewhat quieter pattern during late spring and summer/early autumn pattern. Cold fronts are less frequent and not as strong, and the thermal trough becomes a more dominant weather feature. The thermal trough itself during this particular period will extend further north and be stronger along the California coast, often reaching as far north as interior and coastal sections of Oregon. As this occurs, high pressure remains quasi-stationary in the eastern and central Pacific, extending further north than in earlier weeks and months. The northern extent of the ridge can be found just offshore from the Oregon coast and this keeps gales and storms further north in the North Pacific, generally confined to the Gulf of Alaska, the Alaskan Mainland and points east.

Interaction between the thermal trough and ridge will bring enhancement of northwest to north winds across waters north of Point Conception to Cape Flattery. This occurs when the axis of the trough is further inland, across interior California/Oregon, allowing for a tightened pressure gradient along coastal sections (see figure 3). Wind speeds during these higher wind events can reach as high as gale force (force 8), particularly near any capes and headlands (Cape Mendocino in California and Capes Arago and Blanco in Oregon for example), with northwesterly combined seas often reaching and exceeding 10 feet.

Since the weather pattern is slower and less progressive during the summer season, the enhanced winds and larger seas mentioned above are more persistent. At times, these conditions can last as long as three to four days at a time. Breaks in the enhanced winds will occur as the axis of the thermal trough drifts further west, toward or just offshore from the California coast, inducing a slackening of the pressure gradient along the coast. This will occur as portions of the Pacific high pressure ridge “break away” and move northeast to eastward into and across the northwest U.S.

Breaks in the higher winds can also occur as low pressure in the mid to upper levels of the atmosphere persist or drift southward off coastal waters. This is most likely to occur during the spring and early summer and is most prevalent when larger, stronger upper level lows are present, allowing the circulation to “spin down” toward the surface in the form of very weak surface lows.

South of Point Conception, the weather tends to be quiet for the most part. Episodes of higher west to northwest winds can occur when the axis of the thermal trough is further inland, though these higher winds are more likely to occur in areas favored for local funneling, such as the Santa Barbara and San Pedro Channels. Furthermore, these winds tend to be more “surgy” in nature, mainly occurring during the afternoon hours, with wind surges generally no more than force 5, and west to northwest combined seas, while short and choppy, generally no more than 5 feet.

Aside from the higher wind surges, general wind directions are south to southwest to west during the daylight hours, with speeds no more than force 3-4, with overnight east to southeast winds generally force 3 or less. A persistent light onshore flow across the region will bring nighttime low clouds and fog and reduced visibility, generally burning off or leaving hazy conditions during the mid to late morning and the afternoon.

West Coast Summer

Conclusions:

When one travels across the tropical Pacific during the summer season, tropical cyclones should certainly come to mind, and one must be mindful of the possibility of tropical cyclone development off the Central American coast. A close monitoring of satellite imagery and real time data is absolutely necessary when traveling between the Panama Canal and the Southern Baja, to keep abreast of the situation and to be aware of changes and developments across the region (or any region for that matter).

During late spring through early autumn it is easier to find weather windows along Baja California and the U.S. West Coast. The main thing to look for is the location of the thermal trough axis. A westward progression of the trough offers good news for most in that a slackened pressure gradient near the trough axis will offer lighter winds and lower, longer-period combined seas, more suitable for travel for most mariners. However, an inland progression and strengthening of the trough allows for greater interaction between the trough and ridging (from high pressure) further offshore and higher northwest to north winds and seas, especially between Point Conception and Cape Flattery. In this case, one should consider making transits in shorter hops, stopping along the way when adverse weather is imminent and “waiting things out” (i.e., waiting for the arrival of the thermal trough axis closer to the coast, or a weakening of the trough).

Did you find this article useful? Read General Weather Conditions Year Round From The Red Sea And Across The Northern Indian Ocean by Amanda Delaney, Mark Neiswender and Brian Whitley of WRI..


David Cannon is a Director of Yacht Operations/Senior Meteorologist and Racing & Tournament Specialist, and Jeremy Davis is a Senior Meteorologist. Both are employed at Weather Routing Inc. (WRI Ltd.), which has provided meteorological consultation, including route planning and weather forecasts to private yachts since 1961.

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