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Your exclusive SNN Obs for this Tuesday Sept 16th 630am Update
More cloudy but still calm to light Trades giving way to 11am lite convective onshore seabreezes with more cloud build up into the afternoon. This pattern is set into Saturday. 1.8' High tide at 11am dropping out to a 0.5' Low tide at 630pm.
High Surf advsry for all southern exposures. The epic swell has eased another notch but still good. Buoys are 4' 15 seconds. Inconsistent.
; all over, plus generals and heavy shorepound. East: Makapu'u: Up just a pinch and Holding the small below normal NE and S wrap 0-1.5' maybe a rare 2' set near the shore and semi smooth lite onshores. Great diving all windward. Diamond Head: Down and dropping on the S-SSW at 2-4+' with superb glass to offshores but likely onshores 11am ish then to the evening glass off. (see SNN Cam). Sandy's: Down and dropping on the South at a nice clean 2-4 occ +'
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THE WHERE, WHEN & WHY OF HAWAII’S WAVES & WEATHER: A VIDEO PRESENTATION BY SNN
Winds Statewide in a glance ...go HERE (this link is also the 4th drop down under weather)
Sep 16, 2014 3:38 AM HST
Muggy conditions will continue through the rest of this week. Gentle trade winds will prevail today through Wednesday, but will still allow for localized afternoon sea breezes and interior clouds and showers. The trades are forecast to break down again Thursday into the weekend, leading to more widespread sea breezes along with afternoon clouds and showers.
Scattered showers have developed across the windward coastal waters and around the Big Island during the last several hours, with shower motions and 88D VWP data indicating that gentle trade winds are becoming established once again across the state. Most land areas have remained dry since sunset, with rain gauge data showing just isolated very light amounts. Surface analysis depicts low pressure centered far northeast of the state, with a trailing cold front/trough extending SW to the southern end of the Big Island. Mimic-tpw imagery also clearly shows a stripe of higher precipitable water values along the front/trough, although the mimic estimated values are running a bit high compared to the measured 12Z Hilo pwat of 1.57 inches. A drier airmass continues to spread into the state from the north behind the trough, as shown by the 12Z Lihue sounding with a measured pwat of only 1.15 inches, which is down from 1.77 inches 24 hours ago and below the 25th percentile for this time of year. Another area of somewhat higher moisture is moving southward toward the state, with the leading edge of this moisture about 200 miles north of Kauai as of 11Z. A 1024 mb surface high centered far northwest of the state is building eastward behind the front/trough, allowing the gentle trades to resume over the islands. Aloft, a weak upper level trough lies across the state, with a large deep layered anticyclone centered well to our northwest, just to the east of the dateline.
Surface high pressure will build slowly eastward to the north of the state today through Wednesday, maintaining a gentle but definite background trade wind flow. Trades should remain light enough to allow localized sea breezes to develop each afternoon. Therefore we would expect a hybrid pattern, with showers and clouds focusing on windward slopes during the nights and early mornings, and across some of the leeward and interior areas during the afternoons. The area of higher moisture initially to our north is forecast to brush across at least the western half of the state late tonight into Wednesday. This may help to enhance shower activity a bit in that area. A rather dry airmass is forecast to spread across the Big Island later today through Wednesday, so showers should be less prevalent there with rainfall amounts remaining light. With ocean waters to the north of the state significantly warmer than normal, rather muggy conditions are likely to continue despite the prevailing trade wind flow.
From Thursday into the weekend, another cold front is forecast to drop southward across the north central Pacific. This will likely push the subtropical ridge south and east, disrupting the pressure gradient across the state. Local trade wind flow should weaken further or disappear entirely during this time, allowing for another period of more widespread daytime sea breezes and nighttime land breezes. With more of a pure convective pattern, we should see more prevalent interior clouds and showers during each afternoon, and more clearing across the islands each night. Model consensus eventually stalls the frontal boundary a couple hundred miles north of the state by the end of the weekend and into early next week, with gentle trade winds possibly resuming over the islands by Sunday or Monday.
A high surf advisory remains in effect for south facing shores of all islands through 6 pm today. Surf reports reached high-end advisory levels at many beaches on Monday. Wave heights at the Barbers Point buoy continue to run slightly above wavewatch guidance currently, but peak swell periods are gradually decreasing. Would expect that the resulting surf will remain above advisory criteria at least through today. Will let the day shift evaluate later trends and the surf observations to see if the advisory may need to be extended beyond today. Otherwise, a small northwest swell appears to be near its peak now as indicated by Waimea bay buoy data and wavewatch guidance. This swell will diminish later today, but another couple of small north northwest swells are expected to arrive today and late tonight into Wednesday. Resulting surf should remain well below advisory levels on north and west facing shores.
With either light/gentle trades or a land/sea breeze pattern forecast to prevail through the remainder of this week and into the weekend, winds and seas are expected to remain below small craft advisory levels.
BIG SURF PICTURE
The Jet down under has gone into ‘shutdown’ mode after a large trough last week generated what we’ve been claiming since 2 Friday’s ago…the year’s biggest SSW to south swell. Summer can’t last forever. After this long lasting event we’ll have to hope and pray.
Recent: We’ve been on the down trend to 2’ after a super long run of fun South swells that reached peak hts of 5’ at select reefs but mostly sets averaged 3’ from early Sept to Tuesday the 9th. Last weekend into early last week had a pair of swells from a pair of storms down under. We haven’t counted but since Aug to now there have been at least 8 sources of swell. The last of which will top it all off. Read below.
Next: Last Sunday a powerful Low tracked ENE under NZL with near hurricane force 65kt winds and seas near 50- 60’! The storm broadened further off the coast as the highest winds and seas weakened. But not before setting up a 1200 miles fetch. When storm have such extreme winds they generate long periods as we’ve seen since Friday: long fat 25 seconds leveling off to 22 sec Saturday and 20 sec Sunday. The swell hts will go from 1’ to 4’ and even 12 hours of 5’ 20 seconds. There will be moments at select reefs of warning level hts of 15’ crest to trough or 8’ local. The North shore can get 10’ from such values as the long periods have extreme refraction and shoaling off the bottom. Note: 20 sec period swell can be felt 1000’ down! This shows us how vital periods are to wave measurement.
Last: No sources of swell over 2’ out through next weekend meaning 2 weeks from now. There are marginal fetches in the Taz Monday 15th and another tiny Low off NZL Friday the 19th. Neither source may get here.
Note: High Surf threshold for South swell is 8’ crest to trough and 15’ for NW swells. Why the large threshold difference is likely an extra safety precaution where populations are greatest.
The weak Jet is long and up around 40-50 degrees latitude. There’s some troughs and dips and by next weekend the Jet has a bit more speed and energy going from Japan to the Gulf. We are still in a state of transition.
Currently: the surf is barely 2’ remnants from our 1st real swell of the season that hit near 8’ Tuesday.
Recent: this nice WNW filled late Monday to 6’ and ramped Tuesday from a more NW angle. Source: Storminess Friday the 5th just east of the 180 dateline 1200 miles NW of us. By Saturday she had 30’ seas on the dateline and spun in place (occluded) as she broadened and weakened into Sunday…The result was swell of 5’ 14 seconds.
Next: Former tropical storm Fengshen will bump us up Sunday from the 15 sec WNW at 2-3’ midday and veer NW peaking at 4’ Monday into Tuesday with shorter 12 seconds. The system formed off Taiwan last Sunday the 7th and moved NNE as it broadened and weakend off Japan becoming a cold core Low off the tropics warm core. It then tracked East reaching the dateline Thursday the 11th as it merged and faded with a broad area of Low pressure centered in the Eastern Aleutians.
Last: A tiny weak low tracks east from the dateline this Wednesday and will only pull off some 2’ NW surf Tuesday the 23rd.
Recently and until Friday we’ve see tiny 1’ surf from the light local trades and seabreeze patterns; rare to see it this small this long. Then it’ll kick up from some T Storm action below.
Tropics: T Storm Odile has come into view today Sat. with a NW track and clipping Cabo etc Monday-Tuesday. By Thursday it’s nearly gone but still may shoot out some longer period 3’ East swell around Friday-Saturday the 19-20th.
FETCH: Often called the fetch length, is a term for the length of water or distance over which a given wind direction has blown. Fetch length along with the wind speed (or strength) determines the size of waves produced. The longer the fetch length and the faster the wind speed, the larger and stronger the wave will be and vice versa.
CAPTURED or FOLLOWING FETCH: Not only does the fetch length determine the power and energy of the wave. Additionally, if the winds/fetch are blowing in the same direction as during the wave's or storm's lifetime, the wave will in turn be even stronger. The fetch is related to the orbit of the wave and track of the storm.The longer the wind drags along the sea the more energy the wave will have. This can be seen not just in vertical height measurement but in wave period (the measurement of waves through time).
WAVES PERIOD:Time, in seconds, between the passage of consecutive wave crests past a fixed point. In general the longer the period the bigger the wave. Windswell or 'close interval' swells are under 10 seconds. Big 'ground swells' are 17-20+ seconds (note: a 20+ second swell needs storm force 50+kt winds blowing over a 1500 mile fetch to be fully developed' into 20-25' beasts; these 20 second waves can be felt 1000 feet down!). This is why they jack so much more, pulling large amounts of H2O off the outer reefs.
More generic marine definitions click this link http://www.prh.noaa.gov/hnl/pages/marinedef.php
....All surf height observations and forecasts are for the full face surf height, from the trough to the crest of the wave.
|North-Facing Shores||15 Feet (8')||25 Feet|
|West-Facing Shores - Big Island||8 Feet (4'+)||12 Feet|
|West-Facing Shores - Remaining Islands||12 Feet (7')||20 Feet|
|South-Facing Shores||8 Feet (4'+)||15 Feet|
|East-Facing Shores||8 Feet (4'+)||15 Feet|