October 6, 2017 at 12:01 PM
A week ago, when we published our original October outlook, we knew this month was going to be warm. But we’re realizing now that we didn’t go nearly warm enough.
After looking at recent data, we had to publish this update. The odds are much higher now that not only will this month be warmer than last October, it could challenge the warmest October on record in D.C.
To understand why, we have to look at Alaska, of all places. A massive trough of low pressure is forecast to take shape over the state, which will have ripple effects on our weather downstream. As that trough digs deeper, the jet stream will surge north in the Lower 48.
We saw this pattern many times last year. A large upper-level gyre like this sends a strong warm flow across the Continental U.S. The lack of high pressure around Western Canada or Greenland also prevents cool air from streaming south. In fact, what’s shown on the map above is similar to what we saw in October 2007, which is currently the warmest October on record in D.C. The average temperature that month was 67.1 degrees. An average October is around 60 degrees.
Recent runs of NOAA’s long-range forecast model clearly indicate the effect the Alaska trough will have on our weather. Temperature anomalies in this forecast are 3 to 4 degrees warmer than normal through the end of October.
Perhaps we could see some late-month variability to prevent this extreme warmth from setting up. But that seems unlikely.
In the October outlook we wrote last week, we said, “Overall, we favor October to end up somewhat warmer than normal (by 0.5 to 2 degrees), but the pattern has a chance to change to a cooler one by the middle of month.”
Our revised version: “Overall, we think October will be 6 to 8 degrees warmer than normal and will rival the warmest of all-time.”
What about rain?
Our rain outlook of 1.5 to 3 inches (average is 3.4 inches) looks okay at this point. The remnants of Hurricane Nate early next week should deliver our first 1 to 1.5 inches for the month, but then the warm pattern will shift dry. We can “never say never” with regard to future tropical events, but for now, it looks like Nate’s moisture will provide some much-needed rain.