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Don Paul: How much rainfall is needed to fix drought problem?

That most of WNY is in a Severe Drought is serious but somewhat old news. How much rainfall we need to fix the problem is more complicated, and how much we’re going to get is the most complicated of all.

Here is a Thursday graphic from National Weather Service HQ in College Park, Md. (Actually, the graphic is from the Weather Prediction Center, a branch of the National Centers for Environmental Prediction…only the feds could come up with nomenclature like that!)

rain map

As for how much rainfall we need to fix the problem, it depends on which problem we’re addressing. We’ve already seen a greening up on some parts of the Niagara Frontier where some heavier showers have fallen. Shallow soil moisture has been improved somewhat in certain locations, while other spots remain extremely parched, including Buffalo and parts of the immediate metro area.

It appears we would need a range of 4 to 7 inches of rainfall, hopefully not in blinding, runoff-prone deluges to eliminate a good part of the drought. Less would be needed in southern Chautauqua and southern Cattaraugus counties, where recent showers and thunderstorms have brought some improvement. However, for many interior locations dependent on wells and small streams for irrigation, groundwater levels are low to extremely low. Groundwater takes longer to respond to a drought, and longer to respond to rainfall.

As for how much rain we’re going to get, particularly during Saturday into early Sunday, the above graphic is oversimplified by necessity. We simply lack the ability to pinpoint rainfall amounts 1-2 days out from convective cells that cover small areas at any given moment. In the vast majority of convective rainfall events, coverage will be uneven; often VERY uneven. There can be isolated downpours that produce localized flooding and flash flooding. And there can be locales that get robbed of the benefits of all the water vapor in the lower atmosphere.

That has been the case on the Niagara Frontier many times this summer, with a daytime Lake Erie breeze stabilizing the atmosphere downwind, and preventing vertical development of clouds into big rainmakers. Now that Lake Erie is up to 77 degrees (4 above average), it can have a destabilizing effect later at night when the lake becomes warmer than the air.

In effect, it becomes a source of heat energy and moisture, and can “cook up” late night showers and thunderstorms unlike its cooling effect in the afternoon on hot days. Terrain features can also enhance local rainfall amounts. When water vapor is forced to ascend the hills to the south, it is lifted into slightly cooler air aloft which helps condense and squeeze out some of the water more efficiently than the flat lake plain.

So while the graphic above smoothes out the rainfall (it’s made from a human-machine mix, feeding off computer models and experienced meteorologists entering their own input), it remains likely some spots will get much more rain than others this weekend. In any case, however, I’m expecting next week’s Drought Monitor graphic (released on Thursdays) to show some reduction in Severe Drought coverage from this week.

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