Weather Journal: The day a heat wave began to break, by the numbers | Local News


Twice a day, the National Weather Service issues a daily climate report for its major weather recording stations, once a little after 5 p.m. and then in the predawn hours recapping the entire 24-hour cycle of the previous day.

In the region covered by the weather service’s office at Blacksburg, these sites are Roanoke, Blacksburg, Lynchburg, Danville and the West Virginia communities of Bluefield and Lewisburg.

Often there is more behind the numbers than just the raw data shows. Sunday’s climate report for Roanoke, measured by a weather station on the grounds of the Roanoke-Blacksburg Regional Airport, had some particularly interesting details.

This was Roanoke’s fourth consecutive day to reach 95 degrees or higher, the first time that has happened in three years. It was also the 21st of the last 23 days to reach at least 90.

Sunday’s highest temperature occurred at 11:30 a.m. This is not the way it usually works.

On a typical summer day, the temperature rises through the morning and into the afternoon, peaking somewhere between 2 and 5 p.m. Sometimes it’s even a little later, such as it was Friday, when it climbed to 97 at 5:30 p.m. after the 5 p.m. climate summary reported the high as 95.

On a sunny summer day, we can expect about 5 to 8 degrees more temperature rise from whatever it is at 11:30 a.m. But this didn’t stay a sunny day — clouds increased around midday, and storms initiated not long after.

Without the clouds and storms, Sunday probably would have been Roanoke’s first 100-degree day in seven years. Saturday almost was, reaching 99 degrees.

This occurred in the very last moment of the day, at 11:59 p.m.

Again, this is not when we expect a daily low. Typically, it is within an hour or so of sunrise, when the night’s cooling has maximized and the day’s heating not quite set in.

The lowest it got Sunday morning was just 77 degrees. If that had stood all day, that would have been the warmest low temperature ever recorded on July 21, beating out 75 from 2008.

But it didn’t stand. The downpours and downdrafts from the early afternoon thunderstorm pulled Roanoke’s official temperature all the way down to 72 at 1:23 p.m., which at that point, became the low temperature, supplanting any shot at a record.

We often call it a “morning low,” and it usually is, but it is officially recorded as a daily low and can occur at any point during the day.

Precipitation: 2.76 inches

That is a lot more than most locations in the Roanoke Valley got on Sunday. Some parts of southwest Roanoke and the Cave Spring area got little or nothing.

It’s all the more impressive considering 2.62 inches fell between 1 and 2 p.m. — truly a torrential downpour, and a localized one that just happened to pour into Roanoke’s official rain gauge.

That thunderous cloudburst suddenly reversed a dry July at Roanoke’s official climate station. There had been just 0.36 inches in the 20 preceding days, the third driest July 1-20 period on record. That downpour raised the rainfall to almost four-tenths of an inch above normal for the first three weeks of July, with additional rain both Monday and Tuesday.

Your garden spot may or may not have had enough rain, or too much rain. Spotty, streaky showers and storms have prevailed the last several weeks, though Tuesday morning brought more widespread rainfall.

Seriously? A trace of snowfall on one of the hottest days of the year?

Well, no, it didn’t snow at the Roanoke airport on Sunday. But it did hail a little bit, and that gets recorded in the same line item on the climate report as snow and sleet.

This is an accommodation for automated sensors, many of which can detect some form of ice has fallen, but can’t readily identify the exact form.

The sum of this data was that Sunday was a pivotal day when a heat wave began to break, with powerful storms halting the day’s heating and signaling a change in the pattern ahead.

We’ll be seeing the full effects of this pattern change, with 50s lows in many locations and only mid-70s to lower-80s highs on Wednesday and Thursday.

Weather Journal appears on Wednesday.

2019-07-23 23:00:00

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