Monday 10 a.m.
A loosely organized storm in the Plains will move east and transform into well-organized, gale-producing storm off the East coast. That's the easy part. The challenge is assessing how far north the heavy snow extend. The reason that is so important is that, within a distance of about 150 miles, accumulations can range from a covering to well over a foot.
With many situations, we see a track curving to the northeast as the main storm strengthens offshore. This happens because the upper-air trough helping to support and steer the storm gets deeper, and the flow ahead of it becomes more southerly. Last night's GFS predicted exactly that, and if taken literally, suggests 18 inches of snow for Boston Wednesday night and Thursday after dumping more than 6 inches on New York City.
However, there is a blocking pattern in place over northern New England and eastern Canada. If the block holds, the storm will detour out to sea instead of turning up the coast. That's exactly what the European model predicted last night and why it projects less than inch of snow for both New York City and Boston. Meanwhile, both models predict at least 6 inches of snow for Chicago. Here's my morning video.
This map is a working draft of our snow accumulation predictions. Please check our latest videos and stories to learn more about this developing situation.
A band of heavy rain and locally violent thunderstorms moved from the New York City area at 4 a.m. to just past Boston (distance: 188 miles) by 9:30 this morning. Note the stunning contrast between where it is pouring (dark red) and where it has dried out.
On the map, showers and thunderstorms were located along and ahead of the gray line that cuts through Pennsylvania and along/ahead of the blue line. Both should be off the East coast by Thursday. Drier air from the Upper Midwest should filter into the Northeast later in the week.
The large storm that drenched the Northeast during the weekend has drifted out to sea and somewhat drier air is coming in to replace it. However, another upper air trough extending from Wisconsin to Louisiana is supporting several pockets of showers and thunderstorms.
From northern West Virginia across most of Pennsylvania and western and central New York, there could be several inches of rain with flooded streets and streams.
...will move east to bring rain overnight from parts of Virginia to Southern New England. In the southern part of this area there can be some violent thunderstorms late today and tonight. The rain will depart tomorrow, but a large storm is likely to affect the Middle and North Atlantic states this weekend.
This activity map produced by the National Weather Service Storm Prediction Center overlays existing areas of thunderstorms on the map showing various risk levels.