Tuesday, 11:30 a.m.

Every day you look at and study the weather, you can find something really interesting if you're willing to look for it. If you go back to late last week, the models correctly sniffed out a piece of energy or a storm or whatever you want to call it coming out of the tropics and having an impact on the weather along the East Coast. When I came in this morning, the one thing that jumped out at me on the surface analysis was the fact the winds were roaring out of the southeast at Cape Hatteras with temperatures and dew point temperatures over 70!!! A weak little low had come onshore over the Outer Banks long enough to turn the wind into the east, then the southeast, then, in time, the southwest and west to drop temperatures back into the low 60s.

This little low drenched parts of the mid-Atlantic overnight and this morning, though the rain has pretty much quit along the Delmarva Peninsula and should be exiting the Jersey Coast by the time I'm mounting my bike to ride the Lower Trail early this afternoon. As the low passes south and east of Long Island and Cape Cod late this afternoon and early tonight, it will bring some rain through southern New England for a time as well.

One of the byproducts of this little rogue storm, as JB likes to call them, is that it is pushing a lot of marine air inland to the Appalachians. So, you're effectively saturating the lower part of the atmosphere and readying it for the broader storm to come up from the central Gulf Coast. As surface storms go, this one won't be noted for its barometric pressure or its pressure gradient. It's more a product of an upper-level disturbance that's pulling a ton of tropical moisture from the Gulf into the central and eastern Gulf Coast region. Pensacola, Fla., has racked up nearly 3.50 inches of rain since about 4 a.m. local time, and it's only 10 a.m. there as I write this!

All of this moisture is being directed quickly northeastward up the spine of the Appalachians, but there will also be a decent slug of rain crossing the Tennessee Valley into the Ohio Valley and spreading over the mid-Atlantic states into southern New England. The rain will not be as heavy as what we saw this past Friday and Saturday, but it will continue to fill up the aquifers throughout these areas.

This storm will also fade south of New England as the upper-level ridge strengthens farther west behind it. That turns the flow northwest over the western Atlantic, which, in turn, forces the storm to turn to the right over time tomorrow afternoon and tomorrow night into Thursday.

While all of that happens, a broader upper-level trough will be digging into the Rockies over the next 48 hours. It's not that there will be a lot of arctic air associated with this feature, because there won't be. However, there will be snow in the mountains nonetheless because of where the freezing level will drop to. In most cases across the interior West into the central and northern Rockies, that freezing level will be at or below ground level, so as the west to northwest winds throw some of that Pacific moisture at the mountains, it's going to snow.

The rapid response to this digging trough across the southern Plains will lead to a growing area of rain and thunderstorms later tomorrow night and Thursday, and some of the thunderstorms will become severe on Thursday as a cold front is pushed out of the Rockies and into an increasingly humid air mass.

The front will be slow to move at first through Thursday, but as another upper-level low rolls into western Canada, it will have the effect of tightening the jet stream under it. One of the byproducts of that is to push the upper-level trough along and dislodge it from the southern Rockies and southern Plains going forward. Therefore, the front will pick up momentum Friday as whatever split you mighty see in the jet stream just disappears. By Saturday, this front will be moving quickly over the Appalachians and driving off the East Coast by early Saturday night.

There will be a fair amount of rain with this whole feature but less the farther east you go. Once you're away from the Rockies and parts of the Dakotas, there really isn't going to be much snow, frankly because there really isn't any cold air around east of the Mississippi River.

Even behind the storm and its attendant cold front, the huge temperature departures you'll see over the Rockies and out onto the Plains will fade as they come East. Yes, it will be colder in the East Sunday and into the early part of next week, but not as cold as it was during the first half of October, nor as cold as will be seen the next few days behind the front in the Rockies!

Posted by Joe Lundberg on Tuesday, October 27, 2009 10:30 AM
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