Monday, 11:30 a.m.
Another blast of arctic air is sweeping across the Great Lakes and Ohio Valley into the East today, with snowflakes seen in parts of western Pennsylvania already this morning. It's cold enough for that, but where it managed to be clear with little wind overnight, it got downright frosty overnight. Here's the morning low temperature chart:
Frosty readings are almost a certainty from the central Appalachians and even parts of the Ohio Valley to parts of Virginia, Maryland, interior Delaware and New Jersey into interior New England tonight as high pressure settles in over the East.
Don't look now, but it's getting warmer in the Rockies and Plains! A LOT warmer. Here are the highs from Sunday:
Billings, Mont., was one spot that hit 90 yesterday, but there will be plenty more that do so this afternoon with a decent amount of sunshine from eastern Montana out into South Dakota and down to the Red River Valley. That heat will then surge out toward the Mississippi Valley tomorrow, with 80s to Chicago and southern and western Wisconsin a virtual lock at this point.
If you take the lows this morning across the Midwest and match them against the highs of tomorrow, there is likely to be a 50-degree temperature swing in many places, that's how dramatic of a turnaround there will be over a 36-hour period.
While not that large, it will still be over 40 degrees farther east in the Ohio Valley to the mid-Atlantic states between the lows of tomorrow morning and the highs of Wednesday afternoon. Some of the heat will be trimmed farther downstream as it runs into more and more resistance via a lower height field (which translates to it being cooler aloft), which, in turn, will generate more clouds along with scattered showers and a couple of thunderstorms.
If there's enough sunshine and more of a west wind on Thursday east of the Appalachians, there may be a few places that can reach 90 from Virginia to southeastern Pennsylvania ahead of any thunderstorms, but everything has to come together just right for that to happen. Regardless, the entire region will jump above normal after being so far below it yesterday and today into tomorrow.
There is another front on the way. The disturbance that will send that front sailing downstream will move across the Northwest later this afternoon and early tonight, as depicted here on the 12z 13 May 500mb NAM forecast for this evening:
The feature in question shows up in eastern Ontario by Thursday morning, as seen below on the 0z 13 May 500mb GFS forecast:
Given how virtually every feature like that has drawn very cold air southeastward into the Midwest, Ohio Valley and points south and east so far this spring, one would think it would happen again! This time, however, the connection with all of that arctic air is absent. The key to it is the pressure across Canada. Instead of seeing a large surface high someone between central Canada and Alaska, there's nothing but low pressure. On this morning's weather map, I analyzed one along the border between Alberta, Saskatchewan and the Northwest Territories, with another back in eastern Alaska. That pulls mild, Pacific air into western Canada, and upon downsloping from the Canadian Rockies out onto the Prairies, it tends to warm up and dry out some.
Furthermore, with each passing day, the deep snow pack is being repealed to farther and farther north locations, with virtually all of the area from Alberta into central Ontario now barren. The combination of these two factors leads to the Arctic disconnect that is long overdue!
So, with minimal cooling to or just a touch below normal late this week into the start of the weekend from the Great Lakes to New England, it will be much easier to warm things up behind the trough. And right on cue another system will crash into the Northwest, eventually carving out a low pressure over the northern Plains by Friday. That storm will move relatively slowly to the east-northeast this weekend as another upper-level low forms, this time over the Dakotas.
Underneath that low, there will be a lot of west to southwest winds aloft that will carry more mild air from the Pacific inland through the West. That warm air will become even warmer upon descending from the Rockies, pushing temperatures above normal there. Then, with the storm moving slowly toward the Great Lakes, it will allow more of the warmth to progress into the Ohio Valley and the East later this weekend into the beginning of next week, as the arctic air is completely cut off, at least for the time being, anyway!
A storm heading for the Midwest this afternoon and tonight, while generating snow in parts of the Midwest, will push warm air ahead of it, with little cold air behind it reaching the East.
A snow storm affecting the central Plains and Midwest into the Great Lakes this afternoon into tomorrow night will be followed by the first of two arctic outbreaks around the country before Thanksgiving.
Winter is six weeks away, and there will be a couple of signs this week of the approaching season as a storm develops on the western Plains and heads through the Great Lakes.
A warm pattern is setting up for much of the nation next week.
Tropical moisture associated with the one-time Hurricane Patricia will bring heavy rains to many areas east of the Mississippi Valley between now and Wednesday night.
Patricia, the strongest hurricane on record in the eastern Pacific, will make landfall this evening in Mexico. It will spread torrential rains into East Texas, and impact the weather all the way to the Northeast next week.