Sunday, July 8, 2012

Major pattern shift will provide drought & heat relief this week

It's the week we've all been waiting for... No, not the series finale of American Idol or Super Bowl week. It's "Heat Relief Week"!

For 2 weeks, massive hot high pressure has squashed rain chances and brought eight 100-degree days to the Memphis metro. Drought conditions have become "severe" (by definition) or worse as the last widespread rain fell on June 11-12 from a wind storm that seems like it was months ago.  That ends today.

Upper-level pattern on Saturday morning (July 7) - massive high pressure centered to our north kept the storm track (purple arrow) well to the north and left the Mid-South with very hot and dry weather
A major pattern shift in the upper levels of the atmosphere has weakened the stranglehold of the persistent high the past couple of days, allowing for scattered thunderstorms to form. These upper wind pattern rotates around the high in a clockwise direction, which means our storms the past couple of days have been moving from east to west - backwards from what we typically see! Starting today, the high shifts back to the west and will be replaced by a trough of low pressure forming over the eastern U.S. This will allow a cold front to move into the Mid-South early this week.

Upper-level pattern forecast for Wednesday morning (June 11) - high pressure has shifted west to the SW U.S. and is replaced by a trough in the eastern U.S., anchored by low pressure over eastern Canada. Note that the storm track (purple arrow) takes a dip into the Ohio Valley, much closer to our region.
The front will move slowly, giving us multiple days with good rain chances, before stalling over north MS by mid-week.  Abundant cloud cover and high rain chances will mean high temperatures will fall back to near 90 this week on average, with some locations remaining in the upper 80s early this week if enough rain falls.  Total rainfall amounts this week should be nearly 2" for most locations in the metro, while some areas that get repeated thunderstorms could get more than that.  Though it won't wipe out the rainfall deficit for the year (which stands at about 50% of normal rainfall), it will certainly alleviate it, especially for vegetation as well as agricultural interests.

5-day total precipitation forecast by NOAA/NWS - valid Sunday through Friday morning

Severe weather is possible, though won't be widespread, this week.  The main threats will be localized hail and some strong wind gusts in the stronger storms that form, mainly in the afternoon and early evening hours today through Tuesday.

Looking a little further out, there are some signals that indicate the heat may return for the latter portion of the month, so enjoy the respite from it this week!  Your lawns will appreciate nature's watering can better than yours and your wallet will appreciate your not having to run the sprinklers this week!

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