Thursday, June 23, 2011

Discussion: thunderstorm chances through Saturday

While the humidity has certainly been higher the past few days, cloud cover from distant (and some not-so-distant) thunderstorms have helped keep temperatures in check as we haven't gotten out of the 80s since Monday. The higher humidity and scattered mid-day thunderstorms the past two days have been the result of a cold front that moved into, but not through, the region.

The front is still draped across the area and another weak impulse will move along it this afternoon. Morning high-res computer data is indicating the possibility of thunderstorms forming just south of the Memphis area, across north MS, this afternoon. A Slight Risk of severe weather is forecast for the possibility that some of these storms could contain hail or a high wind gust. In Memphis, I expect a partly cloudy day with slightly lower humidity and highs in the lower 90s. A few thunderstorms could pop up this afternoon in the metro, again mainly over north Mississippi.
Slight risk of severe thunderstorms in yellow area today
As the front starts to pull back to the north tonight and an upper-level ridge of high pressure begins building from the southwest, the Mid-South will be placed in a weak northwest flow pattern for Friday and Saturday. This was the pattern (although somewhat stronger) that brought morning t'storm complexes early last week (for more on this pattern, see this blog post from Monday last week). I believe there is a decent chance of this occurring again, the first of which could be overnight tonight. The most likely track for a potential complex will be from northwest to southeast AR, though there could be a few storms over east AR late tonight or early Friday. During the day Friday, outflows from these early day t'storms could initiate further development in the metro area, so a chance of thunder has been included in the MWN Forecast. In addition, a Slight Risk of severe weather is in place for the area on Friday, with damaging wind and hail being the prime threats.
Northwest flow over the Mid-South  (map above valid Fri PM)
Yet another chance of thunderstorms exists Saturday, with models indicating the possibility of scattered storms developing over the region during the day in persistent northwest flow. By Sunday, the ridge to our southwest builds over the region and continues to dominate through at least the first half of next week.  This will mean higher daytime temperatures, partly cloudy skies, and no mention of thunderstorms early in the week. Highs will reach the mid to upper 90s with heat indices well above 100 thanks to moisture streaming north from the Gulf of Mexico under the ridge.
Upper level high pressure ridge builds in early next week (map valid Monday AM)
Check out the MWN Forecast, the most accurate for Memphis, for all the details.

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4 comments:

Anonymous said...

Thanks for the blog post- I enjoy keeping up to date.

Has the Slight/Moderate/High categories (I guess started by the SPC) for describing weather been in use for a long time or is this fairly new? I don't remember hearing or seeing this terminology in the past, although I haven't followed the weather this closely in the past either. Have just really started paying attn this spring. I know all the news stations are using this way of describing bad weather now. Also, are these "northwest flow" storms what you would call summer storms that we historically have throughout the summer? Really quite ready to get out of "severe" mode! Thanks.

MWN/Erik said...

Thanks for reading!

The SPC categories have been in use for some time. Most people (outside of the meteorology world) probably just didn't know about them or have access to them.

Northwest flow events typically occur during the summer, but are not the "summer storms" that you are probably thinking of, which sort of pop up and die out in the heating of the afternoons. Northwest flow storm events are typically clusters/lines of storms that move through an area, rather than just sitting in one location and raining themselves out. Either type can produce damaging wind and hail though.

Thanks again!

Anonymous said...

Out of pure curiosity, was "Northwest flow" what caused Hurricane Elvis in 2003? I'm sort of getting into this meteorology lingo.

MWN/Erik said...

It was the same kind of system (called a mesoscale convective system, or MCS) only a much stronger and long-lasting one, termed a derecho. Yes, it was a northwest flow event.