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Update Fri 06/16 @ 1:28 PM — The rain moved in about 1 hour earlier than I had forecast.

Some additional showers possible later this afternoon.

An early look at the MRMS shows many areas in Philadelphia and eastward had 1-2″ of rain. The MRMS works on an hour so so delay, so I’ll post it later to provide a meaningful overview of the rain we received.

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Live Blog Experiment

Today’s live blog was an experiment. It’s complicated to do and it won’t be a regular feature. But for some serious future weather events, I’ll consider activating it again.

Friday’s Storms

I’ve made several updates in a in short period of time today. You may need to refresh your browser to see the updates.

Update Fri 06/16 @ 9:28 AM —

Latest water vapor image shows the upper air disturbance in West Virginia that will rotate into our area by 1 PM. Dotted blue contours show areas of maximum moisture convergence—

Update Fri 06/16 @ 9:24 AM — Latest HRRR (12z) just available shows 1PM maximum in instantaneous precipitation rate (PRATE) just west of Philadelphia —

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Update Fri 06/16 @ 8:28 AM — HRDPS forecast instantaneous precipitation rate at 1 PM—

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Update Fri 06/16 @ 8:16 AM — Current Radar with likely track of heaviest rain—

The latest HRDPS has joined last night’s HRRR in its forecast placement of the heaviest rain. It appears to be in South Jersey—

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Updated Fri 06/16 @ 8:02 AM —Fast Update: Not much change in the forecast. There’s still lack of clarity regarding the exact location of the heaviest storms.

For Philadelphia and the immediate western suburbs, the 12-2 PM time frame appears to be the period of maximum storm intensity. A second batch of storms possible 5 PM to 9 PM. There seems to be some emphasis on maximum rainfall just east of the Delaware River into Southern NJ.

I’m preparing some graphics…more info coming.

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Friday’s Storms Update

Update Thu 6/15 11:03 PM — No significant change from the earlier forecast. The action starts as early as 10-11 AM in the western suburbs. Peaks about 1- 4 PM. Tonight’s models haven’t made it any easier to identify the areas that will receive the heaviest rain. Six models, six areas forecast for the heaviest rain.
Trends— the storms may come through in two groups. PWAT values have increased. CAPE and shear values have also increased suggesting stronger storms.

Updates tomorrow morning.

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Friday’s Storms Update

Updated Thu 06/15 @ 5:33 PM—

The high resolution models continue to forecast thunderstorms and rain for Friday, starting late morning (10-11 AM) in western suburbs and peaking between 1-4 PM in the city and adjacent areas of Southern NJ as a result of surface low pressure being spawned by an upper air low pressure system.

Due to low level moisture convergence, areas of very strong vertical motion and reasonably high PWAT (1.7″), the high resolution models crank out very heavy rainfall in some areas, on the order of 1″-3.5″.

(It should be noted that not all the models are predicting very heavy rain: the NBM and the GFS are showing as little as 0.4″ to 0.8″ of rain. )

As always, those models predicting the heavy precip differ in their forecast location of these heavier bands of rain. The HRRR has its maximum from the city eastward into South Jersey while the HRDPS and NAM-NEST have areas just to the west of the city receiving the heavier rain.

Much of this heavy rain will be the result of thunderstorm activity. While CAPE values are in the 1000 joules/kg range (moderate), moderately high levels of vertical shear (24-30/sec) and areas of low to moderate helicity (near 400), there is significant vertical motion dynamics forecast that will result in some strong, possibly severe storms.

NAM-NEST—

HRDPS—

The latest HREF, just available, probably captures the best compromise, but may still be over forecasting the rain—

Rain Friday, Possibly Heavy

Updated Thu 06/15 @ 9:03 AM —

The persistent upper low over eastern Canada is forecast to spawn a surface low near our area on Friday. It looks like rain moves in during the late morning.

The models continue to show low pressure that may cause heavy rain in the immediate Philadelphia area, starting late morning (10-11 AM) and lasting through the afternoon. Instability will add thunderstorms to the mix. Severity levels to be posted later today.

As is always the case with precipitation, the exact location of the heavy banding of rain/thunderstorms isn’t often captured accurately by the models. Over the past 12 hours, there’s been wide range of locations forecast to receive the heaviest rain. (On the order of 2-3 inches!)

I’ve recently been impressed with the Canadian models for this current weather pattern.

Here’s the latest Canadian HRDPS model forecast for total rainfall on Friday—

Updates later this afternoon.

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Thursday – Friday Outlook

Update Wed 06/14 @ 4:31 PM— The system that moved through Wednesday was even slightly faster than the faster HRDPS had predicted. Sunshine broke out about 3-4 PM.

A beautiful day on tap for Thursday with temperatures 80º to 81º.

The persistent upper low over eastern Canada is forecast to spawn a surface low pressure system near our area on Friday. It looks like rain moves in during the morning.

This afternoon’s HREF and NAM-NEST are showing areas of heavy rain possible in and around Philadelphia. I’m not sure if that forecast will hold—

For Saturday, spotty instability showers possible during the afternoon.

Sunday looks sunny.

It looks like the blocked pattern will remain through most of next week. I know I said last week we’d get a weather pattern change by the the middle of this week, but I didn’t think it would be just an eastward shifted block. We have gotten a change in the weather this week with several days of showers and rain.

It now looks like the blocked pattern will flatten by the end of next week, according to the GEFS. Don’t hold me to it.

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Wednesday Radar

Rain on the way. Total rainfall amounts expected to be about 0.25″ in most areas, with some localized higher amounts.

Current radar at 8:42 AM showing RAP model superimposed data—

Wednesday Forecast Update

Update Wed 06/14 @ 8:24 AM — The earlier noontime start to the showers now appears quite likely. In fact, some showers may move into western suburbs after 11 AM. There’s an increased likelihood of showers and some scattered embedded thunderstorms from 2 PM to 5 PM.

The latest HRDPS is faster with this system and has the much of the activity exiting by 4 to 5 PM with sunshine returning, while the NAM-NEST keeps some shower possibility until about 7-8 PM. I’m leaning towards the faster HRDPS.

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Showers & Thundershowers Wednesday

Update Tue 06/13 @ 7:44 PM — An upper low in NY State (part of the new Omega Block) will rotate an mid level (500 mb) disturbance through the Philadelphia area on Wednesday. The upper disturbance will be enhanced by southern jet streak energy—

The combination of the jet streak energy, the mid level (500 mb ~18,000 ft) vorticity disturbance and thermal instability will set off scattered showers and thunderstorms.

Clouds move in late morning. Showers and some thundershowers possible as early as noontime, but most models have scattered showers and thunderstorms most likely at 4-5 PM and possible into the early evening. There will likely be some breaks of sun in the afternoon. Any sunshine will act as fuel for more thunderstorms later in the afternoon.

Right now, severity parameters appear to be mid level to low.

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Yet Another Omega Block Ω

The forecast for this week has changed from just a few days ago. What had been expected to be a flattening of the long standing Omega Block has instead re-established itself with the block’s main axis shifted 30º more eastward than the previous block.

Additionally, a strengthening of the southern jet stream will introduce more moisture into our circulation than the previous several weeks. (The flattening of the block is now expected next week.)

What this means will be another week or so of cooler than average temperatures but with unsettled weather, and with a chance of more rain showers as low pressure at the surface develops where the northern and southern flows come together near the Carolinas—

Between the two low centers, a persistent trough will develop through our area, leading to the development of instability showers. We’ll see the first of these on Wednesday. Details later today

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Rain Totals

Update Mon 06/12 @ 8:16 PM — The rainfall totals were somewhat similar to the model forecasts in a general way. The models captured the rapid decrease of rainfall in New Jersey and this morning’s emphasis on a westward maxima was somewhat correct. Even the rain maxima in Delaware county was somewhat captured, although no model had that rain location as the local maximum.

It was much needed rain!

While it’s still raining in Bucks county and northeastward, here’s the MRMS estimate of rainfall that fell today through 7 PM—

Good news…there’s more rain possible on Wednesday. More about that tomorrow.

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Monday Storm Update

Update Mon 06/12 @ 4:02 PM — The rain arrived before 3 PM and the current radar suggests that the heaviest rain axis may be further east, through Philadelphia as forecast last night’s models, instead of a more westward track, as forecast with today’s models.

Demonstrating this is the current MRMS radar composite with superimposed vertical velocity pressure (white box) from today’s RAP model—

Generalized severe weather still doesn’t appear likely, but some severe localized cells always possible. We could definitely use the rain.

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Monday Storm Update

Update Mon 6/12 12:54 PM — So my 12:12 update indicating a later start and a westward axis of rain appears to be an error of inadequate model spin-up time. I’m going to lean back towards the earlier model run forecasts with a 2-3 PM onset and heavier rains extending into Philadelphia. I guess we”ll see. We need the rain.

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Update Mon 06/12 @ 12:12 PM —The latest models are moving towards a more westward position of the heaviest rain. Additionally, the rain is now expected to move in at 3-6 PM range rather than the earlier start time.

This trend is more in line with the GFS, ECMWF and ICON models, but I’m not sure if the change in the mesoscale models is just a manifestation of inadequate model spin-up time.

Here’s the latest HRRR precipitation forecast —

The latest live MRMS with superimposed RAP model moisture convergence and omega (vertical velocity) supports this westward emphasis—

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Monday Storm Update

Update Mon 06/12 @ 9:08 AM — The trends listed last night continued with the 06z model runs early this morning.

There are large differences in forecast rainfall between the low resolution global models (which tend not to have explicit convection (thunderstorm) parameterization) and the high resolution mesoscale models which do.

This event will have significant convection and I’ll be leaning towards the HRRR, NAM-NEST, HRDPS and HREF models with explicit convection.

Timing: light showers, as forecast by the HRDPS yesterday are moving through the area this morning. Showers and thundershowers move in to western suburbs as early as 11 AM to noon. Heavier activity moves in 2-3 PM and continues with a peak between 5 PM and 7 PM.

Precipitation Amounts: Heavy rain from this system appears to be the major component. Here’s the latest HREF model total precip forecast, which is on the higher end of the HRDPS and HRRR—

Severity— As mentioned yesterday, the severity parameters are in the moderate range with some very localized moderate-severe in a few locations between 5-6 PM. Wind gusts are not forecast to be as high as other recent events. That doesn’t mean severe weather can’t happen. Overall, severity possibilities are lower than recent severe events.

HRRR Severity Parameter	Sept 1 , 2021 Tornado Outbreak (example of highly severe)	Recent April 1st 2023 Tornadoes (Example of severe)	(06-12-23) Today’s 06z HRRR Forecast	Impact
CAPE Joules/kg	3500-4200	900-1100	900-1100	⇩⇩
Helicity m^2/s^2	1350	655	406	⚐
Vertical Shear 1/sec	40-46	40-45	18-20	⇩
Precipitable Water	2.7″	0.83”	2.0″	⇑ ⚑
Lifted Index º K	minus 6	minus 9.3º	minus 3.3	⇩⇩
HRRR Hail inches	1.9	1.4	1.5	⚑
Peak Wind Gusts mph	40-50	40-50	35.5	⚐
Storm Motion Shear Vector Alignment	Aligned – ~ 90º	Almost aligned	non-aligned	↓
250 mb Jet Stream Wind mph	63	135	42	⇩⇩

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Monday — Finally Some Rain!

Update Sun 6/11 10:54 PM — A quick update. Tonight’s early models have the following trends—

1. High resolution models have the heavy rain through Philadelphia.
2. Low resolution models keep the heaviest rain west of Philadelphia
3. As forecast by the earlier HRDPS, some showers possible in the morning.
4. Several models have the heaviest rain in the evening.
5. Several models have significantly less rain in New Jersey.

Updates tomorrow morning.

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Previously Posted Sun 5:56 PM —

Over the past week, the models have been advertising a significant rain event for us on Monday with a frontal passage. If nothing more, one should be impressed that this weather event has been consistently forecast almost a week in advance.

The real grit lies in the details, and there’s increasing evidence that much of the area will get 0.6″ to over 2.0 inches of rain, much more rain than most areas have seen in over a month.

As is often the case, the models’ grasp on the forecast severity parameters (vertical shear helicity CAPE PWAT Lifted Index ) tend to become more meaningful within 18- 24 hours of the event. That’s been the case with tomorrow’s event.

Here are the trends—

• There has been a general trend towards higher amounts of rain, both in eastern PA and NJ.

• There has been a general increase in what I call the severity parameters. However vertical shear values have remained at the low end.

• The actual cold front comes through about 9 PM (Blue Bell) but most of the rain comes well ahead of the actual front.

There’s still a range of model timing with the Canadian model having a line of thunderstorms move through as early as 11 AM to 3 PM, with a second line moving through about 6- 8 PM with the cold front.

Our NOAA models (e.g. HRRR) have scattered storms from about 2 PM through 10 PM with more emphasis on late afternoon and early evening for the heavy activity.

Latest HRRR—

Sunday afternoon’s HREF model—

Check back on Monday for updated forecasts and my “severity table”.

#weather #paweather #wx #pawx #philadelphia #phillywx

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