By Joe D'Aleo
Monday, August 4, 2008
ACTIVE WINTER AND SPRING SHOWED HIGH DEGREE OF PERSISTENCE
Colder weather in Central North America thanks to cold PDO, La Nina and low solar
The winter and spring showed a great deal of persistence which is why anomalies were so large. Persistence tends to be high during stronger La Ninas and El Ninas as we have reported earlier.
Solar minimums also tend to exhibit less variability as the 27 day solar cycle still evident at the minimum is not strong enough to produce pulses in the atmospheric teleconnections like the PNA and NAO as they do when the sun is more active with flares and eruptive disturbances.
Look at the upper level pattern for the last 6 months.
You can see clearly a central North America trough and ridges off the west coast and a weaker one off the east coast.
The corresponding surface pattern has had the polar front in the mean dipping into the central states. This is where the coldest air has been most persistent as would be expected.
Storms moved along the boundary bringing heavy rains and flooding, heaviest from the central states into the northeast.
180-Day Observed Precipitation Valid at 7/23/2008 1200 UTC
This has been 150-200 % or more of the normal in many areas of the central. Thus the flooding and crop problems. High soil moisture provided a positive feedback to the cool and wet pattern just as dryness in the southwest helped perpetuate warmth and dryness there. All this even as La Nina faded,
Current 180-Day Percent of Normal Precipitation Valid at 7/23/2008 1200 UTC
Severe weather also occurred with these storms producing frequent tornadoes and more deaths than in recent years. We discussed earlier in this is story how this is typical of La Nina. 120 have died as of 27th July. This is tragic but falls far short of the well over 300 that died in the big La Nina outbreaks of 1965 (Palm Sunday) and 1974 (the Superoutbreak) or the 800 that died in 1927.
Of course part of this decline is the improved awareness and the onset in the 1950s of severe weather forecasts and warnings. However, if indeed we have moved into a cold PDO era with more La Ninas as we and others believe, we can expect more active tornado seasons.
We can as we also discussed in this April story, expect more cold northern snowy winters, more spring floods, summer heat and drought in La Nina onset years, more Atlantic basin and east coast landfalling hurricanes.