Bob Blog 8 May

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

Compiled Sunday 8 May 2022

BLOCKING

Anyone watching weather maps in this part of the world is familiar with high pressure systems moving from west to east around about 40 degrees south. This motion is due to the stronger westerly winds aloft. These rivers of wind aloft meander north and south as they travel eastwards in a roller coaster fashion and push the surface features around.

To understand how anticyclones form and affect our seasons, we need to quickly recap on the basics. Our global weather engine is powered by evaporation from the equatorial oceans. This causes a zone of rising air around the equator. When this air has risen to the stratosphere it spread outwards and pole-wards, and finally sinks back to the surface at around 30 to 40 degrees latitude. This zone is called the subtropical ridge and is the main latitude belt where anticyclones form. Near the Americas these anticyclones do not migrate but are usually locked in much the same position by mountain and given names such as the Bermuda High and the Pacific/Marine High. In the South Pacific anticyclones and normally mobile.

Sometimes a split develops in this steering field, with some air going north and the rest south, leaving a calm zone in-between. Then the surface high that is underneath this calm zone stays put and is referred to as a blocking high. This “blocking pattern” can last for weeks and affect the weather on a seasonal scale. It is the antithesis of mobility in the roaring 40s.

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In this image from windy.com the jetstreams at the 250hPa height are shown in white with a split in the flow and a calm zone in-between above the surface High centre east of NZ. Further east the jetstream has caught up with the surface high.

In the days after ANZAC day a large region of high pressure (anticyclone) moved over southern New Zealand and since then the same system has been lingering over the South Pacific Ocean east of New Zealand. Trade winds on the north side of a large high were enhanced so much that Fiji Meteorological Service issued heavy rain, flash flooding and coastal inundation warnings last weekend.

This has been a classical example of a blocking high., and by occurring at the end of April and start of May it has held back the normal autumn chilling in New Zealand.

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Time section for the 170E to 150W longitude band from 15S to 50S showing isobars from 21 Mach to 4 May. The normal pattern during April turns into a big block over past two weeks.

This block serves as a reminder of the ten reasons there are to hate a high

Ten reasons to hate a High:

• Near the centre are “dead” winds and usually an area of low cloud called “anticyclonic gloom” or dirty air causing fog.

• Highs intensify the trade winds. It may take about a week for a high to travel eastwards past New Zealand, and during this time the stronger trade winds tend to give night time rain to the eastern side of the larger Fiji Islands…. this is called BOGI WALU (eight nights).

• An intensifying high tends to “squeeze” isobars together on its periphery. If the central pressure is over 1030 there is usually a gale somewhere on the outside of a high.

• As air flows around a High it spins out, and has a speed as much as 20% MORE than that indicated by the isobar spacing.

• Stronger fronts: When an incoming front encounters a strengthening high, its wind and rain may be strengthened, and its frontal movement may stall.

• If a range of mountains blocks the air flowing around a high, the air tends to accelerate around the mountains and through gaps rather than over them. This causes a river of wind at the downwind end of a mountain chain.

• In summer, a high may help trigger inland thunderstorms and hail

• In winter, a long night with the clear skies and light winds of a high may bring frost.

• The bigger they are, the slower they move, blocking the fronts that are trying to follow them and feeding those fronts with warmth and moisture.

• When a new high forms in the Southern Ocean it shovels a polar-chilled southerly flow into the mid-latitudes.

TROPICS

The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

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Tropical Cyclone TWO in the north Indian Ocean is going north and tropical cyclone KARIM (out o season) in the South Indian Ocean is going south.

WEATHER ZONES

Weather Zones Mid-week GFS model showing isobars, winds, waves (green to red +arrows), Rain (Blue), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (pink)

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CAPE mid-week as seen by ECMWF and GFS from Predictwind.com

Different models seem to produce different values.

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SPCZ=South Pacific ConvL1ergence zone.

The SPCZ stretches from Coral Sea to Vanuatu to Samoa A convergence zone is expected between Tahiti and Tonga this week.

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Rain Accumulation next five days from windy.com

HIGHS and LOWS

Low L1 formed at the SE end of the SPCZ last week and is now near 30S to south of the Coos and travelling off to the southeast

High H1 has ben a blocking high east of NZ since late April and is now expected to move off to the east.

Low L2 is crossing the Tasman Sea and should reach central NZ on Monday and then move off to the northeast, following H1.

High H2 is expected to move into the South Tasman Sea by mid-week and then northeast onto central NZ by end of the week. OK for sailing westwards across the Tasman, but not Eastwards.

There is likely to be a squash zone of enhanced trade winds between NZ and Fiji from mid-week.

A tropical low is expected to from off the north Queensland coast by mid-week.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 64277762212

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