Tuesday, 11:59 a.m.
Upper-level lows, or 'storms' if you will, are more common in spring than in summer. In spring, as the jet stream retreats, it's pretty common to see a strong upper-level disturbance separate from the main flow of jet and form a closed circulation of its own, a closed low, or 'storm' if you will. At this time of the year, though, the jet stream is pretty far to the north, and these disturbances don't tend to produce such features nearly as much.
This summer, though, has been anything but typical! There was, of course, the dreaded return of the 'polar vortex' a month ago, with a weaker version of it in late July. And if you look at the upper-level charts for the rest of the week, two more of these closed lows will pepper the charts - one in the vicinity of the Great Lakes and Northeast and the other over the Northwest. Both will bring some cooling and some wet weather, though the one over the Northeast is packing a greater punch.
Look at the 12z NAM 500mb forecast for Thursday morning:
As this closed low is developing this afternoon and tonight, it's pulling tropical moisture northward along and east of the Appalachians resulting in a lot of rain across the mid-Atlantic and parts of New England. And it is making for yet another day with temperatures below normal throughout the region.
A second upper-level disturbance is also dropping in on the back side of all this, and that's what's contributing to all of the clouds and showers over eastern Wisconsin, Lake Michigan and Michigan into northeastern Illinois and northern Indiana. There we're finding temperatures that won't get out of the 60s this afternoon, when normals are generally still above 80! Even where there is sunshine farther south and west, it's cooler than normal, and the nighttime lows will be well below average tonight with a clear sky and light winds. This cooler-than-average air mass will spread to the East Coast tomorrow and Thursday.
Even on Friday, the upper-level low will still be in place just north of New England, and that will keep the air quite cool throughout the Northeast and Great Lakes and even into the Southeast.
The Northwest has been hot and dry much of the summer, so as this feature rolls on through, it will provide a different take on bringing cooling relief to the region. Normally it would be accomplished through a dip in the jet stream pushing a cold front through the area. In this case, though, this upper-level low is pulling in moisture from the Southwest, and it's coming in two pieces. Look at the 700mb forecast of heights and relative humidity for tomorrow morning:
The first batch of moisture is already producing a few showers and a couple of thunderstorms. With the broader area of moisture coming up from the Southwest tomorrow into Thursday, some of this moisture will get wrapped into the circulation of the upper-level low, and that will trigger more showers and thunderstorms right into Friday. The additional benefit of this feature will be to temporarily erase the heat from the region, though by the weekend, temperatures will start to creep upward once again.
Comments that don't add to the conversation may be automatically or manually removed by Facebook or AccuWeather. Profanity, personal attacks, and spam will not be tolerated.