Sunday will be one more day like the last three — hot and humid, highs in the 80s to low 90s, and afternoon storms, some of which could be heavy or even severe. Then, everything changes.
Earlier this summer, seeing 97 for Roanoke and 90s for other regional cities like at left, you would probably have presumed this to be a map of predicted high tempeatures. But it’s not — it’s a map from the Hydrometeorological Prediction Center showing precipitation probabilities forecasted for the period from Monday evening to Tuesday evening. To call for a 97 percent chance of rain some 60 hours out is a huge vote of confidence for expected rainfall — substantial, drought-easing or -ending rainfall, and possibly heavy rainfall. Other maps from HPC show a slight risk for flooding rains – being nearly 3 days out and entering the rainy period relatively dry (a little less so in spots with recent storms) has held that risk down some, but it may yet go up as we get closer and the forecast solidifies – and the potential for 4-6 inches of rain in the next 5 days. All of this is the result of the interaction between Tropical Storm Lee and an approaching strong, summer-breaking cold front crossing the central U.S. The weather map linked here shows how, by Tuesday morning, the remant circulation of Lee will get entangled with the front, which is expected to become stalled very near our region, or just east. We’ll have moisture streaming in from the Gulf of Mexico and the Atlantic, the lift of the front, some of the old squalls from Lee, humid air overriding cooler air, and our mountain topography providing yet more lift. It will indeed be very hard (though never impossible in weather) to miss significant rain with all that, though there remains a good bit of uncertainty about how much will fall in our area, depending on the exact track of Lee’s remnants and the placement of the front. Some of that uncertainty has narrowed as it appears likely that Lee will scoot out a bit faster, with the majority of its effects in the Labor Day to Wednesday time frame. A remnant circulation center may hang back and keep some showers going into Thursday and Friday.
Plenty of models and forecasts show the higher-end 4-plus-inch amounts for our region, but some do take the heaviest core of Lee’s moisture a bit to our west. I’m leaning toward more of the 2-3-inch totals with some locally heavier amounts, but as anyone who remembers the Flood of 1985 knows, a stationary front combined with a slow-moving tropical system can spell major trouble if it connects just right. This is not a situation to get overly nervous about (it would be much different if we were in a period of widespread wetness right now) but certainly one to monitor the next few days, especially if you live near a flood-prone creek or river.
The influx of tropical moisture will somewhat lessen the effects of the encroaching Canadian air mass — we are less likely to see crisp 40s morning than we would have otherwise been this week. Butwith the southerly jet stream dip likely to stick around for many days, another cold front may yet arrive to sweep the tropical moisture away and bring on cooler, fall-like weather in the next couple of weeks. One of these damp days this week could still get stuck in the 60s, and even after Lee clears, near normal temperatures with 70s and low 80s for highs and 50s to low 60s for lows appear to be on tap.