Starting Friday, we talked about a heightened chance of severe thunderstorms Sunday. For much of the metro region, storms failed to materialize, leaving some local residents – who altered their Sunday plans based on the forecast – exasperated.
The ingredients for severe storms seemed to be in place. A strong, energetic cold front would collide with a humid, unstable air mass. At high altitudes, winds were accelerating and changing direction – generating the wind shear necessary for storms to blossom and sustain themselves.
Capital Weather Gang, the National Weather Service and every local TV media affiliate highlighted the elevated storm risk.
So what went wrong? In short, an unexpected pocket of dry air likely suppressed storm development. CWG’s severe weather expert offered this explainer in a post mortem late last night, when it became apparent the storm risk had collapsed:
[L]ack of instability was not the reason. The usual triggers such as heated eastern slopes of Appalachians never fired. The best explanation I can offer is subsidence [sinking air] in the wake of this morning’s shortwave lingered much longer, and dug in its heals. Subsidence dries and stabilizes the middle troposphere. It is the enemy of convection. It cannot be directly measured, only inferred by model diagnostics. A mesoanalysis of vertical air motion (examined after the fact) showed subsidence became anchored in over our region during most of the afternoon. The 18Z special RAOB [weather balloon] revealed a weak “cap” or subsidence inversion had indeed developed.
This dry, sinking air started showing itself in water vapor imagery Sunday afternoon:
Here’s why storms failed to initiate and hold so far…pocket of dry air moved through this afternoon pic.twitter.com/yGWnRnKwrc— TerpWeather (@TerpWeather) July 28, 2014
The dry pocket was a subtle, localized feature that was difficult to anticipate. Note that damaging thunderstorms erupted in northern Maryland and southern Pennsylvania just to its north.
One model used for forecasting thunderstorms – the HRRR – apparently picked up on the feature. It predicted essentially no storminess Sunday afternoon and evening in the D.C. area.
Why didn’t we account for this model in our forecast? Well…we did. “There is some lack of agreement among the models regarding coverage and intensity of storms,” Halverson cautioned in his forecast discussion Sunday morning. “The HRRR does not appear to have the best handle on the situation, as it portrays no convection at all across Maryland and NOVA during the prime time.”
And the day before CWG’s Ian Livingston emphasized a widespread damaging thunderstorm event was no guarantee: “There is plenty of uncertainty as to the evolution of the threat at this time,” he wrote. “Possible morning rain and storms, the potential for cloud cover to remain in the area during the day, boundaries shifting the highest threat south or north, and mistiming of storm development can all hinder reaching potential.”
Ultimately, our forecast was that storms would affect 50-60 percent of the region – conveying that storms were not assured. Even that outlook turned out to be too aggressive; but our forecast was not based on the HRRR model alone.
“I base a forecast on the preponderance of evidence,” Halverson reflected. “Several have pointed out that the ‘HRRR was the model to go with today’…and clearly, in hindsight, it nailed the forecast. But two other trusted models, the WRF-ARW and NAM, portrayed a very active day.”
Halverson continued: “So, new lessons learned. I erred on the side of caution, given the combination of ingredients that seemed to be coming together. I will always try to do my best for our CWG followers, weighing dozens of pieces of analysis and prognostication every time, but will also work hard to learn from the process as well.”
Forecasting summer thunderstorms is fraught with uncertainty and there is large bust potential. Even when the signal for big storms seems strong, I am usually very reluctant to tell people to alter their plans because so much randomness is involved. Better – I recommend that folks stay alert to changing weather conditions and have a plan of action to move to safety indoors if thunderstorms develop.
I understand – especially for readers who get our forecasts on social media or only see the headlines – that perhaps the uncertainty in the forecast wasn’t emphasized enough for this past event. We’ll do our best to communicate this more clearly the next time. Maybe we will develop an equivalent to winter “boom” and “bust” potential outlooks for thunderstorm forecasts. Let us know what you think.