Back Door Warm Front?
Posted November 12, 2007
A term you'll hear from us from time to time on the air is "backdoor cold front" to signify that a mass of cold air will be spreading into central NC not in the usual fashion from the northwest, but instead from the north or even northeast, usually because of an intensifying high pressure system over New England or eastern Canada. This morning, we had a different situation in which it was not cold air but warm air advancing into the region, and while a typical warm front makes its passage from southwest to northeast, we had a warm front-like boundary drift into the region from the northwest, more typical of the approach of a common cold front.
This is not extremely unusual, but is uncommon enough that I've never actually heard anyone use the term "backdoor warm front," though it seems appropriate given the pattern that developed last night and today. We had anticipated an increase in cloud cover associated with the approach of this feature, but the moisture involved was a little deeper than we initially expected, and the clouds became just thick enough to create a few sprinkles and patches of light rain during the morning. Most of us only got a trace of rain, but there were spots where a few hundredths fell, including .04" measured at the Raleigh-Durham airport.
A typical warm front is marked by warm air advancing to replace a cooler airmass, with the warmer (and often more humid) air often lifted gently over the cooler air in advance of the surface level boundary so that the air aloft actually warms up before air at the surface. That was the case this morning for us, both due to the warm front and also to radiative cooling of the surface that occurred before the clouds associated with the front thickened up. We can see the warmer air a few thousand feet off the ground on a radiosonde profile collected from a balloon launched from Greensboro at 7 am (first graphic above, from the Unisys weather site). On the sounding, you'll see that the right hand line (temperature) shows low 40s at the surface at that time, but temperatures in the mid 50s in a shallow layer around 3000 feet up. In addition, the dew point (left hand line) shows a large separation from temperature near the surface (a mark of rather dry air), but is almost the same as temperature above that, indicating the layer where enhanced moisture was moving in from the northwest and being lifted a bit by the colder, drier air below. The sprinkles that occured this morning were generated from within this layer.
The second image, also from Unisys, shows a low level stability analysis from 7 am. This kind of analysis often gives a nice depiction of frontal zones, because they are bands where the stability changes rapidly from one location to another across the front. You can see a typical signature for a warm front moving toward the northeast on this map. The lines that are grouped together along a band from about the Chesapeake Bay northwest into Lake Huron indicate that portion of the front, with more stable air to the northeast transitioning to less stable air to the southwest. However, in this particular case the zone of rapid stability decrease also curves around across our state, running from Elizabeth City southwest to Charlotte and Asheville. The next image is a forecast that shows how the entire warm frontal band is expected to move by 7 pm Monday evening. The "regular" warm front now extends from well east of the NC/VA border up to about Lake Ontario, while the "backdoor warm front" portion has slipped a little south and east of North Carolina.
Behind that front, temperatures had already warmed into the mid 50s early today at Boone, while we were still in the upper 30s and low 40s, and our temperatures should climb into the 60s in many spots this afternoon, warmer than the weekend just past. We'll likely stay in this warmer airmass on into Wednesday, although a cold front to our north may drift down to about the NC/VA border and stall for a day or so during that time. Most of us south of that front will see temperatures climb farther above normal for a couple of day, with low to mid 70s for highs. Normal for this time of year at RDU is 63, with a standard deviation of +/- 10 degrees, so the mid 70s would be notably above normal, though still a little shy of our records for Tuesday and Wednesday of 80 and 81.
Thinngs should change again later Wednesday night and Thursday with the passage of a significant cold front. Unfortunately, it could be yet another front that doesn't have a great deal of moisture to work with, so we may be looking at just some hit andf miss showers or isolated storms during or shortly after it's passage. The front in this case will not be "bacdoor" in nature, but should follow the classic cold front pattern, as seen in the final two images, one of which shows a surface pressure trough over the area Thursday morning, with the other showing the stability pattern. In this case, the front is marked by a band with a rapid transition from less stable (east) to more stable (west) air, and at 7 am Thursday stretches from the eastern tip of Maine to about RDU and on down toward central Texas. If this projected timing holds up, we'll see some morning clouds and maybe a morning shower, giving way to clearing and blustery northwest winds Thursday afternoon, and then seasonably cool temperatures on Friday.