A Developing Problem in the Great Plains as Climate Influences Change with Peak of Summer Arriving

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High pressure ridging across a region of developing shallow soil moisture deficit where deep layer soil moisture deficit already exits indicates a flash drought risk.

A Developing Problem in the Great Plains as Climate Influences Change with Peak of Summer Arriving

Discussion: Concern is increasing in regard to dry-to-drought potential and attendant heat risk as the seasonal climate changes with the arrival of peak summer heat in mid-July. In recent weeks frequent very warm 11-15 day forecasts have verified cooler as remnants from the cold season polar vortex have been able to emit short wave energy out of Canada and into the U.S. However, that capability eases back with the arrival of mid-summer when hot biases gain influence on the general climate.

What are those hot influences? There are two…one very broad and a second very specific. Broadly, typical of summertime during much of the past decade the northern latitude oceans are very warm…Pacific (+0.67C), Indian and Atlantic Ocean (+0.47C). Much of the warm SSTA is in the subtropics. This subtropical warming foreshadows strong subtropical ridge areas many times most intense by late summer and greatly increase heat risk in the middle latitudes. In short, the atmosphere in the middle latitudes wants to go hot after these conditions are established usually after mid-July.

Specifically, there is an area of deep layer soil moisture deficits in the west/southwest Great Plains. This area is about to undergo many days of hot weather and no rainfall which should easily produce a flash drought. This condition emerges beneath a 594+ decameter high pressure area according to ECM ENS in the 11-15 day period. Because of the widening dry soils this high pressure ridge could be stronger and cause wider dryness/heat that currently forecast especially into the 16-20 day period.

The U.S. Corn Belt could certainly be struck by hot weather at least in pulse and possibly for longer duration any time after mid-July possibly extending into August. The tendency for thundershowers to arc through this region shifts north.

Fig. 1: Plot of day 11-15 verification of all models. Note ECM ENS is typically best (red) while GFS OP is highly volatile.

Fig. 2: The ECM ENS day 11-15 upper air forecast indicates a moderate-strength ridge over the southwest Great Plains. The concern is the ridge area could be stronger due to the developing dry soils beneath.

Fig. 3: Shallow soil moisture is drying out in the southwest Great Plains (across areas already very dry in the deep layer).

Fig. 4: The GFS ENS forecasts little to no rainfall in the west/southwest Great Plains the next 15 days.