Rain sets daily record at RDU

Bands and clusters of shower and thunderstorm activity that swept across the region on Tuesday brought widely varying rainfall totals.

Some areas along the Interstate 95 corridor and around the Sandhills received rather small amounts, but others were deluged with more extended heavy downpours, especially just to the north and west of Raleigh.

In addition, the storms produced gusty winds that managed to take down some trees and power lines in a few spots, though severe-level winds were not a widespread occurrence. The atmosphere also proved a bit too warm over a great depth to allow severe-level hail (1 inch or more in diameter) to form.

Heavy rain at the Raleigh-Durham Airport proved to be enough to smash the old daily record there, reaching 4.21 inches, compared to the old July 15 mark of 2.80 inches from 1954.

I've included a couple of images of radar-estimated rainfall totals  from that general area, illustrating the differences in an older, standard method for obtaining estimated rainfall with a newer method that takes advantage of the characteristics of dual-polarization radar.

The newer technique is less affected by attenuation, or weakening, of the radar beam as it passes through heavy rain, which can at times cause underestimates of the rain rate over areas that are blocked by heavy nearby showers. In this case, the old method of estimating rainfall produced estimates around RDU that were shaded yellow (2 inches+) to orange (3 inches+), indicative of lots of rain there but well short of the value actually observed.

The newer dual-polarization method produces both a more detailed breakdown (smoother shading), but when checked against the scale for the vicinity of the airport, shows rainfall amounts in the range of about 3.9 to 4.3 inches, very close to that measured at ground-level by the RDU gauge. Also notable is the red to purple shading a little farther west, where parts of northern Chatham and southern Durham counties may have received around 5 to 6 inches!

In historical terms, the 4.21-inch value at RDU was high on the list, but not an all-time record at the airport. That mark goes to June 14, 2006, when 5.64 inches of rain fell there. Yesterday's daily total comes in at No. 10 on the list of highest daily totals at RDU. In the history of the airport (since 1945), daily rain has reached 4 inches or more on 15 occasions.

One additional effect of the front that brought the storms along is that we will likely see temperatures and humidity levels stay a little on the short side of our normal high of 90 for a significant stretch, with our 7-day forecast showing 80s for highs each day through next Tuesday.

This got me wondering: How often since 1945 have we had seven days or more in a row with highs under 90 in July?

It turns out to be more common than you might think, with 44 such streaks in the history of the airport. For the "meteorological summer" of June-August, such 7-day streaks have occurred 187 times. The longest stretch of sub-90 highs at RDU in July has been 23 days in a row, from 1970,and for meteorological summer we've had as many as 32 straight days, that record being set in 1971.

How long will this streak last? So far, most models and ensemble groups of model results indicate we would be back to low 90s during the mid-to latter portions of next week (and a couple even get us in the ballpark this Friday and Saturday), but there's sufficient variability on the results to keep confidence about the exact timing of our next foray into the 90s a little on the low side.