Joe Lundberg

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Massive Storm Slow to Unwind

April 29, 2014; 10:52 AM ET

Tuesday, 11:55 a.m.

The powerhouse of a storm centered in southeastern Iowa this morning has only moved from eastern Nebraska at this time yesterday. I could probably have ridden my bike that far in the same amount of time! Here's a look at the 11 a.m. EDT pressure analysis:

The storm is co-located with the upper-level low:

When both are stacked on top of one another, there tends to be very slow movement of the storm, as there is not much in the way of a steering flow over the storm to guide it along. Just trace out the 5,700m height line on that chart - it comes in on the north end of Vancouver Island and over the top of the building upper-level ridge, then down through western Montana and Utah before curling to the east and crossing central Texas, then sweeping up across the Appalachians to Lake Ontario into Quebec. There it makes a sharp left turn, back to the west, all the way to eastern Saskatchewan before turning back to the right and heading northward once again! So, the storm is impacting the weather from the Rockies to the East Coast, and from southern Canada to the northern Gulf of Mexico

We've been focusing on the severe weather aspect of the storm for the past couple of days, and rightly so, as it has been deadly. Look at the severe weather reports from Monday and Monday night:

Clearly this has become the most volatile outbreak of severe weather so far in 2014, and we're not yet finished with it. The initial line has all but fallen apart, with the remains of it still bringing rough weather to the eastern Florida Panhandle. However, the dew point temperatures remain relatively high back into Louisiana and much of Mississippi, as well as central and eastern Tennessee up into Kentucky. It is this area that is getting some sunshine, and the atmosphere is becoming more unstable with each passing hour as it heats up. Thunderstorms will form in this area, and some of them are likely to become severe. If you examine the 12z April 29 NAM surface forecast at 6z, you get the idea of where the model thinks trouble will brew into early tonight:

There may still be some strong to locally severe thunderstorms into tomorrow, largely from the Appalachians eastward, as the warm, humid air gets squeezed northward ahead of the cold front. The main threat tomorrow, though, may be that of flooding downpours, and as such flood and flash flood watches have been issued for many states in the East.

There has been some snow, though, mixing in with the rain from parts of Nebraska into the Dakotas and even Minnesota and Wisconsin. It's a wet snow, and it's not accumulating a lot, thankfully, but the air is just cold enough in the heavier bands to cause some of the cold air to mix down to the ground and allow the snowflakes to survive the trip without melting. This can still occur tonight and even into tomorrow across parts of Minnesota and the eastern Dakotas into Iowa, though the precipitation rates will be light enough that most of the snow should melt.

Then, of course, there is the wind. There's been a lot of that, especially on the northern and western flank of the storm, impacting the eastern Rockies and western Plains in particular. The flow is aligned from the surface up to 500 mb and beyond, and with such a tight surface pressure gradient, it's no wonder winds have been clocked well in excess of 50 mph and will be again this afternoon! The wind will grudgingly back down tonight and tomorrow as the storm begins to slowly unravel.

By the time we get to Thursday afternoon, the main low center will be moving across the Great Lakes:

The upper-level low will be opening up, and the heavy rain will move out of Maine in the afternoon. The flow aloft will still be pretty much aligned with the surface cold front that will stretch from New England down the Eastern Seaboard to the Florida Panhandle, and that means the coastal areas will still get some showers and a few thunderstorms. Even on Friday, the Southeast will remain unsettled with a weak wave likely to form along the front and head across the region.

Meanwhile, the upper levels will still be mighty chilly across the Midwest, the Great Lakes and the Ohio Valley, with a lot of low-level moisture trapped underneath. That will translate into plenty of clouds and scattered showers through Friday before the weather makes some turn for the better going into the weekend - finally.

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Joe Lundberg
Joe Lundberg, a veteran forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.