ALERT For Potential Of Severe Thunderstorm Development During Tuesday And Wednesday
Wind Damage, Large Hail, Vivid Lightning and Flooding Rain Will Be The Main Severe Threats ( This Could Result In More Power Outages For Impacted Locations ). This Threat May Extend Into Thursday ( June 23 ).
A stormy weather pattern, anchored by a major heat dome to the west, is taking shape with the potential for strong-severe storms increasing Tuesday into Wednesday.
One or more Mesoscale Convective Systems ( large thunderstorm clusters ) are expected to develop from Tuesday into Wednesday, with some potential for Derecho Formation ( a long-lived wind damage producing system ). Exact location and track of these thunderstorm clusters remains uncertain and will need to be predicted once storms develop on Doppler radar.
More localized strong-severe thunderstorm development will also exist as dewpoints and instability increase from Tuesday through Thursday. Please stay tuned to NOAA Weather Radio for possible watches and/or warnings that may be needed.
Remainder Of Monday Afternoon
Partly cloudy. Small chance of a shower or thunderstorm. Winds W-NW at 5-15 mph, with higher gusts along middle to upper elevation mountain ridges. Temperatures varying from low-mid 70s at highest elevations to the lower-middle 80s ( warmer south toward the Tri-Cities ).
Monday Night Into Tuesday Morning
Partly to mostly clear during the evening, then increasing clouds overnight into morning with a chance of showers & thunderstorms. Winds SW to W at 5-15 mph, with higher gusts, along mountain ridges-plateaus below 2700 feet. WSW-WNW winds 10-20 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures varying from middle-upper 50s to the mid-upper 60s ( coolest in mountain valleys with light winds ).
Showers & thunderstorms becoming likely. Some storms could be strong to severe. SW-WSW winds 10-20 mph, with higher gusts. Temperatures varying from the upper 60s to the upper 70s to around 80 degrees ( coolest at the highest elevations ).
Tuesday Night Into Wednesday Morning
A chance of showers & thunderstorms. Some storms could be strong to severe. SW-W winds 10-20 mph, with higher gusts, along mid-upper elevation mountain ridges. Temps widespread in the 60s.
Wednesday Morning Into Wednesday Evening
Showers & thunderstorms likely. Some storms could be strong to severe. SSW-W winds 10-20 mph, with higher gusts. Temperatures varying from upper 60s-lower 70s across highest elevations to the upper 70s to lower 80s.
The risk for a large thunderstorm complex will be highest from Tuesday Night into Wednesday, with initial development expected to be upstream of the Appalachians followed by movement toward the mountains from the northwest. Timing and trajectory ( track followed by the storms ) will have to be determined as the event gets closer. Stay tuned for updates.
Weather Discussion ( June 18-22 )
A lovely stretch of June weather during the Father’s Day weekend is expected to end abruptly as the risk for showers and powerful thunderstorms increases significantly into Tuesday and Wednesday ( June 21-22 ).
My friend Wayne Riner captured a gorgeous sunrise above Long Ridge, in southern Dickenson County, to begin the work week.
With some vertical cloud development a hit-miss, localized shower or even thunderstorm can not be completely ruled out through this afternoon-early evening. Chances remain low today and tonight in comparison to Tuesday when odds for shower-thunderstorm development ramp upward.
The entire pattern is being anchored by a heat dome centered over the southwest-central portion of the USA.
The initial increases in the risk for strong-severe storms will develop as a cold front sags southward into the region during Tuesday, providing a low-level focus for convection as the upper level flow diverges and streams across the Mountain Empire from the northwest.
However, what is really catching my eye is a surge of blazing heat that will be developing into Wednesday, setting up a huge temperature-dewpoint gradient from southwest to northeast across the Great Lakes and the Middle Atlantic region ( below ).
By early Wednesday a 20+ degree Celsius ( 38+ Fahrenheit ) temp gradient is being forecast between the Upper Midwest and eastern Great Lakes-Northeastern USA. The thermal zone being defined well on the above model ( even though exact placement of its axis may vary some from what any given model is currently showing ).
While thunderstorms Tuesday could become strong to severe amid a general northwest flow setting, and must be respected, the synoptic or large-scale pattern becomes even more favorable for large thundrstorm cluster ( complex ) development by Tuesday Night into Wednesday.
It is during this time that potential for a Derecho or near-Derecho cluster of thunderstorms will increase. One reason for this is the presence of dry mid-level air ( shown above ) which research has shown to be a KEY factor in supporting wind damage producing bow echoes-Derechos. The dry air promotes formation of a strong MesoHIGH or cold pool via evaporation aloft. The cold pool acts to sustain the system by continuous formation and generation of low level convergence relative to the convective system ( evaporation into drier air aloft adding to downdraft momentum by making the air more dense ( heavy ) and aiding its plunge to earth ). Terrain drag at the surface with downward and outward spreading air forces convergence ( or piling up ) of low-level air to drive and renew upward vertical motion ( lifting moist, unstable air to create condensation and complete a cyclic feedback process ).
The horizontal temperature gradient at the surface being a factor to strengthen winds aloft ( dictated by the thermal wind equation and the isallobaric component of the ageostrophic wind equation ) during any time of year, adding to the potential for downward momentum transfer to the surface in organized downdrafts.
It can not yet be known if the Mountain Empire will take a direct hit from a complex. While large storm clusters do tend to move along 1000-500 MB thickness lines, and the 850-300 MB MEAN cloud-layer wind field, the ultimate movement is often dictated by the greatest system-relative low-level convergence ( forced by the cold pool ) which may or may not be in the same direction as the low-level jet.
*This may further be modulated by Supercell’s embedded within the MCS which adds additional complexity to propagation as has been suggested by Stephen Corfidi at the Storm Prediction Center.
Since the low-level jet often streams into such a system from the southwest, it is not uncommon for development to focus on the inflow side of the system which is often the southwest-southern, or forward, flank for a cluster embedded in NW-SE ( downshear ) flow. While this is, of course, very important an assemblage of many MCS’s illustrates that ultimate movement is forced most by the cold pool produced by low-level convergence relative to the system ( and apparently modulated by embedded Supercells ).
Therefore, it may depart from what 1000-500 MB thickness and 850-300 MB MEAN cloud-layer pattern might predict and is best forecast by a Vector approach that takes into account cold pool forced low-level convergence with mean layer fields.
The Bottom Line…while the Mountain Empire is not currently within the slight or enhanced risk regions on Wednesday, it is too soon to know exactly how this will play out and it is likely that this risk graphic will change during the next couple of days as storms form and alter the atmospheric setting over time.
One or more thunderstorm clusters, with potential for a severe weather outbreak, is expected from Tuesday into Wednesday. Localized strong-severe storms may also develop to impact more limited corridors along their paths. Please stay tuned to NOAA Weather Radio and your favorite media sources for possible watches and/or warnings that may be needed from Tuesday into Thursday ( June 21-23 ).