May 2012 Climate Data and MWN Forecast Accuracy

The trend of very warm weather, which began this past winter, continued through the spring as once again another month went into the "Top 10 warmest" on record. The average temperature for May 2012 at Memphis Int'l Airport ranked #4 at 76.3 degrees. There were four daily temperature records tied or broken during the month, including record highs on the 2nd (90 - tie) and 4th (91) and record warm minimums on the 2nd (70 - tie) and 5th (73).

Precipitation was also well below normal with a total of 3.17" recorded for the month - 85% of that amount fell on just 2 days! The metro is now classified as being in a moderate drought, which is a far cry from the past two years when record spring flooding was occurring during the month of May!  In fact, on May 10, 2012, the Mississippi River at Memphis was 34 feet lower than one year ago on that date.

Memphis International Airport, Memphis, TN

The average temperature for the month of May was 76.3 degrees, 4.6 degrees above normal. The average high temperature was 86.4 degrees and the average low was 66.1. The coolest temperature of the month was 55 degrees on the 10th, while the highest temperature was 95 degrees reached on the 28th and 29th. There were 13 days in which the high reached 90 degrees or higher and only 6 days in which the average daily temperature was below normal (and all were within 3 degrees of average).

Precipitation for the month totaled 3.17", which was 2.08" below average. As mentioned above, 85% of the rainfall for the month fell on 2 calendar days (1.78" on the 31st and 0.90" on the 7th).  There were only 7 calendar days with measurable rainfall. The peak wind gust was 29 mph (from the south) on the 24th with an average wind speed for the month of 6.8 mph. Click here for a daily statistical recap for Memphis International Airport.

Cirrus Weather Solutions, Bartlett, TN

Data from Cirrus Weather Solutions in north Bartlett is incomplete for May as the weather station was taken down for maintenance on the evening of the 21st through the end of the month. The average May temperature at Cirrus Weather Solutions through the 21st was 73.8 degrees with a maximum of 96.8 degrees on the 6th and a minimum of 49.8 degrees on the 10th. May precipitation taken from a  manual gauge used for the CoCoRaHS program measured 8.86" - more than 5.5" greater than Memphis International due to the passage of very heavy training storms on the 6th/7th and 31st. 94% of the monthly rain was recorded on just 2 days. The measured peak wind gust through the 21st was 26 mph on the 2nd, though a severe storm on the 31st produced an estimated 50 mph wind. Average relative humidity was 67%. through the 21st. Click here for a daily recap on MemphisWeather.net.

MWN Forecast Accuracy

For the month of May, the average temperature error in all MWN temperature forecasts was 1.75 degrees, besting all available computer model data. Nearly 80% of the MWN temperature forecasts for the month were within 2 degrees of the actual temperature. MWN's forecasts extend out five periods (or 2.5 days). For dewpoint accuracy, the MWN forecast averaged 2.34 degrees of error and fell within 2 degrees of the actual dewpoint nearly two-thirds of the time. Historical accuracy statistics can be found here.

----
For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

Northwest flow to bring thunderstorm chances to the Mid-South

A significant weather pattern shift has taken place across the eastern U.S. as a large upper level low pressure area has set up over southeastern Canada.  Initially, this was evident as a very nice cool down the past couple of days as surface high pressure built in behind a strong cold front. Now, "northwest flow" has set up over the area, which means our mid-level wind (the level that typically steers storm systems) is out of the northwest between high pressure to the west and the low to the northeast (see graphic below).

Mid-level (~18,000 feet) setup at mid-day on Sunday.
The Mid-South is on the periphery of a classic northwest flow regime.

Mesoscale Convective Systems (MCS's)

Typically, summertime northwest flow means somewhat unpredictable storm systems that tend to form during the overnight hours and move along frontal boundaries that are draped from the plains into the southeast.  These storm systems are often referred to as "mesoscale convective systems" (MCS's) and the current setup favors just that type of scenario.  The areas affected by these storm systems vary, but the threats posed generally are damaging straight-line wind and hail.

MCS's in a northwest flow typically develop from clusters of storms that fire after dark as low-level (but above the surface) wind picks up after dark, then organize and follow the mid-level northwest flow downstream.  Many times the storms begin to weaken and dissipate during the mid-morning hours, but occasionally they will continue through the next day.  More often, as the storms dissipate during the morning hours, an outflow boundary from the storms will linger, allowing for the development of additional storms during afternoon heating.

Sunday mid-day surface setup with a warm front just north of  the metro, ample low-level moisture (green shading indicating high dewpoints) and a resultant Slight Risk for the Plains into the Mid-South.

The Forecast

All this talk about northwest flow and MCS activity leads us to the forecast for the next couple of days.  A warm front has moved over the metro, providing southerly flow and warm, very humid air to the region - a far cry from the past couple of days. This morning's scattered storms were the result of the warm front lifting north.  Scattered storms are likely to fire late this afternoon into the evening in the humid airmass near and behind the front.  These should end shortly after sundown.

However, the first mid-level impulse in the northwest flow will provide for the formation of storms after dark not too far to our northwest, likely in northern AR.  As the storms follow the flow in the first graphic above and the warm front, they will likely affect the metro area overnight.  A Slight Risk of severe weather has been identified by the Storm Prediction Center as indicated in the graphic above. The primary risk with any storms today and tonight will be large hail and damaging wind, though storms that cross the warm front this afternoon could have enough low-level wind shear to spin up a brief tornado. The graphic below indicates the NAM model's interpretation of where storms might be early Monday morning (between 4-7am).

As indicated above, northwest flow events are very difficult to forecast precisely.  After tonight's likely storms scenario, there will be additional chances for storms Monday afternoon and evening on any outflow boundaries produced by tonight's storms.  However, it appears that, beginning Tuesday, the warm front may be pushed back to our south and become stationary southwest of the metro.  If this is the case, further northwest flow storms would likely remain to our south for the mid-week and later period.

MWN will be monitoring the situation carefully and provide nowcasting of any adverse weather on our social media channels (Facebook and Twitter links below).  The best way to stay on top of the latest developments is via our MWN mobile app for Android and iPhone, which you can download at the link below.  Be sure to upgrade to include StormWatch+ so that severe weather won't catch you off guard anytime of the day or night.

----
For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.