Thursday, May 27, 2021

Stormy lead-in to a fabulous holiday weekend

The heat and humidity has built this week with spotty precipitation over the past couple of days. High temperatures have generally been near 90 degrees, though as of early Thursday afternoon we had yet to actually reach 90 degrees yet this year. Looking ahead to the Memorial Day holiday weekend, fabulous weather is on tap - low humidity, cool mornings, pleasant afternoons and sunshine! However, we'll first need to deal with a late spring cold front...

Storm chances the next 24 hours

With unstable and humid air in place ahead of a cold front that arrives on Friday, the ingredients are in place for thunderstorms, and those will affect us over the next 24 hours. There are multiple small-scale (or "mesoscale") atmospheric features that are affecting thunderstorm development well to our northwest today - chiefly over the central plains. The development and propagation of those storms will determine when we see storms in the Mid-South, which makes pinpointing the timing somewhat complex. In addition, the timing of the arrival of those storms will also factor into the risk of severe weather. 

Our best educated guess at what to expect is that an initial wave of thunderstorms will move into the area late this evening, most likely after 8-9pm. Models hint at these mainly affecting northern AR and western TN. The Memphis area may be on the tail end of this cluster or line. After what should be a lull of perhaps a few hours, a more substantial threat of storms arrives in the wee hours of Friday morning. The exact timing is again in some doubt, but most model sets suggest that they will roll through at some point between 2-3am and morning rush hour. These storms, due to their timing in the "coolest" part of the day (though it will still be relatively warm and muggy) will likely be weaker when they arrive than when they are moving across AR during the overnight hours and should mostly fall apart as they shift east of the metro. Below is the HRRR model's opinion of how the scenario unfolds, with caveats that it's timing and/or intensity could vary, perhaps quite a bit.

The mid-day Thursday HRRR model showing forecast radar through noon Friday. Model output does not equal gospel. (WeatherBell)

Storm threats and preparation

Our main threats with any severe weather would be strong wind gusts due to the expected linear nature of the storms - in other words a squall line, though weakening. Minor flash flooding or small hail are also possible. The tornado threat appears minimal. Below is our latest impact graphic. Best ways to prepare include tying down anything that might blow away in strong wind and garaging vehicles if able. After a fairly long dry spell, the ground should readily accept most water than falls with little issue as rainfall will average under an inch.

A Memorial Day weekend to remember!

Speaking of the weekend ahead, we'll have quite a nice few days to spend outdoors without too much sweat! Overnight lows behind the cold front are expected to dip into the mid 50s with highs Saturday and Sunday in the 70s! We'll warm up some Monday, but still should feel very pleasant for holiday cookouts. Look for more humidity to build into the middle of next week with daily rain chances by Wednesday.

Erik Proseus
MWN Meteorologist

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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Tuesday, May 18, 2021

Farewell spring, it's been fun...

It has been a wonderful spring so far across the Mid-South, and for many in the Southeast. April was a below average month for temperatures, and that trend has continued into May. In fact, we are 5-7 degrees below average so far for the month of May! With average high temperatures now rising into the 80s, this has given us several weeks of beautiful, pleasant spring weather overall. However, after today, rain chances and cloud cover will decrease, allowing temperatures to quickly rise through the 80s and approach 90 degrees by Saturday. Farewell spring!

This month-to-date (May) high temperature anomaly map for the U.S. shows much of the eastern two-thirds of the country has experienced well below average temperatures for the first half of May. (WeatherBell)

We knew it couldn’t last forever, and sure enough a major pattern shift is coming for us here in the Memphis area. The cool weather was caused by a persistent trough of low pressure over the Mid-South and eastern U.S., which helped to continually funnel cool air down into the south from the northern US and Canada over the last month and a half. The hot weather has been confined to western portions of the country, as a large ridge of high pressure with sinking, hot air has refused to budge for the month of April and into the first half of the month of May. Now, that pattern is finally changing. A trough will dig into the western US, bringing the West Coast and Rocky Mountain region some much needed rain, cooler air, and wildfire relief. 

Unfortunately for us, a large ridge of high pressure looks to build in over the next few days and intensify, ending our very nice spring weather and setting the stage for the arrival of more summer-like weather. With this high pressure center sitting to our south and east, the clockwise flow around the high will pump Gulf moisture directly into the Mid-South, quickly warming our temperatures. Due to the sinking air of the high pressure, rain chances will also diminish, leaving us with lots of hot sunshine and humidity. 

As we go into the latter part of this week, an upper level pattern known as an ‘omega block,’ because the pressure pattern resembles the Greek letter omega, will set up across the United States. This will keep the ridge of high pressure parked over the central and eastern US for at least the next week, if not longer. This is a classic pattern for hot temperatures under the high - a “heat dome” is not a bad way to describe it.

The upper air pattern setting up for this weekend and early next week features an 'omega block' across the country, with high pressure covering most of the eastern half of the nation. (European model, WeatherBell)

The ridge sitting over us for so long will allow temperatures to quickly climb each day, and our first 90 degree days of 2021 are in the forecast for this weekend. In fact, almost every day next week has the potential to reach the 90 degree mark. Luckily, many area pools are beginning to open for the season, and next week is looking like prime time to inaugurate the neighborhood pool for the summer. The center of the high pressure will be to our east, so we should escape the more extreme heat expected for areas like southern GA and northern FL. Some of those areas could approach 100° next week! 

These kind of patterns are notoriously hard to break, and we could be looking at a couple weeks of hot, dry weather. Dry as in lack of precipitation, not in lack of moisture, as there will be continuous southerly flow off the Gulf, keeping our dewpoints high and making for a pretty unpleasant feel outside. Feels-like temperatures will be several degrees higher than actual temps, so find a way to stay cool. Summer weather lovers, your time is coming! 

Another concern for this pattern is the persistent lack of precipitation. With temperatures well above normal and very little cloud cover, evaporation rates will be high, and the nearly 3 inch rainfall surplus for the year could quickly be erased. Models are showing very little in the way of precipitation for the couple weeks. Looking at the European model through next Thursday, only a quarter inch of rain is forecast to fall, with most of that coming today. The GFS model is a little more optimistic on rain chances returning late next week, but that could change as we get a closer in time - definitely something to watch, especially for those with interests in agriculture. 

The European model forecast precipitation from today through the next 10 days. (WeatherBell)

As the heat of summer approaches, it’s a good time to revisit some heat safety tips. Wear sunscreen, especially during the afternoon hours, as UV indexes will be maxed out across the area next week. Stay hydrated, especially if you are planning on being outside in the peak heating of the day, and take plenty of breaks.

If you are planning on working out outside, it would be best to do it in the morning or evening hours. Heat is not something to mess around with, it kills more people each year than hurricanes, floods, earthquakes and tornadoes combined! Stay cool and stay safe Memphis, and get ready - summer is well on the way! 

Christian Bridges
MWN Social Media Intern

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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Saturday, May 15, 2021

Meet the "new normal" in Memphis weather - NWS releases updated data

Most of you are familiar with the weather data referred to as "normals" - the normal high and low temperature for the date, normal precipitation for the month, etc. You see them every night on the TV meteorologist's weathercast and we post a climate summary graphic each evening on social media as well that shows the day's data and our normals for the day. 

[ Personally, though the term normal is generally accepted in the meteorological community, I actually prefer the term "average." However, the statistical calculations made are not precisely the simple mean, or average, of the previous 30 years of data, though they are very close. ]

So what defines "normal" in the meteorological context? The National Weather Service (NWS), as well as the World Meteorological Organization (WMO), use a 30-year average. Why 30 years? Here's how the NWS explains it:
Normals serve two purposes: a reference period for monitoring current weather and climate, and a good description of the expected climate at a location over the seasons. They provide a basis for determining whether today’s weather is warmer or colder, wetter or drier. They also can be used to plan for conditions beyond the time span of reliable weather forecasts. A 30-year time period was chosen by the governing body of international meteorology in the 1930s, so the first normals were for 1901-1930, the longest period for which most countries had reliable climate records. International normals were called for in 1931-1960 and 1961-1990, but many countries updated normals more frequently, every 10 years, so as to keep them up to date. In 2015 this was made the WMO standard, so all countries will be creating normals for 1991-2020. - "1991-2020 U.S. Climate Normals: An Update"
Up until just recently, our daily weather conditions have been compared to the climate normals from 1981-2010. However, as they do every 10 years, NOAA and the NWS has released updated climate normals for 1991-2020, which means we have a new set of normals to compare the day's weather to. It's always an interesting exercise to see how the normals change when a new set of data becomes available. Even though 20 years of data overlap between the two sets of data, in the case of the 1991-2020 data, the 1980s are dropped off and the 2010's are added. 

These two side-by-side maps of the contiguous United States depict the change in U.S. annual mean temperatures (in degrees; left map) and precipitation totals (% change; right map) between the new set of Climate Normals, 1991-2020 (most recent last 3 decades) and the previous set of Normals, 1981-2010. (NOAA NCEI)


So how do the 1991-2020 normals compare to the period from 1981-2010? Glad you asked! I've created a few charts to show temperature and precipitation comparisons. Let's start with monthly mean temperature and mean maximum and minimum temperatures (in other words, average highs and lows).

Comparison of new climate normals vs. previous normals for average, maximum and minimum temperatures at Memphis International Airport. Positive values indicate warmer temperatures in the most recent period.

The most obvious take from this chart is that, in general, temperatures are warmer in the 1991-2020 period versus the 1981-2010 period. The largest change in temperatures are in the cool season, while the summer did not warm as much. The notable values that are cooler are the overnight low temperatures in the heart of the summer (July and August) and in the month of November. The latter skewed the overall November temperature average cooler than the previous normals dataset as the average high temperature did not change. In fact, November was the only month that, on average, was cooler in the new dataset, while December had the largest positive (warmer) change. For the year, the average high temperature rose 0.4 degrees to 63.4 degrees. The average high temperature rose 0.6 degrees and the average low temperature rose 0.2 degrees.


So temperature has  generally warmed in the most recent data for Memphis. What about rainfall? Turning our attention to precipitation data, below are the changes in monthly precipitation from 1981-2010 to 1991-2020.

Comparison of new climate normals vs. previous normals for total precipitation by month at Memphis International Airport. Positive values indicate wetter conditions in the most recent period

The main takeaway of the new data from a precipitation perspective is we're generally receiving more water from the sky. In fact, for the year, average rainfall for the year increased 1.26" to just shy of 55" per year (or a bit more than 2%). The increase can be attributed to positive change from early spring through late summer. There was almost no change in September and October, while November and December saw rainfall totals fall, particularly November. This results in the wettest month of the year shifting from December to April. The driest month of the year moved later by a month, from August to September. 


Let's look into more precipitation data, specifically of the frozen variety. (Snow and sleet are lumped into "snowfall" data by the National Weather Service.)

Comparison of new climate normals vs. previous normals for monthly snowfall at Memphis International Airport. Negative values indicate less snow in the most recent period

The snow season for Memphis ranges from late November through early March with the highest totals typically in January and February. With the warmer temperatures discussed above (and to the surprise of no one who has lived in the Mid-South for an extended period of time!), average annual snowfall continues to decline. In the new data, the annual average has dropped from 3.8" to 2.7", or about 30%. The largest change has occurred in January, falling by an inch in the new normals. February's total also fell a bit. Interestingly enough, though the amounts are very small, average snow for November and March increased just slightly. The takeaway here seems to be that, although snowfall totals have fallen, the "winter season" seems to have expanded just a bit. In fact, the cooler month of November described above may have contributed to the slight increase in average snowfall in that month.

Recapping the 1991-2020 Normals

So, to wrap it up, our daily and monthly weather data will now be compared to a more recent set of average, or "normal" data. Overall, that data indicates a trend towards warmer and wetter conditions in the Memphis area, though there are exceptions such as a cooler and drier November. The largest positive change in temperatures occurred in the cool season, while precipitation increases were spread over many months, with the exception of late autumn to early winter. Snow lovers may want to look further north!

Erik Proseus
MWN Meteorologist

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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Saturday, May 8, 2021

April 2021 Climate Report for Memphis, TN

April Climate Recap

While March was much warmer than average, April cooled down a fair bit relative to average, ending almost two degrees below normal. While there were several days of above average warmth, only a couple were excessively so. On the other hand, there were several days were it was abnormally cool, including a few days with morning lows in the 30s to start the month, and another particularly cold shot centered around the 21st, when frost occurred again and high temperatures were quite cool. A daily record low was set on the 22nd with a minimum temperature of 37 degrees. As usual though, a wide range of temperatures occurred, from a few lows in the mid 30s to a few days with highs in the mid 80s.

Precipitation ended well below average (56% of normal), even though there were several days with rain, as only one day recorded more than an inch of rain. In fact, two-thirds of the total April rainfall fell in the last week of the month. Surprisingly, in what historically is one of the busiest months of the year for severe weather, no severe weather reports were received and only a couple of weather warnings were issued in the metro during the month, on the evenings of the 7th and 9th.

Memphis International Airport, Memphis, TN

Average temperature: 61.0 degrees (1.9 degrees below average) 
Average high temperature: 71.3 degrees (1.7 degrees below average) 
Average low temperature: 50.6 degrees (2.3 degrees below average) 
Warmest temperature: 84 degrees (12th) 
Coolest temperature: 35 degrees (2nd, 21st) 
Heating Degrees Days: 166 (29 above average)
Cooling Degree Days: 51 (24 below average) 
Records set or tied: April 22 - daily record low minimum (37 degrees)
Comments: There were no days with high temperatures above 90, nor any days with low temperatures below freezing.

Monthly total: 3.10" (2.40" below average) 
Days with measurable precipitation: 7 (2.6 days below average) 
Wettest 24-hour period: 1.17" (29th) 
Snowfall: 0.0"
Records set or tied: None
Comments: One day recorded precipitation of more than an inch, 1.0 day below average for April.

Peak wind: Southwest/40 mph (22nd) 
Average wind: 8.1 mph 
Average relative humidity: 61% 
Average sky cover: 48% 

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

Cirrus Weather Solutions /, Bartlett, TN

Average temperature: 60.7 degrees 
Average high temperature: 72.8 degrees 
Average low temperature: 49.1 degrees 
Warmest temperature: 85.9 degrees (9th) 
Coolest temperature: 29.2 degrees (2nd) 
Comments: None 

Monthly total: 3.72" (automated rain gauge), 3.90"(manual CoCoRaHS rain gauge) 
Days with measurable precipitation: 8
Wettest date: 1.69" (29th) (via automated gauge) 
Snowfall: 0.0"
Comments: None

Peak wind: Southwest/29 mph (8th)
Average relative humidity: 68% 
Average barometric pressure: 29.98 in. Hg
Comments: None

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

MWN Forecast Accuracy

MWN average temperature error: 2.36 degrees 
MWN forecast temperatures within 2 degrees of actual: 64% 
MWN average dewpoint error: 2.90 degrees 
MWN forecast dewpoints within 2 degrees of actual: 55% 

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

Climate Outlook - May 2021

The May climate outlook for the United States from the Climate Prediction Center is shown below. Above average temperatures are forecast for all of the southern US., with highest probabilities from the southern Plains to Desert Southwest. A low probability of below average temperatures is forecast in the upper Great Lakes. Odds favor above average temperatures for Memphis (41%) versus only 26% chance of below average temperatures. The average temperature for April is 71.7 degrees.

Precipitation is expected to be below normal southwestern U.S. in May. Above average precipitation is forecast for much of the Mississippi River Valley east to the Appalachians. For Memphis, odds favor above average precipitation (41%) versus only 26% chance of below average precipitation. Rainfall historically averages 5.25 inches.

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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

With apologies to all the mommas, more severe weather could be on the way

It's been a fairly active spring weather pattern this year in the Mid-South and the trend continues this weekend. The last few days have been beautiful, interrupted only by a strong, but brief round of storms on Thursday afternoon. Today we transition into the warm sector of the next weather system. A warm front moves through during the day, having brought a round of showers this morning. By afternoon, a bit of sunshine is possible with increasing southerly wind and warmer temperatures that top out in the upper 70s.

Mother's Day severe weather

By Saturday night, the warm sector is fully established over the area with gusty south wind and temperatures that remain in the 60s all night long. An isolated shower is possible. This pattern continues into Sunday morning with a few more showers possible, particularly northwest of the Bluff City. Strong wind in the low and mid levels of the atmosphere, coupled with increasingly unstable air by early afternoon as temperatures rise through the 70s, sets the stage for late afternoon to evening thunderstorms - with our apologies to all the mothers out there! As the front approaches from northern AR, thunderstorms become likely ahead of it and across our region. 

The HRRR extended model showing simulated radar from 10am Sunday through midnight. Note the increase in coverage and strength of echoes during the afternoon and evening hours. A few storms in the late afternoon/evening could be severe. (WeatherBell)

The ingredients mentioned above will allow some storms to become severe, mainly from mid-afternoon through early evening. The Storm Prediction Center (SPC) currently has the entire Mid-South included in a Slight Risk (level 2/5) for severe weather. The primary hazards will be damaging wind gusts, as well as large hail due to cold air aloft. Though the wind shear profiles are not ideal for tornadoes, the fact that the wind will be quite strong means we can't rule out the potential for a couple tornadoes somewhere across the region.

The SPC outlook for Sunday shows a Slight Risk (level 2) of severe weather (upper left), a 5% probability of a tornado within 25 miles (upper right), and a 15% risk of large hail and damaging wind (lower left and right, respectively). (PivotalWeather)

The cold front will pass through by early evening, bringing the threat of severe weather, and most storms, to an end. Showers will be possible into the late evening behind the front.

Early next week

With the front well to our south and surface high pressure building in from the north, Monday appears to be dry with sunshine, especially in west TN and east AR, but cooler temperatures again - back in the mid to upper 60s for highs. Overall a great start to the week!

Models are having some difficulty with the upper level pattern for the Tuesday-Wednesday timeframe. Despite high pressure at the surface, a large trough of low pressure aloft over the eastern U.S. puts our area in northwest flow. A passing disturbance aloft could bring a few clouds and perhaps a few showers during this period though I believe we'll have many more dry hours than wet ones, and a decent amount of sunshine. The trough aloft means below normal temperatures - highs in the 60s and lows near 50 or so.

Mid/late week - Memphis in May

The pattern begins to trend a bit warmer, but still cooler than average, for the latter half of the week with high pressure moving to the east of us. That should mean dry weather for the majority of the World Championship Barbecue Cooking Contest - a welcome sight after having not had the contest last year! We expect highs in the 70s and morning lows in the 50s. 

Erik Proseus
MWN Meteorologist

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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder