By Kate Bilo
Here’s the quick update:
Be prepared for a significant rain/severe weather event Thursday across the region. Even 1″ of rain could lead to flooding, with the ground saturated and the creeks and streams running high. This is already the 10th wettest June on record and we’re only a dozen days into the month. While this is a developing situation with the exact location of highest risk yet to be determined, areas mainly from the city on southward must prepare for severe thunderstorms with flooding rain, damaging winds, large hail and isolated tornadoes.
Have a severe weather/tornado action plan at the ready in case severe weather strikes, as it did in Delaware on Monday. An interior windowless room or basement is the safest place to be. We will provide constant updates on-air, online at CBSPhilly.com, on our CBS radio stations, and via Twitter and Facebook. Stay alert and stay safe.
As always, prepare for the worst and hope for the best.
More than you probably wanted to know version:
A strong system diving down from the northern Plains will spread rain and severe weather all the way to the eastern seaboard through tomorrow night, and our area is in the main threat zone.
Today, the Storm Prediction Center upgraded the categorical risk for severe weather to HIGH across portions of the upper Midwest, including the city of Chicago. A “high risk” zone is relatively rare and indicates the potential for widespread damaging severe weather, especially, in this case, damaging wind. The severe weather will begin as a number of discrete supercells this afternoon, capable of producing tornadoes especially in eastern Iowa and northwest Illinois. Then, these cells will cluster into a complex known as an MCS (mesoscale convective system), which will blast across the midwest with very strong winds – there is even the possibility of a derecho.
Overnight, that MCS will continue to progress eastward, and portions of our area may experience thunderstorms with heavy rain overnight and into the very early morning hours on Thursday. But this will not be the main threat for severe weather for us, as the storms should be weakening. We will rely on new convection on Thursday afternoon to get going in an environment that is ripe with severe weather potential. We are currently in a “moderate” risk category here in the Delaware Valley, something that happens only a couple of times per year around here.
A few things have to come together. First, today there is a stationary frontal boundary stalled just over portions of the Delmarva. Overnight, that front will begin to activate northward as a warm front. The position of the warm front is CRUCIAL to forecasting tomorrow’s threats. Any area south of the front will be in what’s known as the “warm sector” of this advancing storm, and this is the area that is most susceptible to severe weather. With very moist air in the boundary layer and a south wind at the surface promoting shear, the areas just south of the warm front will be in the greatest tornado risk zone. The actual surface low will ride right along that boundary, and we expect the heaviest of the stratiform rain to fall north of the low center.
So for example, if the warm front sets up right over Philadelphia, the heaviest rain will fall across northern Pennsylvania and near New York City, and areas across Delaware and South Jersey will be in prime position for severe weather, including the potential for tornadoes. But the warm front setting up slightly further north OR south could bring a different set of problems. A southerly solution (as posited by the 12z GFS) would bring the risk for heavy flooding rain to the city of Philadelphia, but would shift the severe weather risk a little further south, toward the shores of MD and VA. A more northerly solution, with the warm front laying over Central New Jersey or Long Island, would put the city of Philadelphia right in the threat zone for severe weather, but would shift the heaviest rainfall further north over portions of NY and coastal New England.
The 12z model suites were hinting at a more southerly, progressive solution, meaning that the storm would move a bit faster and put us more in the heavy rain zone than severe weather, but this is not a solution that I’m prepared to embrace fully at the moment. Current thinking is that we will start the day with possible rain/thunder, especially to our south along the stalled front. Then, perhaps some clearing or even brightening during the day – any sun that breaks out will enhance the risk for severe weather, but we don’t even need that much sun to get things going with plenty of available moisture and uplift. It will be in the mid-afternoon to early evening that things really fire up across our region, with heavy rain to the north and the risk for supercell thunderstorms to the south – these storms could bring the potential for large hail, wind gusts over 65 mph, and even the threat for tornadoes. Currently, it’s looking like the areas most susceptible to severe thunderstorms will be southern New Jersey and Delaware.
We’ll also have to keep an eye on how the relatively cool ocean waters impact things down the shore. Depending on the wind direction, this may not be a factor, but we’ve seen before how a sea breeze front can throw a cap and inhibit convection closest to the coast.