Wednesday, 11:20 a.m.
Those lazy, hazy, crazy days of summer that Nat King Cole made immortal are now behind us for the most part. That doesn't mean there won't be pockets of heat and humidity in the coming weeks, but they will be fewer in number, not nearly as expansive, and of much shorter duration, plain and simple. The inexorable march of the seasons is underway, and it cannot be stopped.
One evidence of that change is the observation I noted walking out to my car on a quick break just ahead of sunrise. I went out to get the numbers from yesterday's ride off my bike that I had left strapped to the back of the car, so as I wiped off the moisture that had gathered on my bike computer, it felt odd. It felt like it was a little frozen! After I got what I needed, I ran my finger across the top of car, and sure enough, it was frozen dew! I have seen reports of frost from parts of central and northern Pennsylvania this morning, a little too early for my taste, but there's not much I can do about it.
It isn't just the overnight lows that are highlighting the changing of the seasons. Even the late afternoon and evening hours are vastly different now. Again, while riding with the Tuesday evening group last evening around 5:45 - 6:45, there were distinct differences between those areas still in full sunshine, and those where the sun was blocked by either a canopy of trees or a hillside. The former still felt relatively warm, while the latter would send a few chills up and down your spine!
Such is typical of a sprawling September high pressure area like the one over the Northeast and mid-Atlantic states this morning. Here's the 0z Sept. 18 GFS surface forecast for this evening:
Underneath these large highs, the air is stable and calm. With nearly 12 hours of darkness now, it's much easier for the air to cool to the dew point temperature, especially in places where the cold air can collect, such as in river valleys or low spots in the terrain. That allows fog to form, and it was quite notable across the river valleys of the Northeast this morning. It will be even more prevalent tomorrow morning as the air mass becomes a little more 'stale,' if you will. Then, by day, the sun still gets high enough to mix out the low clouds and fog, revealing plenty of sunshine for the rest of the day with a warming trend during successive afternoons.
That's in play for the Northeast through Friday. Another similar high is building into the interior Northwest and northern Rockies later today and tonight behind a cold front and will lead to another chilly night tonight and a cool day tomorrow. That air mass, though, is not as chilly as the one currently over the Northeast. As it slides eastward, it will bring cooling across the northern Plains behind a cold front tomorrow and tomorrow night and into the Midwest later Friday and Friday night. That cooler air will also slide down the Plains and bring some cooling into the Great Lakes and Ohio Valley later this weekend.
The front that's moving out of the northern Rockies will have some weather with it. Thankfully for Colorado, the monsoonal moisture that directly contributed to the colossal flooding of last week is no longer present. Therefore, the spotty showers and thunderstorms that erupt with and ahead of the front this afternoon and early tonight will carry little moisture over the flood-ravaged areas.
Showers and thunderstorms will become more numerous from Kansas, western Oklahoma and the northern Texas Panhandle up into the Midwest late tonight and tomorrow as the front encounters a much more humid air mass. Some of these thunderstorms are likely to be severe, producing damaging winds and hail along with some brief downpours. The Storm Prediction Center severe thunderstorm outlook for tomorrow:
The front will begin to slow down a bit as it crosses the Great Lakes and Ohio Valley later Friday and Friday night, so the rain totals could exceed an inch from the mid-Mississippi Valley into the Great Lakes and Appalachians. Here's the 12z Sept. 18 NAM total precipitation forecast through Saturday:
The final hours of summer on Sunday are apt to be wet from New England down through the mid-Atlantic states into the Southeast with that front moving slowly through.
What may complicate matters over the weekend is tropical moisture getting involved near the Gulf Coast. If you look back at that image above, one thing that really stands out is all of the expected precipitation over the Gulf of Mexico. This appears likely to be stemming from a tropical low that is about to emerge over the Bay of Campeche from off the Yucatan Peninsula. The computer forecasts are all over the place with this feature, but if there's a broad consensus, it's to bring the low slowly northwestward to a point off the lower Texas coast, somewhere southeast of Brownsville early this weekend. Some models bend it more toward the northeast coast of Mexico after that. Others stall it for a time. And some others start to turn it north and northeastward and have it take aim at the central Gulf Coast late in the weekend or early next week.
Clearly the verdict on this storm is in doubt, but at the very least it appears very likely that there will be a notable increase in wet weather over the central and probably eastern Gulf Coast this weekend, with some of those heavier rains spreading inland across the Deep South toward the Southeast.
Lastly, another upper-level trough will take aim at the Northwest late this week. It will drive a cold front into western Oregon and western Washington and across Vancouver Friday afternoon and early Friday night, accompanied by some rain. Much cooler air will again follow the front into the Northwest this weekend, with lots of clouds and a few instability showers likely on Saturday as the upper trough passes. And it will be followed by a fairly robust storm later Sunday and Sunday night that will bring heavy rain and strong winds into western Washington and southern Vancouver.
That may be the stronger signal that we're now moving into fall - a potent storm and cold front bringing rain back to the Northwest. By this time next week, the Northern Hemisphere will have slipped into a sunlight deficit, with the winter solstice a mere three months away.
The heat and humidity will be erased from much of the East later this week, but warmth will spread from the Plains eastward over the weekend. The tropics could still play an important role in the weather along the Eastern Seaboard this weekend.
A dominant ridge will keep it hot from the Ohio Valley to the East into next week, while the disturbance north of Cuba is slow to develop as it approaches the southeastern Gulf of Mexico.
Two systems will delay the onset of warm weather in the Ohio Valley and the East over the next week or so, but then it should get warm all across the country heading into the Memorial Day weekend.
A turn to much colder air over the Northeast and mid-Atlantic states will set the stage for a rain and snow storm later this weekend before it turns much warmer later next week.
It's warm now, but will turn much colder this weekend, with a storm threat later Saturday into Sunday. Warmth will return by the second half of next week.
Though it is cold now east of the Mississippi, with a couple of opportunities for snow into the weekend, a blast of warmth is due for much of the country east of the Rockies next week.