Grib Practice 01 (From the Starpath Grib School)

Use grib file: GFS-sample-01.grb2. We use decimal Lat-Lon. N and E are positive. We use all true directions. Underway you may make other choices, or even use all compass on deck, and all true in the nav station. We will use all UTC for times, but underway you will have to go back and forth from UTC to ship's time.

You can use any Grib viewer for this exercise, XyGrib, qtVlm, OpenCPN, or the fine commercial products from LuckGrib and Expedition. Other nav programs such as Coastal Explorer or Time Zero also show grib files with convenient measurement tools. Don't forget the meteogram/meteotable option that many offer. Each Grib Viewer has its own way to set the forecast time to a specific value, but all let you do this. For more practice, consider our online course in marine weather.

(1a) What is the interval time step in this set of forecasts? (1b) What is the file size for this forecast set?

(2) What is the time and date of the first and last forecast in this set of forecasts?

(3) At the time of the first forecast, the Pacific High has two peaks. What are the peak high pressures in each of these Highs?

(4) what is the distance between these Highs and the bearing from northern most to southern most?

(5) Set the isobar spacing to be temporarily 1 mb (reduced from the normal 4 mb, which is typical default). Then looking at 12z on Feb 23 off the coast of California there are strong winds offshore and weak winds over the land of California, separated by a prominent pressure pattern.

(5a) Is that pattern a trough or a ridge? Notice that this pattern is much more prominent at 1 mb spacing than at 4 mb.

(5b) how do you convince yourself this is a trough and not a ridge? After looking at the map with this pressure setting, you might want to return to the standard of 4 mb or maybe 2.

(6) This is a very unusual winter pressure in the Pacific in that we have an exceptionally strong Pacific High, with ocean-wide circulation around it. What are the typical trade winds south of this High on Feb 26 00z

(7) On Feb 26 at 00z, what is the pressure at (24, -144)?

(8) What is the pressure tendency according to this GFS forecast underneath the Golden Gate Bridge at 00z on Feb 26?

(9) This is an exercise in locating a place on the forecast map and using a measurement tool to find a location. Viewers do this in different ways. Once you find the point you care about you might put a marker there if your program allows that.

(9a) What is the wind speed, wind direction, and pressure 2 miles west of Pt Reyes Lighthouse (38.0, -123.02) at Drakes Bay on 21z, Feb 25?

(9b) What is the wind at that same point 6 hr later?

(10) We often observe that when sailing in good weather from the NW quadrant on the West Coast, the first signs of approaching bad weather will be the wind backing down toward the south. Often we can spot that before we actually see the pressure drop or the clouds lower. Refer to the discussion surrounding Figure 2.1-9 in Modern Marine Weather. In these cases we expect the wind to back around to the south and build in speed as the pressure drops. Another common behavior is the pressure will continue to drop but the wind dies down as it backs around and then builds rapidly from the south. In this exercise we want to look at this behavior as bad weather replaces good. Assume we are sailing south on Feb 24 at 06z located at (47.7, -128.8).

(10a) What is the wind speed, wind direction, and pressure at that time and place.

(10b) At that location when do we first see the wind back around to the south and start to build?

(10c) Has the pressure changed much during this period?

(10d) We have a guideline in the textbook that notable pressure changes that can warn us of increased wind are recalled with "4-5-6." This means a drop of 4 or 5 mb in 6 hr is our clear warning to pay attention. From the pressure we have been observing, when do we first see this criteria triggered? That is, when is the end of the first 6h period when the pressure has dropped by at least 4 mb? You can also see this guideline with related trend information the Help file to our marine barometer apps, which can be read online.

 

Answers:

(1a) One forecast every 3 hr. (1b) 4.8 MB. In short, this is way larger than you would download at sea, but in preparing a voyage on shore we can use any size we like.

(2) 12z Feb 23 to 12z Feb 27.

(3) 1038.6 mb (43.9, -137.5) and 1037.0 (37.3, -141.8)

(4) 438 nmi 207 T from 38.6 to 37.0

(5a) It is a trough (trof)
(5b) Put the cursor off the coast of San Diego and move up the coast offshore to watch the pressure rise as you move away from the Low below. Likewise, at any point along the trough move inland and then offshore to show that the lowest pressure is along the trough.

(6) 20 to 25 kts from 068 to 072

(7) 1023.4 mb

(8) Use meteogram: Pressure at 00z = 1023.0 and 3h earlier at 21z on Feb 25 the pressure was 1025.0, so the tendency is - (25.0 - 23.0) = -2.0 mb/3h

(9a) 11.5 kts at 325 1025.7 mb (2/25 21z)
(9b) 33.1 kts at 322 1023.7 mb (2/26 03z)

(10a) 15.9 kts at 334, 1032.9 mb.
(10b) Wind backs down and goes slack around 15z, then starts to build about 18z.
(10c) No! At 15z it is 1033.0 and at 18z it is 1032.5 mb

(10d) We have been sailing along in about 1033 mb so when do we first see 1029 mb? This happens about 22z, but at this point we already have 20 kts of wind. In a case like this when we have witnessed the wind back around and then the pressure start steadily going down, we should indeed be alerted well before this full criteria is triggered. We would also be seeing in this case the clouds lowering. This type of sailing and many others is much benefitted by a good calibrated barometer, which most mariners have in their phones already! See our marine barometer and barograph apps for tools designed for just this type of analysis. Remember to check the meteogram for a pressure plot, which can be analyzed with the 4-5-6 guideline.

Many thanks to racing navigator Andrew Haliburton for his careful review of this exercise.