Wednesday 10 a.m.
Colder air is advancing from the Appalachians to the East Coast today and tonight, triggering snow showers (and in places not cold yet) rain showers. Temperatures from Boston to Philadelphia will be at least 10 degrees lower tomorrow than today. Meanwhile, mild air will replace cold air in the central Plains. At Omaha, where it won't get past the 30s today, it will be in the 60s on Friday. The mild air will only be able to advance so far to the northeast before the next in a series of low pressure areas races by and sends cold air back south. During (at least) the next week, low pressure areas will move along the boundary zone. How far mild advances and the extent which cold air does likewise will then set the stage for who gets snow, rain or some combination with the next storm.
Actually, since cold air is more dense than warm air, the warm air cannot advance until cold air retreats. If warm air tries to advance but cold air holds its ground, the warm air is forced to rise over the cold. As air rises, it cools and can no longer accommodate as much water vapor as before, so clouds and precipitation break out. Cold air can advance no matter what the warm air is doing. This direct action helps trigger lines of showers and thunderstorms.
This video shows a forecast through the middle of next week. The pattern we are in is one of the most challenging that forecasters face, because the outcome of subsequent storms is so dependent on what their predecessors do. Any model or human error gets magnified. You will see on this video two totally different outcomes for Middle Atlantic states later this weekend.
The basic storm track across the U.S. is being forced south of where it might otherwise be by a blocking pattern aloft. Individual high or low pressure areas get cut off from the main current and act as roadblocks or wrecks do on the highways. Until the block breaks down, storms take a detour.
The Northern Hemispheric map below shows the setup. In the southern branch of the jet stream (black arrows), the flow takes a series of twists and turns from the central Pacific to the middle of the Atlantic Ocean. Across much of Asia, the flow is much more straight west to east. The red arrows trace the meandering path of the northern circulation. Notice the two areas marked "split flow." At these locations, the northern current detours way to the north while the southern branch runs well to the south. This setup works to block or detain anything that is upstream. When the two branches merge, the combined currents are said to phase, and this can help generate very strong low pressure areas.
Over the Ohio Valley, rain is starting nudge northward again, prodded by one of a series of upper air disturbances embedded in the flow. It appears the rain will advance across Pennsylvania overnight, reach the area from New York City to Boston tomorrow... then head out to sea.
Farther north, the area from Chicago to Boston looks most likely to have sunshine and dry weather. Cloud cover will vary during the weekend, and I focus on that aspect of the weekend forecast in this video.
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.