Local Severe Weather Event, Monday June 26th
Created by: Ben Tomfohrde, Kaitlyn Potucek, Denys Khrulov
Synopsis
A rather robust setup will be impacting our region early this week, which will spark the potential for severe weather. Multiple low pressure systems will develop along a stationary front across the Great Lakes region. A low pressure system that will develop over Michigan and surrounding areas will be the one we will be closely watching. A cold front will stem from it, which may become occluded closer to the low, which will force for thunderstorms to develop across the region. Additionally, a surface trough east of the Appalachian Mountains will serve as another focal point for thunderstorm development.
Ingredients
For thunderstorms, a variety of ingredients within our atmosphere are expected to be present. Moisture, instability, shear and lift to name a few. Will these ingredients be available for this upcoming storm threat?
Moisture – As the cold front begins to approach our area, we will have a strong southerly flow advecting rich moisture from the Gulf of Mexico and the western Atlantic Ocean. Dew point values across the severe weather risk zone will range from the low-to-mid 60s, adequate for the formation of thunderstorms.
Instability – Based on the latest guidance, there will be plenty of instability within the severe weather zones. However, the greatest factor that may limit the severe weather potential will be residual cloud cover and rain showers from the Sunday night complex. As of right now, these clouds and showers should clear out in time for the atmosphere to destabilize during the afternoon hours. CAPE values may reach as high as 2500 J/KG across portions of central and eastern Pennsylvania, which is plenty of juice needed for thunderstorms.
Shear – Especially for a large severe weather threat, wind shear needs to be present. Wind shear measures the difference of wind speed and direction as you head up further in the atmosphere. Higher values of wind shear help to fuel severe thunderstorms as the downdraft and updraft of a thunderstorm are able to remain separate, keeping its lifespan going. Additionally, higher amounts of wind shear are able to transport the winds down to the surface, which may lead to straight-line wind damage. Based on the current guidance, around 30 to 40 knots of wind shear should be present between 0-6km of the atmosphere, adequate for severe thunderstorms.
Lift – At last, some sort of lifting mechanism is needed to initiate the thunderstorms. Synoptic-wise, there will be an upper-level low moving into the Great Lakes region, allowing for a cold front to develop and move through the Commonwealth. This front will form one of the main mechanisms for thunderstorm development. Additional leftover boundaries from previous thunderstorms from Sunday evening may serve as focal points of thunderstorm development, which will be needed to be closely monitored short-term.
Forecasted Timing
Late Sunday Night into Monday Morning
Storms will begin to form in western Pennsylvania, including the Pittsburgh Metro, ahead of a bowing line of storms around 5 pm, rather scattered in nature and intensity. Current guidance shows that the stronger line of storms will enter the western border of the Commonwealth around midnight, racing through the Pittsburgh area and weakening as the line approach central PA by 5-6 am. The main severe threat, especially in the bow echo, are damaging wind gusts to 60 mph.
Monday Morning to Early Monday Afternoon
As many begin to wake up on Monday, storms will begin to fire back up in central parts of the Commonwealth, and track northeastward. These storms will increase in coverage in southeastern portions of Pennsylvania, affecting the Philadelphia and Allentown metros, before exiting the state around noon. These storms are not expected to reach severe limits but heavy rain and lightning can cause flash flooding on low-lying roadways and it will be best to limit any outdoor plans.
Monday Afternoon through Monday Evening
Around 2 pm, storms will begin to form as single-celled showers in the center of the Commonwealth and begin to move eastward. As the storms enter a more unstable environment, these showers are expected to strengthen into a well-defined line of thunderstorms, some severe as they move into eastern Pennsylvania by 5 pm. The thunderstorms are expected to move through the southern portions of the state, including the Philadelphia and southern Allentown areas between 8-10 pm before finally subsiding and exiting the state around midnight. These storms will pose a more intense severe threat than Sunday night, especially in southeastern PA, including Harrisburg, Philadelphia and Allentown. Damaging wind gusts to 70 mph and isolated areas of large hail are possible among the strongest to severe thunderstorms.
Storm Prediction Center
As of right now, the best chance for severe weather, highlighted by the Storm Prediction Center, is across much of central and eastern Pennsylvania.
Marginal (1/5) – Isolated severe thunderstorms are expected. The strongest storms may produce damaging winds and hail.
Slight (2/5) – Scattered severe thunderstorms are expected. The strongest storms may produce damaging winds, large hail, and the threat of an isolated tornado.
Bust Potential
We are closely monitoring the potential for a “bust” that may lead to an underperformance event severe weather-wise. This will be highly dependent on the overnight convection with the leftover thunderstorms across the Ohio Valley due to Sunday’s storms, which will move over into our area. If widespread cloud cover and leftover rain showers and general thunderstorms plague the region for too long, there may not be enough recovery time for the atmosphere to destabilize, leading to very little severe weather. As of right now, we are expecting enough clearing in the afternoon for destabilization to occur, although this will become more clear tomorrow morning. Stay tuned for updates!
Conclusion
The best way to be prepared ahead of tomorrow’s severe weather event is to be prepared! Know the difference between a watch and a warning. A watch signifies ingredients are available for severe thunderstorm development. Keep in mind that this does not guarantee your appropriate location will see severe weather. For a watch, it is best to keep an eye out for any updates and be alert for rapidly changing weather conditions. A warning signifies severe weather has been observed either by spotters and/or the radar and you will be impacted within a very short period of time. For a warning, it is best you take shelter indoors.
Stay tuned for updates throughout tomorrow regarding this severe weather threat!
– CWS Extreme Weather Group
