To my dismay, I read a short-range discussion about the imminent winter storm along the Atlantic Seaboard tomorrow night and Tuesday. I quote: "A low pressure system crossing the Midwest states is expected to phase (sic) with another low off the southeast U.S. coast." I don't know about you, but the word, phase," in this context suggests two low-pressure systems merging along the East Coast. It's just not true.
For the record, I usually reserve "phasing" to describe the merging of two 500-mb short-wave troughs. The idea that low-pressure systems "phase" is a new one on me.The 36-hour forecast from this morning's GFS 12 UTC run of 500-mb heights (in meters) and large values of 500-mb absolute vorticity (color-filled) greater than 20 x 10-5 sec-1. Valid at 00 UTC Tuesday (Monday evening at 8 P.M. EDT). Courtesy of Penn State.
Given that 500-mb data are obligatory in any analysis and forecast, let's start there.
The 36-hour forecast from this morning's 12 UTC run of the GFS (above, valid at 00 UTC Tuesday, which is Monday evening at 8 P.M. EDT) showed a compact but feisty 500-mb short-wave trough over Mississippi and Alabama. The solid contours are 500-mb heights (in meters) and values of 500-mb absolute vorticity greater than 20 x 10-5 sec-1 are color-filled.
In time, this 500-mb short-wave trough is predicted to move eastward to the Atlantic Seaboard, and then become negatively tilted as it moves northward along the coast (see animation of the 27-hour to 60-hour 500-mb forecasts below (every three hours). The forecasts are valid from 15 UTC Monday to 00Z Wednesday (11 A.M. EDT Monday to 8 P.M. EDT Tuesday). An animation of the 27-hour to 60-hour GFS forecasts (every three hours) of 500-mb heights and 500-mb absolute vorticity, Valid from 15 UTC Monday to 00 UTC Wednesday (11 A.M. EDT Monday to 8 P.M. EDT Tuesday). Courtesy of Penn State.
At the surface, the GFS 36-hour forecast, valid at 00Z on Tuesday, shows two centers of low pressure...one just west of the southern Appalachians and an incipient low off the Southeast Coast (below). The 36-hour forecast of MSL isobars from this morning's 12 UTC run (valid at 00 UTC Tuesday, which 8 P.M. EDT Monday evening). Courtesy of Penn State.
Those two lows have as much chance of "phasing" than I do of winning the Tour de France. For starters, it would be curtains for a surface low to cross the Appalachians to the "Lee side" (Get it? :-). Indeed, a low-pressure system crossing the Appalachians would have to lift cold, very stable air associated with cold-air damming. In effect, cold air damming puts the brakes on low-pressure systems attempting to cross the Appalachians. Instead, the low tends to migrate northward along the western foothills of the Appalachians, where easterly winds downslope and create some slightly negative pressure tendencies (an inverted trough). But the writing is already on the wall...with the short-wave trough heading toward the East Coast, the low west of the Appalachians will eventually dissipate.An animation of the 27-hour to 60-hour GFS forecasts (every three hours) of MSL isobars, Valid from 15 UTC Monday to 00 UTC Wednesday (11 A.M. EDT Monday to 8 P.M. EDT Tuesday). Courtesy of Penn State.
Meanwhile, the 500-mb short-wave trough will move toward the Southeast Coast (revisit the 500-mb animation
), where the temperature contrasts between land and sea provide fertile grounds for cyclogenesis. Indeed, divergence ahead of the advancing 500-mb short-wave (and over the low-level baroclinic zone) will pave the way for cyclogenesis and, eventually, a nor'easter. For proof, check out the rapid development of the coastal low on this animation of 27-hour to 60-hour GFS forecasts from the 12 UTC run this morning (above).
In this way, the low west of the Appalachians is sometimes said to "transfer its energy to the coast." Any way you slice it, the imminent winter storm in the Northeast Monday night and Tuesday will initially consist of two lows...one dissipating low west of the Appalachians and one rapidly developing low along the Southeast Coast.
Such a scenario is described as a Miller-B type storm system. There is nothing resembling a "phasing" of low-pressure systems as the short-range discussion I read early this morning pontificated. Just more sloppy language and science in the age of fast-food meteorology.The 54-hour GFS forecast for 850-mb isotachs (in knots) and 850-mb streamlines, valid ay 18 UTC on Tuesday. Courtesy of Penn State.
Looks like there is a potential for really heavy snow across interior New England and parts of the northern Middle Atlantic States as a low-level jet stream rapidly imports moisture from the Atlantic. Check out (above) the 54-hour forecast for 850-mb isotachs (in knots) and 850-mb streamlines (valid at 18 UTC on Tuesday, which is 2 P.M. EDT Tuesday). Hmmm...greater than 60 knots at 850 mb...now that's a low-level jet stream!!
Look out Loretta!