The pattern shift that finally balanced things out a bit this month after the prolonged stretch of extremely warm temperatures in March, is now ready to show a new face. In the East, a few chilly days this week will give way to a short burst of very warm air late in the weekend. And a large spring storm will soon move into the West and notably increase chances for severe weather, including tornadoes, in the middle of the country during the next few days.

High-altitude temperatures and winds right now. Red shading indicates anomalous ridging/warm-air aloft. Blue colors likewise denote upper troughs and cool air. Yellow dashed line identifies the upper trough that will directly impact U.S. later this week (Penn State)

The northwesterly flow aloft currently over the nation’s midsection is associated with a kind of temperature distribution we haven’t seen much of this year. As shown in the image above, a very cool air mass is now in place in the Northeast (blue shading), just on the northeast side of the jet stream axis (black arrow).

In the heart of that cold pool, temperatures are low enough for snow in a few places. Indeed, snow showers are expected across much of Northern New England tonight and tomorrow morning. And National Weather Service (NWS) forecasts for places as far south as southern New York State include the mention of snowflakes tonight.

This time around, the chill won’t spare the mid-Atlantic or Deep South, either. The frosty weather expected tomorrow night and Thursday night in Raleigh, NC may even extend southeastward from there across the coastal plain to the coast. In North Florida, temperatures in the low 40s will be common Thursday and Friday mornings as well.

Yet, on the other side of the jet stream, in the Pacific Northwest, there has been a nice little run of mild weather the last couple of days. Good for them, because the West Coast completely missed out on last month’s near-nationwide warmth. Seattle, WA (SEA-TAC) recorded a record high on Sunday (70°F), and Portland, OR has been in the 60s since last Saturday.

However, we appear to be in an atmospheric mode right now that, unlike last month, discourages lengthy spells of anomalously warm or cold weather. That means a few days of much cooler weather will soon come back to the West and, likewise, a few days of some really warm weather will overtake the East late this weekend –particularly in the Mid-Atlantic. And all of this will begin to unfold when that next upper trough, still off the West Coast, (identified by the dashed yellow line in the image above) moves onshore.

Meteorologists in the Plains will likely keep a wary eye on this trough as it moves over the Rockies in the coming days. That’s because large weather systems like this one are quite capable of supplying the ingredients for the production of severe local storms this time of year, as they sometimes juxtapose air masses of very different properties into a potentially explosive mix. Present indications suggest this upper trough will do just that.

By Friday, the trough will indeed be over the Rockies (blue shading in the image below). The severe weather threat will ramp up across the Plains (encircled area in the image below) as southerly winds near the ground (white arrow) bring in warm and humid air from the Gulf of Mexico underneath increasingly windy conditions aloft (black arrows).

Winds and high-altitude temperatures expected late Friday. Encircled area outlines highest severe-weather risk. Black arrow is jet stream and white arrow is low-level wind (Penn State)

This encircled zone covers essentially the same areas outlined by the Storm Prediction Center (SPC) in their outlooks for Friday and Saturday. From this early viewpoint, the greatest severe weather potential appears to lie in a swath across Oklahoma and Kansas. SPC will refine their predictions as we move forward, so please defer to for their latest expert analyses.

Some severe weather is expected between now and then in roughly the same region of the country, as the atmosphere adjusts from the current quiet pattern to the approach of the upper trough. However, it seems that the main show/bigger storms will accompany the system itself when it moves across the Rockies and over the Plains late in the week.

Its slow movement is expected to keep the chance for big storms in place for a few days. In fact, it’s not out of the question that the threat for severe weather will continue into next week. A piece of the upper trough might very well get left behind, only to move eastward across the South in about a week and cause problems there.

In a larger context, the initial penetration of this system into the West later this week represents a reorganization of the jet stream that will flip the temperature pattern across the country. The northwesterly winds aloft right now over the nation’s midsection will transition into a southwesterly flow late this week (black arrows in the image above). This setup will deliver much cooler weather to the West (blue shading), and much warmer conditions to the East (red shading).

What makes this arrangement of weather systems noteworthy, aside from the severe weather potential in the Plains, is that by late in the weekend the winds at many altitudes east of the Appalachians will have a westerly component to them (like the black arrow in the image above). Air flowing in this way typically compresses and warms on its descending transit toward the coast in lee of the mountains. Consequently, very warm air will not be far off the ground in many locations from New England to the Carolinas on both Sunday and Monday.

A conservative prediction would place highs near 80°F in D.C. and in the low 80s in the Carolinas both days. Yet only a slight underestimation of the overturning in the lowest thousand feet or so would significantly underestimate the maximum surface temperatures. We shall see, but I wouldn’t be surprised if those numbers aren’t high enough along that section of the I-95 Corridor.