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Don Paul: May not so hot, but don’t take that the wrong way

Of course May will be warmer than April. It doesn’t take a degree in meteorology to figure out that one. On the other hand, we had forecast March would be colder than February, thanks to the very warm last two weeks in February and a coming pattern change that verified that  forecast.

With the sun coming up to a higher angle in the sky, it would be much more difficult for May to be colder than April than it was for March to be colder than February. Through the end of the third week in April, this month has been a doozy across much of the nation, with unseasonably cold and snowy conditions from the northern plains to New England. Locally, the Buffalo temperature had been running a huge 8.4 degrees below average, and measurable snow had fallen on an April monthly record-tying 11 days. These numbers all will be changing for the better during the last week of the month. It’s not that we’re headed for unseasonable warmth; it’s more a matter of heading for seasonable temperatures, with a couple of days above average. The daily average will be creeping up toward 60. That should feel positively delightful for most of us.

The National Weather Service Climate Prediction Center issued its May outlook. It views a weak tendency for our temperatures to run above average for the month when all is said and done:

 

That is not a probability outlook brimming with confidence. As for even harder-to-pin-down precipitation, its confidence of wetter than average is a little higher:

I have a couple of problems with the above-average temperature forecast, albeit a low confidence one. We have wet soil in place. It will dry a little this last week of the month, but there will not be enough drying to get rid of all the mud in yards and fields.

 

The air above moist soil is more difficult to heat than air over dry soil. This is not an unusually large soil moisture surplus, but it’s sufficient to cramp the style of lower atmospheric heating. If we were likely to have a long period of dry weather to allow more evaporation of that moisture, that would change. But confidence for a wetter pattern is higher than for a warmer pattern. The trends for warmer than average and wetter than average are at odds, if they hold true.

Our best hope for more persistent warming is depicted in this National Weather Service headquarters upper air ensemble:

It shows a flattening of the pattern as we move toward May with fewer cold troughs/areas of low pressure allowing Canadian air masses to drop in, and more influence from Pacific air masses. The European ensemble allows for some decent warming during the first 10 to 15 days of May, but then more instability returns to the pattern with more troughs traversing the lower 48 states by mid- and late month.

So we have the Climate Prediction Center showing a slightly better than even probability for above average temps and a slightly higher probability for wetter than average conditions for the month of May. And we have me being suspicious, due to the wet soil and the persistence of the pattern we’ve had in some weakened form staying with us in parts of May for temperatures. Worst case scenario: Slightly below average temperatures would still seem awfully mild compared to most of April, and wetter than average means wet some of the time, and white none of the time. Allow me to finish with CPC’s almost unbridled optimism about the period of May through July for temperature probabilities:

 

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