Friday PM weather map 071219

By Friday evening a cold front will have passed our region, pressing showers and storms south of us. Meanwhile, what is expected to be Hurricane Barry is forecast to be nearing the Louisiana coastline.

UPDATE 7:50 PM, 7/11/2019: Showers and storms have moved east of the Roanoke and New River valleys and nearby areas, and the setting sun will cut the fuel source off for the showers and storms still back in West Virginia and Ohio.  A cold front pushing through early Friday will bring us drier, more stable and (for a day) slightly cooler air, before we heat back up over the weekend. Saturday also looks dry, with some low to mid 90s highs possible in the Roanoke Valley and points south and east, with maybe a hint of scattered afternoon showers and storms returning by Sunday afternoon. Tropical Storm Barry is barely a tropical storm, with 40 mph winds, and likely headed for a late Saturday to Sunday landfall in Louisiana as a stronger tropical storm or possibly a low-end hurricane. As noted below, its main inclement weather concern will be intense rainfall of many inches into already high rivers and streams. Its main effects will stay far away from Southwest Virginia, perhaps spraying a little moisture our way by mid to late next week. END UPDATE

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A low-pressure system in the northern Gulf of Mexico is expected to become a tropical depression later on this Thursday and, quite likely, a tropical storm named Barry by Friday morning. (UPDATE 4:30 PM, 7/11/19: The system was officially upgraded to Tropical Storm Barry earlier today. END UPDATE) The system may be a hurricane by the time it makes landfall, probably in Louisiana on Saturday. Regardless of the final strength of its winds, this could be a devastating flood disaster for much of Louisiana, including New Orleans, as rivers are already extremely high

The tropical system will only have an indirect effect on Southwest Virginia weather this weekend, and probably only minor effects, if any, into next week.

The first order of business locally is a cold front that will press eastward overnight, passing through our region from the northwest early Friday. Ahead of the front, heat and humidity will bubble into some afternoon and evening thunderstorms. Every location in our region may not see a storm, but those that get in the core of the strongest storms will see torrential rain for several minutes, some potent cloud-to-ground lightning and possibly locally damaging wind gusts and/or hail. 

This is something of a "Trojan horse" cold front, as its ultimate result for us will not be cooler weather, but quite the opposite. Temperatures may back off a few degrees and the humidity will drop on Friday, but the front is actually the leading edge of high pressure that will build over us during the weekend and early next week and lead to hot, dry weather. Low to mid 90s highs are likely Saturday through Tuesday in the Roanoke Valley and points south and east, with 80s generally in the New River Valley and points west. I wouldn't rule out some upper 90s to near 100, especially Southside to Central Virginia, a day or two in that stretch.

The atmosphere will become strongly capped by warm, dry, stable air over the weekend, so development of showers and thunderstorms will be extremely limited, if it can occur at all. My rule of thumb is that anytime we are above 85 degrees, there is at least a 5% chance of an isolated shower or thunderstorm, owing to daytime heating, terrain effects and difficult-to-detect subtle atmospheric boundaries, disturbances, etc. That said, the general expectation over the weekend in our region is for dry weather.

What could then be Hurricane Barry is an indirect factor in our expected dry weather, as odd as that may sound. We will be in a broad zone of sinking air, or subsidence, that exists some distance around the intense convective updrafts of the tropical system, retarding storm development. Also, a closed tropical cyclone often tends to consolidate the moisture flow in its circulation and feeder bands, which will be moving away from us, not toward us, so there won't be a dense flow of moisture emanating from Gulf and overspreading us this weekend.

At some point next week, the remnants of the tropical system will likely curve into the Ohio and Tennessee valleys, possibly scooped up by a cold front and/or low-pressure trough. It is not out of the question that we could get some moisture from the system next week, but it would probably be diffuse and not an organized, widespread flooding rainstorm in our region. Rather, it might augment the showers and storms that would occur anyway. This won't be factor any earlier than maybe Wednesday next week. 

The Gulf tropical system is not going to be our problem, but it will be a huge problem for some places that have suffered major tropical and flooding anxiety in years past.

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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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