My Fall Forecast

5:19 pm

If you ask an astronomer when summer ends, they’ll tell you “September 23.” That is the date of the autumnal equinox, the date when the day and night are approximately equal in length (the day is always a couple minutes longer because the sun does not set all at once and therefore radiates light until the very last part of it has set below the horizon). Ask a meteorologist though, and they’ll tell you that autumn begins September 1, as that is the beginning of ‘meteorological autumn,’ which goes until November 30. During this time period, our nights get longer while our days get shorter, our daytime and nighttime temperatures plummet, stronger storms start to enter the region, and people become more and more obsessed with the Seahawks. Our earliest lowland snows have generally appeared the last two weeks of November, and our stormiest week is Thanksgiving week. I will have an exclusive winter 2015-2016 forecast on WeatherOn that will be released November 1, so be on the lookout for that. For now, I thought I’d give a more informal autumn forecast.

This has been one of the more abrupt summer-to-fall transitions that I can remember. June and July both broke their respective all-time average temperature records and were extremely dry as well, while August was our 4th wettest month on record and was capped off by a historic windstorm at the end of the month.  Now, it’s mid-September, and according to the National Weather Service, showers or rain are forecast for the area for the entire week, and highs likely will not reach 70 in many locations.

We’ve gone from a pattern of persistent ridging directly over our area to one of persistent troughing, with the ridge having retrograded much further west. Now, storm systems and “shortwave troughs” are undulating on the east side of this ridge and sliding down from the northwest into our area, giving us cooler temperatures and precipitation than anything we experienced during June, July, and even much of August.

Valid 05:00 am PDT, Mon 14 Sep 2015: Retrieved from UW Northwest Modeling Website

So far, this pattern has been pretty consistent. Will the mega-ridge return? I don’t think so. That isn’t to say we won’t have some sunny days, but we won’t be seeing the mid-80s until 2016. In fact, we may be done with the 80s for all of 2015. Time will tell.

I’ll get into a brief month-by-month breakdown soon, but first let’s take a look at some of the major factors that are influencing my forecast. The main factor I’m looking at is El Niño, not only because it is an incredibly influential phenomenon for our weather around here, but because this El Niño could end up being the strongest El Niño ever recorded, perhaps even surpassing the 1997-1998 El Niño. Right now, sea-surface-temperatures in the tropical Pacific are around 2 degrees Celsius above normal in the “Niño 3.4” region, which is the region of the eastern/central tropical Pacific that is most telling for the status of an El Niño. In addition, the trade winds have weakened and convection that typically forms over Indonesia has shifted westward, two things that are also consistent with a strong El Niño.

El Niños tend to give warmer temperatures and drier conditions to northern states and cooler temperatures and wetter conditions to southern states. These effects are most pronounced in winter, when El Niño is typically strongest. However, this pattern manifests itself at many other times during the year. During the spring, we had a persistent trough off southern California that not only gave them desperately-needed rainfall but gave us quite a bit of lightning, especially over the Cascades and in Eastern Washington.

Credit: University Corporation for Atmospheric Research

But remember… just because you have an El Niño does not mean that the above scenario will come true. It only means that the probability of it becoming true is higher. Also, just because a location is forecast to be warmer and drier than normal does not mean it can’t have any cold and wet days. This week doesn’t exactly look “warm and dry” to me.

SST Anomalies 9/14/2015. Retrieved from NOAA Office of Satellite and Product Operations (OSPO)

I’m also looking at the “Blob,” which is an area of anomalously warm water in the Northeastern Pacific. The Blob was quite strong during the summer, but with the cool and wet weather the past few weeks, it has decreased in ferocity. I predict that El Niño will ultimately destroy the Blob due to increased storm activity into California, but it is still around for now and will influence our weather by warming us up slightly. I could look at the Pacific Decadal Oscillation and many other oceanic and atmospheric factors, but with the El Niño being so obscenely strong, it should overwhelm any of those other factors, including our beloved Blob. Just take a look at the SST anomalies right now in the tropical Pacific. Incredible.

September:


September is halfway complete, so we don’t need to use El Niño to try and predict what you should wear on Wednesday (you should wear a raincoat!). The models are painting a cool and rainy scenario for us the next two weeks… the above forecast provides evidence of that for this upcoming week, and the graphic below shows that we are expected to see below-normal temperatures and above-normal precipitation next week as well.

October:


Our longest range deterministic global model, the GFS, has been consistent in keeping our current pattern, with a ridge offshore and trough over the Northwest. As a result, we will likely start off cooler and rainier than normal. However, El Niño will continue to strengthen, and as a result, our precipitation and temperature patterns will likely begin to resemble those found in the strongest of El Niños, meaning southern regions like California and Texas will be wetter and cooler than normal while we will be warmer and drier. I should note that the precipitation signature over our region with an El Niño is weaker than the temperature signature, so while I am confident that the late autumn and winter will be warmer and drier for our region (the models have been advertising this for a long time), I am less confident about the precipitation than the temperature.

November:


By this time, El Niño has kicked into overdrive. We will almost certainly be warmer than normal, and we will probably be drier than normal as well. November is our stormiest month, and El Niño years tend to bring less of the big storms, so it is less likely we will see one of those. It’s also less likely we will see any of those early lowland snows I was talking about. I’m making it out to be a pretty lame month for us weather-wise, and it very might well be. But I want to emphasize this: it is unlikely that this winter will be as warm as last winter. Last winter was crazy warm, and our snowpack paid the price. The culprit? A consistent ridge of high pressure, pushing our storms to the north and leaving the entire West Coast warm and sunny. That is not predicted to happen this year. Even during the record-breaking 1997-1998 El Niño, snowpack was 70-80% of normal throughout most of the mountains here. Last year, our snowfall was around 25% of normal, with some places in the Olympics not breaking 10%.