We’re over halfway through “winter” as defined by snowfall accumulation and precipitation for the water year across much of the northwestern US. With news of the monsterzilla of an El Nino back in the fall all signs pointed to a warm, dry winter for the NW that would hasten drought conditions. El Nino has not disappointed and has rivaled the historic 1997-1998 El Nino in the various decathlon of metrics.
After a dismal October, abundant precipitation occurred across much of the NW in November and December effectively ending significant “drought” from the vantage of the US Drought Monitor.
And unlike the toasty winter last year, temperatures so far this winter have been just a bit warmer than normal (warm Oct, followed by ) allowing our mountain snowpack to be fairly close to normal (within 15% of normal for most basins).
That’s a good thing, right?
Expecting to be down by 15 points at the half of a basketball game and being tied is a good thing. No complaints here. But there is likely so explaining to do as to why forecasts haven’t panned out so far.
Let’s have a look at a few maps that show the average response for a strong El Nino year for Oct-Jan precipitation and temperature as well as the North American Multimodel Ensemble average forecast from September 2015 and compare these to observations. For those interested, my group at the University of Idaho, specifically Dr. Katherine Hegewisch, has been statistically downscaling NMME output from these models to enhance decision making for rangeland restoration projects in the Great Basin.
Past El Nino fuertes and the seasonal forecast: Precipitation across the NW has been on average below normal from Oct-Jan during past strong El Nino years as indicated in the figure above which shows standardized precipitation anomalies. The signal is a bit more pronounced over Idaho and western Montana and less so in the Cascades. The seasonal forecast from the North American Multimodel Ensemble project issued in early September 2015 called for subpar precipitation in the NW.
What we got: The El Nino moisture train has left its typical track across the western half of the US so far this year. Instead of the typical dipole where the northern half of the western US is dry and southern half wet, [almost] everyone’s been getting a bit more than normal.
While news headlines have exploited using El Nino as an adjective (e.g., El Nino fueled storm), it’s not clear whether the wet spell in December had much to do with El Nino. I don’t think we know why El Nino hasn’t panned out on the moisture side so far this winter. It might be due to an enhanced Hadley circulation that has pushed the subtropical jet further north than its typical spot during an El Nino year, or we might just have to wait and revisit this in a few months.
Past El Nino fuertes and the seasonal forecast: El Nino’s typically bring warmer than normal winters to the northern tier of the US and Canada, and some cooler than normal tempertaures to the southwestern US tied with the more active storm track in that area. Forecasts basically called for this and them some.
What we got: Temperature forecasts have largely verified when you look at the aggregate of the past four months. Yes, we had a cold December, but the warmth of October and relatively warm January temperatures above inversions have combined to bring temperatures for the northern half of the region above normal.
The months ahead
The seasonal forecasts haven’t wavered from their projections of a dry winter for most of the NW. We’ve faired well in spite of the strong El Nino, but know comes the more reliable part of the winter as far an ENSO is concerned.
An analysis of statistical relationships between ENSO and 61-day averaged temperature and precipitation for selected stations in the NW shows a couple important features.
- All stations show a significant (indicated by bold markers) positive correlation between an ENSO index (here I use the Multivariate ENSO index, positive values = El Nino) and late winter (61-day window centered in mid/late February) temperatures.
- Things are less clear with precipitation although a couple sites show negative correlations with Feb-Apr precipitation.
And for those keeping score, the groundhog failed to see it’s shadow today in Punxsutawney – meaning an early spring…somewhere. The forecasts have been worse than a coin flip in recent years, but don’t tell that to the groundhog or this might happen.