THIS WEEK’S WEATHER

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—

MRMS rainfall through 7 PM (Click on image for a larger view.)

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


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—

RADAR at 3:45 PM with superimposed vertical velocity pressure from RAP model (Click on image for a larger view.)

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


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.


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 —

12z HRRR total rain forecast. These model distribution of the rain should never be taken literally. The immediate Philadelphia area is in the red circle. (Click on image for a larger view.)

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


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—

06z HREF precipitation totals. (Click on image for a larger view.)

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 655406
Vertical Shear
1/sec
40-4640-4518-20
Precipitable Water 2.7″0.83”2.0″⇑ ⚑
Lifted Index
º K
minus 6minus 9.3ºminus 3.3⇩⇩
HRRR Hail
inches
1.91.41.5
Peak Wind Gusts
mph
40-5040-5035.5
Storm Motion Shear Vector
Alignment
Aligned – ~ 90ºAlmost alignednon-aligned
250 mb
Jet Stream Wind
mph
6313542⇩⇩
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity
↓ indicates works against Severity ⇩ Significant, but less impact


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.


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’s 18z HRRR total rainfall forecast for Monday. The exact placement of these heavier rain bands should never be taken literally. (Click on image for larger view.)

Sunday afternoon’s HREF model—

18z HREF total rainfall through 8 PM Monday. (Click on image for a larger view.)

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

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


WEEKEND WEATHER FORECAST

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

Sunday Forecast Update — Monday Outlook

Update Sun 06/11 @ 9:01 AM — Today, we’ll finally see some summer-like weather. High temperatures 89º (Blue Bell) to 91º (Philadelphia). Some areas will experience some cloudiness at times during the afternoon today, but “partly to mostly sunny” is the standard terminology.

Dew points, according the the NBM, will be in the upper 50s instead of the upper 60s (as forecast yesterday), so it won’t be as humid as previously predicted.

Monday looks to be “interesting”, as a cold front moves through with much needed rain. Things are looking better for a good soaking in many (but not all) areas. The Canadian models are treating this as a warm front in the morning, followed by a cold front in the evening.

As a result, the RGEM and HRDPS have been predicting some shower activity as early as the late morning, followed by thunder showers from the afternoon through 9 PM.

Our NOAA models are underplaying the warm front and emphasizing the strong cold front, with showers and thunderstorms, from mid afternoon into the evening.

06z HRRR total accumulated rain forecast by Tuesday morning. Every model will show a very different range and areal coverage of precip, so don’t take the locations of the higher totals literally. (Click on image for a larger view.)

Precipitable water (PWAT) is in the 1.7″ to 2″ range, so some areas may get rainfall in that upper range. Generally 0.6″ to 2+ inches of rain is expected.

No tornadic activity is expected, and while strong storms are possible, many of the severity parameters are in the moderate range.

(I’m sure there will be official “severe thunderstorm warnings”, since there are liability issues in not issuing these. If you or your house gets hit by lightning, I guess it’s ‘severe’. )

Severity parameter forecasts have a strange way of dramatically increasing the day of the event, so I’ll be keeping an eye on this.


Saturday-Sunday Forecast Update

Update Sat 06/10 @ 9:28 AM — There’s some uncertainty with the amount of cloud cover for Sunday afternoon. Last night’s models showed only high level cirrus cloudiness, but several models are suggesting some periods of some mid and low level clouds (with sunny breaks) by mid afternoon.

The Canadian HRDPS is also showing the possibility of widely scattered showers/thundershowers far north of the city by 3-4 PM

Today’s 06z HRDPS forecast rain rate for 3-4 PM Sunday. (Black-grey = clouds). (Click on image for a larger view.)


Previously Posted Fri 5:30 PM —

Following the passage of an upper air trough through our area Friday evening, high pressure in the western Atlantic will circulate warmer air into our area over the weekend—

Today’s 12z ECMWF shows the departing upper low (blue L arrow) and the Bermuda High (H) in the Atlantic and its southwesterly flow bringing in warmer (and cleaner) air for the weekend. Out in the Midwest is our next system to affect us on Monday. (Click on image for larger view.)

As mentioned last week in several posts, there appears to be a breakdown happening in the long-standing Omega block (Ω) that has been responsible for our cooler and dry weather for the past 5 weeks.

The new pattern, expected by the end next week, will likely be much warmer for us, but it’s unclear that it will become much wetter as well.

Indeed, Monday’s cold front that should bring some significant showers and thunderstorms to areas west of us, but may not bring all that much rain to Philadelphia and New Jersey. We’ll have to see.

Saturday

Mostly sunny.

High temperature 80.4 sd 2.0º ( NBM model— location Blue Bell, PA)

Sunday

Mostly sunny in the morning, gradually increasing cloudiness in the afternoon as some moisture from the system in the Midwest moves in aloft.

High temperature 88.0º sd 1.5º ( NBM model— location Blue Bell, PA)