Category Archives: Severe Weather


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Tues 10 AM Update: This morning’s RAP (Rapid Refresh) model has thunderstorms breaking out 3-4 PM around Philadelphia and continuing on and off until 8-9 PM. Over past weeks, the morning RAP model has had a fairly good track record.

from earlier Tues morning:

Tues 8 AM Update: Last night’s models have backed away from the severe weather in the immediate PHL area. The severe parameters (Helicity, Vertical Shear) have reduced from yesterday’s model runs; the expected lack of sunshine will reduce the chance of severe weather in our immediate neck of the woods.

One can still expect showers and thunderstorms to develop as early as noon and continue into the afternoon. Some of the rainfall may still be locally heavy in spots. It may be very windy in some thunderstorms.

As mentioned yesterday, the heavier dynamics now appear to develop to our south and east.

from yesterday…

Tuesday has been in the crosshairs for severe weather for several days.  The bullseye for the severe weather had been the immediate Philadelphia area, but the latest models show that the most extreme aspects looks to occur to the south of us, near the Washington DC area.

That said, there are plenty of ingredients that are poised to come together in our area.

A warm front will pass through Tuesday morning, possibly causing showers and thunderstorms before daybreak with another possibility around noon.  A cold front associated with a low pressure system moves through during the late afternoon and early evening hours.

Thunderstorms with heavy rain and high winds are possible both during the afternoon and early evening.  Some of the severe weather parameters (helicity, vertical shear) are highly elevated giving us a  risk of high winds and slight risk of tornados.  Luckily, we may not have that much sunshine, limiting the available energy and reducing the potential for very extreme weather.

The models are still evolving and have changed their forecast considerably over the past 24 hours.  The NWS does a great job with these events and it’s suggested to stay tuned to their forecasts.

Thunderstorm Outlook- Wednesday

The frontal passage and thunderstorms in Philadelphia expected today will be well-handled by the NWS.

So, what value-add can I offer this early in the day?

Reviewing the models from earlier this morning, the following characteristics and trends exist:

There’s a possibility of some earlier scattered showers and thunderstorms as early as 12-2PM.

Increasing  areal activity is expected between the hours of 4 PM and 8 PM.   A line of storms should develop by 5-6 PM and move through the Philadelphia area, according to the latest RAP and HRRR models.

The severe weather parameters (cape, helicity, shear, precipitable water) are all elevated, but not as elevated as seen a few weeks ago with the tornado development.

Some low pressure centers are expected to develop in this line of storms and additional activity may linger until after 8 PM.


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This afternoon’s models have backed off on the extreme weather just a bit.  Most of the severe activity will move in between 8-12 midnight.  Accumulated precipitation values have come down a bit to 1-2 inches of rain with additional rain on Thursday afternoon and evening.

Shear, Helicity and Vertical Velocity values have reduced a bit from yesterday’s posted graphics with the heaviest activity now predicted to occur in the northern suburbs of Philadelphia, Berks, Lehigh and Bucks counties and into central/ northern NJ and New York City.

HRRR Model Maximum Upward Vertical Velocity 9 PM (black shading)

That said, these locations are hard [impossible] to pin down precisely in advance.  There are differences in the models.

It’s a good thing I don’t usually “do thunderstorm forecasts”.  This would have to become a full-time job!  


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11PM Update. Tonight’s NAM continues to show most of the heavy/severe thunderstorm activity between 7 and midnight Wednesday evening. However, tonight’s  WRF models suggest the possibility of scattered showers and thunderstorms any time during the afternoon as well, with the heaviest in the evening. It will be an active day weather-wise.

For tonight (Tues), the models have no showers moving into Philadelphia, but current radar shows some storms in upper Montgomery county moving eastward.

Looking at the latest Rapid Update Model, there are enough parameters that would support the showers moving into the immediate PHL area.  I guess we’ll see whether they fall apart as predicted or hang together.  Nothing major expected.

As for tomorrow, Wednesday, the remains of Barry will be moving through our area late afternoon and evening.  Some of the severe parameters are fairly impressive and the models have been predicting that the Philadelphia area will be a bullseye for the heavy activity.

Some models have storms breaking out in the northwest suburbs as early as 4-6 PM.   Most of the severe weather and heavy rain looks to occur between 7 PM and midnight.  Models are predicting about 3 inches of rain with locally higher amounts.  Some severe parameters (like “helicity” and “vertical shear”) become very impressive in central New Jersey during the late evening hours. Tornados are associated with these levels of shear and helicity.

NAM predicted vertical shear 12 midnight Wednesday. (Dark Brown >20 vertical shear)