Although we’re currently entrenched in a wedge of cool, clammy air, a strong warm front lurks to the west and southwest. Temperatures stuck in the mid-to-upper 40s in the metro region quickly warm to near 70 around Staunton, Va. It’s looking less likely that warm air to the southwest will make it into the metro region (though it’s a close call). But even if it stays cool at the surface (where we live), the atmosphere aloft (above the surface) is warming while a strong cold front approaches from the west.
This is likely to create an unstable situtation - but rather than at the surface, at the upper levels of the atmosphere. We refer to this as “elevated instability” which can be conducive to “elevated convection” that produces hail especially during spring time. Hail formation requires a lot of lifting in the atmosphere, as hail stones grow when falling water droplets get thrust back up into the cloud where they freeze around ice crystals at high altitudes. Elevated instability is what provides the lift, as light warm air is sucked up into the denser colder air above it.
An elevated convection scenario arose just this past Monday morning prior to rush hour when hail was reported in many locations as a warm front came through. Bob Ryan wrote a nice summary of the circumstances that supported those hail-producing storms on WJLA’s StormWatch7 Blog. The website HabyHints has a nice overview of elevated convection if you want to learn more, or read the journal article “Elevated Convection and Castellanus: Ambiguities, Significance, and Questions” if you really want to get into the technical nuts and bolts .
The National Weather Service in Sterling, Va. has included the possibility of large hail in its forecasts for the region this afternoon and evening. In its afternoon forecast discussion, it stated the following:
HAVE LARGE HAIL WORDING IN MOST OF THE ZONES THIS AFTERNOON...EVEN A LITTLE FURTHER NORTH OF WHERE THE WARM FRONT MAY SET UP. SURFACE BASED STORMS DO HAVE A GOOD CHANCE OF GOING ELEVATED NORTH OF THE WARM FRONT...THUS THE POTENTIAL OF LARGE HAIL WILL CONTINUE TO EXIST.
And as we mentioned this morning, the Storm Prediction Center in Norman Oklahoma has indicated there is a slight risk of severe storms in the area. In extreme western Maryland and into West Virginia, a tornado watch is currently in effect through 9 p.m..
We’ll keep you posted about any severe weather watches or warnings that are issued throughout the evening. The most likely time for storms to break out around here is between 6 or 7 p.m. and around midnight.