Shortwave generating machine

The rest of May is shaping up to be unsettled across the central United States as a perfect setup for shortwave generation in is place.

The model output for this afternoon shows what the synoptic pattern will bring for at least the next several weeks. Shortwaves (mid-level vorticity) are formed either from speed shear or curvature in the 500 mb flow. An effective way to generate shortwaves is to have a broad cyclonic flow with pockets of fast moving air (jet streaks). Since jet streaks are formed from tight baroclinic gradients that result in highly sloped constant pressure surfaces aloft, an ideal situation would be to have a sub-tropical high directly south of a polar trough. This is exactly what you see from the Thursday afternoon model output. Note the anticyclonic flow directly beneath the broad trough over southern Canada. What you see is the remnants of an anomalously cold winter hanging on in the face of approaching summer warmth. This type of pattern will produce vort maxes in the flow every 2-3 days which will lead to a very unsettled pattern.

Note how little the overall synoptic pattern changes as we head towards day 4 (Sunday), day 7 (Wednesday), day 10 (Sunday, May 18), and day 14 (Wednesday, May 21). While the timing of the storms over the next two weeks cannot be taken directly from the model runs, there is definitely the potential for unsettled weather throughout the period. Showers and thunderstorms will occur roughly every 2-3 days along this general storm track.

Confidence in the overall pattern is high, as shown by the high confidence levels in the shading in temperature and precipitation anomalies for both the 6-10 day and 8-14 day forecasts. Even the May outlook shows the same thing.

This is the type of pattern that can result in late-season snows for the Northern Plains, Great Lakes, and New England. The severe potential in this type of pattern is tricky, since the broad cyclonic flow prevents maritime tropical air from getting north of the Gulf Coast states and Southern Plains. Without the deep low-level moisture in place, instability can be too marginal to get much in the way of severe weather for the Mid-South.

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