Category Archives: Summer Storms

Thursday Thunderstorms

Update 2 PM: The latest 1PM Rapid Refresh (RAP) model has plenty of rain here and in NJ this evening. The trend is for much of the rain to come down during the evening hours.

Update 4 PM: The latest HRRR is consistent with the HREF graphic below, showing the heaviest rain centered around the Philadelphia area.

The showers and thundershowers that moved though during the dawn hours associated with a warm front mostly affected areas of NJ. Some of last night’s models had forecast this scenario, but not all.

For today, the warm front continues to move very slowly to our north and clouds should break for some sun. An upper trough moves through later today with showers and thunderstorms, preceding a ‘cold’ front that passes through around noon on Friday, possibly with more showers/thunderstorms.

The amount of sunshine today will determine the amount of energy available for thunderstorm development.

As was the case last night, there are differences regarding the timing and placement of thunderstorms today.

The three new variations of the High Resolution Models (HIRESW) have somewhat different forecasts. The HIRESW-MEM2 (my preferred model for thunderstorms) has a line of thunderstorms moving in between 5 PM and 7 PM, but has it dissipating after it passes through Philadelphia.

(A few earlier scattered storms can’t be ruled out.)

Additional scattered shower activity continues through at least midnight.

The HIRES-ARW is somewhat similar to the HIRES-MEM2.

The new HIRES-FV3 has these storms making it into NJ, but dissipating as they get to the shore.

The NAM-NEST is similar to the HIRESW-MEM2 with the storms dissipating as they make it through NJ.

The HREF (High Resolution Ensemble Forecast) puts this all together—

HREF total 3 hour precip forecast 7 PM, showing the highest rainfall to occur in the city.   (Click on image for a larger view.)

 

The severity parameters are in the moderate range, not overly high. Heavy rain (1-1.5 “) with wind gusts and lightning—the usual strong thunderstorm issues — are expected.

Regarding severity, what’s missing is a strong adjacent jet flow presence and jet level vertical motion. CAPE (Convective Available Potential Energy) values are moderate. Additionally, the “shear” vectors and the storm motion vectors are not parallel as the line of storms approach Philadelphia at 7 PM

HIRESW-MEM2 Storm Motion Vectors and Shear streamline not parallel with Simulated radar at 7 PM tonight.  This may limit severity of the storms.  (Click on image for a larger view.)

While available energy is forecast to be high (CAPE values > 1000 joules/kg) , CAPE values associated with very severe storms can approach 4000 + J/Kg. Not the case here.

Shear values and thermodynamic instability values also in the low- moderate range.

Maximum Precipitable Water values are fairly high ~1.7″

As mentioned, additional storms possible mid-day Friday as the actual front moves through.

THUNDERSTORMS THIS EVENING & WEDNESDAY

Forecast Updated Tue 5:40 PM — This afternoon’s HRRR and HREF models have the heaviest activity to our northwest—

This afternoon’s HRRR shows the storms mostly to the northwest at 8 PM. with storm motion vectors and shear vectors most aligned in these areas. We’ll see if the earlier HIRESW-ARW-MEM2 forecast from this morning does a better job with the thunderstorm placement than the more recent HRRR. (Click on image for a larger view.)

Tue 5:35 PM —Updated with storm motion/shear vector graphic from this morning‘s HIRESW

An upper air disturbance will approach Tuesday evening ahead of a cold front Wednesday.

Several parameters that are used to predict severity of thunderstorms are not all that impressive in the immediate PHL, in the low-moderate range. (CAPE values in the 600s)

Nonetheless, there are several factors including a strong jet streak to our northwest and considerable daytime heating to 82º that can create some interesting weather. Additionally, the storm shear vectors align with the predicted storm motion vectors, especially north west of our area, resulting in some stronger storms north and west.

Look for thunderstorms developing between 6 and 8 PM tonight.

This morning’s HIRESW simulated Radar with shear direction and motion direction vectors. at 7 PM and 8 PM Highest support is north and west of the city. (Click on image for a larger view.)

More on the thunderstorm threat Wednesday with a later post.