Grib Practice 02 (From the Starpath Grib School)

These practice exercises are best done after working those in Grib Practice 01. In both of these we 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.

(1) On 2/26 00z the wind on the approach to La Paz Mexico, south of Isla Del Espiritu Santo (24.37,-110.34), is over 22 kts from the north. What is the time and date when that will drop to 12 kts or less?

(2) On 2/26 00z, you are stuck with no wind in the center of a Pacific High at (43.6, -148.0).

(2a) When (time and date) do you expect to get wind of at least 10 kts at that position?

(2b) What will be wind speed and direction at that point at the time of your answer?

(3) Looking at isobars

(3a) On Feb 27 at 00z what is the latitude of the 1028 isobar at Lon -145.0?

(3b) What is the wind speed and direction on the 1028 isobar at Lon -145.0?

 

(4) If you were sailing about 45 nmi NE of Maui (21.3, -155.4) on the 24th or 25th of Feb, 2021

(4a) What time of day (UTC) would you expect squalls on each of these two days at that location?

(4b) Which day would likely have the more intense squalls at that location?

Answers:

(1) Wind at 00z 2/26 is 22 kts at 359; dropping slowly to 12 kts at about 07z

(2a) 2/26 1800z or a bit earlier.

(2b) 10 kts at 191 or so

(3a) 27.11

(3b) 24.3 kts at 072

(4a) Use REFC display. On 24th showers start around 9z and peak about 18z and gone by 21z.  On the 25th start about 9z, peak around 14z, and gone about 18z. Note that both of these times are mid morning local time.

(4b) Squalls of the 25th seem more widespread and a bit more intense. At that location the 24th peak at about 26 dBZ and on the 25th this is about 29 dBZ. Recall this is a log scale so that difference is a bit more than it might appear.