WRAL WeatherCenter Blog

Isaac and the rain...

Posted September 5, 2012

Rainfall estimate, based on a combination of radar and gauges, for the 7 days ending at 8 AM Tuesday Sep 4, 2012.

A little over a week ago, Hurricane Isaac made landfall in Louisiana as a slow-moving storm with Category One winds and a combination of storm surge and heavy rain that brought serious flooding impacts to parts of Louisiana and Mississippi. Since that time, we've seen the system track north through the western Mississippi Valley as a tropical depression, then join with an old frontal boundary as a post-tropical low drifting east into Kentucky, and finally elongating into a north-south trough and band of enhanced moisture that will continue to bring some clouds and scattered storms to the eastern U.S. for another couple of days. In addition, at the northern and southern ends of that trough, new low pressure centers have formed, one of which is heading through the northeastern U.S. while the other has pushed off the Gulf coast and has a slight chance of developing into a tropical cyclone in the next few days.

All told, the system has been quite a rainfall maker (or at least, contributor) for many locations in the eastern half of the country over the past week, as seen in the 1st graphic above, which is a contour map from the NWS Precipitation Analysis site showing estimated rainfall across the country during ht e 7-day period from 8 am last Tuesday until 8 am yesterday (Sep 4th). The impact of the storm is quite obvious in both this image and the 2nd one, which shows the departure from normal rainfall for that same time frame. For large swaths from Alabama to Arkansas and north into Missouri and Illinois, rain totals through the period have ranged between 3 and 8 inches, while more extreme values between 10 and 20 inches were observed across southeastern Louisiana and southwestern Mississippi.

While that was simply too much rain too quickly there, the storm did have a more positive nature for some drought-stricken areas farther north. The third image is a map of drought conditions issued by the U.S. Drought Monitor last Thursday, based on data through last Tuesday. The map highlights a very large area with severe to extreme drought that has parched the Plains and middle Mississippi Valley, and a comparison of the 7-day precipitation map shows that Isaac may have proved quite helpful to about the eastern third of that large area, while providing little relief at all to much of the Plains. The new Drought Monitor that is released tomorrow will take into account the rains from Isaac, and it will be interesting to see how much week-to-week improvement is assessed for some of the areas in the Mississippi Valley, after having seen very little change in that drought pattern over the past three or four weeks.

It hasn't been as direct a result of Isaac, but a broad flow of moist air from the south on the east side of that system has given our state several rounds of locally heavy rains in the past week, and of course even before that, none of our state has been designated as a drought area for the past few weeks. For us, the last of the band of moisture and lift associated with remnants of Isaac will pass across the area from late today through Thursday evening or so, helping to keep some scattered showers and storms going at times, a few of which may still produce some localized downpours. There is a bit of a pattern change in our future, though, with a cold front this weekend (right now, passage would most likely occur sometime Saturday night or Sunday morning) likely to bring some more showers and storms, but then usher in high pressure that will bring lots of sunshine and much less humid air to the region for early next week.

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