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Spring-like temps to come to an end;
t-storms Saturday break the warmth
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EPAWA Autumn 2015 Outlook

After a warmer than average summer and a rising El Niño upon us, it is time to examine the Autumn projection from EPAWA. It is a time of transition from the hot and humid summer months to the cold and snow waiting on the other side. It is also a time for capturing and enjoying some of nature’s most beautiful moments, as well as the painstaking fallen leaves cleanup.

autumn

Below we take a look at the technical discussion that made up the EPAWA Autumn Outlook for 2015, and below it, a month by month breakdown of temperature and precipitation departures by location:

Technical discussion follows:

Almost everyone has heard by now that we are in an El Niño currently, and that it is expected to become quite strong this Fall, and peak sometime (projected) in mid-Fall before subsiding. ENSO (El Niño Southern Oscillation) is a periodical climate change caused by variations in sea surface temperatures over the tropical eastern Pacific Ocean. The warming phase is known as El Niño, and the cooling phase as La Niña.

Currently, these equatorial waters in the eastern Pacific are WARM as shown in the image below. This is El Niño, and the moving graph shows sea surface temperatures that are increasingly above normal in this region:

The model projection for El Niño is for it to peak in mid-Autumn, and then decline from its peak as we head into winter. Below is the latest model plume for mid-August, forecasting the upcoming year of ENSO:

enso

September:

September will start out on the warm side, and we expect that to continue through most of of the month.  The warmer than normal waters off the coast will support ridging backing in over our region providing some warm weather.  While this is going on across the Eastern U.S., we will be seeing changes over the center of the U.S.  The increasing El Niño will begin to bring in cooler weather to the Central US.  We can already see this showing up with the increasingly strong El Niño, and the projected Southern Oscillation Index (SOI) drop.

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The image on the left is the latest SST (sea surface temperature) anomalies, with the image on the right showing the change in departures over the last month.  The increasingly warm waters off the Eastern US is building the confidence of a warm month. Also, the colder North Atlantic waters off the UK/Ireland are showing up on one of our analogs that matches up the best in October.  Our two favorite analogs going into the Fall will be the years 1957 and 1987.

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The first noticeable difference with the 1987 analog is the lack of warm water in the Eastern Pacific.  However, the warm phase of the PDO is impressive in 1987, but it reaches the positive values by having a very impressive cold pool of water in the Central/Western Pacific. The month of September of 1987 featured a PDO value of 2.44 compared to 1.84 for July 2015.

Both years are too cold with the waters off the Middle Atlantic/Northeast coast. This is likely underestimating the warmth in the Eastern US when you combine both analogs:

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Combining both analogs, the chilly than normal air in the center of the U.S. extending back into the SW U.S. is supported by the strengthening El Niño.  Latest SOI value projections show an impressive drop as we start the month.  This amplification should put us on the receiving end of some warm air before a transitional cooler period.

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October:

1987 has warmer waters along the Western U.S., with the cold pool over the North Atlantic in a very similar spot to current observations.  If we don’t see much of a transition going into October, then 1987 may overtake 1957 as the analog of choice.  But both do show cooler than normal conditions moving into the eastern half of the U.S.

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November:

The 1987 analog does not fit well beyond October, as the ENSO peaked. Forecast models currently show the ENSO peaking a little later.  Most analogs do show warming in November, but 1957 is much more up and down.  1957 featured warm conditions but the transitional cold shots were impressive enough to balance out the month to near normal.  With the lack of snow cover build up expected, based off of a positive Quasi-biennial Oscillation (QBO) through Fall, we do not think these cold shots will be as impressive.

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1957 has similar QBO values to our current cycle, and could very well allow more colder episodes in the eastern U.S. in November. Other analogs such as 2002 show much more colder air available for November.  However, this was a positive QBO that was dropping towards neutral in the Fall. 1963 was a strengthening El Niño with a rising QBO with similar values expected, the end result was a very warm November.

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EPAWA month by month breakdown of temperature/precipitation departures:

Temperatures:

september

Precipitation is expected to be slightly above normal in September in PA, near normal in NJ/DE.

EPAWA expectations for temperature departures, October 2015:

octobertemps

Precipitation is expected to be near normal in October in our entire coverage area.

EPAWA expectations for temperature departures, November 2015:

novembertemps

Precipitation is expected to be near normal in November in our entire coverage area.

About EPAWA seasonal forecasting:

We take great pride in our seasonal outlook product accuracy, and take it very seriously. Many argue that seasonal forecasting is done without a high degree of accuracy, but we have now successfully verified 12 of the last 13 seasons, missing only Summer of 2014 temperature departures during that stretch, a streak of success which began in the Spring of 2012. Research is compiled using a combination of analogs, current climate, observational data, and lastly, model projections. Seasonal forecasting with more detail is offered through our private client services… please visit http://epawaweather.com/private-forecast-services/ for more information.

Forecaster: Meteorologist Mike DeFino
Contributions to the outlook by Meteorologist Bobby Martrich
August 26th, 2015