Roanoke River forecast 052020

Wednesday morning forecasts project the Roanoke River to rise more than 4 feet above flood stage at the Walnut Avenue bridge to a crest of 14.4 feet, which would be the 11th highest crest on record since 1901.

Mountains don’t move — at least on any timescale we notice in our lifetimes.

Problems develop in our region when weather systems also don’t move much for a few days.

That is the dilemma we are in this week in Southwest Virginia.

We are in the middle of a multiday episode of frequent and sometimes heavy rainfall. Inches already have fallen at many locations in and near the Roanoke and New River valleys, and inches more are likely on the way. Most locations will easily top 3 inches total for the week (many already have), some will top 6 and a few may go over 10.

The rain gradually will become more intermittent Thursday and Friday. Sun and warmth will return for Memorial Day weekend, but with so much moist ground to evaporate, there still may be some showers and storms at times, especially in the afternoons.

The troublemaker for this soggy week is a “cut-off” low-pressure system just to our west.

The core of the jet stream flow, the river of air 5 to 8 miles above us that steers weather patterns, is shifting northward with the advance toward summer in the Northern Hemisphere.

On a more short-term scale, high pressure bulged northward over the eastern U.S. last week, rapidly pulling us from frosty mornings to summerlike afternoons, while also pushing the jet stream farther north into Canada.

A southerly dip in the jet stream winds, called a trough, began nosing into the U.S. over the weekend. That trough, however, has now become dislodged from the main flow of the jet stream over Canada. The higher speed winds aloft are circling in a loop, effectively “cut off” from the jet stream.

A low pressure system has been marooned just west of our region, generally over Kentucky and Tennessee, left to wobble only very slowly eastward and essentially spin in place.

The counterclockwise flow around the low is causing a fetch of moisture off the western Atlantic to be pulled northwestward toward the higher terrain of the Appalachians.

And that is where the immovable mountains become a factor. Sources of lift are very important in meteorology, determining where moisture can be raised, cooled and precipitated. Atmospheric sources of lift move around and can be difficult to fully detect. Mountain ranges don’t move and their location is obvious.

Day after day this week, this moisture off tropical oceans — the Gulf of Mexico and Caribbean are ultimately contributors, as well — is being lifted up and over a cooler air mass trapped against the mountains, and also up the gradually rising terrain from the coastal plain across the Piedmont, then the sharply rising terrain of the Appalachians.

This orographic lift, that resulting from rise in terrain, is enhancing what would already be a rainy scenario from the stalled upper-level low.

Mountains move perceptibly only in the course of millenia, but even a stuck atmospheric pattern will wobble loose in a few days.

As the week progresses, the deep low-pressure system will “fill in” and weaken as higher pressure controls the pattern around it. It will also start to gradually wobble northward, slowly bumped by subtle movements in the atmospheric pattern to the west.

The gaps between showers and rain bands will be come wider on Thursday and Friday. This will be a mixed blessing: More warming between periods of rain may allow more convective growth in showers and even some thunderstorms, possibly producing heavy downpours on top of saturated soils from days of prior rain.

Flooding will become a greater concern as the week progresses. We entered this week with May being pretty dry, but the soil is becoming increasingly saturated. Any heavier periods of rain could trigger localized flooding, and many area rivers may rise to near or above flood stage by late week

Temperatures have also become quite chilly for May, hovering near 50 as the rain pours down and locks in a cool, damp air mass against the mountains.

Warmth will come back quickly as the sun pops out over the weekend, with highs shooting back into the 80s just in time for the Memorial Day weekend. But as already noted, that warmth may trigger afternoon showers and storms.

The area’s rivers will be moving pretty vigorously after all this rain, so be aware of that if you have socially distant watery fun in mind for the holiday weekend.

Weather Journal appears on Wednesdays.

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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