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Severe weather to continue unabated from March into first week of April » Yale Climate Connections

A month that’s brought more than its share of meteorological travail ended with another day of trouble on Friday, March 31. Beyond that, yet another strong severe-weather producer may take shape in the first week of April.

On Friday morning, in its Day 1 outlook extending into early Saturday, the NOAA/NWS Storm Prediction Center placed an arcing region from Arkansas into eastern Iowa under a moderate risk of severe weather — the second-most serious of the center’s five risk categories. The moderate-risk zone was expanded, and two focused areas were upgraded to high risk, with the center’s outlook issued at 12:30 p.m. EDT Friday. These are the center’s first high-risk areas anywhere since March 25, 2021.

“At least a few long-track, strong to potentially violent tornadoes are probable, particularly over portions of the Mid-Mississippi Valley to the Mid-South,” the center warned. “Swaths of intense damaging wind gusts along with very large hail are expected as well.”

A map of the U.S. showing high risk of severe weather along much of the Mississippi River. Highest risk zones are in Iowa, Illinois, Indiana, Kentucky, Tennessee, Mississippi and Arkansas.
Figure 1. The Day 1 severe weather outlook for Friday, March 31, 2023, issued at 12:30 p.m. EDT Friday. (Image credit: NOAA/NWS/SPC)

The northern area of high risk was just south and east of a surface low set to intensify and move from the Central Plains toward the western Great Lakes. Behind a warm front, ahead of a surging cold front, and southeast of an intensifying surface low, supercells could erupt as soon as early afternoon across the warm sector southeast of the low from southeast Iowa into northwest Illinois. Surface temperatures may be 70 degrees Fahrenheit or less in some areas, but cold air aloft should more than compensate to create the instability needed for severe thunderstorms. Extreme wind shear will be capable of supporting strong to violent, fast-moving tornadoes, and very large hail could develop as well. By late afternoon into evening, the storms are expected to congeal into fast-moving lines or clusters packing fierce downburst winds.

Farther south toward the Mississippi Valley, the second high-risk area extended from roughly eastern Arkansas into western Tennessee and northern Mississippi. Supercells were expected to erupt somewhat later Friday afternoon, as a “cap” of warm air roughly a mile above the surface will keep storms from developing earlier. However, once the cap breaks, there will be more than enough instability to produce strong to violent, fast-moving tornadoes, given the exceptionally strong wind shear.

In between the high-risk areas, across far eastern Missouri and Illinois, storm coverage may not be quite as dense as further north and south. However, any isolated supercells that do develop could produce intense long-track tornadoes.

By late Friday night, the thunderstorms will likely grow into a broken squall line extending along the length of the advancing cold front, pushing well into Illinois and Indiana, western Kentucky and Tennessee, and northern Mississippi and Alabama. Short-lived but still-dangerous tornadoes will be possible, along with widespread damaging thunderstorm winds.

Even as the severe weather subsides by early Saturday, the surging cold front will bring powerful winds into the central Appalachians, where high wind watches and warnings are in place.

Tornadoes within squall lines: the quarry of a second-year experiment

The prospect of line-embedded tornadoes (or QCLS tornadoes, for quasi-linear convective systems) means that Friday could be a key day for the NOAA-led PERiLS Experiment (Propagation, Evolution and Rotation in Linear Storms), now in its second year. The multiagency project has set its fourth intensive observing period of the season for Friday night in the project’s Tennessee Valley subdomain.

More than two dozen mobile teams are participating in PERiLS. They’re deploying equipment that ranges from mobile radars (including Doppler on Wheels, or DOW) to truck-mounted sensors and uncrewed aerial systems.

“As recent tragic events have revealed, Southeastern tornadoes can be particularly violent and deadly, Kosiba said in a National Science Foundation news release in February. “If we can learn how, why, when and where they will form, then we can make better predictions, more precise and longer lead-time warnings and save lives.”

More severe weather expected next week from a relentless pattern

Yet another round of significant U.S. severe weather will likely erupt in the first week of April. An unusually warm Gulf of Mexico will help generate a very sultry flow of surface air by early in the week, with temperatures above 80 degrees Fahrenheit and dew points (an index of moisture) approaching or topping 70°F. As the next strong Pacific upper-level low sweeps inland, it may well encounter a richer surface air mass in the south-central states by Tuesday than the one expected on Friday.

Some severe weather could break out as soon as Monday night in parts of Texas and Oklahoma, but the most threatening day appears to be Tuesday, April 3. In its Day 5 outlook for Tuesday, the Storm Prediction Center has a broad zone in the central U.S. under a slight risk, and parts of Missouri and northern Arkansas under an enhanced risk — the highest possible classification five days in advance. Already, the center is flagging the risk for strong tornadoes, provided that any early-morning thunderstorms do not hinder instability.

A map of the U.S. showing that for most of the western U.S. into the Midwest, March temperatures were unusually cold. Meanwhile the Gulf Coast and Eastern U.S. had an above-average warm March.
Figure 2. Departures from average temperature for the period March 1-29, 2023. (Image credit: NOAA/NWS Climate Prediction Center)

A month of intense U.S. surface temperature contrasts draws to a close

Tornadoes rely on much more than strong fronts at the surface. In fact, an especially strong cold front may outrun a supercell thunderstorm, pinching off the unstable air needed to keep the storm alive.

However, the same multiweek patterns that tend to favor tornadic storms — especially a strong upper-level jet stream with distinct impulses progressing through it — are often accompanied by persistent temperature contrasts at the surface.

As shown above, the contiguous U.S. has been a tale of two nations in March when it comes to temperature. Unusually warm air, so characteristic of the winter of 2022-23, continued to blanket the Gulf Coast states, extending north into New England. Meanwhile, the northwest two-thirds of the nation has been unusually cool for March, especially from the Great Basin into the Northern Plains, as the relentless storm track slamming California has pushed chilly air and cloudy, moist conditions across much of the Intermountain West.

Jeff Masters contributed to this post.

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