Thursday, 11:30 a.m.
Another hot day prevailed in the West on Wednesday, as temperatures soared well into the 90s east of the Cascades with a few places reaching the century mark in Idaho. The record setting heat in June farther south across California and Nevada has eased, so the 109 in Las Vegas yesterday was merely 5 degrees above average.
We are starting to see a little more moisture filter into the southern Rockies, and that is contributing to some thunderstorms. Look at the 24-hour precipitation chart, ending at 12z this morning:
Most of these totals are not noteworthy, with two exceptions. Flagstaff, Ariz., picked up nearly an inch of rain, while a slow-moving thunderstorm near Colorado Springs, Colo., triggered flash flooding over the burn area near Manitou Springs. The video footage of a reporter getting trapped in his car and caught in that mudslide was scary!
The whole area from the central and eastern Rockies to the Southwest deserts will be fascinating to watch into and through the weekend for daily changes. If you look at the center of the upper-level ridge as it appears today, it is one entity over Colorado:
If you then look at where the thunderstorms are most likely to occur this afternoon and evening, you'll note they will largely be north and west of the center of that upper-level ridge, which means areas from western Colorado and Utah into Nevada and points south will see most of the activity.
That doesn't change a lot tomorrow, but as we go through the weekend, the upper-level ridge will get pinched to the southwest to some degree. More on why that will happen in a moment. Regardless, the impact of that will be to send the sinking air associated with the upper-level high farther west and south and force the moisture farther west and north over time. Thunderstorms by Sunday afternoon will be more of an issue for Utah and especially Colorado up into southeastern Wyoming and western Nebraska. Part of the reason for this is a fairly weak upper-level disturbance darting across the northern Rockies toward the northern Plains, pulling a weak cold front for the ride that can then help to trigger some of the mainly afternoon and evening thunderstorms. That will also mean less activity down across southern Arizona and parts of southern New Mexico.
I mentioned the pinching process of this high. That is likely to be accomplished by an upper-level low rolling westward across the Ohio Valley and beyond the Mississippi Valley. It actually gets all the way out to Kansas and eastern Oklahoma on some of the latest model runs:
This little feature is now a digging upper-level trough over the Midwest. It was originally supposed to split in two pieces, with the northern part of the feature moving farther downstream across the Northeast. Then in the past couple of days, that went to an upper-level low closing off over New England, to now that upper-level low forming tonight and tomorrow over Pennsylvania, then rolling southwest and west over the weekend!
As this feature moves from Pennsylvania tomorrow to Kansas and Oklahoma by Sunday afternoon, it will trigger some mainly afternoon and evening convection along the way. Tomorrow that will be in the central Appalachians. Saturday that could be in parts of the Ohio and Tennessee valleys. By Sunday, look for scattered convection from the Red River Valley to the middle and lower Mississippi Valley associated with this feature. Much of it will be south of the path of the low, as the air north of it will be relatively dry going into the weekend thanks to the cold front now limping over the Appalachians and pressing into the Tennessee Valley.
And why is this upper-level low suddenly forming so far west and retrograding back to the west over the weekend? Look no farther than that image above, and you'll see why - the Atlantic ridge is ready to flex its muscles yet again, just as it did a week ago! When it did, it cleared out much of the mid-Atlantic into southern and eastern New England for a day or two. I was in the middle of that 'clear out' on Saturday, riding in Total200 all day long in a baking sunshine with high humidity. Guess what - that's coming again.
Going into the weekend, some of the remnants of Chantal will be pulled northwestward toward the Carolinas on the southwest flank of this expanding upper-level ridge, leading to a continuation of this moist weather pattern from Maryland on south. The humidity that will briefly go down tonight and tomorrow will start going right back up on Saturday. By Sunday evening, with the upper-level high clearly over the northern mid-Atlantic (farther west than it EVER got last weekend, areas north of the Mason-Dixon Line will see less thunderstorm activity, though it won't be totally absent.
After that, the first half of next week looks hot and steamy throughout the mid-Atlantic and Northeast, with only token relief from any pop up thunderstorms during the afternoon and evening hours. Temperatures will easily be well above normal throughout the region. There may not be any 100-degree readings in this pattern, but consistently 90 and above with high humidity and an almost daily threat of a thunderstorm, mainly during the afternoon hours.
I might have to get out the carbon paper and just run a few copies of the same forecast through the machine when we get to Monday!
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