As predicted, La Niña is collapsing, which should be good news for those worried about water supplies.
La Niña is associated with cooler-than-normal surface ocean temperatures in the central tropic Pacific: what is known as the Nino 3.4 region (see below).
The latest NOAA forecast of temperatures in the Nino 3.4 area shows rapid warming, with the cool surface waters gone by April. Neutral conditions will be in place then.
So why should you care?
La Niña tends to produce big ridges of high pressure in the Gulf of Alaska, with occasional breakthroughs of moisture to the south (see schematic below). We saw a lot of this configuration after January 1; however, the pattern shifted a bit to our south.
In January, the coastal high pressure produced dry conditions over the NW, and the precipitation was mainly directed into central and southern CA. A modest shift in the pattern has made a big difference for the Northwest.
But as the La Nina weakened this month, the ridge weakened, and low pressure replaced it, producing the cooler/wetter conditions of the past weeks.
You can see this transition in the figure below, which shows upper-level maps for January 4 to a few days ago. Red indicated higher than normal neights (pressures)
January 7-21 had a huge ridge located off of southern BC....no wonder we were dry!
January 11-Feb 9 was transitional, with the ridge weakening.
Feb 6-21 was VERY different, with troughing (low pressure over the West Coast).
Thus, no reason to expect a return of the high-pressure ridging that kept us dry and warm.
Obviously your team has looked at this deeper, but i pulled the Yakima reservoir since the date all 5 have records (caveat i didn't try to fix some of the periods where quality improvements in dam height skewed things), but over the past 101 years, i tried to find when do you have very low reservoir levels following very high winter volume. A few days ago the reservoirs were 134% of historic average, i tried testing some 120% at an imaginary key day of day 90 of the year, or about March 31. Then hows the level on day 288, the historic low point. Seems pretty rare but not impossible to see a "high level" early followed by really low later". I am not sharing the data since surely you have better. I cannot past, but i found out of last 101 years we've seen 30 years with day 90 being 120% or more of historical. Then for those same years 10 were at only 80% of normal on day 288. So only 10 years out of 101 is about 10% of the time we've see high spring followed by lean summer (as i defined things). (wish i could paste this, but again you have the data).
ReplyDeleteIf this is the case, I hope the forecast models get the memo. Because right now, I see an annoying lack of precipitation out to day 11. If I wanted this kind of crap I would live in San Diego.
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