WRAL WeatherCenter Blog

Active Pattern Again

Posted November 28, 2005

Not unlike early last week, we find ourselves in another active pattern that should be helpful in terms of chipping away at the drought conditions over central North Carolina. While last week's system involved a deep cutoff low pressure area aloft that swept into and across our state, this week's is organized a little differently, and rainfall totals in a general sense may not be quite as high, but should be substantial all the same.

Over the weekend, we developed a surface trough off the southeast coast of the U.S. that interacted with both a weak upper level disturbance approaching from the southwest and the left exit region of a mid-level jet streak just south of us. Back on Friday and Saturday, and even to an extent early Sunday morning, the precise rainfall patterns that would result from those interactions remained difficult to pin down, with a variety of computer models generating similar forecast patterns in most respects, but with subtle differences that led to large differences in the placement and intensity of precipitation. We settled on the idea of forecasting patchy light rain and a few heavier showers, focused mainly over central and eastern NC, with an expectation of unevenly distributed rainfall amounts for Sunday and Sunday evening ranging anywhere from a trace to around a tenth of an inch on the low end and around 3-4 tenths the expected upper end. For those in the western Piedmont or the central and northern Coastal Plain, this turned out to be quite correct. However, a corridor of more persistent and more intense rainfall developed over Robeson and Scotland counties in the morning and spread north right through the middle of our viewing area, bringing many of us totals that pretty well doubled those forecast values. I'm not complaining too much, since we really needed it - however, it's always a little hard to hear the rain continue to fall (and rather intensely at that) long after it's surpassed your forecast! Many of us ended up with amounts in the range of .4 to .9 inches, while parts of Scotland and Robeson counties reached the 1-2 inch range. You can get a rough overview of how the rain was distributed at a new NWS Precipitation Analysis site that allows you to review one day, 7-day, 2-week, month-to-date or year-to-date rainfall patterns for the U.S., and you can zoom to a view of a single state by clicking on that state. Archived data is also available, so if you're reading this a few days later, you can go to the site and select the date you'd like to review. Here's the adress:

http://www.srh.noaa.gov/rfcshare/precip_analysis_new.php

While some sprinkles or brief showers today may add a bit to our totals, it appears a slow moving cold front on Tuesday will likely bring a band or two of rather heavy showers, with the added potential for an intense thunderstorm or two. Rainfall amounts with the frontal passage could easily reach an inch or more in many spots, with the added potential for an intense and gusty thunderstorm or two midday into the late afternoon, when a lack of instability across the area may be overcome by strong wind fields and wind shear through the lower and middle atmosphere. We'll be watching it closely...
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