Although the poem does seem to have an inconsistent number of syllables in each line, the scansion of its rhythms and meter are actually fairly consistent if you emphasize the appropriate words. Sometimes with a poem like this, you have to read it aloud to find the appropriate emphasis, as it does not just fall in line with the number of syllables. The emphasis in terms of scansion is on the words that create the image or meaning in each line. Modern poetry often does not conform precisely to these meters in the same way poetry from earlier centuries does.
If one considers Masefield may be paying homage to Percy Bysshe Shelley's poem "Ode to the West Wind," which is written in iambic pentameter, then we can see Masefield's poem may be scanned as if it is also in iambic pentameter.
In this way, we san see a reading of the first line of Masefield's poem should emphasize the words "warm," "west," "full," and "cries." This gives the poem an iambic meter and feeling and is appropriate to its structure.