Monthly Archives: September 2024

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

Leave a comment

#Philadelphia #weather #PAwx

Huge Forecast Change?
Tue 06:02 PM Update — The NAM-Nest still has the storm further away off the coastline and continues with little rain for us. The latest Canadian RGEM keeps the rain away and off the coast. The HRRR also has almost no rain here. The AI version of the ECMWF has only light rain here. It looks like forecast of the ECMWF posted at 4:56 PM with the heavy rain is an outlier.

Here’s the latest GFS 30 hour forecast—

18z 09-17-24 GFS forecast accumulated rain through 30 hours -8 PM Wednesday. Rain misses our area. (Click on image for a larger view.)

Posted Tuesday 09/17/24 @ 4:56 PM — It’s been clear that this week’s weather would be low confidence/high uncertainty. The latest ECMWF has just come in and it has a huge change in the forecast. I wanted to get this posted; many other models have not yet become available and I’d like to compare it with the latest GFS, which won’t be available until around 6 PM.

The latest ECMWF completely changes its forecast. The coastal low, expected to be off the coast is now forecast to be right on the coast. Rainfall totals over 3 inches are now forecast by Thursday!

12z 09-17-24 ECMWF total rainfall forecast by Thursday morning. (Click on image for a larger view.)

I’ll be reviewing other models and will update this forecast early this evening.

This Storm will Bypass our Area

Posted Tuesday 09/17/24 @ 9:23 AM — As has been the trend over several days, the main area of rainfall from the remnants of the tropical system will bypass the Philadelphia area, and a second low pressure system develops somewhere off of the NJ coastline.

The reason for the storm missing us— high pressure is expected to nose down in western PA and block its movement towards us.

The latest GFS has almost no rain in Philadelphia through Thursday, while the ECMWF and several other models show as little as 0.10 to 0.20″ .

The latest forecast models not only block the movement of the storm towards us, but they also block the exit of the storm from the western Atlantic. As a result, the storm is forecast to rotate and meander around itself off of the NJ Coast for several days.

The caption on the graphic below explains the situation—

06z GFS forecast for Wednesday evening. Low pressure off the Delmarva coast will rotate around itself and meander through Friday. It is forecast to retrograde back towards the coast. Meanwhile, high pressure over western NY and Pennsylvania will nose down into Virginia, blocking the rain shield from our area. (Click on image for a larger view.)

With a meandering system rotating around itself, uncertainty with this forecast is high regarding the location of the low.

The ECMWF shows high uncertainty in the position—

00z ECMWF ensemble surface pressure standard deviation. Uncertain position of the low late Thursday. (Click on image for a larger view.)

With uncertainty in the position of the low, any forecast through Friday has lower than usual confidence.

Posted Monday 09/16/24 @ 6:19 PM — The forecast track for this storm has changed and a secondary low is forecast to develop somewhere off the coast. The latest GFS reverts back to a minimal rain event for our area—

18z 09-16-24 GFS total rainfall (Click on image for a larger view.)

The latest ECMWF is somewhat similar. Some of the higher resolution models have about 0.40 to” 0.70″ of rain (RGEM, NAM-NEST, HREF)

The good old NAM still is forecasting heavy rain here, likely not correct—

09-16-24 18z NAM model total rainfall forecast (Click on image for a larger view.)

Still an uncertain forecast with lack of consistency. Stay tuned.

Posted Monday 09/16/24 @ 8:36 AM — The storm track of Tropical Cyclone 8 (not yet named a tropical storm) has changed and slowed somewhat from yesterday, according the the National Hurricane Center. As a result, the forecast for our area remains uncertain.

The “ensemble” models statistically capture uncertainty as an increase in standard deviation of their model members. The latest ECMWF ensemble shows high uncertainty in the position and track of the expected extra-tropical low on Wednesday.

While several models still maintain our area in a precipitation ‘hole’, the latest GFS has shifted some of the heavy rainfall back into our area—

06z GFS total rainfall by Wednesday 2 PM (Click on image for a larger view.)

Further changes are expected. Stay tuned!

Posted Sunday 09/15/24 @ 7:17 PM —The latest GFS has become available and the trend is for much less rainfall here. In fact, for much of our area, the GFS has almost no rainfall this week.

I heard the 6 PM forecast on TV, however, I don’t see “flooding rains” in the immediate Philadelphia area. (The TV weather people are big ECMWF model fans, which is still predicting somewhat heavy rainfall here.)

18z GFS forecast rainfall through Thursday 2 PM. (Click on image for a larger view.)

When it comes to tropical systems, forecasts often change. Stay tuned.

Originally Posted Sun 5:42 PM —It’s been many weeks since any substantial rain has fallen here. The tropical system off the North Carolina coast (Tropical Cyclone Eight) had been expected to bring us much needed heavy rainfall, however, recent trends, as discussed in this past Weekend Weather, show the rainfall forecast in the immediate Philadelphia area to fall short of what’s needed.

Here’s the current water vapor image—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Based on the models, I’ve drawn the approximate track of this storm. The storm is blocked from northward movement by persistent high pressure. (Click on image for a larger view.)

There remains a range of rainfall totals, with each model having a somewhat different take on things.

The latest model blend (NBM) captures the trend, with the heaviest precipitation bypassing the immediate Philadelphia area—

18z NBM forecast total rainfall through Thursday at 2 PM. (Click on image for a larger view.)

The time frame of the storm is outside of the higher resolution model ranges at this time (60-84 hours). Changes in the forecast are still likely.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

Storm Update

Posted Sunday 09/15/24 @ 9:55 AM — A change in the forecast appears to be developing. First, cancel the GFS forecast from yesterday; it wasn’t believable and now most of it is no longer in-play.

The majority of models have come together (finally) with the an inland, northwestern storm track, but then have the storm blocked from further northward movement due to persistent blocking high pressure.

I’ve drawn the consensus track on the current water vapor image—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. HIgh pressure blocks the northern movement of the low pressure system. (Click on image for a larger view.)

Unfortunately (and incredibly), several models have the main areas of rain blocked from reaching the Philadelphia area! At best, only 0.30 to 1.0 inches of rain will fall in our almost drought-stricken area, with most areas in the lower range.

Here’s the latest GFS total rainfall through Thursday morning—

06z GFS total rainfall through Thursday morning. (Click on image for a larger view.)

Undoubtedly, the forecast is likely to change again but this trend towards less rainfall here has been showing in an increasing number of models.

It should be noted that the ECMWF supports this trend for less rainfall here. The latest ECMWF-AIFS still has considerable rainfall for us but at this point, it’s an outlier.

Stay tuned

Potential Storm Update

Posted Saturday 09/14/24 @ 9:02 PM — I’ve rarely seen the model forecasts as strange as those for this upcoming week. A tropical system, maybe semi-tropical, will develop in the Atlantic and move into North Carolina. The speed of movement, the path, the intensity and the forecast beyond that point is a full range of possibilities. No single model forecast really is fully believable.

Case in point is the latest GFS which has it moving inland Monday morning, then dissipating, then re-forming again off the Delmarva coast, then moving westward again Friday evening.

GFS Monday Morning—

GFS forecast as a tropical system due to a warm core (orange thickness circle in center of low shows warm core)

GFS Friday Evening Forecast—

GFS storm path from Monday morning to Friday evening. (Click on image for a larger view.)

Other models end up with an inverted trough, a closed upper low, or another surface low pressure system or combinations of those three.

What the models have in common:

  • Blocking high pressure will influence the path of this storm and slow its exit
  • Rain starts in our area Tuesday afternoon or evening.
  • Heaviest rainfall Wednesday.
  • Rainfall may linger on again off again into Friday.
  • Total rainfall here may be as little as 0.75″ or 2″-3+”

More info tomorrow.

Rain Next Week!

Posted Saturday 09/14/24 @ 10:45 AM — Following a wonderful weather weekend, we may finally be getting some much needed rain next week.

The models are coming somewhat closer together in forecasting a semi-tropical system to develop off the North Carolina coast and then slowly move towards us.

However the track, speed and development of this system still varies from model to model. Most models, despite differences in speed and track, have rain here by Tuesday evening into Wednesday and possibly beyond that.

The latest GFS has the storm moving inland—

06z GFS forecast for Monday. (Click on image for a larger view.)

The latest ECMWF-AIFS has joined the rest of the models with more of a coastal storm and a coast track—

06z ECMWF-AIFS forecast for Wednesday at 8 AM (Click on image for a larger view.)

The latest German ICON model is fastest and has a track between the ECMWF-AIFS and the GFS—

06z ICON model forecast for Tuesday at 8 PM (Click on image for a larger view.)

Either way, it’s looking more likely that we’ll get some much needed rain this coming week.

Stay tuned for updates.

Food for Thought

Update Fri 9/13 9:27 PM — The forecast for a tropical storm affecting our area next week remains highly uncertain. Here’s the latest ICON model. Just food for thought at this time—

18z ICON model forecast for Wednesday 5 AM. Just food for thought.

Originally Posted Fri 6:00 PM —

High pressure continues to block the rain and moisture from what was hurricane Francine from moving towards us. Sunny skies and warm temperatures to continue here for the weekend.

Meanwhile energy and moisture associated with Francine is forecast to become incorporated in a potential new tropical system that moves towards North Carolina early next week—

12z GFS forecast for Sunday at 2 PM. Tropical system is expected to move northwestward, according to the GFS. (Click on image for a larger view.)

The models are all over the place regarding this forecast, with the ECMWF, ensembles and the Canadian Global only showing an inverted trough with some rain here by Wednesday, while the GFS and the German ICON model show a distinct tropical-type system to possibly affect our area.

The ECMWF AIFS. which I started using this past week, has been very inconsistent and shows a distinct system far off the coast. So much for artificial intelligence.

The GFS is known to over-forecast tropical development, so there remains much uncertainty with this forecast. I don’t see any unifying forecast at this time, but many models have some rain here by late Tuesday into Wednesday. Low confidence.

I’ll be updating over the weekend. Stay tuned.