Chance of rain from a front and Hermine?
Posted September 7, 2010
Parts of the area are running rather dry despite a couple of episodes of localized heavy rain in the past few weeks, with much of our viewing area simply designated as "abnormally dry," but with much of the northern coastal plain in moderate drought and a parts of Halifax and Northampton counties in the severe drought category on the most recent U.S. Drought Advisory. Hurricane Earl brought significant rain to the northeastern quarter or so of the state last week, but really notable amounts were somewhat restricted to places within a county or two of the coast.
A map of the region from the Southeast Regional Climate Center shows a rough distribution of rain over the last 30 days, highlighting the heavy precipitation "bullseye" over Johnston county contrasting to fairly low amounts (the orange colors) surrounding it. The second map shows the percentage of normal rainfall for the same period, with the red and dark red areas showing under 50% and less than 25% of normal rain over a good part of the region. In addition, much of the rain that has occurred has been concentrated into a couple of heavy convective events, not really an unusual situation around here in the summer.
This week, we should have a couple of opportunities for rain, but at the moment each one appears it may be limited in terms of the coverage and amount of rain produced. First up is a "cold" front that will traverse the area from northwest to southeast late Wednesday or Wednesday night. Some scattered showers appear possible, but models are also indicating some factors that may inhibit rain, including a modest amount of moisture, a lack of significant upper level support, and winds that largely blow from the west through the lowest several thousand feet of the atmosphere. On the other hand, a wild card may be the possibility of some mid-level moisture flowing ahead of Tropical Storm Hermine becoming caught up with the front more than models are currently accounting for. By Thursday morning, much drier air should rush in with dew points perhaps falling into the 40s as they did early in the Labor Day weekend.
Later in the week, late Friday into Saturday, a remnant upper level circulation from Hermine will have tracked north, then northeast, and finally southeast around an upper level ridge to impinge on the mid-Atlantic and our area. On its own, this system looks awfully depleted of energy by then. However, the front that moves through Wednesday night may stall south of us, then lift back toward the area and interact with that upper system. The combination may be enough to enhance rain chances Friday night into Saturday, but again confidence on how much we might receive is fairly low due to the complexity of the interactions involved and the distance into the future.