Sunday, August 1, 2021

A reprieve from mid-summer heat and humidity this week

The first three weeks of July featured temperatures largely below average, but the past week made up for that, reminding us just what Memphis summers typically feel like! The daily temperatures for the past eight days averaged nearly 4 degrees above normal with high temperatures consistently in the mid 90s and feels like temperatures reaching heat advisory criteria (105°+) on multiple days - a few even topping 110°! So what does the next week hold? A nice break from the heat!

Overall pattern

The upper level atmospheric weather pattern over the past week featured a large ridge of high pressure that built over the region, resulting in those above average temperatures. However, with the passage of last night's surface cold front, the upper level pattern also shifts. A trough of low pressure over the northeastern quarter of the nation dips south, then into the week lingers over the Ohio and Tennessee Valleys before finally weakening and lifting out by the end of the week. 

Lower pressure aloft typically means cooler temperatures at the surface. Coupled with the passage of a series of upper level disturbances rotating through the upper level trough that result in periods of cloud cover, temperatures will be well below average for much of the upcoming week. 

The European model ensemble of upper level height anomalies for the coming week. In short, lower than average pressure aloft is shown in blue and higher than average is in yellow. The blue colors that move over the Mid-South  mean low pressure aloft, which translates to cooler than average temperatures. (WxBell)

Early-week cool temps

In the wake of Sunday's cold front and arrival of the upper level trough, we'll see a fair amount of cloud cover Sunday night into Monday, then partly cloudy skies on Tuesday. A north to northeast wind, dry conditions, and dewpoints falling into the low to mid 60s will set the stage for very pleasant conditions for early August with highs in the mid 80s and lows dropping to around 70 Monday morning and the mid 60s Tuesday morning.



Mid-week persistence

We'll see more sunshine on Wednesday with just partly cloudy skies as highs remain in the mid 80s. Morning lows start in the mid 60s in the city but could drop a few degrees cooler in outlying areas. A bit of warming occurs Thursday with highs in the upper 80s, but with humidity still remaining below average, there still won't be much to complain about!

More warming into the weekend

By the end of the week, temperatures start to climb further as the upper level trough pulls out and higher pressure aloft results in warmer temps below. A passing upper disturbance could mean a few showers Friday, but that is still too far out for anything conclusive. Highs will be at or just above 90. Sunshine returns Saturday with highs back near average in the low 90s and lows returning to the low 70s. Sunday into the following week (as schools return for many students) could be back above normal with highs in the mid 90s, so enjoy this coming week while we have it!

Forecast temperatures for the next 10 days from the NWS model blend. (WeatherModels)

Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: MemphisWeather.net on the mobile web or via the MWN mobile app
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MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

Saturday, July 31, 2021

"Destructive" severe thunderstorms to trigger wireless alerts

You are probably familiar with Wireless Emergency Alerts (WEA), the push notification-based alert you receive on your smartphone when a Tornado or Flash Flood Warning has been issued in your area. These alerts are defaulted on from the cell phone companies, but can be disabled in your phone's alert settings if you wish. They are triggered from cell towers to those within their broadcast area whenever the tower's coverage area (roughly) overlaps the weather-warned area. Thus, they are a little more precise than county-based alerts (like those from NOAA Weather Radio), but not quite as precise as a well-designed weather alert app that pinpoints your specific location. 

A sample WEA for a "destructive" Severe Thunderstorm Warning.

Beginning Monday, August 2, the National Weather Service will make a nationwide change to "tag" Severe Thunderstorm Warnings into one of three categories based on expected impact. The highest of these alert levels will also be sent to smartphones via the WEA system. All three categories will continue to be called "Severe Thunderstorm Warnings," but there will be additional information in the warning as to their degree of severity. The three categories are:
1. Base (or baseline) - These warnings follow the existing criteria for a severe thunderstorm: 1.00 inch (quarter-sized) hail and/or 58 mph thunderstorm winds are expected. These are the lowest level of severe thunderstorm and are a threat to those not properly sheltered and will not trigger a WEA.

2. Considerable - Warnings tagged as having the potential to cause considerable damage are expected to contain at least 1.75 inch diameter (golf ball-sized) hail and/or 70 mph thunderstorm winds. These also will not trigger a WEA, but pose a higher level threat.

3. Destructive - Storms that pose a threat of destructive damage will be tagged as such and will trigger WEA alerts. The threshold for a destructive storm is at least 2.75 inch diameter (baseball-sized) hail and/or 80 mph thunderstorm winds. These storms are estimated to comprise less than 10% of all severe thunderstorms. Extra precaution is warranted (similar to actions taken during a Tornado Warning) given the level of damage they can cause - similar to a low-end tornado, particularly to weak structures.

So while additional alerts will be generated to your smart device, the number will be small and they will be high-end storms capable of significant damage. As a rough measure of their frequency, the NWS office in Memphis issued 223 Severe Thunderstorm Warnings in 2020. If ten percent of those were for "destructive" storms, that would be 22 warnings. And remember that is for all of west TN, north MS, east AR and the Missouri Bootheel! For your location, perhaps you could expect to receive ONE of these warnings per year. And remember that they are in essence based on your neighborhood - based on a cell tower location - not your county. This move by the National Weather Service and Federal Communications Commission is a good one and will hopefully draw more public attention to, and therefore result in a better rate of taking action, for the most destructive storms.

Want even more precise alerts and more control over those alerts you receive? Those that ONLY alert when your specific location is expected to be impacted by the type of alerts you select? Then StormWatch+ Alerts is for you! Add this upgrade to your MemphisWeather.net app and take control over which alerts you receive and for where (select multiple locations anywhere in the U.S.).



Erik Proseus
MWN Meteorologist

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Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: MemphisWeather.net on the mobile web or via the MWN mobile app
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!
MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder