Friday, September 19, 2014

Mid-September weekend weather - not quite to "summer" levels

I promised that summer was over with our last big cold front. I still maintain that, though it'll get within a few degrees of my "summer threshold" this weekend ahead of another cold front that moves through Sunday!

There are a lot of outdoor activities this weekend, including the Memphis Tigers football game Saturday evening vs. MTSU (Go Tigers!), the start of the Mid-South Fair at the Landers Center in DeSot County, and the 3rd annual Best Memphis Burger Fest at Minglewood Hall among others. Normally mid-September highs are in the mid 80s and lows in the mid 60s. A warm airmass ahead of the cold front will mean temps slightly above average. In fact, we could push 90 degrees Saturday afternoon with upper 80s on Sunday as the front moves through the region. Humidity levels will also be elevated  with dewpoints in the mid 60s, but not quite to the "summer" threshold.

Dewpoints this evening are high enough to notice some humidity in the air.

The cold front Sunday will bring a chance of afternoon thunderstorms, but right now we put that rain chance at just 30%. No severe weather is expected and it should not deter you from carrying on with your plans. You might just keep an umbrella close by in case a passing storm moves overhead!

Average rainfall expected on Sunday as the front moves through. Scattered t'storms are expected mainly in the afternoon.

By Monday, cooler and drier air seeps in with dewpoints falling into the comfortable range in the 50s, mostly sunny skies, and highs near 80. The rest of next week looks mighty fine with abundant sunshine through at least Thursday, highs in the low to mid 80s and lows generally in the 60s, though many areas will see upper 50s on Tuesday morning.

Dewpoints on Monday evening are much more comfortable behind the front.

Hope everyone has a fantastic weekend! And in honor of "International Talk Like Like a Pirate Day"...

Erik Proseus, MWN Meteorologist

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Thursday, September 11, 2014

Recap of the September 11th Flood of 2014

While most readers of this blog have clear memories of 9/11/2001, many of you will now remember 9/11/2014 for some time to come as well. The flooding that took place today may be as bad as some of you have witnessed, or at least rank in the top couple of events that you have personally experienced.

It began innocently enough last night with a forecast of showers and thunderstorms in the early morning hours that would likely impact rush hour. But the bigger story was clearly going to be the arrival of autumn air and promise of a pleasant weekend that many have been looking forward to after a late-arriving summer. Or so we thought...

As storms well to our north moved towards the region overnight, an outflow boundary moved slowly into the metro from north to south. Behind it, storms rapidly fired on a southerly low level jet stream and plenty of warm, moist air. The storms moved east, but continued to fire behind the outflow, resulting in a "training" effect. Storms fired near the Mississippi River repeatedly and moved east across Tipton, then Shelby and Fayette, and finally DeSoto Counties.

The hardest hit area during a couple hour period during rush hour was a stretch from Frayser to Raleigh to Bartlett. Later, during the mid-morning hours, DeSoto County, particularly the center of the county near I-55, saw repeated storms. Rainfall totals of 6-10" were found in these corridors, though widespread 2-5" readings were found throughout the metro.

Below you'll find a video loop of the NWS Doppler Radar from 4am until noon. Feel free to play it a couple of times and watch the storms fire behind a southbound "front" (outflow boundary) and train over the same areas.

Next, we've produced a Storify that describes the events using social media posts and images, many of which are ours. It summarizes the events quite well I think. (It's long. Just click "Read Next Page" a couple of times as you read down the page. At the end, you'll return to the closing paragraph from this blog.)

As mentioned in the Storify, the rainfall total of 7.23" in just 3 1/2 hours at MWN headquarters in Bartlett is considered a 1,000 year rain event. It's not the same magnitude everywhere in the metro, but given the amount of rain in the duration of time that it fell, you'd be hard pressed to see another event like this in your lifetime. (For comparison, former Olive Branch mayor and meteorologist Sam Rikard's rainfall in Olive Branch was about a 400 year event.)

Hopefully you have a story of your own to tell without having been harmed in some way by the power of Mother Nature. I think I'll remember this one for a LONG time to come!

Erik Proseus, MWN Meteorologist

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Monday, September 8, 2014

Awaiting the arrival of a season-ending cold front

Despite a very late start, summer did end up rearing its ugly 3-H head (hazy, hot, and humid) the past few weeks. Though there were a few brief respites in August, the end of last week into the first half of the weekend did me in. I am over the heat and especially the humidity.

We are now in the midst of a short respite from the humidity, but temperatures are already rebounding behind Saturday's cold front and humidity will also make a return tomorrow, and especially Wednesday. As highs reach back into the 90s those days, we'll see the heat index back near 100 on Wednesday afternoon.

HOWEVER, I'm now highly confident, perhaps even giddy (much like a Memphis Tiger football fan after a very solid moral victory), that a season-ending cold front will kick summer to the curb later this week. A strong autumn front will dive south this week, passing through the Mid-South on Thursday. By this weekend, high pressure originating in Canada will dominate the U.S. from the Rockies to the east coast. The first freezing temperatures of the season will be found in the northern U.S. Snow will fall in the northern Rockies. And fall will begin! If you don't have outdoor plans this weekend, make them.

Saturday morning's projected low temperatures according to the GFS computer model.  These numbers are backed up by the European model as well.

Saturday's projected high temperatures according to the GFS computer model.

With a front of this magnitude, it's worth considering the possibility of severe weather, especially considering we go from a heat index near 100 Wednesday to lows near 60 less than 48 hours later. In this case, the passage of the front appears to be early in the day on Thursday, which is not the best timing for a severe weather threat. Also, with the main dynamics of the low pressure system going by to our north, that is where the main severe weather threat will be focused, as per the Storm Prediction Center severe weather risk for Wednesday shown below. By Thursday, thunderstorms are likely along the front, but severe weather is not expected, including here in the Mid-South.

The fall cold front will bring a risk of severe weather to our north on Wednesday, but no severe weather is anticipated Thursday in the Mid-South.
Beyond this week, the long-range outlooks favor near to below normal temperatures well into September. In fact, below normal temperatures are highly likely into the first half of next week according to NOAA's Climate Prediction Center, as shown below. By the time we start seeing any chance of above normal temperatures again (and I'm not predicting this), the average highs will be in the lower 80s. So while highs well into the 80s are still entirely possible, anything higher than that looks unlikely after this Wednesday. Thus my proclamation that with the arrival of this week's major cold front, summer is done - put a fork in it. Check out our forecast for the details on what I expect with the cold front, and more importantly, behind it.

The NOAA temperature outlook for Sep 13-17 (Saturday through  Wednesday) indicates well below average temperatures east of the Rockies, excluding the FL peninsula. For the Memphis area, there is a 70% chance that temps will average below normal. Average highs during this period are in the mid 80s and lows in the mid 60s.
Are you ready for fall or do you wish summer could hold on a little longer?

Erik Proseus, MWN Meteorologist

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Friday, September 5, 2014

August 2014 Weather Recap and MWN Forecast Accuracy

August Recap

Through July, each month in 2014 had experienced below normal average temperatures. In fact, it had been 10 months since Memphis had an "above average" month temperature-wise. That streak ended with an average temperature for the month of August  0.5 degrees above normal. The highest temperature for the month (100, which tied a record high) was the first 100-degree reading in just over two years. Despite the slightly warmer than average month, the average temperature for the year through August ranks 22nd coolest on record (a span of 140 years).

Precipitation-wise, the dry pattern from July continued into August. In fact, until the last two days of the month, the rainfall total for the month was less than 1", resulting in dry vegetaion and browing Bermuda across the region! For the year, precipitation is still 124% of normal, or over 8" above normal. The only severe weather in August was wind damage in DeSoto County (Horn Lake) on the 24th from a downburst produced by a summer thunderstorm.

For the period June-August, which is defined as "meteorological summer," temperatures were cooler than normal by 1.5 degrees (79.9 degree average) and precipitation was well above normal (17.76", +6.66" compared to normal) fueled by a record-breaking June.

Average "meteorological summer" temps by year for all years on record for Memphis, TN. 2014 continued the downward trend of the past 3 year and was near climatological average.

Memphis International Airport, Memphis, TN

Average temperature: 82.5 degrees (0.5 degrees below average)
Average high temperature: 91.7 degrees
Average low temperature: 73.4 degrees
Warmest temperature: 100 degrees (24th)
Coolest temperature: 63 degrees (14th)
Records set or tied: Tied record high of 100 on the 24th and tied record high minimum of 79 on the 25th.
Comments: 23 days had maximum temperatures at or above 90, four above the climatological average. The record high of 100 on the 24th was the first 100-degree reading in 25 months, last recorded on July 30, 2012 (101). For the year, the average temperature at Memphis is 62.5 degrees, which is 2.5 degrees below average.

Monthly total: 1.80" (1.08" below average)
Days with measurable precipitation: 8
Wettest 24-hour period: 0.85" (August 30-31)
Records set or tied: None
Comments: Zero days had more than 1" of rain and only one day recorded more than 0.50". As of the end of August, the yearly precipitation has been 43.72", which is 8.34" above (or 124% of) average.

Peak wind: West/37 mph (7th)
Average wind: 6.2 mph
Average relative humidity: N/A
Average sky cover: N/A

Click here for a daily statistical recap for Memphis International Airport.

Daily August temperature readings (blue bars) overlaid on the normal temperatures (brown) with record highs (red) and lows (blue).

Cirrus Weather Solutions /, Bartlett, TN

Average temperature: 80.3 degrees
Average high temperature: 91.9 degrees
Average low temperature: 69.9 degrees
Warmest temperature: 99.7 degrees (23rd)
Coolest temperature: 57.5 degrees (14th)
Comments: 22 days had high temperatures at or above 90. The warmest low temperature was 77.6 on the 20th.

Monthly total: 1.49" (automated rain gauge), 1.46" (manual CoCoRaHS rain gauge)
Days with measurable precipitation: 7
Wettest date: 0.96" (31st) (via automated gauge)
Comments: 1.27" of the 1.49" recorded this month fell in the last 3 days of August. The 31st was the only day of the month with more than 1/2".

Peak wind: Southwest/21 mph (7th)
Average relative humidity: 77%

Click here for a daily statistical recap for Bartlett, TN.

MWN Forecast Accuracy

MWN average temperature error: 1.27 degrees
MWN forecast temperatures within 2 degrees of actual: 88%
MWN average dewpoint error: 1.42 degrees
MWN forecast dewpoints within 2 degrees of actual: 80%

MWN's forecasts extend out five periods (2.5 days, or roughly 60 hours). Historical accuracy statistics can be found here.

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