Category Archives: Summer Storms


Thu 10:06 PM Forecast Review — Despite severe weather having broken out in Bucks county and central NJ in the expected time frame, I don’t consider today’s forecast a success.

The actual weather in Philadephia and most of the adjacent counties was far different than had been forecast by a multitude of models including the Canadian and German models.

Incredibly, the GFS (where convective activity is not its forte) did the best with timing and precipitation placement. Something to consider going forward.

Thu 07:30 PM Update — Severe weather has broken out in Bucks county and areas north and east into NJ.


Philadelphia, Delaware and Chester counties have not (yet) seen any severe weather.  In my neck of the woods, we haven’t had much of anything except for some rain earlier.  

Satellite water vapor imagery suggests something might be developing just to our west that may move within the next two hours.

Water Vapor Satellite imagery showing developing disturbance just to our west at 7:30 PM

Thu 05:06 PM Forecast Review — This is beginning to look like a bombed out severe weather  forecast, where only the GFS model (not known for its thunderstorm acumen) is correct. 


Last night’s 7 PM update mentioned that the GFS was downplaying the severity threat.  But, with all the high resolution models showing severe weather, I would never have just gone with the GFS for today’s forecast.  

Here’s the current radar which shows the heavier action going to our north and south.

MRMS precip image  4 PM.  (Click on image for a larger view.)

There’s still some time for the other models to redeem themselves.

Updated on Thu 3:31 PM: This morning’s GFS model with its earlier precip arrival and its less impressive, less severe weather may be on-target. The lack of sunshine this afternoon has kept a lid on some of the instability. I guess we’ll see.

Thu 12:34 PM Update — The latest GFS continues to be in it’s own camp with the main activity coming through about 3-6 PM.  I guess we’ll see.  
Thu 10:39 AM Update — The morning’s high resolution models are in (HIRESW, HRRR, NAM-NEST) .  Like the NBM meteogram just below,  they show two peak probabilities in rain/severe thunderstorms.  Rainfall will be very heavy, hail is possible along with damaging winds.  Helicity values are high enough to support the possibility of tornadic activity.  Any sunshine in the afternoon will assure more intense development. Peak times are 4-5 PM and 8-9 PM.
Thu 09:45 AM Update —This morning’s latest NBM ( 12z ) has the following timing for today’s storms—
NBM 12z  1 Hour probability  of thunderstorm and rain Meteogram for Blue Bell, PA (Click on image for a larger view.)

Thur 7:30 AM Update — Showers had broken out at the airport at 6 AM,  as predicted by last night’s HRRR.  Here’s the latest HREF meteogram for maximum simulated radar echo and the severe weather parameters, helicity and vertical shear.  The 5-7 PM timeframe is the time to watch. —
HREF meteogram (Blue Bell, PA) for simulated radar, helicity and vertical shear

Wed 09:58 PM Update — Tonight’s early models just becoming available. The latest HRRR still shows some impressive, severe thunderstorm parameters in the 5 PM to 7 PM time frame with gusty winds, hail, heavy rain and possible tornadic activity.


Tonight’s 00z NBM rain/thunderstorm meteogram for Blue Bell—

NBM 00z   (Click on image for a larger view.)


Wed 7:00 PM Update — The afternoon models just available have backed off on the magnitude of the severe weather parameters.  The GFS, in particular, has much of the activity moving through during the afternoon and early evening. This earlier afternoon activity appears to reduce the amount of sunshine and daytime heating resulting in lower instability CAPE and LI values and lower severity.  Helicity and shear parameters are coming in lower too.  The Canadian High Resolution model and Canadian GEM have strong storms 5-7 PM.   The peak time remains 4-7 PM, but as shown in the earlier posted NBM meteogram (below), some activity expected in the morning and early afternoon.
Wed 05:00 PM Update —I have used the words “strong”  and “very strong” to describe forecast thunderstorms this season.  I have rarely used the word “severe” this season. 


Tomorrow’s thunderstorm strength potential will likely fall into the severe category.  Some “severe weather” parameters are forecast to be the highest I’ve seen so far this summer season.  One parameter, “helicity” related to potential tornadic activity, is forecast to be substantially elevated at various times during the day.  Another parameter, “shear velocity”, is also highly elevated. 

Some of the other parameters that that have been elevated in previous storms this season  (CAPE, Lifted Index) are not as high as they’ve been previous thunderstorm days, so we’ll have to see how it all plays out. 

The latest HRRR suggests we might get some thunderstorms in the early morning (7 AM) , hinted at by the NBM.   The heaviest activity is likely between 4 PM-11 PM, but other activity is possible around noon or 1 PM. 

The general timing of all of Thursday’s storms is still best depicted by the NBM model meteogram posted below. 

Updated- Added 16z NBM meteogram

First, most of this morning’s (Wednesday’s) models have backed off on the thunderstorm potential for Wednesday afternoon and evening. Some models maintain a slight chance from just south of the city into Delaware and south Jersey.

Thursday looks to be much more active, as a strong cold front approaches and upper air disturbances move through in advance of the cold front.

A warm front moves through early Thursday morning. Several models forecast showers and thunderstorms early Thursday morning with this warm front.

Additional showers develop early afternoon and then again in the late afternoon and evening. Some of the dynamics may be very strong late Thursday afternoon, as the forecast jet stream energy has increased from previous model runs.

NBM model meteogram for Wednesday into Thursday thunderstorm probability & rain probability Blue Bell, PA. (Click on image for a larger view.)

The actual front moves through around midnight and before dawn Friday.

I’ll be updating later this afternoon and evening with more specific information. Stay tuned.


Update Thu 6:09 PM : Tonight and Friday should be a very low thunderstorm probability timeframe. The later part of Saturday and Sunday look to be active weather days.

Thu 06:05 PM Forecast Review —Some thunderstorms did develop around the region Wednesday evening, but not where the models had forecast; instead of south and west of the city, they were mostly north and east. 

Updated Wed 10:19 AM : As mentioned yesterday, the models didn’t do too well with thunderstorm prediction on Monday and Tuesday.

For today, Wednesday, things are a little different. An upper air trough (see graphic below) moves through about midnight and this upper air support along with several areas of vorticity ahead of it are expected to trigger thunderstorms in advance between 4 PM and 11 PM tonight. Currently, the models have the heaviest activity to our west and south, but show scattered activity elsewhere as well. Precipitable water about 1.8″, CAPE = 1800, Lifted Index= -6

RAP model upper air trough prediction for midnight tonight. (Click on image for a larger view.)

Updated Tues 7:30 PM: I’m glad I’ve been taking a break from posting any mid-week weather forecasts over the past two days; most if not all of the models have been wrong about the high temperatures and the thunderstorm development today, Tuesday.

An unexpected change in the position of a frontal boundary gave us an easterly stable wind flow, lower temperatures, and as of this post, no thunderstorms.

The boundary is south, across Delaware, and it’s unclear when it moves north past us, whether we’ll have and thunderstorms later this evening or on Wednesday. The numerical weather models so often impress me. Not today.

It shouldn’t be any news to anyone that we’re entrenched in a hot and very humid air mass.

High instability (Lifted Index = -6 and lower) and high CAPE (>3000 J/kg) along with some upper air disturbances have set off thunderstorms. Precipitable Water values are more than 2.2 inches.

While my immediate neck of the woods hasn’t seen more than a few drops of rain, areas just a mile or so down the road have had severe thunderstorms with very heavy rain.

Current Radar with super-imposed Water Vapor image and RAP model upper air flow at 5 PM. (Click on image for a larger view.)

Thunderstorms are expected to continue in some areas past midnight.

More storms expected after 1-3 PM Tuesday.


Wed 09:39 PM — The westward path of Elsa, as indicated in Monday’s post (“If-Else(Elsa), then Rain”) continues to hold.   Latest models have 1.5-2.5 inches of rain for areas in Philadelphia and NJ.  More info tomorrow… 
Wed 05:55 PM Forecast Comment —  So why little thunderstorm threat today vs yesterday?  Despite high predicted CAPE and hugely negative Lifted Index values, there is little vertical upward motion trigger today in the upper atmosphere.   Wind flow in the upper half of the atmosphere is slightly anticyclonic today, resulting in sinking airflow compared to yesterday’s cyclonic flow.    Some widely scattered storm activity is possible, but nothing like Tuesday night.    

Updated 1:08 PM highlighted.
This morning’s models have the strongest vertical motions and dynamics from Allentown northward. While some scattered storms are possible in this area, the heaviest activity will be in the Allentown area.

Another quick post- Last night’s models show significant instability and CAPE values between 5 PM and 11 PM Wednesday evening. Thunderstorms, heavy to severe, will likely develop again this evening from 5 to 11 PM. (GFS timing shows peak about 10 PM)

The NBM and GFS keep the heaviest activity to the north and west, but the higher resolution models forecast some impressive numbers in our area. Here’s the latest RAP model—

RAP model CAPE at 9 PM. (>3900!!!!) (Click on image for a larger view.)

RAP model Lifted Index at 8 PM (-9!!!) (Click on image for a larger view.)