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    Developing Weekend Storm to Dominate the Weather Deep Into Next Week

    4/25/2014, 8:25:19 AM

    Friday, noon

    Rain is falling in California this morning, a noteworthy item in and of itself amidst a long-term drought. San Francisco has already picked up a tenth of an inch of rain as I write this, and a band of heavier rain is moving through the city now. The rain is spreading south and east toward Los Angeles, and they'll get some late today and tonight before this storm moves on downstream.

    If this were the middle of winter, that storm would be to the East Coast Monday night or Tuesday at the latest, and after a cool, wet period, it would quickly moderate on the back side of it. However, it isn't the middle of winter, it's the middle of spring, and a developing high-over-low block will trap this storm and cause it to only creep eastward Sunday into next week. That's going to mean an extended period of cloudy, wet weather for much of the country and once on the back side of the storm, a few days of below-normal temperatures. It may not be the same setup that brought cold wave after cold wave through the heartland throughout the winter and early spring, but the end result will more or less be the same - another period of chill as we approach the mid-point of spring late next week.

    Let's start out by looking at the 12z April 25 NAM 500mb forecast for tomorrow morning:

    There are three striking features on that forecast chart. One is the intense upper-level low rolling through southern California. A second is a similar looking feature crossing the Great Lakes, and the third is the upper-level ridge over the Plains in between. There's a lot of warm air from the eastern Rockies out across the Plains and Mississippi Valley right now, and it will hold into tomorrow across the South and up into the Tennessee Valley, even up into parts of the Ohio Valley. The chilly air being pulled out of central Canada into the Midwest and Great Lakes will be aimed at the Northeast and northern mid-Atlantic states, but from southern Virginia on south, it will largely stay warm.

    Here's the same model forecast for Sunday afternoon:

    The lead upper-level low is rolling away from New England. The upper-level ridge is pushed downstream over the Ohio and Tennessee valleys. And the much bigger upper-level storm is already out into the plains. Here's the accompanying surface forecast for Sunday afternoon:

    Masked by the expanding area of precipitation being forecast from Iowa to eastern Texas is the initial cold front pushing out across the Plains. While there will be some severe thunderstorms late Saturday and Saturday night from central and especially north Texas all the way to Nebraska and western Iowa, the severe weather threat will become larger Sunday afternoon and Sunday night as this front plows into an increasingly moist air mass out ahead of it.

    If you go back to the 500mb forecast for Sunday afternoon, you'll also note a feature coming through the Northwest, generating a lot clouds and some showers. This will move swiftly east-southeastward Sunday night, feeding into the growing upper-level low over the Plains. At the same time, the upper-level ridge downstream will become pinched off, with a piece of it over the Southeast up into the eastern Great Lakes, while at the same time an upper-level high tries to form over northern Ontario and Hudson Bay. This latter feature will force the storm to stay beneath it, and more or less cast it adrift apart from the main jet stream flow.

    The slow movement of this storm will mean areas from the eastern Rockies through the northern Plains and Midwest are going to be very wet. The chance of snow is relatively low, though some of the higher ground in the Rockies then out into the Dakotas are at risk for accumulating snow late this weekend and into Monday. The bottom line is that it will be very wet, and 2-3 inches of rain may contribute to flooding.

    The rain won't stop falling there, though. As the storm inches eastward, the rain will spread across the Great Lakes, with rain and thunderstorms in the Ohio Valley. The rain will reach the mid-Atlantic late Monday and Monday night, with showers and thunderstorms spreading across the South into the Southeast later Monday into Tuesday. Some of the stronger thunderstorms could not only contribute to flooding downpours from the Ohio Valley on south to the Gulf Coast region but will likely also be severe with hail and damaging winds, and probably some tornadoes.

    It promises to be a stormy week, and one that won't be warm. Look at the forecast temperature anomalies for next Wednesday:

    Then for Friday:

    If you want a taste of summer warmth, head to Florida, or wait a few days and head to the West Coast. As an upper-level ridge of high pressure balloons over the Northwest, it will quickly warm up after Monday, boosting temperatures into the 70s and 80s over parts of Washington and Oregon and probably some 90s in parts of central California.

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


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