Interesting Weather for Monday

Forecast Review Mon @ 7:26 PM — So, in my book, these were typical thunderstorms with typical outflow wind gusts. Strong, yes. Severe, hardly. (I’m sure there will be some TV footage of a tree down somewhere.)

As mentioned the HRRR, HIRESW and the NAM-NEST did very well with timing and the handling of the front. The RAP was less impressive. Here are a few takeaways for the lack of severity—

•Temperatures were in the 70s, not the 80s or 90s
•Shear values were in the regular range one sees before thunderstorms. They weren’t over 30.
•Helicity values were relatively low. My software settings don’t even show helicity contours until they reach 400+.
•Shear and storm motion vectors were not aligned.
•Barometric pressure tendencies were flat before the frontal passage
• Jet streak position was west of the front and PHL was not in the entrance or exit regions to enhance the storms.

Update Mon @ 5:46 PM — Well, some severe strong activity has popped in upper Montgomery County. The NAM-NEST looks to be correct, as did the earlier HRRR. Expect some strong storms about 6-7PM

Update Mon @ 4:57 PM — The latest HRRR model has really backed off on the extent of [severe] storms coming. I can understand how it’s difficult for TV weather and the NWS to back off on a [hyped] forecast, especially when people’s safety might be at risk.

Nor did I want to stick my neck out too far with my post earlier today with a forecast saying these are going to be just the usual thunderstorms. But with my commentary on the overuse of word ‘severe’, it was my way of hinting at the possibility.

Anyhow, the latest NAM-NEST is still predicting a significant line of storms about 6-7PM . The latest HRRR not so much.

Any barometer fans out there? Maybe you’ve noticed the barometric pressure is already rising flattened? The front has already close to moving through!

Update Mon @ 3:01 PM — Newest models have the front moving through earlier- 4-5:30 PM (RAP Model) 5-7 PM (HRRR model)

Greatest shear is in northern NJ, not in this area.

CAPE is in the 1500-2200 range which is reasonably high. Latest HRRR has helicity in the 100-160 range in some areas.

There are differences in the two short range models. The RAP has several areas of storms moving through this afternoon into this evening The latest HRRR has storms ahead of the front with the main storm line moving through 5:30-7:30 PM (Philadelphia).

Updated Severity Parameter Table: (17z = 1 PM EDT)

Plenty of ingredients available for strong storms. However, shear vectors and storm motion vectors not aligned, suggesting difficulty in creating/maintaining severe storm cells. I guess we’ll see.

Update Mon @ 11:57 AM — Some storms have broken out ahead of the main front, as mentioned previously.

Of interest is the current water vapor image with superimposed NAM and GFS potential vorticity. There’s poor line up of these features, suggesting that these current models aren’t capturing reality that well.

The RAP model has a better conformity to current water vapor imagery. The RAP has significant showers/thunderstorms during the afternoon, breaking out ahead of the main front that moves through at 6 PM—

This morning’s HRRR and HIRESW show the main area of thunderstorms moving through between 4-6 PM. Here’s the 12z HRRR at 6 PM—

I’ve also updated the table below with the latest HRRR representative values. Notice that the severe parameters have decreased, especially shear values.

One parameter that stands out today is the hail parameter, unique to the HRRR model. It’s a parameter that can’t be always taken literally, but today’s hail parameters show the possibility of large size hail—

Update Mon @ 8:37 AM — Just a few thoughts about today’s weather. Can anyone remember an outbreak of thunderstorms where the term ‘severe’ wasn’t used?

From a liability point of view, I think the NWS and TV weather people use the term just a bit too frequently. (But who’s not going to say when there’s any lightning and wind involved that somewhere, it might be severe?) I think you get my point. Let me be more specific regarding today’s forecast.

I see a high probability of strong storms and maybe even ‘severe’ weather today. But let’s compare a few severity parameters to those from the relatively recent tornadic outbreak of this past September 1st.

As you can see, there’s ‘severe’ and then there’s SEVERE of last September. Today will be different.

It’s sorta like the radio world where all children are ‘above average’ and all thunderstorms are severe.

I’m sure there’s going to be some big storms today. But I think it’s important to keep it all in perspective. While anything is possible with thunderstorms, based on today’s current forecast parameters, I don’t think there will be any tornadoes. This will likely just the first of many summertime ‘severe’ thunderstorm days. I’m glad I got this off my chest. 🙂

Updated Mon 7:36 AM —Unstable air and a cold front will combine to form strong thunderstorms today, Monday.

Additionally, low pressure systems are expected to form ahead and along the front, enhancing the duration of the rain. While the main front is expected to pass through about 5:30 – 6:00 PM, low pressure will develop causing thunderstorms to break out as early as 1-2 PM; additional rain may last until 8 PM. (This event will not be a single thin line of storms, based on current models.)

The morning high resolution shorter range models 24 hour forecasts will available shortly after 10:30 AM. I’ll update this post later this morning.

UPDATE 9:50 PM – HUGE FORECAST CHANGE: Tonight’s 00z models are becoming available. (The 00z models is one of the two model daily runs that include direct upper air measurements done with weather balloons [radiosondes].)

If you’ve been following the forecast, the timing of the frontal passage has been pushed further up with each model run. The latest HRRR and RAP 00z models shows the front significantly stalling as it tries to move through the Philadelphia area tonight.

The latest: low pressure is expected to develop along this stalled front and move up over Philadelphia and southern NJ overnight and through tomorrow. Most of the thunderstorm activity may stay to our west overnight or just make it into Philadelphia about 1-3AM.

Additional rain expected to develop with this low pressure system over our area in the early morning hours and through much of tomorrow. The forecast has changed significantly. Too early to put it all together. But the RAP and HRRR area forecasting rain for much of early Thursday! (This afternoon’s GFS had hinted at this change.)

The higher resolution HRRR keeps the heavy storms just to our west tonight. The RAP (which uses GFS input data), has the storms making it to PHL, followed by significant rain on Thursday.

Update Wed @ 3:53 PM — Latest HRRR 18z available. Forecast still on track. A couple of isolated storms late this afternoon. Rain and thunderstorms, possibly severe, move into the immediate PHL area around 11PM – 2 AM, earlier north and west, in line with this morning’s NAM and HIRESW models. Rain and storms last through the night. Helicity and shear values lower than previous model runs, making the chance of tornadoes in our area unlikely. Still strong/severe storms possible with total rain amounts somewhat higher, near 1 inch, possibly 2 inches.

Strong storms are expected tonight. Most of the forecast remains unchanged from last night’s post. As usual, there are differences in the models from this morning.

To make things easier, I’m going to do a straight HRRR model forecast and then indicate where things don’t all agree.

Most of the thunderstorm activity breaks out between 9 PM and 1 AM with the maxima around 11-1 AM. (The latest HRRR does not show much activity in the afternoon, BUT the HIRESW models continue to show scattered storms this afternoon after 3 PM.)

The difference in forecasts between having strong/severe thunderstorms and extreme events like tornadoes are physical dynamics that are parameterized as helicity and vertical shear in numerical weather prediction models.

During the period between 9 PM and 2 AM, the HRRR has notable areas of moderately high helicity and high wind vertical shear. (As per my post last night, the helicity values are 350 m2/s2 compared to last week’s 1000 m2/s2 values; these are significantly less.) My personal feeling is that strong/severe thunderstorms are likely, but tornadoes are possible but not likely.

As for wind shear, there is a concerning level of high vertical shear about midnight in areas north of the city. This would translate into severe thunderstorms with high straight line wind gusts, hail and heavy rain.

While I have boxed in the area of highest shear, strong storms could form in other areas.

Heaviest rain areas—

It should be noted that the NAM and HIRESW version models don’t agree on the above HRRR forecast. The principle differences for these models is that they have the heaviest rain and storms staying mostly to our north and west and have weaker showers/thunderstorms moving in after midnight and continuing until about 3-4 AM in the PHL area. They also have some scattered storms in the late afternoon today.

Stay tuned.