This blog post goes back to a short one I wrote in January “2016-01-08 Life Goes On“. That post was done on my phone so let me elaborate a little. As I was gathering material regarding anchor bridles I realized how little I had documented about January’s Storm in the Bahamas. Many in the Washington Capitol Region remember the Derecho that struck in June of 2012. As I recall there was more damage from that storm than from Irene or Sandy. As it happened I was alone aboard Magnolia at a dock in Galesville MD. I was literally tossed out of the centerline berth because of the significant roll Magnolia took in her slip…btw I do not attribute at all the 2 scotches and a cigar to me popping to my feet and asking myself “When did we get underway?” True story….
June 2012 North American derecho – This link documents the June 2012 event.
Fast forward to the January 2016 Georgetown Exuma Event. Magnolia was at anchor in Georgetown on the eastern side of Elizabeth Harbor at Sound Dollar Beach on the morning of event. Note: Generally speaking the prevailing trade winds come from the east but often times bad weather is coming from the west.
The Red mark indicates Sand Dollar Beach
The Blue mark indicates Kidd Cove
The Fuchsia mark indicates where we dropped our second anchor
There had been a front predicted to come through the area the evening of the January 6th so earlier in the day we relocated Magnolia from Sand Dollar Beach to Kidd Cove, essentially moving 1.25 miles to the W-SW to get in the lee. Magnolia anchored in 15 feet and deployed about 100 of our 225 feet of 5/16 chain. During the storm our bridle came loose allowing the strong winds to pull the additional 175 of chain through the gypsy and breaking safety line at the bitter end of our chain. Essentially setting Magnolia a drift in 50 knot winds. With the engine already running Annette managed to get the bow through the wind, avoiding many boats and not striking the point as we rounded. We managed to get our secondary anchor down in the middle of Elizabeth Harbor a short time later. In my next post I will try and explain why I think our old bridle system failed and what modifications we have done to alleviate the problem in the future. Please note this is NOT a discussion of anchors or “we should haves.” It is what happened to us, why we think it happened what what we are doing to hopefully avoid in the future. Below are a couple of videos as well as a description from out weather router that re wrote the day after.
YouTube — Squall or derecho hits Normans Cay anchorage
YouTube — George Town Derecho
Wx Update, Bahamas & Florida, Thu7, 3pm
Chris Parker [firstname.lastname@example.org]
Sent: Jan 7, 1:15 pm
Wx Update, Bahamas & Florida, Thu7, 3pm
10a ASCAT: SE Bahamas-Jumentos WSW-W@10-14 / C Bahamas W-WNW@12-18, highest E of 76W / Abaco WNW@12.
BUOYs: Canaveral 310-360@8-18g21, 8′<11’/8-12secNE-E / WEnd 340-020@11><7k / KyLargo 350@12><020@6 / KyW 050@8-11<080@5-7.
IMAGERY: isolated showers & mild squalls along Crossing area, greatest coverage N Route, drifting SSE-S, do not look significant.
Parts of Bahamas experienced an interesting weather event yesterday.
StanielCay reported (confirmed independently from multiple sources) W-NW@45-50 with gusts at least into the 60s from about 6pm-7:30pm EST.
CambridgeCay (about 20mi N of StanielCay) recorded a gust which registered 106.2k on an anemometer. Even if not precisely correct, there were almost certainly Hurricane Force wind gusts.
Most reports were a bit less…mostly W-NW winds in 30-40k range, gusting 50k+ persisting about an hour+/- generally between the hours of 5pm-8pm in RoyalIsland & RockSound Eluthera, various locations near GreatExuma/LittleExuma.
Vessel overnight near 24N/73W reported an hour of S-W@35.
Our forecasts the past couple days were for squalls to 40-50k generally predicted for Wed6 afternoon-evening in C Bahamas. In the 1pm Wed6 forecast I refined that to say squall risk would end in most of C Bahamas Wed6 evening, and that areas which had been seeing mild wind S of TROF Wed6 still had chance of squalls until FRONT passed and we saw steadier WNW wind establish N of TROF in the evening.
While our forecasts missed the intensity of squalls (some of which were 50-70k, possibly a bit higher especially in gusts…versus the 40-50k we predicted for this event over the previous several days)…did capture the timing, with activity not ending in C Bahamas until sometime in the evening / I think activity weakened some as it swept thru SE Bahamas later in the evening and overnight.
In 13 years, I don’t think I’ve seen an event like this in the Bahamas. Although there was some fairly strong convection (Lifted Index -4 to -6, and CAPE 1000 to just under 2000), there were no very tall (cold) cloud tops. Infared Satellite cloud top temps were only about -20C TO -30C, suggesting cloud tops probably in the range of 20,000′ to 25,000′ or maybe a bit higher. In order to generate observed winds, I would expect cloud top temps below -50C, and cloud tops well above 40,000′.
UofWisc analysis shows a large pool of cold air aloft near & W of the squall event.
Yesterday we discussed the 10am Wed6 ASCAT: TROF lies from 20mi S of Nassau-N BightAndros-N side of CaySalBk-Veradero, with SW-WSW@20-45 (sustained) within 120mi SE of TROF / NE-ENE@25-45 (sustained) within 120mi NW of TROF. Lightning strike data shows a band of intense lightning was along TROF.
So here’s my ANALYSIS: I believe the same TROF/convergence persisted from before 10am Wed6 morning until well after 10pm Wed6 evening, (at 10pm Wed6 it was along 25N/73W-Acklins-21N/75W, with one of the most intense bands of lightning strikes I’ve seen)…and it is this line that swept thru much of C Bahamas just before Sunset Wed6.
If we pick a point along TROF W of Andros (24N/80W) at 10am…and follow TROF E thru Bahamas (to 24N/73W) at 10pm, it covered about 400 miles in 12 hours, moving about 35k. As TROF began rotating around the LO which was developing just NE of Eluthera, the S portion of TROF/convergence moved more rapidly than the N part (closer to the developing LO)…and TROF/convergence gradually became more NNE-to-SSW-oriented. But throughout the day our TROF/convergence spanned about 300-400 miles from NE-to-SW (or NNE-to-SSW).
One unusual wind event which can persist for a long interval of time and move across many hundreds of miles, and lies along an axis hundreds of miles long is a Derecho.
Here’s a pretty good discussion of Derecho:
A Derecho is essentially “a self-sustaining linearly-organized storm”. A Derecho often starts as a series of outflow boundaries/gust fronts extending from squalls/T-strms, advancing ahead of a pool of cold air aloft. Over time, these outflow boundaries/gust fronts can merge into a long line, and be self-sustaining.
To meet the definition of a Derecho, the wind event must extend more than 240mi (from end-to-end), include wind gusts of at least 50k, and have several, well-separated 65k gusts. Winds are “straight line” in nature (rather than circular like in a tornado or hurricane), and typically blow perpendicular to the motion of the Derecho. Winds are supported not by the collapse of towering cumulonimbus clouds (as re typical squalls/T-strms), but rather by the inflow of warm air from ahead of the Derecho inward & upward into the pool of cold air aloft behind the Derecho…and fast-moving down-rushing air from the cold pool sustains the progressive gust front with the Derecho.
Derechos are thought to occur less often in moist environments, where inhibiting factors include abundant low-level clouds and less-cool air aloft. Derechos typically form on the equatorial side of the JetStream, with strong wind-shear. The leading edge of a Derecho is often marked by some sort of a shelf cloud.
Our event seems to meet all these criteria…we saw an event:
–over 300mi from end-to-end, and persisted along a path over 400 miles
–widespread wind gusts 50k+ along most of the line, with well-separated areas of 65k+
–straight-line winds, with reports of mostly uniform W-NW wind direction (perpendicular to the squall line)
–pool of cold air aloft located behind the squall line
–relatively-dry (cloud-free) conditions ahead of the squall line (at least in some areas)
–some sort of a shelf cloud was clearly visible in many of the photos I saw taken in Georgetown just before the event
–this occurred along the SE side of sub-Tropical JetStream, in an environment of strong wind shear
I can’t be sure what we saw was a Derecho, but it was certainly (thankfully) a rare event.
REST OF SYNOPSIS/FORECAST hopefully by about 6:30pm, but I don’t think there are any big changes from yesterday’s forecast/discussion for the timeframe from tonight thru Sat9…ChrisP.