Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 2:23 PM GMT on April 17, 2013
The 13Z Convective Outlook issued by the Storm Prediction Center this morning included a moderate risk of severe thunderstorms over parts of the Southern Plains for this afternoon and this evening (graphic). It's a difficult forecast, primarily because of the uncertainty associated with the disposition of a shortwave in the subtropical jet stream over the Southwest. Nonetheless, discrete to semi-discrete supercells (storms with rotating updrafts) will likely develop, producing damaging winds, large hail, and some strong tornadoes.
The low-level jet stream, which will reach speeds in excess of 50 knots at 850 mb (roughly 1500 meters), will likely play a pivotal role in the outbreak of supercells in the area at moderate risk for severe weather later today. Check out the 12-hour NAM forecast of 850-mb isotachs (color-coded in knots) and 850-mb streamlines below (larger image).
The 12-hour NAM forecast (valid at 00Z this evening) of 850-mb isotachs (color-coded in knots) and 850-mb streamlines. Larger image. Courtesy of Penn State.
Before I elaborate, you might be interested to know that I use the term, "low-level jet stream," to indicate that the speedy, southerly winds are a consequence of the large gradient in heights at 850 mb. In my view, the driving force for a low-level jet stream is different than the physics associated with a nocturnal low-level jet...decoupling (fodder for another blog). The bottom line here is that, in my world, there's a fundamental difference between the underlying meteorology associated with a low-level jet stream and a low-level jet.
At any rate, the low-level jet stream will heighten the risk of supercells later today by increasing the vertical wind shear in the lower troposphere. Of course, the large vertical wind shear between the ground and an altitude of six kilometers is more important because it provides an environment that prevents updrafts and downdrafts inside individual thunderstorms from interacting, paving the way for longer-lived, more organized storms. But the large vertical wind shear in the lower troposphere, say from the ground to an altitude of two kilometers, paves the way for convective updrafts to rotate (see the annotated photograph of a supercell below).
Vertical wind shear in the lower troposphere produces spin around a horizontal axis. When this spin encounters a convective updraft, it tilts the spin into the vertical, causing the convective updraft to rotate and setting the stage for a supercell. Courtesy of Jessica Higgs.
Referring to the photograph above, vertical wind shear in the lower troposphere produces spin around a horizontal axis. When this spin around a horizontal axis encounters a convective updraft, it's tilted into spin around a vertical axis, causing the convective updraft to rotate and paving the way for a supercell to develop.
Okay, that's my stripped-down explanation. If the complete truth be told, the more technical version involves storm-relative winds and storm-relative inflow. Interested? Check out my more scientifically sophisticated explanation (it's rather brief, so don't get nervous).
When I was teaching mesoscale meteorology, I always likened the storm-relative inflow associated with supercells to a spinning noodle of spaghetti. The rate at which the convective updraft of a developing supercell "ingests" or "slurps up" the spinning spaghetti noodle can be likened to storm-relative helicity, a topic I will reserve for a future blog. The bottom line is that storm-relative helicity will be high over the moderate risk area (15-hour SREF forecast of probabilities of storm-relative helicity in the lowest kilometer greater than (or equal to) 150 square meters per square seconds, valid at 00Z this evening), so some strong tornadoes are likely.
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