Let's start out by considering the big picture today. Across North America, the 500 mb (about 22,000 feet in altitude) chart shows two strong upper lows and resultant troughs - one on the west coast, and one meandering southeast along the Ontario/Quebec Provincial line, en route to Northern New England. The size of each of these upper lows is rather impressive, and this is creating a narrow ridge over the Central United States. Of course, that ridge carries ample heat with it, a product of the searing Southwest U.S. heat of the past few days, now displaced by the falling heights and strong storm over the Western U.S. that has brought a change from 90s to mountain snow in the Rocky Mountain states. In time, the longwave pattern across the Lower 48 will stabilize as the Eastern Canada upper low continues its eastward migration that is already evident in satellite imagery today, and ends up east of Nova Scotia with a trough-ridge-trough pattern setting up as we head into the weekend.
With the approach of the upper low and its associated mid and upper level cold pool, scattered convection has continued developing this afternoon (can be monitored via the radar links at right by clicking on the thumbnail imagery for your area) and with the cold pool aloft, some hail is possible in the deeper convection, though the heart of the cold remains to our northwest. Cloud to ground lightning strikes are likely in the healthier cells, but parameters for severe convection are rather dull. Wind is unidirectional and while 850 mb wind is 25-30 kts, 700 mb winds are weak and surface instability is about 300 J/kg.
Convection dies out overnight with loss of diurnal heating but upper low moves directly overhead and this means not much clearing of the skies. Any breaks of sun on Thu are self-destructive with the cold pool overhead and this should mean numerous afternoon showers and embedded thunder with hailers as plenty of cold air aloft will be available regionwide, but especially in Central and Northern NewEng. By Friday, upper low pulls out and there's still another lobe of vorticity that needs to swing through, and this may spark a few more showers upon passage. The ridge builds in for the weekend but we remain on the cusp, which means we hold northwest flow aloft - a favorable flow for shortwaves from Canada that will spark thunderstorms along the edge of warm advection, which should be across Northern Maine Saturday and Sunday, meaning some thunderstorm complexes are possible there. Otherwise, cirrus debris from the passage of vorts above dry air, and a gradual warming by about 5 degrees per day until a late day cold frontal passage with possible thunder on Memorial Day, when highs in Central and Southern New England should touch 80 as the air over Nebraska yesterday moves through with assistance from downsloping.
Matt
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Posted by: Nurseconsider | Tuesday, December 08, 2009 at 08:33 AM