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    Split Disturbances Will Keep Heat Down From Northern Plains to Northeast

    8/14/2014, 8:17:10 AM

    Thursday, 11:45 a.m.

    Don't trust the models these days. It is sometimes laughable to see the changes in the forecast models from one day to the next, or even one run to the next. And often there are major differences in the models, adding the challenges of making a forecast of almost any length.

    Let me cite just one example here before delving into the rest of the overall forecast. Over the weekend and even into Monday, it was looking like several days of sunny, dry, gorgeous weather from the Ohio Valley to the mid-Atlantic states, from today through Sunday. Then the European model began to diverge from the general consensus and bring a feature across the northern Plains and zip it east-southeastward, accompanied by clouds, showers and thunderstorms. This would mean wet weather for at least a part of the weekend, first in parts of the Midwest, Great Lakes and Ohio Valley, and then, eventually, farther downstream over the eastern Ohio Valley and central Appalachians into the mid-Atlantic states and parts of New England by Sunday. The next iteration sped things up a bit, so that drying would move into Pennsylvania and upstate New York later Sunday on some of the models.

    The latest NAM model has come up with still a new twist - split the energy, with one piece attacking the departing cool, dry air in the Northeast late Saturday into Saturday night and Sunday morning, with the main piece lagging far behind, crawling across the Plains into the Midwest later Saturday and Saturday night into Sunday. Between the two, not a lot would happen, especially in central and eastern Pennsylvania into New Jersey and down into Delaware and Maryland. Here's a look at the projected 48-hour precipitation totals from Friday evening through Sunday evening:

    I am certain the forecast will change a few more times before we get to Saturday and Sunday as the models continue to try and figure out what to do.

    Where is all of this coming from?

    Look at the late-morning water vapor image:

    The feature you see in Wyoming is the main disturbance in question, and it's not embedded in a very fast flow aloft, as the main jet stream is well to the north. Look at the afternoon 500mb forecast:

    As you can see from the water vapor imagery, there's a lot of moisture being tapped all the way from the Eastern Pacific. Slow-moving showers and thunderstorms this afternoon can trigger flash flooding in parts of the Northwest around the upper-level low there, as some of this rich moisture supply is pulled back into the circulation of that upper-level storm. However, there can also be flooding downpours over central and western Colorado this afternoon and evening since this feature is pulling all of that water through the Southwest.

    It gets even more interesting tomorrow and tomorrow night into the Plains. There's even more water to tap by this point, as evident by the forecast of precipitable water tomorrow evening:

    Again, this upper-level feature is caught in a weak steering flow, and as such, the showers and thunderstorms that do develop won't move very fast and can lead to other incidents of flooding and flash flooding.

    Then, over the weekend and into early next week, the latest model trend is to peel a piece of this disturbance off to the east and send into through the Midwest and Great Lakes toward the Northeast in some for over the weekend, generating lots of clouds along with some showers and thunderstorms. The bigger piece remains in the much weaker steering flow and drifts southeastward toward the middle Mississippi and western Ohio valleys by the end of the weekend. If that turns out to be the case, then some of the heavy rains will likely follow that feature over the weekend, then try to come east through the Ohio and perhaps the Tennessee valleys on Monday.

    One thing out of all this is fairly certain - it does not bode well for heat from the northern Plains and Midwest to the mid-Atlantic and especially New England through the middle of next week.

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com


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