Radar storm totals 0930

Doppler radar estimated rainfall as of early Friday evening indicates many locations east and north of Roanoke got 4+ inches of rainfall, while a streak to the west got 2+ inches with localized 4+ inches. This is an "estimation" and may not perfectly grasp rainfall at any given location, but it gives a general idea of where it was heaviest and less so.

The nearly stalled upper-level low-pressure system still spinning over the Ohio Valley completely rewrote the script on how this week was originally supposed to go for us, weatherwise. Though it is gradually losing its influence, it may already be playing a role in deciding how next week goes, too.

About a week ago it looked as if a cold front would push well past us by the middle of this past week with a few days of cool, crisp, dry weather. But energy rotating through the dip of the jet stream pushing the front spun up the upper-level low wobbling in the Ohio Valley for the past 60 hours or so. This completely changed everything about the late-week weather, as the cold front stalled just east of us, and the upper-level spun moisture and easterly to southeasterly wind flow over Virginia. This led to training bands of storms that dumped heavy rain totals on some streaks (while somewhat less for others) and even sufficient shearing winds for rotating storms producing microbursts, enough tight spin for a few tornado warnings (though no confirmed tornadoes as of yet) and, combined with low freezing levels, some substantial and large hail reports. 

The preceding dryness -- the U.S. Drought Monitor still shows most of Virginia in "abnormally dry" conditions, as it is based on Tuesday data before the downpours -- turned out to be something of a blessing in disguise, as it prevented what could have been serious flooding with the levels of rain that occurred in some areas over the past 3 days. The Roanoke River at the Salem Pump Station came within a foot of flood stage on Friday.

The upper-level low is very slowly unwinding and getting nudged northward, feeling the influence of another low-pressure trough behind it. It began rotating a patch of dry air into Southwest Virginia on Friday, cutting off most of the rain west of the Blue Ridge, though bands of storms continued to the east. As it leaves high pressure will replace it, likely leading to several dry days with seasonable temperatures, 70s highs and 50s lows, for the most part. In time it will gradually warm to the point where we may see some 80s again, but just as the cooldown was short-circuited by the upper-level low, its lingering presence is delaying this warmup a few days.

More importantly, the upper-level low is a factor in an expected turn of Category 5 (as of 11 p.m. Friday) Hurricane Matthew,  currently in the Caribbean. This storm poses a major threat to Jamaica and Cuba over the next few days as it curves northward.  The nudge of the upper low, building high pressure, a stalled front just offshore and the influence of a new trough moving into the central U.S. may be just enough to bump Matthew east of the continental U.S., though there is at least some risk of a westward curve at some point. As this last week has shown, the weather can go off script from relatively minor influences, so it's important to keep an eye on Matthew and its progress in the days ahead.

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Contact Kevin Myatt at kevin.myatt@roanoke.com. Follow him on Twitter @kevinmyattwx.

 

Since 2003, Kevin Myatt has penned the weekly Weather Journal column, and since 2006, the Weather Journal blog, which becomes particularly busy with snow. Kevin has edited a book on hurricanes and has helped lead Virginia Tech students on storm chases.

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