First 90 of the season - what's 'normal?'
Posted May 12, 2015
After reaching 87 degrees at Raleigh-Durham International Airport on Monday afternoon, a warm and humid air mass just ahead of an approaching cold front may push us up to 90 degrees Tuesday afternoon, barring a well-timed cooling shower or thunderstorm outflow.
If we make it to the forecast high of 90 degrees at RDU, it will be the first time in 2015. It raises the question – when does the Triangle typically reach that initial 90-degree mark in a statistical sense?
A check of "official" observations for Raleigh, stretching back to 1887, shows that the temperature has topped out at 90 degrees or higher as early in the year as March 12, in 1990, and as late as June 27, back in 1983.
So there can be a good bit of variation. But when we average things out over the years, we find that we're currently right in the ballpark of the most common time of the year for this threshold to be reached. To illustrate, if we include all the years back to 1887, the average date for the first 90-degree day is May 10, and if we look at all the years of observations from the RDU airport only (starting with 1945), that average is May 12. Finally, if we look at the "normal," meaning the 30-year average ending with the most recent "zero" year, in the same way we'd calculate a normal high or low temperature or normal precipitation amount, we find the date that arises from the 1981-2010 time frame is May 15.
Whether we make it to 90 degrees today or not, it appears to be our last chance at a temperature that hot for a while, since the cold front that crosses the area this evening should leave us with noticeably cooler, and substantially less humid, air in place for the remainder of the week.
Dew points in the somewhat muggy 60s today will give way to pleasantly dry 40s for the next two or three days, and our high temperatures will only reach the mid-70s to around 80 from Wednesday through Saturday.