Baltimore, MD - Powerful storms raced across the Mid Atlantic and Northeast yesterday prompting the National Weather Service to issue bold warnings that a "Life Threatening Storm" was headed in.
ABC 2 was the first to promote these storm warnings on ABC 2 News at 5pm on Monday calling for the potential for powerful thunderstorms some in which could bring 50-70mph winds. The end result was thousands of damage reports, injuries, and the tragic loss of life in Carroll County Maryland.
The storm yesterday was not a derecho by definition however, beared a similar resemblance in respect to winds.
Around 3pm Tuesday July 8th a line of storms formed over western Maryland. This line was slow moving at first as it sat over the western Maryland mountains producing winds around 40-60mph and lots of lightning.
At 5pm the line of storms started its march east. As the line moved over the mountains it began to accelerate rapidly moving at speeds of 55 mph. The line formed a squall as low level air rose from the ground and down drafts occurred on the front side of the line. The atmosphere was extremely unstable allowing for the line to sustain itself coming over the mountains. As the line rushed east it began to bow out turning into a "Bow Echo". The momentum of the line increased the down draft of the storm increasing the winds at the surface.
As the line of storms and bow moved east the national weather service issued a Severe Thunderstorm Warning around 6pm for Carroll County. It was at this point a "Bulge" in the line formed propagating out from the main line of storms. This bulge or wave that extended out from the center is known as a "Line Echo Wave Pattern" which can enhance winds further known as a "Microburst". So instead of 55-65 mph winds we were looking at with the initial line this "Microburst" was capable of producing 80 mile per hour winds due an enhance outflow boundary. This was the reason some parts of the ABC 2 viewing area saw BIG damage while others just saw gusty winds and rain.
The "Line Echo Wave Pattern" is a pattern we see on radar used to classify it.
The line of storms that whipped through Maryland at intense rates was the product as an area of low pressure rapidly developing over PA/NY causing numerous tornadoes. This is known as a meso low feature which is where the greatest directional shear (differing directions at height) and speed shear (differing speeds at height) occurred causing more tornado damage vs straight-lined wind damage we saw in Maryland.
This is what the storm looked like coming through Taneytown, MD VIA Lori Evans Butterfield who posted to my facebook page.
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Here is the meteorological setup behind squall lines/bow echos.
- SURFACE PATTERNS
- Generally east-west frontal boundary (genesis area frequently north of front).
- Strong surface convergence (near genesis area).
- High dewpoints pooled near front/genesis area; maximum values just south of front.
- Surface equivalent potential temperature (theta-e) values very high along derecho track.
- Bow echo often moves parallel to front with slight component toward warm sector.
- UPPER-LEVEL PATTERNS
- Straight or anticyclonically curved mid/upper-level flow near a ridge axis.
- Weak shortwave trough located near/upstream from genesis region.
- Moderate/strong warm advection at 850 and 700 mb present near genesis area with weaker advection downwind. Neutral or weak cold advection noted in mid/upper levels over and downwind of genesis area.
- 850 mb moisture very high and pooled just south of bow echo track; drier air can be present at 700 and 500 mb enhancing damaging wind potential.
- Bow echo moves generally along 850 and 700 mb thermal gradient (along or north of thermal/theta-e ridge axis).
- THERMODYNAMIC AND VERTICAL WIND SHEAR PROFILE
- Long-lived warm season bow echo events associated with very unstable air mass.
- Average maximum convective available potential energy (CAPE) values in genesis area roughly 2400 J/kg with even greater instability downwind where average maximum CAPE is about 3500-4000 J/kg (range of 2500-6000 J/kg).
- Extreme instability due to pooling of moisture near front, so surrounding upper-air soundings may be unrepresentative of true local instability.
- Winds at 850 and 700 mb show good directional shear (veering) near genesis area, and often mainly speed shear parallel to storm track downwind.