Sunday, August 20, 2023

Late-summer heat and dry weather this week; tropical update

The wet and stormy pattern from late July through mid-August has evaporated as we head through the latter half of August. Several very comfortable days last week had most of us grateful for the arrival of #FalseFall. But a massive ridge of high pressure above us (a.k.a., a Heat Dome) will dominate a large portion of the country from the Rockies to the east coast, including the Mississippi Valley this week. 

The European model ensemble data shows a gargantuan high pressure system aloft over the easter two-thirds of the nation midday Tuesday. This high will result in hot weather and dry conditions for most of the coming week. (WeatherBell)

That will mean very hot temperatures under little cloud cover and increasing humidity this week. We'll likely see our first string of 100-degree temperatures since July 2022 as #SecondSummer arrives with a vengeance. 

Fortunately, it appears that a cold front will move through next weekend as the high pressure ridge is suppressed to the south, allow temperatures to drop back to near average values around 90 degrees in about a week.  Even with the cold front arriving, precipitation chances look to be very low, and on Saturday.

With heat indices near 110 degrees for a good part of the week, heat safety will be a necessity, especially with morning lows near 80 degrees providing little overnight relief. Heat Advisories will likely become Heat Warnings before it cools down a bit next weekend. Avoid the outdoors in the hottest part of the day, stay hydrated, take frequent breaks if you must be outdoors, and check on neighbors, the young and elderly, and your pets too!

Tropical action

Meanwhile, the Atlantic Ocean tropical basin has sprung to life after a month of quiet conditions, and just as we begin the peak of the Atlantic hurricane season. Fortunately, only minor impacts are expected in the next several days as developing storms well out to sea will not affect the U.S. or Caribbean. A little closer to home, a system in the eastern Gulf of Mexico doesn't have optimal atmospheric conditions for development, but could become a tropical depression or weak storm as it moves towards south Texas in the next few days. That area of the country could use rainfall though, so rain would be beneficial in that region. Another system worth watching in the Caribbean is heading towards Hispaniola (Haiti and the Dominican Republic) and is likely to become Tropical Storm Franklin within the next day. It appears to miss Florida on an eventual track north.

Finally, you've likely heard about now-Tropical Storm Hilary, which is moving into southern California this evening and dissipating tomorrow, but not without catastrophic flooding impacts across that region, extending north into Nevada. Many desert areas of southern California will receive as much rain in 48 hours as they typically see in an entire year. The current Tropical Storm Warning for San Diego and other areas of southern CA is the first ever issued for the state!

Sunday afternoon forecast map for Tropical Storm Hilary (NHC)

Flash flood potential is high across southern CA into southwest NV. (NHC)

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, August 8, 2023

Details on the severe weather threat for Wednesday

As if we needed even more proof that this one of the busiest Mid-South summers for severe weather, yet another threat presents itself on Wednesday. And this one could be muti-pronged. 

This calendar for June and July shows the severe weather risk level by day for the Mid-South. 35 days in June and July presented some degree of severe weather risk. The trend is continuing into August unfortunately! (via Iowa Environmental Mesonet)

The unknown is a given...

The first thing to know about tomorrow is that there is simply a lot we do not know yet, even the night before. We are looking at the potential for multiple rounds of thunderstorms crossing the Mid-South, but believe there will be two main "windows" for severe weather. However, within those, multiple storms are possible. 

One of the reasons for the uncertainty is that each wave could have an effect on what happens in its wake. If predicted storms do not materialize, or are brief, it could keep open the possibility of the next one being a bit stronger. However, if a larger complex of storms were to move through early, then it could stabilize the atmosphere in its wake and result in a weaker round later. 

...but honestly it is, Mitch. So let's talk about what we know and what we think we know.

What we know

The Storm Prediction Center believes that the threat of damaging wind is high enough (as of Tuesday evening) between 7am Wednesday and 7am Thursday to include areas roughly north of the TN/MS line in an Enhanced Risk of severe weather, level 3 out of 5. Areas south of the state line are in a Slight Risk (level 2). 

Damaging wind is the main threat with any storms that move through tomorrow. Yes, again. By now you know how to prepare for that - secure it, stow it, and charge it. Heavy rain and lightning are also likely, probably multiple times. If storms move over an area repeatedly, flash flooding could result. It's not likely from a single storm, but a couple back-to-back could overwhelm storm drains or small creeks and tributaries. Secondary threats, which are not as likely but also cannot be ruled out, include hail and an isolated tornado. Tornadoes in August?? Do I have to remind you this is not your typical summer? Yes, it is in the realm of possibility that the strongest storms could be capable of spinning one up. Not likely, but not zero chance.

The probability of severe wind (58+ mph) occurring within 25 miles of Memphis is about 30%. (SPC)

The probability of a tornado occurring within 25 miles of Memphis is about 5%. (SPC)

What we think we (might) know

There will likely be multiple windows when storms are expected, but the two main ones appear to be mid-morning through early afternoon and evening through the early overnight hours. In Shelby County and immediate vicinity, we think that means about 9am-2pm and about 5pm-2am. But check back again regularly tomorrow because that could, and probably will, change! 

A 24-hour forecast loop of simulated radar from the Tuesday mid-day HRRR model, starting at 7am Wednesday and ending at 7pm Thursday. Times don't matter a whole lot, because this isn't exactly right. But you can see how there are multiple waves, and the stronger storms tend to be mainly north of Memphis. Take it with a grain of salt and prepare as you should. (WeatherBell)

In addition, the potential for the strongest storms appears to be with the last wave to move through, which we think will be around midnight or shortly thereafter. That is, if the atmosphere is not so worked over that the storm fuel is tapped out. Additional showers and probably thunderstorms will be possible even after that, but we believe that by then, the severe threat will be quickly declining.

 Most of the strongest storms will probably be north of the TN/MS state line, in northern AR and west TN. Memphis sits on the southern edge of the Enhanced Risk and while there is little the high-resolution models agree on, one is that the best severe parameters currently are north of Memphis. However, again, that could change. All it takes is one butterfly to flap its wings, or one big mean supercell, to change that. And we know that a Slight Risk is not nothing. How many trees and power lines have been downed in Slight Risk or lower this summer?

How we prepare

By now, we hope that everyone has learned their lesson. It's not like we haven't seen this before, recently, multiple times. Charge devices and auxiliary power banks overnight tonight and keep them charged tomorrow. Anything outside that could lift-off without so much as a countdown clock should be tied down or brought in. That might include Fifi the poodle, and it certainly includes lawn furniture, patio cushions, etc. If you have a place to park under cover, use it. Program whatever devices you use for severe weather notifications as well.

And of course stay plugged in, not just to your outlets, but to your no-hype weather peeps. They'll keep you sane and informed. If storms move through and the sun comes right back out, expect the next round could be severe. We'll keep social media posts going regularly throughout the day starting early tomorrow morning with the latest adjustments to the forecast as needed. Be prepared, not scared.

Erik Proseus
MWN Meteorologist

Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: 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

Sunday, August 6, 2023

July 2023 Climate Report for Memphis, TN

July Climate Recap

July started and ended with above average temperatures, but the majority of the middle of the month was cooler than typical. So on average, the mean for the entire month ended just below average, much like June. Still, the number of days in which the high temperature met or exceeded 90 degrees was just one day below normal at 21 days. The below average temperature for the month was driven by the average high temperature being just over one degree below average. Overnight low temperatures actually ended just slightly above normal. 

July precipitation was well above normal, and in fact almost record-breaking. The month ended as the 4th wettest July on record, almost four inches above normal. 

Training storms on one particular day (the 21st) resulted in 94% of an average July's precipitation in a single day. That particular day ended as the second wettest July day on record. The Bartlett observing site also recorded over 2.5" of rain on the 21st, but both sites experienced multiple days of heavy rain and storms, resulting in over 8" of rain for the month. However, not far away in southern Crittenden Co and the Mississippi Delta, rainfall was actually below normal for the month.

Departure from average rainfall for the Mid-South in July 2023. Much of the metro finished 2-4" above normal for the month. (NOAA)

As far as severe weather, July was abnormally active. Nearly a month of repeat storms in northwest flow started with the June 25 wind storm and continued for the first few of weeks of July, culminating in severe storms and very heavy rain on the 21st (described above). Multiple wind storms resulted in mass power outages due to downed trees and power poles, including - to varying degrees - the 2nd, 3rd, and 5th, followed by two larger-scale events on the 18th and 21st. In all, hundreds of thousands of electrical customers lost power at some point in the month, many on multiple occasions. Tree and utility infrastructure damage was extensive, resulting in clean-up from one storm rolling right into clean-up from the next. After the 21st, the severe weather pattern calmed as heat built back in under upper level high pressure.

Memphis International Airport, Memphis, TN

Average temperature: 82.3 degrees (0.5 degrees below average) 
Average high temperature: 90.7 degrees (1.2 degrees below average) 
Average low temperature: 73.8 degrees (0.2 degrees above average) 
Warmest temperature: 96 degrees (1st) 
Coolest temperature: 68 degrees (10th, 11th and 23rd) 
Heating Degrees Days: 0
Cooling Degree Days: 543 (7 below average) 
Records set or tied: Record high minimum tied (81 degrees on the 1st)
Comments: 21 days reached 90 degrees, which is 1.1 days below average for the month of July

Monthly total: 8.62" (3.80" above average) 
Days with measurable precipitation: 14 (4.5 days above average) 
Wettest 24-hour period: 4.53" (21st) 
Snowfall: None
Records set or tied: 4.53" on the 21st set a daily rainfall record and was also the 2nd wettest July day and the 17th wettest overall day on record
Comments: July 2023 ranked as the 4th wettest July on record

Peak wind: Northwest/69 mph (5th) 
Average wind: 6.6 mph 
Average relative humidity: 73%
Average sky cover: 52%

 Click here for a daily statistical recap for Memphis International Airport. Headquarters, Bartlett, TN

Average temperature: 81.2 degrees 
Average high temperature: 92.1 degrees 
Average low temperature: 72.3 degrees 
Warmest temperature: 99.9 degrees (1st) 
Coolest temperature: 63.9 degrees (11th) 
Comments: None 

Monthly total: 8.40" (automated rain gauge), 8.38" (CoCoRaHS rain gauge) 
Days with measurable precipitation: 14
Wettest date: 2.65" (21st) (via automated gauge) 
Snowfall: None
Comments: None

Peak wind: West/35 mph (21st)
Average relative humidity: 78% 
Average barometric pressure: 29.97 in. Hg
Comments: None

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

MWN Forecast Accuracy

MWN average temperature error: 1.93 degrees 
MWN forecast temperatures within 2 degrees of actual: 71% 
MWN average dewpoint error: 1.79 degrees 
MWN forecast dewpoints within 2 degrees of actual: 76% 

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

Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: 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