Posted: 9:00 am Thursday, April 20th, 2017
By Kirk Mellish
Using the atmospheric signal from the expected evolution of an ENSO base state forms the basis for the early thinking on the coming summer.
To repeat for those new to it, the analog method of long-range forecasting seeks matches from past weather history to current and expected trends to extrapolate the future.
Pacific ocean is coming off a La Nina and has been hovering around neutral for a couple months now. A weak to moderate El Nino is expected to emerge by late summer or fall.
The warmest departures from normal are expected out West and in the East with the coolest readings compared to normal near the northern tier states and Great lakes. The best summer rains are expected in the middle of the country.
Whatever regions are the driest as we exit spring will have an enhanced risk for heat and drought. There is still some uncertainty as to where those areas will be.
Normally as we transition from La Nina to El Nino much of the country has a coolish summer. However, the global playing field is starting from a warmer than normal point.
So at this point much of the country is expected to be warmer than normal on average but the summer heat may back off some compared to climate normal late in the season.
We have to monitor the rest of this month and May for a positive drought feedback loop. Often a high pressure heat ridge surface and aloft in the summer will become established where spring drought was the worst.
The waters in the Gulf and off the east coast are above normal that can help build ridging as well but can provide humidity to fuel scattered thundershowers to prevent drought from getting out of hand, but only IF the ridging is not too strong.
Later in the summer there is a tendency for a trough in the nations mid-section when El Nino comes on the scene.
The current analog list: 1963, 1999, 2002, 2007, 2009, 2014, 2016.
Model outputs favor warmth for the summer like the analogs, but model outputs favor above-normal rainfall UNLIKE the analogs so the rain outlook is more mixed for summer as of now.
PRELIMINARY SUMMER TEMPERATURE OUTLOOK:
PRELIMINARY SUMMER RAINFALL OUTLOOK:
Take note that the analog years differ some from the composite of all past El Nino summers (differences arise from strength and timing and other influences):
EL NINO SUMMER TEMPERATURES WITH DECADES LONG TREND:
EL NINO SUMMER RAINFALL WITH DECADES LONG TREND:
AVERAGE OF ALL EL NINO SUMMERS REGARDLESS OF MATCH TO THIS YEAR:
El Nino projections:
El Nino in the summer and autumn also correlates with a reduction in the total number of tropical storms and hurricanes from the long-term average in the Atlantic and Caribbean.
I will have a final summer outlook by early June. Follow me on Twitter @MellishMeterWSB.