February 6, 2002
Things appear to be following along with last night's forecast thus far. The intense upper low
closed off near Denver, CO this afternoon and has now moved into Northern Texas. Shortwave number two had entered the Southwest Canadian Coast and is headed Eastward, almost riding the Canada/US border. The big question remains: Do both shortwaves meet up and phase or do they remain separate? For now, I will stick with my track from last night, in regards to the movement of the surface low (125 miles South of Houston-->50-75 miles South of New Orleans, LA-->Pensacola, FL-->75 miles Southeast of Macon, GA-->Charleston, SC-->50-100 miles South of Cape Hatteras). The one thing I might tweak is to bring the low a bit farther North near Haterras. Beyond that, the track is still sketchy. However, there are a few clues which can lead us to a fairly solid forecast.
There are a few differences that I noticed tonight between the Aviation and ETA schemes. Rather than jump on every model that shows cold and snow, it seems more reasonable to choose the two or three models that appear to have the most diverse solutions and analize the differences therein which may or may not be conducive to wintry weather. At the surface, we have the ETA which follows my track closely, but takes on more of a Northerly component near the Panhandle of Florida. So, the low tracks farther West and doesn't reach the open water until it nears the DelMarVa Peninsula. Suffice to say, heavier precipitation falls in the area on Thursday. On the other hand, the AVN follows our track from last night almost to a tee. With the AVN scenario, we get clipped with the Northern and Western fringe of the precipitation as the low heads ENE off the coast near Haterras.
So why the difference in track? At the 500mb level, the Southern shortwave looks similair on both. One minor difference is the AVN keeps the upper low, associated with shortwave, closed just a touch longer while the ETA opens the low by 9Z Thursday. The AVN's scenario would keep the shortwave a bit farther South and East allowing for less of a chance of interaction with the Northern shortwave (which should be near the upper peninsula of Michigan around 9Z Thursday). Also, looking downstream, there is a powerful shortwave streaking across Canada behind the shortwave North of Michigan. The ETA is a bit slower with this feature, while the AVN acts like a 'kicker' to shear out the first shortwave (the one in Michigan if you're still with me) and allow no phasing between jet branches. Here is where the ETA is different. The ETA focuses less on the 'kicker' shortwave and allows more amplification upstream with the Michigan vortmax, thus allowing a VERY close call between phasing or not between the said shortwave and the one opening up in the Tennessee Valley. It comes close, hence it's more Westward track of the storm.
At the 850mb level, the main difference that is noticeable right off the bat is the lack of any 850 low on the AVN North of Michigan into Canada. Meanwhile, the ETA (and others) have a strong 850 low in that vicinity. The lack of a low on the AVN allows the redevelopment of the 850 low near the coast to happen sooner and farther South, thus, it locks colder air in place. This is reflected well on the H5 maps by the AVN shearing out the energy North of Michigan due in part to the 'kicker' shortwave.
In short, I have not changed much with my thinking from last night. My storm track I have kept the same except brought it just a hair closer to Haterras(125 miles South of Houston-->50-75 miles South of New Orleans, LA-->Pensacola, FL-->75 miles Southeast of Macon, GA-->Charleston, SC-->50 miles South of Cape Hatteras). Three scenarios seem possible in terms of apparent weather outside: 1)Cloudy and cool, 2)Flurries/light snow/light mix or 3)Light snow/mix to rain. I will warn that if a trend starts that shows a phase between the two shortwaves (one currently over N. Texas and the other North of Minnesota) there is the chance that we could see snow or a prolonged mix from this sytem. However, the chances seem reasonably low enough at this time to keep that scenario on the back burner for some other time. On the same token, the chances appear better tonight to throw out the idea of just cloudy for Thursday. Some precipitation will likely fall, just how much is still uncertain. For the time being going with increasing clouds Wednesday night. Chance for light mix to showers or rain starting mid morning on Thursday. Highs in the 34-38 range. Things clear out for Friday. Another threat late in the weekend with another cold shot by early in the week. So, tricky weather on the horizon, but it does not appear to be a big deal at this point for the Philadelphia region. Another update likely Wednesday night. Take care!
S.B.
