10 Questions: Sunday afternoon update on Groundhog Day storms

A windy and unseasonably warm day is concluding across the metro. Temperatures climbed to 70 degrees with wind gusts measured as high as 40 mph thanks to the sun poking through between the clouds, mixing strong wind at a few thousand feet down to the surface. A cold front will move into the area tonight, stalling just to our south and resulting in weakening wind and a shift to the northeast for Monday. Scattered showers will dot the area overnight near the front but precip amounts remain light overall. Temperatures remain mild.

At midnight tonight, a cold front will be draped across the area with scattered showers the result. As the low pressure in the southern Rockies moves east, it will begin to pull that front back north Tuesday night. Graphic courtesy NWS.

On Monday, despite cloudy skies and northeast wind, temperatures will still likely make it to the mid 60s as cold air remain positioned well to our north and the front hangs around the area. By Monday night, the cold front becomes a warm front as it retreats north back through the metro in response to a deepening (strengthening) low pressure system that will move across the Plains. Temperatures will remain mild as southerly wind re-establishes itself across the area, setting the stage for a potentially stormy Groundhog Day.

By Tuesday morning, the Mid-South will be firmly entrenched in the warm sector of the approaching storm with a warm front to the north and southerly wind escorting Gulf moisture and warmth north. Graphic courtesy NWS.

Regarding Tuesday, additional model data is available now that we are within 3 days of the event, including the North American Model (NAM), which offers a little greater insight into the details of the atmosphere than the Global Forecast System (GFS) and European models do. There are many similarities between the various models, but still some fairly important differences, especially with regards to the storm mode – squall line versus discrete (single cell) storms. How the storm mode plays out will ultimately determine a few important factors, including whether everyone in the metro sees storms (the coverage), how strong they may be, and perhaps just as important, the tornado threat.

Here are answers to 10 questions you might be asking:

  1. What time are we looking at for storms to affect the metro? Models are narrowing their focus on the afternoon hours. Ultimately the storm mode could determine this as well.
  2. So are we talking squall line or individual supercells? And why does it matter? That’s the million dollar question. There are factors that support both and differences of opinion that are based on good evidence both ways. Based on the consistency of the NAM model over the past day, which tends to have a little better handle on the mesoscale (local scale) details, I’m starting to give as much credence to the supercell scenario as the squall line scenario. However, there is still a good amount of evidence that supports a squall line forming rather quickly in far eastern AR by mid-day and moving across the metro.
  3. Why does it matter what the storm mode is? Squall lines tend to result in more of a high wind threat (50-60 mph+) with isolated weak “spin-up” tornadoes given enough low level shear, while supercells are more likely to produce larger, strong tornadoes that stay on the ground longer since they don’t have to complete with neighboring cells for energy. 
  4. Is there any good news on the supercell front? Yes. If supercells are the result, they are more likely to form further east than the squall line, according to the NAM model data. The key factor here is a “cap,” or atmospheric lid, between about 5-10,000′. A stronger lid keeps the storms from forming until the bubbling airmass below (measured by instability) is strong enough to blow the lid off. Storms form and can start rotating fairly quickly given the atmospheric shear we’ll be dealing with. The NAM model favors supercells, but doesn’t form them until the late afternoon when they would likely be east of the metro (barely) given the current timing of the approaching front. If this were to happen, it’s conceivable that most of the metro escapes with no storms.
  5. The NAM modeled CAPE values (an indicator of instability) approach 1000 J/kg mid-afternoon Tuesday. Values this high are more than sufficient to produce severe thunderstorms in early February, when instability is typically the main factor that is missing to produce strong storms. Graphic courtesy WxBell.
  6. How bad will it be in my neighborhood? You know I can’t answer that. 🙂 As described, there are multiple scenarios with outcomes that range from nothing happening to a squall line to a supercell that moves over your house. As always, we root for you, not the storms.
  7. Should I change plans? Not yet. Hopefully we’ll know better at this time tomorrow what the storm mode will be and the likely initiation area (west or east of the metro), which can then guide that decision making. Most likely, we’ll either get a squall line, which you would want to be cognizant of and ready to shelter if very strong wind occurs, or supercells, which could form to our east but could fire over the metro. Stay tuned.
  8. Could we completely miss out on all storms? See #5. Yes, it is possible. That’s why we will have rain chances that aren’t 100%. It’s not a done deal that everyone gets storms at this point.
  9. What factors are still uncertain that would contribute to a higher confidence forecast? A few key ones, namely the amount of instability and the strength of the cap. In addition, there are indications that surface wind could veer to the southwest in the afternoon, which would reduce low level shear and result in a lower tornado threat even if storms do form. The real wild card appears to be instability and low-level moisture – dictated by how warm we get during the day ahead of the other factors coming together to promote storms. Morning showers could limit instability and thus storm strength, but models are still indicating more than sufficient instability for severe storms despite some morning rain. Breaks in the clouds during the afternoon would also allow additional warming in the low levels, which would cause more instability to be realized. These are the type of factors that simply aren’t well known 48 hours in advance. One factor the models do show fairly uniformly is abundant low level moisture. Dewpoints in the mid 60s are forecast, which is more than sufficient to fuel strong to severe storms this time of year.
  10. The GFS (American) model depicts dewpoints in the mid 60s as far north as Memphis by Tuesday mid-day, providing the fuel for storms to feed on. Notice how quickly the dewpoints drop off behind the front in central AR as much drier air moves in quickly. Graphic courtesy WxBell.
  11. Could this storm be remembered years from now as the Groundhog Day Storm of 2016? That might be a little dramatic, but if supercells form and drop tornadoes, there’s a decent chance, given the favorable dynamics in play, that they could be strong. Again, we root against severe weather. However, the setup is similar to previous events that have produced severe weather, including January 19, 1988, which produced multiple significant tornadoes in west TN and north MS. 
  12. What do I do now? First of all, stay tuned. The details that really matter have yet to be determined. You should also start preparing for the possibility of severe weather Tuesday afternoon and early evening. Where will you be and what will you do if a warning is issued? Review your safety plan. Know how you’ll get warning information. I highly recommend our MemphisWeather.net app with the StormWatch+ upgrade that customizes watches and warnings the way you want them, and only delivers them to you if you are in the path of the storm. See the links below. Some additional tips are also shown below. Be prepared, not scared!

Here’s your graphical takeaway:

Once the severe storm threat ends early Tuesday evening, much drier and cooler air infiltrates the region. A dry forecast is anticipated for the rest of the week with seasonal temperatures. On a side note, this same system will be dropping a significant amount of snow from the Front Range of the Rockies across the central Plains and into the Upper Midwest Monday through Wednesday. If you’re traveling that direction, keep an eye on forecasts for the areas you’ll be heading to.

Erik Proseus
MWN Meteorologist

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