A forecast for La Nina conditions in the
Pacific Northwest has been raised to an advisory, with colder and wetter
weather seen in October now expected to continue throughout the winter.
During a teleconference Thursday, NOAA
forecasters showed how colder and wetter conditions prevailed through much of
October, but not all of the Pacific Northwest. For example, those conditions
did not extend into most of the Northern Rockies.
Temperatures in the Pacific Northwest states
of Washington, Oregon, Idaho and parts of western Montana ranked in the top 10
coldest for the month of October.
Nationwide, however, October averaged out to
be warm and wetter than normal. “It is unusual to see a warm year that is
really wet,” said Jake Crouch, a NOAA climate scientist based in Washington,
D.C. Much of the precipitation nationwide average can be attributed to very rainy
weather in the Northeast toward the end of the month.
The long-range December forecast predicts
colder and wetter-than-average conditions in the Pacific Northwest, and those
conditions are expected to continue through January.
Weak La Nina conditions emerged in October, as
reflected by below-average sea surface temperatures across most of the central
and eastern equatorial Pacific Ocean.
Stephen Baxter, a forecaster with the NOAA
Climate Prediction Center in College Park, Maryland, said a second winter of La Nina was somewhat unexpected. “Six or
eight months ago we expecting El Nino and it actually turned out to be quite
different,” Baxter said.
An El Nino influence tends to deliver warmer
and drier conditions to the Pacific Northwest.
“La Nina is likely to affect temperature and
precipitation across the United States during the upcoming months,” the Climate
Prediction Center advisory issued this week states. There is now a 65 to 75
percent chance of that outcome, when the confidence for a weak La Nina was only
at about 55 percent just in the last couple months.
Generally, the advisory translates to a
prediction of warmer and drier weather across the southern tier of the United
States, and colder and wetter weather across the northern tier of the
Forecast often caution that another La Nina
does not necessarily mean this winter will be the same as last winter; but it
is likely to be similar with some differences.
Meanwhile, scientists say there is a greater
than 50-percent chance La Nina will also be in place February through April
2018, says NOAA.
This is the second winter in a row with a La
Nina, and like last year, forecasters expect this one to be weak. Last year,
this weather phenomenon extended from July 2016 to January 2017 before a return
to neutral El Nino Southern Oscillation (ENSO) conditions.
La Nina (translated from Spanish as “little
girl”) is a natural ocean-atmospheric phenomenon marked by cooler-than-average
sea surface temperatures in the central Pacific Ocean near the equator, the
opposite of El Nino (“little boy”).
Typical La Nina patterns during winter include
above-average precipitation and colder-than-average temperatures along the
northern tier of the U.S. and below-normal precipitation and drier conditions
across the South.