Although summer does not officially start until next week, this week will feel much more like summer.
Contrasted with last week’s Andrea-induced gloominess, this week will feature warmer and more humid air, but a chance of thunderstorms along with that as well.
The week will start off with an area of Low Pressure developing over the Ohio Valley and moving northeastward across the Great Lakes. The associated warm front will bring heavy rain to the Delaware Valley today, with the cold front then introducing a chance of thunderstorms, some possibly severe, Tuesday, before a pattern known as a “ring of fire” sets up mid-week, which will continue the chance for evening and overnight thunderstorms through Thursday night.
Today’s concern will be heavy rain and a renewed threat for flash flooding. Plotted below is the Sunday evening (Monday 00z) run of the NAM, showing forecast Mixing Ratio at 850 mb for this evening.
Mixing Ratio is a measure of moisture in the air, much like Relative Humidity or Dew Point, although many meteorologists prefer Mixing Ratio to the other two because it more readily shows where moisture is “pooling,” which indicates areas where heavy rain is likely. The numbers you see plotted are the actual amount of water vapor contained within a unit of air, in this case, in a ratio of grams of water vapor per kilogram of air.
You may need to squint a bit to see this, but notice that there is a circular area of darker green located over northeastern Pennsylvania on the map. That circular area forms over the mountains of West Virginia this morning, according to the model, and then moves northeastward to the position you see by evening. This indicates moisture pooling over those areas, which also means that those areas are the most likely to see the heaviest rain today.
With the ground still saturated from Friday’s flooding rains, it will not take much additional rain to cause more flash flooding.
In addition to Weather Forecast Offices, like Mount Holly, the National Weather Service also staffs several River Forecast Centers. River Forecast Centers release a product called Flash Flood Guidance (FFG), which is an estimation of how much rainfall within a certain amount of time would cause flooding.
These products from the Middle Atlantic River Forecast Center indicate that for areas north and west of Philadelphia, just one inch of rain in one hour will cause flash flooding, and that for the same areas, just one to two inches of rainfall over a six-hour period will cause flooding.
The GFS and Euro agree that most of the Delaware Valley will see at least an inch of rain today. With these amounts falling near FFG, some minor flooding is possible, though it is not expected to be widespread. That said, any areas that see more than an inch will deal with flood issues again. The flood advisories that have been issued this morning are evidence of how little rain it will take to cause flooding.
Once the warm front clears, it leaves us primed for potential severe weather Tuesday.
Winds veering from southwest to west-northwest between the surface and 500 mb, and increasing in magnitude from around 15 knots at the surface to potentially as high as 45 knots at 500 mb, will produce vertical shear that is uncharacteristically high for summer in the Mid-Atlantic, with surface-500 mb shear vector magnitudes as high as 40 knots expected.
In addition, shear vectors will cross the cold front at almost a 90-degree angle, which is also somewhat unusual for our area. This means that as storms build taller in the atmosphere, they will shear away from the cold front, instead of along it. The difference here is that storms will remain somewhat discrete, instead of immediately congealing into one line, which may allow them to become stronger than the average “garden-variety” thunderstorms.
However, with moisture so abundant, storms may eventually congeal into one or two large clusters, regardless of the vertical wind shear present.
The big question, however, is just how much the area destabilizes ahead of the cold front. As is common with Mid-Atlantic severe weather threats, the atmosphere will be nearly saturated ahead of the cold front, which means that lapse rates will be rather unimpressive, hence limiting instability. However, should enough pre-frontal destabilization occur, Tuesday afternoon and evening may feature clusters of thunderstorms moving northwest to southeast across the Delaware Valley.
That cold front will then stall out Wednesday and begin to take on more of an east-to-west orientation, which will be the catalyst for more chances at heavy rain and thunderstorms Wednesday and Thursday.
Ryan Connelly is the newest addition to Phillyweather.net. Ryan is a Bucks County native and attends Valparaiso University in Indiana, studying meteorology. He also writes for Valparaiso's student newspaper "The Torch."