Unfortunately, life is never quite so simple in the real world. First, what are these “resources” referred to here? We have more details elsewhere, but they would include the weather maps (surface analysis and forecasts), and they would include the GRIB files of numerical weather predictions such as those from the GFS.
We shall see in the videos below, however, that the maps themselves are often not detailed enough if we happen to indeed be inline with, or close to, a TC. The GRIB model predictions on the other hand, give us very detailed forecasts for wind and pressure, but we must be aware that these unvetted model predictions could be wrong! This is actually more likely with a TC than with more climatically normal forecasts, and indeed more likely to be wrong during the first day or so of formation.
Thus it turns out that perhaps the most crucial official weather information we have underway in a case like this are the email text or radio broadcasts that give the official NWS numerical information on the location, motion, and extent of the systems. These are available at least 4 times a day, and often hourly on the storm warning broadcasts segments of the WWV and WWVH HF radio. We can pick these up with a standard, battery-operated SW receiver. The voice broadcasts are usually a voice synthesized rendering of the text reports. Here is a note on how to get these text forecasts by email.
From these text reports we learn the Lat-Lon of the system at a specific time, and we learn the speed of the system in kts, and the true direction it is moving—this might be stated as “northeast or 050,” in which case the numbers are the more specific value; this phrase is not a range of directions or an uncertainty statement.
Instruments
For the best work here we need an accurate barometer and calibrated wind instruments. Many places elsewhere we discuss barometers and how to check that they are reading properly. This is fundamental to ocean voyaging.Wind instruments are frankly harder to calibrate properly, but the more accurate they are at this point, the better off we are. The crucial point here is we need to know true wind speed (TWS) and true wind direction (TWD). We get this from the apparent wind speed (AWS) and apparent wind angle (AWA) along with our COG and SOG from the GPS. Some instruments will make this conversion for us, which is a great convenience once it is tested to be working properly. Otherwise, we can use a calculator to convert the AWS and AWA to TWS and TWA, then we can combine the TWA with COG to get the TWD.
Without any wind instruments, you can look into the waves to estimate the true wind direction. The TWA will always be aft of the AWA. That is if you have instruments telling your the AWA is near the beam; the TWA will be more on the quarter, etc. Then add this to the true heading of your vessel at the time, being the compass heading corrected for variation.
The barometer is dropping and the wind is building. Which side of the storm am I on?
Simple enough question, but it needs all kinds of clarification. First of all a reminder that we are talking primarily about “tropical” systems, either within the tropics, or those that have moved out of the tropics…. and we are discussing another rarer type of system which is a “tropical storm” that formed well outside the tropics. An example of the latter is TS Claudette from July 2015 that formed off the coast of NC and moved NE.
The key characteristic we are calling “tropical” means a small and well defined system with closed isobars. A typical extra-tropical storm in the North Atlantic or North Pacific does not qualify. Which side of these is rarely a question we face—the other side could be more than a thousand miles away. A tropical storm is typically a few hundred miles across, or less. These are the ones whose sides we care about in this discussion. We can also have the barometer dropping and wind building with an approaching frontal system, but this discussion would not apply to that situation at all.
When sailing in the tropics, however, we are not often confused by what is going on. We are used to glorious trade wind sailing, with the wind and barometer pretty stable. In fact, the barometer is actually more stable than the winds. We can see the trades vary from 10 to 20 kts with not much change in the barometer. Throughout the tropics summertime pressures will be 1011 to 1014, depending on month and location, and more to the point, the standard deviation (SD) in this pressure will be only 2 or 3 mb. In the absence of a tropical system, a pressure drop of 2 SD occurs only 2 or 3% of the time. Thus if you see the mean pressure drop 4 or 5 mb then you have a very good warning that a system could be approaching, even if the wind has not started to build. Note we have to say mean or average pressure because within the tropics there is a semi-diurnal variation of the pressure with an average amplitude of 1.7 mb. Worldwide pressure statistics and how to interpret them is covered in the Mariner's Pressure Atlas.
Now a video review of the key issues of wind shifts as a closed system of isobars approaches and passes.
Sides of a Tropical Storm, Part 1. Background and References (10 min)
For full screen, double click and wait a few seconds for focus.
Step by Step Procedures
(2) Using the forecasted course and speed of the storm and your known position now, plot your present position as well as the DR position of the storm at the present time.
(3) Now plot the forecasted track of the storm and its forecasted locations. We are getting this information from the text reports if we have them (see below).
(4) If we have this data, then we have a picture of what is taking place and where we are relative to the storm. More specifically, you see how far away the storm is and how close it will pass if you do not move.
(5) Now the crucial check to see if the forecasts are correct by recording our barometer and wind speed and direction at least every hour. With a good barometer, you might learn something on an hourly basis. (Examples in the videos below.)
(6) You might also at this point do a reality check with the Buys Ballot law (refer back to Video 1). Namely, with your back to the true wind direction, i.e. true wind blowing against your back, put left arm out and rotate it forward by 25ยบ. Now check the compass to see which way you are pointing and record that, then convert that compass direction to a true direction. Then you can go back to your plot and draw a line from the boat to the storm and see if that direction is the same as you just measured with Buys-Ballot. If yes then you can try this again once the wind direction changes enough to detect it. If still pointing to the storm, then you are indeed inside the closed isobars of the storm.
If the Buys Ballot direction does not match your plot, and you checked both, then chances are you are not quite into the closed isobars. This could also be indicated by a pressure that is not dropping or dropping very slowly.
Now we have a couple specific examples.
Sides of a Tropical Storm, Part 2. Hurricane Guillermo 2015 (17 min)
For full screen, double click and wait a few seconds for focus.
Sides of a Tropical Storm, Part 3. TS Claudette 2015, Value of a Good Barometer (15 min)
For full screen, double click and wait a few seconds for focus.
Now that you see the pressure dropping and wind maybe building, and you have from Buys Ballot a good check on the direction to the storm, you are prepared to consider a course of action. They key issue is almost always getting as much separation from the storm center as possible at the time it passes you… or with confident knowledge, the possible choice of getting across the front of it before it gets to you. This is especially true with these TC that have strong winds. These strong winds are usually located fairly close around the system… that is, we see that 80 or more nmi gain is usually a huge safety factor.
The video examples we have show this, but we can use the data from a typical TC to see this for other cases. You can also go to http://www.nhc.noaa.gov/archive/2015/ and choose any of hundreds of storms to check this.
Here are a couple examples. For tropical storms or for TS stage of a hurricane, the safety radii are smaller, so it is easier to get out of the way of them.
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HURRICANE GUILLERMO FORECAST/ADVISORY NUMBER 18
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI EP092015
0900 UTC MON AUG 03 2015
HURRICANE CENTER LOCATED NEAR 16.2N 145.4W AT 03/0900Z
POSITION ACCURATE WITHIN 20 NM
PRESENT MOVEMENT TOWARD THE WEST-NORTHWEST OR 300 DEGREES AT 9 KT
ESTIMATED MINIMUM CENTRAL PRESSURE 990 MB
MAX SUSTAINED WINDS 65 KT WITH GUSTS TO 80 KT.
64 KT....... 30NE 25SE 20SW 20NW.
50 KT....... 70NE 40SE 25SW 35NW.
34 KT.......115NE 100SE 50SW 70NW.
12 FT SEAS..300NE 200SE 270SW 310NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.
(skipping a few reports)
FORECAST VALID 04/0600Z 17.5N 148.1W
MAX WIND 55 KT...GUSTS 65 KT.
50 KT... 50NE 30SE 15SW 30NW.
34 KT... 90NE 80SE 40SW 60NW.
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TROPICAL STORM CLAUDETTE FORECAST/ADVISORY NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL AL032015
2100 UTC TUE JUL 14 2015
TROPICAL STORM CENTER LOCATED NEAR 42.5N 59.8W AT 14/2100Z
POSITION ACCURATE WITHIN 30 NM
PRESENT MOVEMENT TOWARD THE NORTHEAST OR 50 DEGREES AT 18 KT
ESTIMATED MINIMUM CENTRAL PRESSURE 1005 MB
MAX SUSTAINED WINDS 40 KT WITH GUSTS TO 50 KT.
34 KT....... 60NE 70SE 60SW 0NW.
12 FT SEAS.. 60NE 120SE 60SW 0NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.
__________________________
Once you see where you are and how the storm is moving, you can then figure the best plan of action. Here is a video showing a couple choices. It uses the program Expedition, which does automatic routing, but that powerful feature is not required for a good judgement. Also since the sea state will likely be much larger than usual, chances are any polar diagram you have might be an overestimate of performance.
Sides of a Tropical Storm, Part 4. Maneuvering from a Known Side (15 min)
For full screen, double click and wait a few seconds for focus.
Before making a major decision such as trying to cross in front of a system you are nearly in line with, it is best to run for a while in the chosen direction to know true SMG and CMG possible. In some cases, like the example shown, it will be pretty clear if you can or cannot make it across to the navigable side.
That choice, however, is crucially tied to the proper identification of where you are relative to the storm path… and projected storm path. Recall if you do not have any data at all from the NWS, we must rely to some extent on the known average trend of a storm turning poleward at some point. The statistics of storm speeds and directions as a function of age is discussed in the text book, and hundreds of actual examples are online at the NHC to test these averages.
Please post your questions or past related experiences as comments and we can follow up here with that discussion.
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