Saturday, November 19, 2011

The latest on an extended wet weather pattern

Clouds have been on the increase this Saturday with moisture levels on the rise under breezy southerly wind. Moisture is increasing ahead a cold front moving southeast out of the Central Plains states, headed for the Mid-South. The front will approach the region Sunday, but like we saw with our previous cold front earlier in the week, it will not sweep through very quickly at all. In fact, the front will stall just north of our area, resulting in extended rain chances that will continue through late Tuesday, with periods of heavy rainfall and thunderstorms possible.

Though the regional radar is dry as of Saturday afternoon, expect that to be changing soon. Rain will begin breaking out ahead of the front after midnight across Arkansas and Missouri, slowly pushing southeast. This first round of rain will impact the metro during the day Sunday. At the same time, the front is expected to stall just north of the region. As this happens, repeated upper level disturbances will track along the stalled surface boundary, resulting in additional rounds of rain that will continue into Monday.  After a brief break late Monday into early Tuesday, a final and much stronger upper level disturbance will move in from the southwest, resulting in the development of surface low pressure that will move just north of the region, finally sweeping the cold front through by Tuesday evening. This will lead to the final round of rain for the metro area.

Though confidence is high that the front will stall somewhere just north of the area on Sunday, exactly where that happens remains uncertain, which will be key to what parts of the metro see the heaviest rainfall amounts during this period. At this time, the latest computer model guidance indicates the front’s stall will occur near the Arkansas/Missouri border. Should this happen, the steadiest and heaviest rain would be north of Interstate 40, while areas to the south may have more extended dry periods and thus lesser rainfall amounts, especially between Sunday and Monday when the front is stalled.  This would be quite similar to the situation earlier in the week with our previous system.

The most significant hazardous weather threat during this period will be heavy rainfall. In general, 1 to 4 inches of rainfall can be expected across the metro between Sunday and Tuesday. However, areas north of Interstate 40, closer to the stalled front, may see even higher amounts, with totals up to 6 inches possible.  In these areas, specifically across Crittenden and Tipton Counties (which also saw the heaviest rainfall earlier in the week), a threat of flash flooding may develop. Therefore, a Flash Flood Watch has been issued by the National Weather Service from Sunday night through Tuesday afternoon. Across the remainder of the metro, even though a widespread flooding threat is currently not anticipated, some issues with standing water may still develop, especially in flood-prone areas. No matter where you are, if you encounter a flooded roadway, as the National Weather Service says: Turn Around, Don’t Drown!

National Weather Service forecast precipitation between Sunday and Tuesday, indicating rain amounts of 1 to 3 inches in the metro. Higher amounts are possible north of Interstate 40.
In addition to the heavy rain and flooding potential, thunderstorms are possible as well during this period. The best threat for storms is expected to be with the final round of rain on Tuesday, with a band of thunderstorms likely to sweep through ahead of the cold front. As is typical during the fall season, the amount of expected instability is in question, but strong wind dynamics will be in place. Nevertheless, some strong thunderstorms may occur. A better assessment of this potential will be possible closer to Tuesday itself, as more computer model data will be available.
GFS model forecast for Tuesday morning (11/22), depicting a low pressure area to our NW, which will help sweep the cold front and a final band of rain and thunderstorms through the region.
Stay with MemphisWeather.Net for the very latest information on the heavy rain and thunderstorm threat as it evolves over the next few days. Our MWN Storm Center and forecast page will have full and updated details, with social media nowcasting on Facebook and Twitter expected as well.

--Kevin Terry, MemphisWeather.Net

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