Have you had any cold weather? maybe some damp cold weather?

I dont think its a Magnesium deficiency. That usually shows signs of yellowing and doesn't effect the perkiness of the fronds.

Im guessing something is wrong with the growing point, bud rot maybe?

goodluck

Perhaps the crown is being watered to often...similar effects-
cold damage?

It's 85 today, and I think the coldest it's been this fall is around 45. This palm has been through sub-freezing weather numerous times in years past.

We had a couple of desert downpours with flooding in the last six weeks, but it is sited near a block retaining wall on one side with lawn on the other with a great drainage system, and it gets no water spray from any irrigation source.

It has maybe four 1/2 GPH drippers watering it, though when I returned for the winter the idiot landscapers had it dripping for 50 min 5 days a week, which is a lot of water. I cut it way back about a month ago -- normal for this time of year is about an hour a week, though I'm a bit higher this year because of the unusually warm weather (40 min x 2/week). Excess water appears to be the most likely cause. It no doubt has it's "toes in the water" under the lawn now and I suppose I could shut down both the drippers on that side.

The frond that has just opened (right of the center point in the photo) DOES have yellowing in a band on it (see attached), which is what made me wonder about magnesium.

My first inclination looking at the picture was root damage. Sounds like root rot is the culprit. Too much water thanks to the landscapers.

Washingtonias are generally desert palms are they not? I don't think they really need watering at all.

Here where we get about four inches of rain a year, believe me, everything except for a very few native plants needs some watering to survive! (Mesquite, Nevada).

Magnesium deficiency always affects oldest fronds first, never the youngest ones. For Washingtonia palms, this would be orange leaf tips on the lower fronds. That tree looks like a victim of phytophthora root rot to me.

Thanks everyone for the opinions. I dug around the root ball at the surface & the roots look healthy there. I've shut down the drippers & will withhold water for quite a while & see what that does. To add to the excitement, it's supposed to be a low of 26 to 32 degrees tonight! Not much else I can do but hope now!

Did the yellow leaf pop out fast or just recently open?

May have been stuck closed....

Yes. They all appear to be "stuck" and having a hard time opening. That one opened when the wind blew and forced it open. The others following it look the same way and appear to still be stuck. I wonder what causes that?