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September Slowly Warming Up
No rain in sight

Dan Stillman @ 10:10 AM

As Matt mentioned yesterday, September got off to a rather warm start before cooling off during the past week. Temperatures are now back on the upswing, but along with humidity should stay in most people's comfort range over the next several days. Partly to mostly sunny skies are in store for most locations through the weekend, with virtually no chance of rain.


Nice Day StampPartly sunny, pleasant. With a broad area of high pressure remaining in control, the metro area should be partly sunny today with low humidity, highs in the mid-to-upper 70s and light winds at 5-10 mph. Toward the Chesapeake Bay and on the Eastern Shore, a light onshore breeze and in some places more clouds will probably limit highs to the low 70s. Tonight, mostly clear with lows ranging from the low-to-mid 50s in the suburbs to near 60 downtown.


Forecast Confidence: High-Very HighPartly to mostly sunny, warmer. The air mass warms a bit, allowing highs to reach the low 80s under partly to mostly sunny skies. Like temperatures, the amount of moisture in the air will also be on the rise, but I still wouldn't call it humid. Again, onshore flow and more in the way of clouds may keep high temperatures near the Bay and on the Eastern Shore a bit lower, in the mid-to-upper 70s. Overnight, mostly clear with lows ranging from the mid 50s in the suburbs to the low 60s downtown.


Forecast Confidence: HighSimilar to Thursday. Partly to mostly sunny and low-to-mid 80s should do it for just about everyone on Friday. Again, humidity should for the most part remain in check. Overnight, clear with lows in the low-to-mid 60s.

The Weekend

Forecast Confidence: HighSunny and warm. The upcoming weekend looks like it will lack the fall crispness of this past weekend. But a Saturday and Sunday with lots of sunshine, highs in the mid-to-upper 80s, and maybe a touch of humidity ain't too shabby. Saturday night, partly to mostly clear with lows in the 60s.

New Heat Index: It's All Relative

In Washington, oppressive heat and humidity are par for the course come July and August. But the same conditions can be a shock to the system when they occur in May or late September. Meanwhile, the dry heat that is so typical of the Southwest feels quite unusual when it occurs in the MidAtlantic.

Keeping in mind the various factors that determine how heat affects people in different places and at different times, the National Weather Service has been testing a new type of heat index this year called the "heat stress index," which characterizes heat relative to location and time of year rather than in absolute terms.

The heat index currently in use expresses a "feels-like" temperature as a function of temperature and humidity alone, and does not take into account geographic location or time of year. In other words, a temperature of 95 and dewpoint of 65 yields the same heat index value in Washington in June as it does in Phoenix in September.

The new index factors location and time of year into the equation, and also incorporates variables including cloud cover and the number of consecutive days of extreme heat. The resulting value is not in the form of a temperature, but rather a percentage indicating how stressful conditions are compared to average for a given place and date. For example, a heat stress index of 97% indicates that in a sample of 100 days (all on that same date), only 3 would be expected to produce more stressful heat.

Still in developmental phase, the heat stress index has been tested in 10 National Weather Service forecast offices this year, with the goal of assessing how well the index meets the needs of forecasters and users such as emergency managers and local health officials. Potential benefits to the general public are also being considered. In 2001 and 2002, a version of the index converted to a 0-to-10 scale was shown on air by TV stations in Philadelphia and Baton Rouge, La. A majority of viewers responding to an online poll said the index was "very easy" or "easy" to understand and that its use should continue.

The heat stress index was developed at the University of Delaware's Synoptic Climatology Laboratory by Jill Derby Watts under the direction of Laurence Kalkstein. Thanks to Katrina Frank for providing background information for this story. More info in this PDF.

Graphic courtesy University of Delaware

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