Monday, March 23, 2015

Spring thunderstorms precede a weekend "blackberry winter"

9:45am Tuesday - SPC graphics updated for enhanced severe weather risk to our west.

Signs of spring have taken hold in the Mid-South, such as blooming dogwoods and daffodils and the smell of fresh mulch. It has been an unusually quiet severe weather season nationwide featuring no severe weather watches or tornadoes so far this month. That could change in the next couple days as a large-scale upper level trough and associated cold front march through the Plains into the Mississippi Valley through Thursday. The Storm Prediction Center has placed Enhanced Risk (category 3 of 5) areas in the Ozarks Tuesday and over the southern Plains Wednesday, while a Slight Risk (category 2 of 5) extends east to areas just north of the Mid-South.

Tuesday's severe weather risk areas as posted by the Storm Prediction Center

Wednesday's severe weather risk areas as posted by the Storm Prediction Center

Mid-South Impacts

The system responsible for the possible severe weather will move into the Mid-South late Wednesday night into Thursday morning. The ingredients for severe storms will diminish some as the cold front approaches early Thursday morning, thus severe weather is not expected in the Mid-South. However, there will be a decent chance of thunder overnight Wednesday. Behind the cold front, rain will be likely on Thursday, tapering by afternoon. Temperatures well into the 70s ahead of the front will remain in the 50s on Thursday, some 20° cooler than Wednesday.

Once the frontal system clears, the large trough will settle over the eastern U.S., not unlike a good part of the winter. This will mean very cool temperatures for late March with highs again in the 50s Friday and near 50 ° on Saturday. Mornings will also be unseasonably cool Saturday and Sunday in particular with lows well into the 30s expected! Some would call this a "blackberry winter" (not the troubled Blackberry that you use for work e-mail, the fruit...) or a "dogwood winter," referencing the flowers in bloom when the cold snap hits. We'll just call it too cold!



Frost is probable especially outside the city and a few hours of sub-freezing conditions are possible in the coldest outlying areas. We would suggest holding off another week if you haven't already planted plants susceptible to frost. Fortunately, it appears the cold streak will be quick-hitting. We should be back into the 60s to start next week. (By the way, the average last freeze in Memphis is March 22 and the latest 32° reading ever recorded was April 16. These dates are later outside the city and in rural areas - for instance at the Agricenter the average is March 28 - so this weekend's cold snap is not totally outside the norm, especially with a late winter this year.)

Side note on crap apps

A couple of people have mentioned seeing a snowflake in the extended forecast on their iPhone weather app today. This is why we recommend using human-powered weather instead of an app driven by a computer model. YES, one of the computer models dropped some light snow over us on Saturday on one of its four runs today. It was gone with the next set of data. We're not fickle enough to put that in the forecast then take it right back out and HIGHLY DOUBT we'll see any snowflakes this weekend. We recommend you...



Erik Proseus
MWN Meteorologist

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Changes to severe weather products as spring storm season approaches

March has started off very quiet from a severe weather perspective nationwide. In fact, as of today (March 23) there have been no severe weather watches issued for the United States and no tornadoes reported. The Storm Prediction Center noted last week: "Since the beginning of 2015, the SPC has issued only four tornado watches and no severe thunderstorm watches, which is less than 10 percent of the typical number of 52 tornado watches issued by mid-March. The approximately 20 tornadoes reported since January 1 is well below the 10-year average of 130 for that time period." However, a slow start doesn't necessarily translate to a quiet spring, or year for that matter.

There are several recent and upcoming changes to severe weather products issued by both SPC and local National Weather Service (NWS) offices, including Memphis, that you need to be aware of.

SPC Convective Outlooks

We've talked about the changes made last October to SPC's Day 1-3 convective outlooks in a previous blog. To recap, the three-tier system of Slight, Moderate, and High risk categories has become a five-tier system, adding Marginal and Enhanced categories on either side of a Slight risk. We invite you to read the linked blog above for details on these changes. We'll pair the 5-point numerical value of the forecast category when discussing the risk for the Memphis metro to help eliminate confusion associated with the new names.


In addition to the Day 1-3 outlooks, the Day 4-8 experimental outlooks issued by SPC previously only displayed areas with a 30% probability of severe weather during this period. In December 2014, these outlooks were also modified to display areas under either a 15% or 30% risk of severe storms, which provides meteorologists and the public with a heads up to lower risk severe weather areas in the day 4-8 period that weren't previously highlighted.

Impact-Based Warnings (IBW)

While SPC is responsible for nationwide issuance of Tornado and Severe Thunderstorm Watches, local NWS offices issue the warnings for their area of responsibility. Starting April 1, the Memphis office of the NWS will join many other offices across the nation in an experimental change to the format of Tornado and Severe Thunderstorm Warnings that began a couple of years ago with a handful of offices in the central U.S.

Areas where impact-based warnings will be used in 2015

Called "impact-based warnings," the format change was made in response to the findings of a service assessment done after the Joplin, MO tornado in May 2011. That assessment found that "credible, extraordinary risk signals prompt people to take protective actions." One of the intended outcomes of IBW is to motivate proper public response to warnings by distinguishing between low and high impact events. In addition, it allows the warning system to be optimized within the existing infrastructure, realigning the message in terms of societal impacts, thereby communicating recommended actions more concisely.

The changes will have no effect on the mechanics of warning polygons, but will provide more detailed hazard and action information within Tornado and Severe Thunderstorm Warnings. That detail will include the impacts the severe storm could cause, which will be reflected in the updated format of the warning bulletins. The changes include the addition of "event tags" at the bottom of each Tornado and Severe Thunderstorm Warning. These tags include "Tornado" and "Tornado Damage Threat," which are described below. In addition to those tags, Severe Thunderstorm Warnings will carry maximum hail size and wind speed tags. Tornado Warnings will also carry a maximum hail size tag.

The Tornado tag will have three possible options:

  • "Possible" - Reserved for Severe Thunderstorm Warnings, indicating that the storm has some potential to produce a tornado, but forecaster confidence is not high enough to issue a Tornado Warning.
  • ""Radar Indicated" - Evidence on radar and near storm environment is supportive of a tornado, but there is no confirmation.
  • "Observed" - A tornado is confirmed by spotters, law enforcement, etc.
The Tornado Damage Threat tag will also have three options:
  • No tag - Used most of the time when tornado damage is possible, but the tornado is expected to be short-lived.
  • Considerable - Used rarely, when there is credible evidence of an ongoing or imminent tornado capable of producing considerable damage. Tornado is expected to be long-lived.
  • Catastrophic - Used exceedingly rarely, when a severe threat to human life and catastrophic damage is occurring. Only used when reliable sources confirm a violent long-lived tornado. This tag will trigger the "Tornado Emergency" warning.
The example below summarizes the possible tag options and also shows the Impact bullet in the warning that describes the threat to life and property.


Some additional examples of warnings using the new tags are shown below (changes are highlighted in red).

Severe Thunderstorm Warning with new tags.

Tornado Warning for radar indicated tornado.

Tornado Warning for an observed tornado producing considerable damage

Tornado Emergency for an observed tornado producing catastrophic damage.

More information from the NWS on the 2015 IBW Demonstration is contained within the YouTube video below and on this website.



Severe weather watches add "Summary" section

Finally, beginning April 14, all Tornado and Severe Thunderstorm Watches issued by SPC will contain a "Summary" section. According to SPC, the summary will provide a "general 1-2 sentence statement of the severe weather expected in and close to the watch area. This new section facilitates consistent, forecaster-driven, concise communication for public consumption." For more information on this change, please see this page from the SPC.

Erik Proseus
MWN Meteorologist

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Wednesday, March 18, 2015

MWN Lightning Round: A trio of astronomical events on Friday!


The MWN Lightning Round returns! This Friday, March 20, will feature not one... not two... but THREE astronomical events!


Vernal Equinox - SPRING!

The official first day of spring is Friday, March 20. Specifically in Memphis, the equinox occurs at 5:45pm CDT, when the sun crosses the equator from south to north. For the next 6 months, the sun will remain north of the equator, resulting in the usual hot, sultry summer in the Mid-South!

The orientation of the sun to the earth on the vernal equinox

In an odd technicality, while equinox means "equal parts day and night," the length of the day in most places is not actually a precise 12 hours on March 20. In Memphis, our 12-hour day was on March 17, when the sun rose and set at 7:09 (am and pm respectively). The reason for this is because the earth's atmosphere refracts, or bends,  sunlight, such that we can actually see the sun rising (and setting) a little before (after) it technically occurs. For more details, we invite you to check out this article from TimeandDate.com.

The Missing Supermoon

A supermoon occurs on Friday! But don't get too excited... A supermoon, or perigee moon, occurs when a full or new moon makes its closest approach to Earth, thus making it look larger than usual. By a looser definition, it occurs "with the moon at or near (within 90% of) its closest approach to Earth in a given orbit." By this definition, there are 6 supermoons in 2015. However, this month's supermoon, which also coincides with the equinox and an eclipse (see below) is a NEW moon, meaning it is 100% obscured by the Earth's shadow. Thus, the "missing supermoon!"

Total (solar) eclipse of the heart

The third in our astronomical trilogy will require an expensive plane ticket to be able to see in person, as it won't be visible in the Americas either. (Basically, we realized we're telling you about a bunch of stuff you won't be able to experience firsthand...) A total solar eclipse will occur in the wee hours of the morning Friday with best viewing over northern Europe and into Greenland.


A solar eclipse occurs when the Earth, moon, and sun all line up in such a way that the moon blocks the sun for viewers on Earth, either partially or totally (thus the type of eclipse). The next eclipses visible in Memphis will be a partial lunar eclipse in the early morning hours on April 4 and a total lunar eclipse on September 27. The next solar eclipse viewable locally occurs August 21, 2017 - a near total eclipse! (We'll be sure to remind you...)

Cool astronomy stuff like these events (at least the supermoon and eclipses) can be watched live, or later, on the internet if you can't shuck out the cash to fly to where they are visible. There are several good sites, but we especially like Slooh.com.

So there you are - 3 astronomical events all occurring on the same day!

Erik Proseus
MWN Meteorologist

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Monday, March 16, 2015

The Results Show: Reviewing our 2014-2015 Winter Outlook

Now that it is mid-March with highs in the mid 70s, we can probably safely safe winter 2014-'15 is over for the Mid-South. That doesn't mean we won't see temps drop below freezing for a couple of hours some morning or frost on the ground. In fact, on average the last freeze date is still about a week away (March 22 at Memphis International Airport, later in outlying areas), but we don't see it getting that cold this week!

Methodology behind this year's outlook

So, in the name of full transparency (and perhaps curiosity more than anything), I thought it would be educational to re-visit the winter outlook I prepared early last November. As I said then, "if I got paid for accurate long-range outlooks, my family would be hungry!" As a reminder, my outlook was based on a few key pieces of data:
  • 10 analog years with weak El Nino conditions (1952, 1953, 1954, 1959, 1970, 1977, 1978, 1988, 2005, 2007)
  • Recent research by NWS-Memphis on El Nino impacts in the Mid-South
  • Research and 2014-'15 winter outlooks composed by WeatherBell, WeatherTrends360, and NOAA
Going into this winter, there was a better than 50/50 chance we'd see weak El Nino conditions before the season was over. As it turns out, NOAA declared just this month that El Nino had officially arrived. So while it didn't start until March, it actually takes a few months of above average Pacific sea surface temps to make it official. We were basically dealing with weak El Nino conditions in the Pacific for at least part of the winter.

The MWN Winter Outlook

Here's the outlook I made November 10, 2014 for the upcoming winter:
  1. Temps slightly below to below average (possibly as cold as 2013-'14)
  2. Precipitation near average
  3. Snowfall near to above average (average is about 4")
  4. Periods of large temperature swings and possible severe weather
For the sake of this discussion, since we had some of the earliest measurable snowfall on record last November and the biggest snow/ice events into the first week of March, I'm calling "MWN Winter" the period from Nov 10-Mar 10. I'll also look at the period meteorologists define as winter, which is Dec 1-Feb 28.

The Results... drum roll please

Let's look at each point in succession:
  1. "Temps slightly below to below average" - CORRECT - The most recent 30-year average temperature for Nov 10-Mar 10 is 45.6°.  This year's average was 42.0°, which is more than 3.5° below normal, but not quite as cold as last year (41.3°), which will be remembered for a January that was colder than any since the mid 1980s. This year, February was #7 coldest on record. In 141 years of record-keeping, this period ranked #26 coldest, or in the top 20%. For meteorological winter, this year's average was 41.1°, which was 2.7° below the most recent 30-year average of 43.8°. 
  2. "Precipitation near average" - INCORRECT - Total precipitation for the Nov 10-Mar 10 period was 13.22", which was 6.28" below the latest 30-year average. In fact, parts of the metro were classified as being in "severe drought" during this winter. For meteorological winter, precipitation totaled 8.25", or 5.63" below normal.
  3. "Snowfall near to above average" - CORRECT - Snowfall (which includes sleet, but not freezing rain) averages 3.8" a year in Memphis based on the 30-year climatological record used by the NWS. For the entire winter (our Nov 10-Mar 10 window), total snowfall was 6.1". Since some of that was outside the traditional "meteorological winter" period, the total is higher for MWN winter than meteorological winter (2.3", or 1.1" below average). We firmly believe that the 3.1" in March in 0.2" in November (which was some of the earliest on record) should count though!
  4. "Periods of large temperature swings and possible severe weather" - PARTIALLY CORRECT  - Starting with the second point, there were no severe convective storms this winter, which was what we were alluding to in this outlook. However the series of ice and snow storms that affected the Mid-South from mid-February to early March arguably had a wider impact on the entire metro than a round or two of severe thunderstorms would have. I believe these were "severe" for winter, but will only take half-credit for them! As to the point of large temperature swings, I think I nailed this one. Looking strictly at daily average temperatures (which averages the high and low for a day) between Nov 10-Mar 10, there were a total of 30 instances in which the daily average temperature varied by more than 10°F on consecutive days (many of them were much more than 10°). This is not a swing of 10° between one day's high (or low) and the next day's high (or low), it is the average daily temp moving by that much, which is harder to accomplish. All in all, despite the general cold weather pattern, we saw frequent large temperature swings over the course of the winter, from warm to cold, or from cold to colder!
In sum, 3 of the 4 points made in this year's winter outlook were correct, which surprised even me when I went back and re-visited the winter numbers. I can't guarantee that kind of accuracy on winter outlooks going forward, but this year I seemed to be on the right track!



What were your overall impressions of this past winter? Comment below or drop us a Facebook comment or tweet! We'd love to hear from you!


Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook, Twitter, and Google+
Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!