Our region’s first heat wave is in progress. With high temperatures and humidity, an unstable air mass often spells afternoon thunderstorms. Will any of these become severe?
The short answer: We have a marginal risk of severe storms for the next 48 hours, with the greatest likelihood north of the Mason-Dixon line. On Thursday, the risk may shift towards the D.C. region. In terms of timing, the late afternoon and evening hours are most likely for storm activity all three days. The chance of storms – in general – is 20 percent today, 30 percent Wednesday, and 40-50 percent Thursday.
A corridor of thunderstorm activity today
It’s instructive to look at the large-scale pattern setup. The eastern U.S. is sitting beneath a heat dome – a large, summertime area of high pressure, or anticyclone. A weak jet stream flows from the Great Lakes northeastward into Canada. Along its southern margin lies a stationary frontal boundary. Figure 1 below shows the synoptic forecast map for later this afternoon.
As you can see by the green shaded regions, the front to our north will serve as the focus for most of the thunderstorm activity. This defines a corridor along the northern edge of the heat dome, where unstable air is forced to ascend. Packets of energy in the jet stream, called shortwaves, will ripple through over the next three days.
One of these will cross NY-PA late today, triggering a round of thunderstorms. The Storm Prediction Center (SPC) is calling for a 30% chance of severe storms, mainly wind and hail, to our north, as shown in Figure 2 below.
We are on the southern periphery of this outlook zone, where our probability of severe lies somewhere between 5%-15% (5% in the vicinity of D.C., increasing to 15% along the Mason-Dixon Line).
The high resolution prediction models portray a generally inactive day across our region. The reason: Even though instability is predicted to become significant, sinking motion in the anticyclone’s core will suppress activity over a widespread area.
The models suggest that an east-west band of convection across the PA could sag south and cross the Mason-Dixon overnight, approaching the D.C. region in the very early morning (Figure 3, below). However, such a line will encounter relatively stable air, and should be weak as it crosses through.
With a moderately unstable air mass in place this afternoon-evening, in spite of being close to the anticyclone’s dynamic core, a few pop-up thunderstorms could get going. Sources of localized uplift include lifting along ridgetops, along Bay and river breezes, and along a surface pressure trough over the Piedmont. But with very weak wind shear, these should be slow-moving and short-lived storm cells. A few, however, could “pulse” briefly to strong-severe levels – so expect isolated intense lightning, small hail, heavy rain and possibly a wet microburst or two.
A corridor of thunderstorm activity again, tomorrow
Wednesday’s pattern is essentially a carbon copy of today: Stationary boundary to our north, heat dome over the Mid Atlantic, and SPC portrays a slight-risk corridor to our north (Figure 4).
For tomorrow, models portray the atmosphere lighting up with convection north of the Mason-Dixon, with remnants sagging south across the Washington region tomorrow night (Figure 5).
Like today, local lifting mechanisms including terrain, Bay breeze and lee trough could organize a few regions of thunderstorms during the late afternoon-evening. The instability is predicted to be a bit stronger tomorrow, as is the shear. However, lacking large-scale, organized lift, I do not anticipate widespread severe activity across the Washington region. Isolated pulse-type severe storms remain on the table, however.
Thursday: A transition day
The stubborn front to our north will begin sagging southward through the day. With increasing cloud cover near the front, instability will not be as strong as today or tomorrow. However, wind shear will creep up a bit, as the core of the anticyclone weakens across our region. One of the models (the NAM) is hinting at an enhanced level of convective organization, with one or more lines of convection tracking along the frontal boundary (Figure 6).
Of the three days, this may be the one to watch most closely. At this point, SPC does not depict a risk of severe activity across the Mid-Atlantic. However, there may be just the right combination of instability, shear and sustained uplift for a slight risk, particularly if a shortwave comes through during the afternoon or evening.
CWG will keep you apprised of any changes to our thinking as we draw closer to Thursday.