Forecast Blogs

Thursday’s Severe Weather Threat Growing, Closer

Current Radar

Updates reflecting new information from this afternoon are written below in blue.

Today: Clouds Rolling In, Windy – High 75°

We’ve already begun to see the clouds moving into our area ahead of the storms we will be seeing tonight into tomorrow. Even with the clouds, temps are expected to climb very quickly into the 70°s.

Visible Satellite Image showing the cloud cover building to our west and moving in:

The winds could be annoying at times today with gusts up to 25 mph. Sustained winds should remain between 5-15 mph the rest of the time.

Tonight: Heavy Rain Late – Temps only Dropping into the 60°s

The HRRR Model is still a little too far out for us to see exactly when the non-severe storms will come tonight. We expect to see light showers beginning as early as 10 PM tonight. Heavier rainfall will not come until the early AM hours tomorrow.

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Thursday: Heavy Rain Early, Severe Storms in the Afternoon – Wake Up 61° High 74°

Summary

The lunchtime severe weather outlook from the Storm Prediction Center has arrived. Compared to this morning, this outlook represents a heightened severe weather concern that’s getting closer to us:

 

On David’s unofficial SPC outlook translator, this is a “you have my attention” event, right on the border of “getting concerned”:

Let me add something else — this isn’t anything like the last two events we’ve had here, where there was a conditional “slight” risk. In the first event, the condition was instability/CAPE — none developed, nothing happened. In the second event, the condition was shear (and CAPE), we got little of each, not much happened.

In this event, unlike what we’ve seen so far this storm season, SPC expects a significant severe weather event to be ongoing to our west, then racing into Middle Tennessee Thursday late afternoon/evening. This system should have at least twice as much instability/CAPE, and as much, if not a little more, shear, than those previous non-events. In other words, it has a lot more “stuff” to work with. So, while we are not guaranteeing severe weather Thursday (see below to read why), do not dismiss this event, and stay weather aware. 

We expect 2 rounds of rain/storms. First, in the early morning tomorrow, main concern is flash flooding. Then, a break in the action, to allow for the atmosphere to recharge. Second, later in the afternoon/early evening, our main concerns are damaging winds, more localized flash flooding, hail, and a tornado or two.

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Round 1 — Early Morning Outlook

Rain will continue through the overnight hours into the early morning hours tomorrow. Our first threat comes around the rush hour time frame. Periods of heavier rainfall will be impacting us then.

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These periods of heavy downfall could cause localized flash flooding. Be advised that your drive to work tomorrow may take longer than usual. Be safe and aware of your surroundings. If you see a flooded roadway, “Turn Around Don’t Drown!”

Lunchtime-ish Break in the Action

When the morning rain ends, weather nerds everywhere will be rushing to view the satellite trends (will the sun come out?), special weather balloon data (if NWS launches some), and the SPC’s RAP Mesoanalysis page, all of which will indicate whether the atmosphere is filling back up with instability/CAPE, shear, supportive wind fields, LCL heights, and other ingredients that will have to come together to set us up for severe weather ahead of an approaching cold front expected later in the day.

For example, here’s the NAM4 model for 4 PM Thursday, showing some sunshine, which suggests we’ll be recharging our CAPE/instability.

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Round 2 — Late Afternoon/Evening/Late

ETA and storm strength is conditional on what happens in the morning. Models vary as to when the next round will arrive.

In the lunchtime update, the Storm Prediction Center emphasized that Thursday morning rain/storms will lay down outflow boundaries and cold pools which can feed and otherwise enhance the storm concern for later in the day. After that morning rain, a strong southern wind will shove reinforcing warm air to recharge our atmosphere; however, if pockets of the region remain clouded over, or ongoing storms to the south cut off the good southern moisture transport, the threat will diminish. In addition, the upper level support of severe weather should be higher nearer to the Ohio River. The whereabouts of these gaps in co-located severe weather ingredients present the forecaster with the most uncertainty here — no one knows where they will develop — so absent that certainty SPC did not feel it appropriate to expand its risk area across I-65. I also think our risk is slightly lower because models suggest the storms will show up after dark.

The Euro and GFS models delivers this rain well before midnight.

However, the NAM4 shows this second round — the “main event” — reaching us around the midnight hour:

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If the storms develop sooner, they should be stronger. This is because as we go through the day, the highest shear will be escaping north, chasing the system’s parent low pressure center racing off to the Great Lakes. But, if the storms can get rolling late afternoon/early evening, there should be sufficient shear and helicity to set off some supercell and quasi-linear thunderstorms. If this verifies – and there remains uncertainty about whether it will – our tornado threat will rise; however, the main threats will be damaging straight-line winds from storms that appear to form a bow on radar. Hail is also a concern.

All that said, it is expected there will be plenty of instability, shear, and upper level support in west Tennessee to develop significant severe potential with large hail and perhaps a strong tornado or two. As a southwesterly low level wind jet intensifies and nears into the evening hours, “storms may congeal [sic] into an eastward propogating bow/mesoscale convective system …. impacting … Middle/Western TN.” (SPC Forecast Discussion).

Perhaps the most dangerous part of these storms may turn out to be flooding during Round 2. If we get one of those supercells “training” over our area, the already-saturated ground from the morning rain could set the stage for localized flooding. Most weather deaths and injuries occur from people in cars around flooded roads.

All that said, we do not expect to be in the epicenter of severe weather activity Thursday, but the expected epicenter has moved much closer to us in this latest outlook. The bigger risk exists to our west, including almost all of Mississippi, extending into west Tennessee (including Memphis), then into W Kentucky and southern Illinois.

One forecast model, the usually-over-dramatic NAM4, is painting a grim, and frankly scary, severe weather setup tomorrow. The details of that aren’t helpful. It looks way too unrealistic; however, the other models are building a consensus that the storm potential is real. So, stay tuned.

We will be updating you throughout the day on Twitter at @NashSevereWx. Consult multiple reliable weather sources. 

When Will It End?

Rain should be gone by your drive to work on Friday.

Rain totals are expected to be around 1′ to 2″ when all is said and done. Local areas (especially in Williamson County) could potentially see up to 3″ of rain through this event.

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Friday: Sunny – Wake Up 52° High 66°

After the rain, clouds will be clearing throughout the day and we should see partly cloudy skies through the night. Temps will also stay relatively warm. Very nice start to the weekend.

Weekend Outlook:

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We are expected to have an amazing weekend with clear skies and warmer temps. Next shot of rain isn’t until the beginning of next week.

This website supplements @NashSevereWx on Twitter, which you can find here.

Categories: Forecast Blogs