The numbers are in for April nationwide. They show a continuation of the same pattern we’ve seen since mid February. That would be a cool Pacific Northwest & north-central states, but warm south-west, south-central, and eastern-states. So folks we are closing in on 3 months of gloomy, cool, and rainier than average weather. Wayne Garcia says he’s had enough, I shudder to hear what Shauna thinks about it when she gets in later (…if Shauna ain’t happy, ain’t nobody in the news studio happy…or something like that). Actually she’s real nice, but gets annoyed about the weather like anyone else. The image on the left shows the one month period, and the right shows February through April. It’s somewhat unusual to see a pattern locked in for such a long period. I would be REALLY surprised to see cool AND wet continue all the way through June…that would be 4+ months.
Now check out the reason, it’s real clear when you look at the 500 millibar maps (around the 18,000′ elevation). Think of it as a topographic map, showing the height of a certain pressure surface above the earth. Warm colors indicated a higher height, like a hill (we often refer to these areas as ridges or upper-level highs/ridges). The blues indicate lower heights, like a valley or dip (we call it a trough). Notice we have seen below-average heights over the Pacific Northwest and/or the north-central states for the past 2+ months, while ridging has been hanging on over northern Mexico, and the southern USA. This pattern is conducive to a stronger than normal jet stream across the middle of the country, possibly helping out with the record tornado month in the southern states. Compare the pattern the last few months with May 2009. Temps were definitely above average that month (not too extreme with no 90s), but almost all days in the 2nd half of the month were in the 70s and 80s. Upper level heights were a bit above average over us, keeping weather systems away and thus a much warmer atmosphere overhead.
So our very chilly and wet spring is a result of a persistent trough of low pressure near or over the northwest USA. Most likely it’s directly related to the end of the past year’s La Nina episode. Conditions in the Equatorial Pacific are about to turn “neutral” (no La Nina OR El Nino), but technically we are still in La Nina, and lots of La Nina springs are cool, but not always wet.
The big question is…do we see any change in the next 7-10 days? That takes us into the 2nd half of May, and unfortunately models don’t show a big change.
How about the past few La Nina springs? When did the weather return to normal?
2008: Other than a 5 day hot spell in mid-May, it stayed chilly, but not unusually wet, through mid-June. Then summer kicked in with no rain the 12th of June through the end of the month.
2000: This one didn’t get the “La Nina Springs are Cool” memo. April-June temps were above average. There was no long period of spring rain/chill. Then July-August were slightly cool.
1999: April through June were below average, but drier than average as well. You could argue that warmer/sunnier weather arrived the 3rd week of May, then off/on all through June. Almost no rain for two weeks in late May and early June. Then July was slightly on the cool side.
1996: Classic cool and wet La Nina spring! But a sudden change on June 1st. Much warmer and sunnier for June. The rest of the summer was a scorcher.
1989: This one seemed similar to 2000. No long period of spring rain/chill. April-June was above average. Then July-August were below average.
Whew…a long post, but lots of good info. Looking forward to comments below!
Chief Meteorologist Mark Nelsen