Thursday, 11:45 a.m.
Record heat is blistering the West right now, though the heat has peaked. The nation's high on Wednesday was 106 in Santa Ana, California, and no doubt you have seen some of the footage of the fires in southern California as a result of the dry and hot offshore winds. It's been warm in the Northwest, too, and that warmth will linger this afternoon into tomorrow before it gets erased.
We have also seen a fair amount of warmth in the eastern third of the country, but the area of warmth is shrinking, and will be gone tomorrow. Here are the highs from Wednesday:
In between the two, it's more like early April rather than mid-May! The snow may be over, but the chill is not, and as long as this upper-level low or upper-level trough remains so strong coming through the Great Lakes and Ohio Valley, it will stay below normal in the Plains. The 12z May 15 NAM 500mb forecast shows the trough still pretty deep, with the jet stream digging deep into the South and Southeast Saturday afternoon:
That translates to yet another cool day for this time of the year from the Plains eastward:
Just as this lead upper-level low begins to open up into an upper-level trough, another will form and step in to replace it. Initially the air reaching the mid-Atlantic coast this weekend won't be too far below normal, and in eastern New England, it could just as easily remain above average right into Sunday. Going into next week, however, that will change. With a new upper-level low likely to form right over New England Monday, it signifies a fairly dramatic cooling of the atmosphere aloft.
You can look at that several ways, but I'll take the easiest one for me to look at, and that is 850mb forecasts. For the next 24 to 36 hours over the coastal mid-Atlantic and New England, the temperatures are generally between 10C and 15C. By contrast, those very same temperature forecasts are closer to 0C to 5C by Monday night and Tuesday, or approaching degrees Fahrenheit cooler! And once the cooler air is in, it's likely to stay in place, as this upper-level low impacts the weather into Wednesday. Another cold front may well slip through the area late next week, so that any warming that does take place into the Great Lakes and Northeast gets trimmed back again for the Memorial Day weekend.
So, if you look at the seven-day ensemble means from tomorrow through next Thursday, it's easy to see how dominant the cool air is and will be:
That gets us to the Memorial Day weekend, the unofficial start to the summer season. It does appear as if warmth, while not as extreme as we have seen this week, will be showing up in more places next weekend, especially in the northern Plains and Upper Midwest.
Slow changes mean more wet weather across parts of the southern tier of states through the weekend, while a faster jet stream will bring more swift changes to the northern tier states.
Odile will bring flooding rains to parts of Arizona and New Mexico over the next three days, but most of the rest of the country will rather quiet weather.
Much of the country from the Plains to the East Coast will have several more days of very cool weather. Meanwhile, Odile is threatening Arizona and New Mexico with flooding rains later this week.
A strong storm in Quebec combined with a very large high near Montana will result in sweeping changes across the Ohio Valley into the mid-Atlantic and Northeast in the next 24 hours.
The latest surge of heat and humidity into the Midwest will spread to the East tomorrow but be trimmed back to the South later this weekend by the passage of a cold front.
Summer's heat and humidity will still have a period of time to sizzle the rest of the week into the start of the weekend, but a fall-like air mass will invade the northern Plains and Midwest this weekend.