Precipitation threshold for semi-arid climates

The winter peak climate would be more arid, as summer is (obviously) hotter and so evap rates are much higher. Higher precip in summer mitigates the evap rate, so winter peak climates would be more arid.

No such thing as "arid" in colder climates. Dead, is NOT the same as arid!

I'd say it depends on the severity of the winter. In a climate with a mild winter (say with a mean of 6C or more) many plants still actively grow in winter and will accordingly use water in the soil. So, more water would be available for plants to use in a wet winter compared to a wet summer climate, since a larger proportion of the soil moisture would evaporate in summer.

If, on the other hand, winters are very cold and most plant life is dormant/dead, a lot of winter moisture will be unused. In these sorts of climates, a winter precipitation peak might correspond with aridity, especially if plant growth is limited in both winter and summer.

I used plant growth as a metric for aridity since plants (or the lack of) are generally what determines how arid a place seems. I voted for summer precipitation peak because most climates with winter wet seasons have mild winters.

I personally associate winter precipitation with semi-aridness. Many semi-arid climates (that verge on Med) in West Asia and Northern Africa have winter rainfall peak. So that seems like the 'norm' to me.

A summer rainfall peak always has some sort of a humid subtropical vibe to me, really.

Sorry, but cold climates can't be "arid". End of story.

With the evaporation rate being so much lower in winter than summer, you'd need a lot less precip to get the "arid" status. And at a certain point, no matter how low the precip, the climate cannot be called…"arid". Say, a winter averaging –11° / –2° C; grass will die regardless of precip, giving a faux-"arid" look.

It all depends on the avg temps of said summer peak and winter peak climates in their high-precip months.

Not according to the OP's examples. Mount Locke is strikingly cooler than LA, and has more reliable rainfall year-round (an extra 100 mm annually).

Even if we used Trewartha’s, LA has a threshold of 117 mm. 117*2 = 234 mm so it’s solidly Mediterranean.

Mount Locke has a threshold of 284 mm. 284*2 is 568 mm so it’s solidly semi arid.

Yes it’s cooler and has more rain, but there’s similarly warm summers and summer would have a much higher evaporation rate in both places so than mount Locke would have the equivalent of much less rain if it fell mostly in winter.

The same temp patterns as well? As in: same winters and same summers (like both 15° / 28° C in summer vs 1° / 11° C in winter, for instance)?

If so, then I'll change my vote.

I know, but doesn't the subtropical zone feature Dry-summer climates as well? I was thinking of places like Perth, LA, San Francisco, Turlock, Beirut, Adelaide, Rabat, Casablanca, Athens, Tel Aviv, Sicily (and many parts of the Mediterranean Basin), etc and etc, that look bone dry in the summer and relatively lush in the winter. 

I do notice that Wet-summer climates look greener in the summer, but in their dry winter they just don't appear as dry as these aforementioned Med areas do in the summer. At least, they're not that striking. And, if anything, they just look 'dead' or bare in the winter. Not dried out. So that's why I think the extreme Mediterranean rainfall pattern takes the cake for being more arid-ish, considering the places I mentioned above.

Mediterranean precip patterns in the temperate zone don't seem as pronounced (looking at central/northern Europe and western Canada) as they are in the south and western Asia/Northern Africa/SW USA, but I may be wrong. I think Central Asia is the only region in the temperate zone where winters are rainy and summers mostly dry (like Tashkent), though you can argue that they would border a continental climate.