By Chris Martz | July 26, 2019Follow @ChrisMartzWX
While my winter outlook from last year was a fail, I’m not going to lose any sleep over it. Long-range forecasting is not easy. So, I’m back again and am issuing a fall forecast. The only way to get better is with practice and persistence.
There isn’t a whole lot you can do when it comes to long-term weather forecasting, it’s pretty much a guessing game based on the data you see and the factors you consider. It’s far from an exact science.
August is rapidly drawing to a close, and that means that autumn is on the way, SOON! September 1st marks the beginning of meteorological fall. That means temperatures, on average, start dropping off. Hooray!
Meteorological autumn often starts off fairly warm and hot across much of the U.S. with average highs in the 80s and 90s, with the exceptions being the Northeast, upper Plains, upper Midwest, and Rocky Mountains.
By the end of November, highs are bone-chilling cold across the Great Plains, Midwest, Northeast, and Rockies. States where U.S. Route 50 runs through (mid-section of the U.S.) have highs that are fairly chilly while it’s still very mild across the southern tier of the Lower 48.
However, there are always exceptions to the rule. With that in mind, what am I looking at now that will have major implications for autumn this year?
1. El Niño Southern Oscillation (ENSO)
One of the main things that weather forecasters monitor for long-term weather forecasts is the El Niño Southern Oscillation, or ENSO for short.
You may have heard of ENSO before. It’s essentially an irregular alternating weather pattern of sea surface temperatures (SST) and wind direction over the Equatorial Pacific Ocean.
Trade winds generally blow from east to west across the Pacific which pushes warm water on the sea surface westward, and as it gets pushed west, the water is forced to accumulate in depth in the ocean. As warm water accumulates in the West Pacific, the cooler water is displaced eastward, upwelling towards the surface in the East Equatorial Pacific.
The warmer water in the west adds more heat and water vapor to the atmosphere, and because warm, moist air rises over cooler, drier air, clouds form, causing unsettled weather and lots of rainfall over parts of Asia. Because all rising air must be balanced by an equal or near equal amount of sinking air, we see cooler, drier air sinking over the Eastern Pacific.
This sets up an atmospheric circulation, which enhances the east-to-west trade wind flow.
If the winds blow faster across the Pacific, causing more upwelling of cooler water off of the South American Coast, then the cool phase “La Niña” begins. If the trade winds weaken, or even reverse, and the warm water dominates the Equatorial Pacific, then “El Niño” begins.
So, what’s going on currently?
Sea surface temperatures in the Equatorial Pacific’s “Niño 3.4” region have been steadily cooling since June. Since August 12, sea surface temperatures have been cooler than average. Despite the fact that sea surface temperatures have been cooler than average recently, they are not yet cool enough to be considered a La Niña. We have to wait until they fall to 0.5°C below average, or cooler.
Latest model guidance suggests that the fall will have neutral conditions. The only model that is strongly suggesting we’ll see a La Niña is the NASA GMAO model. While this shouldn’t be completely ruled out, it is a statistical outlier in the dataset, so it’s not likely that the NASA model will verify.
I did some looking, and based on ENSO data provided from the Golden Gate Weather Service, and temperature data from NOAA, neutral ENSO events tend to favor cooler than average temperatures across the eastern U.S., especailly in the Midwest and Ohio Valley with warmer conditions in the Southwest.
Autumn in the Midwest, Southeast, Mid-Atlantic, and Northeast tends to be very dry during neutral ENSO phases.
2. Buildup of Warm Water in the North Pacific
While ENSO is certainly important, it is not the only factor that I consider in my long-term forecasts.
The sea surface temperatures in the North Pacific Ocean, especially around the Gulf of Alaska and off of the west coast of the U.S. and Canada can have a very large impact on weather patterns in the Northern Hemisphere, especially when sea surface temperatures are anomalously warm or cool, rather than near average.
There is currently very warm water in the eastern North Pacific and Gulf of Alaska, and a cool pool of water in the Central, North Pacific, which means that the Pacific Decadal Oscillation (PDO) is positive.
The warmer water off the west coast of Canada and in the Gulf of Alaska tends to produce a negative EPO, and a positive PNA, causing a ridge to build over western Canada, and a trough to form over the eastern Lower 48.
Because water temperature doesn’t change very quickly relative to air temperature, I do not see the North Pacific and Gulf of Alaska cooling down anytime soon.
Autumns with a negative PNA tend to favor a cool Southeast and warm Northwest in the Lower 48, while much of the country remains dry, with the notable exception of Florida.
3. Solar Activity
As all of us are aware, the sun is spotless and is going into a quiet phase.
Before everyone blows up the comments section with claims of a grand solar minimum, I want to make it clear that it is still uncertain at this point in time whether or not the sun is going into a prolonged (decades) period of low activity.
It should also be noted that average tropospheric temperatures are not affected by low solar activity during regular minimums. While the stratosphere and mesosphere do see temperature changes, it is only pressure patterns and the jet stream that are affected in the troposphere, which is where most weather occurs.
During periods with low solar activity, blocking patterns are enhanced causing the polar jet stream to become wavy. This causes both cold and heat extremes.
With that being said, let’s take a look at some of the years with low solar activity and the winters that followed.
The sun goes through an [on average] 11-year cycle of low solar activity to high solar activity and back to low, or vice versa. It takes about 5 1/2 years for the sun to go from an active state to a quieter state.
Many of the coldest and snowiest winters and autumns on the east coast that we have seen in recent memory have occurred during times when the sun was quiet. Some notable ones include 1965-1966, 1976-1977, 1995-1996, and 2009-2010.
Overall, autumns in the U.S. are quite cool during solar minimums, with the exception being the west coast.
As far as precipitation goes, the east coast is pretty wet. This is a big signal for mid-latitude cyclones, which can mean S.N.O.W. if cold air supply is sufficient. Areas like the Deep South, Midwest, and Southwest are typically drier in fall during solar minimums based on analog years.
4. Big Analog Years
One of the things I like to do when making long-term weather forecasts is to look at years in the past with similar atmospheric and oceanic conditions and compare them to what we are seeing now. A lot of the time, the past is the key to the future; Joe Bastardi has taught me that and it usually works pretty well!
We have already looked at similar years with corresponding ocean temperatures and solar activity. Now it’s time to see which years have the strongest connection to the present and one way of doing that is to see which years frequently pop up. The most frequent years are 1966, 1985, 1996, 2008, and 2009.
During those autumns, the Great Plains and Midwest were the coolest relative to average while the Mid-Atlantic, Deep South, and Southeast were also a bit cool. The Southwest and Northeast remained closer to average during those years.
These analogs also suggest that the Ohio Valley and Mid-Atlantic are typically wetter than average while it stays pretty dry in the Gulf States.
6. Long-Range Model Forecasts
While I don’t like to use long-range model forecasts that often, they do provide useful insight. I like to see how the model forecasts stack up against analogs.
Lucky for us, the models are mirroring the analogs pretty well.
The last few and latest runs of the European (I can’t show without a license), CFS, and Canadian models are all showing a trough over the east coast of the U.S., while ridging builds over the Arctic, Greenland, western Canada, and Alaska.
So, without further ado, here is my 2019 autumn forecast.
TEMPERATURES: I think that there’s a pretty good chance that the Great Plains and Corn Belt will have a pretty chilly fall this year, with temperatures averaging 3-5°F below average.
States like Wyoming, northeastern Colorado, Kansas, Oklahoma, Arkansas, Tennessee, northern Mississippi, Alabama, and Georgia, western Virginia and North Carolina, and Pennsylvania, and southern New York should expect temperatures to be 1-3°F below average.
The west coast and Southwest should see warmer than average conditions.
Now, this does not mean there won’t be periods where it’s opposite, but overall, I think autumn will shape up to be cool in the central, eastern U.S. and warmer in the west.
PRECIPITATION: Overall, I think much of the country will be see normal precipitation, give or take a few inches. The driest places will likely span the southern tier of the Lower 48, with the drought conditions over Texas, Oklahoma, Arkansas, and Louisiana worsening – something I’m concerned about.
The Mid-Atlantic and Northeast will likely see above average precipitation, but it shouldn’t be a “soaker.”
We will see if this verifies as the next three months unfold. There are NO guarantees that my forecast will be accurate, as there is high uncertainty in long-term weather prediction.
However, based on what I’m seeing, I think that there’s a good chance that we’ll see what’s in forecast.