Saturday high temperature 071819

A projected high temperature of 92 at Blacskburg on Saturday would be a new record for July 20. Roanoke is projected to hit 97, though 100 is possible. Heat index values will range 5 to 10 degrees above these numbers with dew points in the mid 60s to lower 70s.

It is possible Roanoke will see its first 100-degree high temperature in seven years this weekend. It is also possible the high will fail to make 80 degrees by Wednesday.

Probably, it will be a hair less than 100 this weekend and little more than 80 for high temperatures by the middle of the coming week after a strong cold front passes, but the change from the year's most extreme heat to a few days of relief from summer heat will be stark. 

Thick high pressure aloft building over our region from the west and northwest, essentially covering all of the central and eastern U.S., will allow temperatures to spike upward Friday through Sunday, likely into the 95-100 range for Roanoke and points south and east, with a few spots topping 100, and widespread 90s in sub-3,000-foot elevation areas to the west, including most of the New River Valley. 

Dew points running in the mid 60s to lower 70s will push heat index values  -- or an approximation of the impact of combined heat and humidity on the human body -- above 100 for the Roanoke Valley and low-elevations areas to the south and east. That moisture in the air may also allow for a few pop-up afternoon storms that will, locally and briefly, bring cooling downpours and wind gusts, but any rain that falls will only ramp up the humid feel after the storm passes.

A strong cold front will begin pushing through on Monday, ramping up the showers and thunderstorms, and leading to cooling northwest winds from an air mass originating in Canada that will drop temperatures about 20 degrees by Wednesday, with some areas seeing lows in the 50s and most high temperatures staying below 80, though probably creeping a little above that in the Roanoke Valley and areas south and east. A slow moderation back to near-normal 80s-lower 90s for highs/60s for lows will occur the remainder of next week, though there may be some reinforcing cool shots down the pike to keep the end of July relatively cool compared to the sticky June and first half of July we have had.

You may have seen me mention here or on social media before that the heat wave that builds in from the west or northwest is usually worse for us than the one that builds in from the south or southwest. Part of that is the development of downsloping westerly to northwesterly winds, compressing and heating as they glide down the slope of the Appalachians. Winds from this direction often bring us cold weather behind Arctic fronts in winter, and cooler weather as with the coming Canadian cold front, but even then, those temperatures are modified a bit by the downslope effect. With high pressure aloft and heat parked to the west and northwest, this becomes a hot wind.

Roanoke has not hit 100 degrees officially since July 8, 2012, when the high hit 102 on the next-to-last day of the heat wave associated with the June 29, 2012, derecho. It has only got as high as 98 once since July 26, 2012, that occurring exactly four years after that date in 2016, part of a heat wave with four days of 96-98 temperatures that became a national news story when then-presidential candidate Donald Trump complained about the heat in the Hotel Roanoke during a rally, to jog your memory.

It takes a little memory-jogging to recall heat waves since the "derecho heat wave" of 2012 because the four days in 2016 are really the only period that meets my definition of heat wave locally, which, for Roanoke, is 3 or more consecutive days of 95-degree temperatures. I know some use 90 degrees as the marker, but that's only 3 degrees above the normal high temperature for this time of year, and to me, a "heat wave' should imply something more extreme.

My definition is arbitrary, of course, but the point is that it has been a long time since we've had heat like we are going to see this weekend.

Roanoke probably will not establish any new record highs during this heat wave, as they run 101 (1977), 104 (1930) and 104 (1926), respectively, for July 19, 20 and 21, period of record dating to 1912. Blacksburg may have a shot at the July 20 record on Saturday, with the current mark standing only 91 from 1983. The July 19 record of 93 from 1977 and the July 21 record of 94 from 1983 probably won't be breached at Blacksburg, where official weather records start in 1952.

Something that could be set are records for warmest low temperatures, which are 76 (1996), 74 (2015) and 75 (2008) at Roanoke and 71 (1986), 69 (1996) and 68 (2015) at Blacksburg, respectively, for July 19-21.

This weekend's heat spike is part of a reorganization of the upper-air pattern that will center the "heat dome" in the West and therefore allow a southerly dip in the jet stream over the eastern U.S. by next week, radically altering the temperature profile at least for a few days.

So to get that summer relief, we have to go through summer's worst heat first. 

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Contact Kevin Myatt at Follow him on Twitter @kevinmyattwx.


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