Severe Weather Discussion for June 17 - June 20

This is a severe weather discussion focusing on the time period of June 17 to June 20.

Long range forecasts from the Weather Prediction Center show an upper level low entering the Pacific Northwest, provoking ridging in the northern Plains, which is further amplified by an additional trough in the Great Lakes. This ridge then progresses eastward and strengthens, as the upper level low in the Northwest also shifts east, and the Great Lakes trough recedes into Canada. This opens the door for a substantial ridge to push northeast into the Southeast US, as the forecast for early next week shows. At that point, warm and unstable air will flow north into the Central and East US, which will eventually be utilized by an approaching weak trough seen best in Montana by the Day 7 forecast (bottom middle panel).

Projections of convective available potential energy (CAPE), a crucial ingredient for thunderstorms, put instability values over 5000 j/kg. When one considers that 2000 j/kg of CAPE is sufficient for the development and propagation for potentially severe thunderstorms, concern quickly rises over this forecast of incredibly high instability.

Lower level wind speed projections for Wednesday, the same time period as the above image, show a band of enhanced winds extending from Oklahoma into southern Canada due to influence from a storm system draped across the North Plains. Wind speeds inside this band look to meet and exceed 50 knots, meaning any thunderstorms that may form on Wednesday have great potential to be sustained. If there is little wind shear in an environment, a thunderstorm updraft can become entangled with the downdraft, which then weakens the storm. This phenomenon is what creates 'pulse storms', or those small-celled thunderstorms that form and then die off just as fast. However, with forecasts showing substantial lower level winds for Wednesday, more organized, sustained convection may be anticipated.

Additional updates concerning this situation will be published in the near future.

Andrew