A Turbulent Weather Day

2016-07-19 14:15:45.000 – Mike Carmon, Senior Weather Observer & Education Specialist

Yesterday, we were all thrilled with the prospect of severe weather in northern New Hampshire. All signs pointed to an eventful afternoon for most of New Hampshire and Maine, with the summit of Mt. Washington smack in the middle of the action.

Yesterday’s severe weather risk from the Storm Prediction Center

Forecasted values of CAPE (Convective Available Potential Energy) were exceptionally high (by New England standards), signifying a good deal of instability in the air. That’s ingredient number one.

Due to the income of a warm, moist air mass from the southwest, plentiful moisture was available, which is necessary for convection to occur. This is ingredient number two.

And finally, a strong cold front was approaching from the northwest. A frontal boundary such as this provides the third and final ingredient needed for convection: lift. A source of jump-starts the convection process, prompting air parcels at the surface to begin an ascent into the atmosphere.

With these three components in place, thunderstorms began to fire across northern New Hampshire around midday on Monday. It didn’t take long before Severe Thunderstorm Warnings were posted for these thunderstorms blossomed into supercells.

The first thunderstorms to fire yesterday afternoon.

Supercell thunderstorms need additional atmospheric conditions in order to flourish, and the balance of these ingredients is much more critical. First, sufficient wind shear must be in place moving upwards in the atmosphere. Wind shear is what prompts thunderstorms to rotate, which allows the feeding updraft to separate out from the thunderstorm downdraft, or the exhaust of the storm. The separation of these two phenomena is critical for the health of a storm; it allows the cell to continue its intensification without a cold downdraft cutting off its inflow of warm, moist air at the surface, which fuels the storm. However, too much wind shear can be an issue too. An abundance of wind shear will effectively rip a storm to shreds as it grows upwards in the atmosphere. Yesterday, however, the amount of shear present in the atmosphere was right in the sweet spot, allowing developing thunderstorms to blossom into supercells.

Structure of a supercell thunderstorm (Note the separation of the updraft & downdraft)

Lines of thunderstorms continued to form throughout the afternoon and evening, bringing several rounds of turbulent weather all across the region. In fact, several tornado warnings were issued for counties in northern New Hampshire and Maine, although no tornadoes have been confirmed as of yet.

Being stationed on the summit of a mountain, receiving a direct hit from a supercell thunderstorm is a rare occurrence for summit weather observers. Mountains disrupt wind flow and can present a literal physical barrier to developing thunderstorms. Unfortunately, passing thunderstorms succumbed to these effects yesterday, as any and all storm that formed either went around us to the north, to the south, or hit the summit and promptly weakened.

However, we were treated to a spectacular show yesterday afternoon around 4PM as a particularly intense supercell hit the Presidential Range and skirted around to the north. Simultaneously, the fog cleared from the summit, allowing us to witness this menacing storm as it passed to the north. Check out our Facebook Live video as we followed this storm while it passed us to the north yesterday.

Mike Carmon, Senior Weather Observer & Education Specialist

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