Category Archives: Philadelphia Weather Outlook

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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#Phillywx #Philadelphia #weather #PAwx

Week’s Weather Update- Philadelphia Marathon Outlook

Updated Thu 11/16 @ 5:22 PM — The approaching cold front Friday will bring clouds and should be considerably cloudy (high level cloudiness- cirrus) between 1 and 3 PM from the city westward.

High temperatures on Friday will be 65º (Blue Bell) and 67º (Philadelphia) with a higher than usual uncertainty of ± 2.4º due to differences in cloud cover forecasts.

As has been the forecast trend, our immediate area will receive little to no rain from this system. Here’s the latest NBM rain totals—

Today’s 18z NBM accumulated rainfall (Click on image for a larger view.)

The forecast for the Philadelphia Marathon on Sunday is mostly unchanged.

Sunday Philadelphia Marathon Outlook — Mostly sunny early. Temperatures somewhat colder at the start- 37º early, rising to 46º in the city between 8 AM and 11 AM. Highs low to mid 50s. Winds 11 mph gusting to 20+ mph by late morning from the W.

The longer range forecast shows a good chance of rain on Tuesday.

Something that I’ve mentioned over the past year: NOAA has been developing a new model, the RRFS (“Rapid Refresh Forecast System“) which is going to be beginning of a major advance in high resolution weather forecasting. The model will run hourly, will have a 3 km resolution, and its forecasts will extend out to 60 hours. It will also have an statistical ensemble component.

The RRFS takes much of its physics from the HRRR and will use the FV3 core used in the recently updated GFS.

This RRFS is going through a series of testing and fine tuning and the groundwork is being laid for it to be operational around September 2024.

Of great relevance, about 6 months after it is released, many of the models I look at regularly (NAM, NAM-NEST, RAP, SREF, all three flavors of the HIRESW) will cease operation.

My mantra “Never Ignore the NAM” will become a thing of the past.

I’ve always felt there are “too many models” and it appears that the RRFS is incorporating all the best approaches to numerical weather modeling that have been distilled out from these earlier models.

NOAA/NWS has been making the development version and their testbed variants available on a NOAA- Amazon Web Services site (AWS) for some time.

In recent weeks, I’ve been experimenting with downloading these RRFS runs. AWS has certain restraints that the main NOAA NOMADS site doesn’t have, along with different directory structures (buckets). Nonetheless, I hope to be downloading and using the RRFS model in some winter forecasts soon.

The RRFS version 1 release will truly be a big deal. I’ll be posting more information in coming weeks.

Week’s Weather Update- Philadelphia Marathon Outlook

Updated Wed 11/15 @ 8:56 PM — We had a bit more cloudiness than forecast today.

Thursday should be quite nice, with sunny skies and warm temperatures, 63º in Blue Bell and 65º in Philadelphia.

Friday will be mostly cloudy by noon but still mild.

As discussed previously at length in previous updates, it does appear that there will be little rainfall in Philadelphia with the passage of a cold front Friday night. At most, the models, including the ensembles, crank out 0.10 inches and many show even less rain.

Saturday clears in the morning. Very windy conditions expected.

NAEFS forecast for 7 AM Saturday with previous 6 hour accumulated rainfall. (Click on image for a larger view.)
Sunday Philadelphia Marathon Outlook — Mostly sunny early. Temperatures somewhat colder at the start- 37º early, rising to 46º in the city between 8 AM and 11 AM. Highs low to mid 50s. Winds 13 mph gusting to 20+ mph by late morning from the W.
Week’s Weather Update- Philadelphia Marathon Outlook

Updated Tue 11/14 @ 5:57 PM — No change in the forecast since this morning. Thursday and Friday now look dry. The “ensemble” models (GEFS, NAEFS, GEPS) still crank out about 0.20″-0.40″ of rain Friday night into Saturday morning. The “deterministic” models (ECMWF, GFS) have the energy transferring to the low pressure system east of us, with little or no rain here. The forecast of the Philadelphia Marathon on Sunday remains unchanged.

Updated Tue 11/14 @ 9:12 AM — As has been the case many times over the past several months, the dip in the jet stream forecast for this weekend is now expected to be less amplified and the coastal system (and its associated rain) may totally miss us as the energy from the approaching cold front skips over us to our northeast. The latest ECMWF and ICON models support this scenario—

00z ECMWF shows the storm to our east with later secondary development to our northeast. The rain misses us! (Click on image for a larger view.)

We could really use the rain. If this keeps up, could we have another almost snowless winter?

Sunday Philadelphia Marathon Outlook — With a less amplified trough, expect more sunshine and less wind than my previous forecast. Mostly sunny early. Partly cloudy in the afternoon. Temperatures 39º early, rising to 46º in the city between 8 AM and 11 AM. Winds 10mph gusting to 20mph by late morning from the WNW.
Week’s Weather Update- Philadelphia Marathon Outlook

Updated Mon 11/13 @ 7:33 PM — The latest models have slowed down the movement of the coastal system as it interacts with the trough and cold front. It now appears that clouds, originally expected to move in on Thursday will move in on Friday.

With less cloudiness, high temperatures for Thursday will be in the low 60s and probably mid 60s by Friday.

The amount of rainfall with the next system, Friday into Saturday, now appears to be less than previously thought, with some models having the rain skip past us. The Ensemble models are forecasting about 0.40 inches of much needed rain, but the GFS is less impressive and the Canadian even less so.

Here’s the latest NAEFS with GEFS precip, just available, for Friday night (1 AM Sat) —

Combined 18z NAEFS (mode version) forecast with Bias-Corrected GEFS model 6 hour precip for Friday night 1 AM Complex interaction of coastal system with approaching upper trough and cold front (blue line) (Click on image for a larger view.)

The interaction between the trough and coastal low appears to be be a challenge and it’s too early know how it will play out.

Sunday Philadelphia Marathon Outlook — Here’s the current outlook for Philadelphia, Sunday morning. Partly sunny early. Increasingly cloudy by late morning. Temperatures 39º early, rising to 46º in the city between 8 AM and 11 AM. Quite windy, especially by late morning. Chance of a sprinkle early afternoon.

Updated Mon 11/13 @ 9:09 AM — Nice weather continues through Thursday with a slow increase in high temperatures—

Today’s 12z NBM shows high temperatures somewhat steady until Thursday. (Click on image for a larger view.)

It looks like clouds move in Thursday afternoon ahead of the next system and cold front. There’s a range of model forecasts, depending upon how deep the plunge of cold air makes it southward—

Today’s 06z NAEFS forecast for Saturday morning shows a moderate plunge of cold air (red arrow and red 540 thickness contour) and the track of the coastal storm (white arrows) (Click on image for a larger view.)

The sharper the trough dip, the more westward the storm track and the longer it affects our weather on Saturday. It will also greatly affect whether we get a soaker or another light precipitation event.

Too early to see a trend with this storm, but the recent history over past months is for the storm to be eastward, therefore, less rainfall and some clearing on Saturday afternoon.

Previously Posted Sun 6:52 PM —

Large sprawling high pressure will dominate our weather through late Thursday. Nothing much is happening here, weather-wise. Continued fair with gradually moderating temperatures into the end of the week.

Commentary — As much as I enjoy nice weather and I enjoy following the weather, it’s periods like these where I’d hate to be a full time meteorologist, on-TV or otherwise. (Ok, they make good money.)

There’s simply nothing happening; the weather has been uneventful for several weeks and will remain that way this week.

How long can one harp on normal-range colder temperatures or want to be thanked for forecasting a warmup later in the week?

Now that I got that off my chest, our next weather event is Friday, as a cold front moves through. It may become interesting where a semi- tropical system affecting Florida this week moves up the coast, causing rain to linger into Saturday. Lately all of those scenarios have fizzled, with the storm staying east in the Atlantic.

We could really use some rain.

Here’s the latest NAEFS forecast for Friday—

Today’s 12z NAEFS forecast for Friday, shows low pressure off the Florida coast with a sharp dip in the jet flow and a cold front whose general position is the red 540-thickness line. Notice how the warm air is all the way up into Alberta Canada in this highly amplified jet flow. (Click on image for a larger view.)

So no rain until later on Friday and mostly sunny skies.

Winter Outlook Discussion —Which brings me to the winter forecast. Frankly, I think we’re going to see large swings in temperatures here with primarily mixed precipitation events here. Not necessarily large accumulation snow storms.

The jet pattern these past several months has been somewhat different than last year. With almost no accumulating snow last year, it’s unlikely to be similar, but it doesn’t mean we’re “due” for big snow storms.

Last winter, a persistent dip in the jet had the main axis of the trough directly over us, causing storm formation to occur in the Atlantic. This year, I ‘m seeing more favorable positioning of the axis of the jet dips (more to our west), but favoring warm air preceding any storm. The major storm developments over recent months have all been to our north and east, missing us. Will that continue?

Patterns change; what we have now could be different in January. But the trend towards warmer temperatures may be an increasingly large factor when it comes to getting large snow storms here.

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Phillywx #Philadelphia #weather #PAwx

Friday

Updated Thu 11/09 @ 10:50 PM —An interesting Friday forecast. All of tonight’s earlier models have the light showers well to our south. Interestingly, the Canadian HRDPS and tonight’s GFS just came out and both show very light showers extending north of Philadelphia. The HRDPS has been consistently forecasting this. Will see what happens.

Updated Thu 11/09 @ 7:59 PM — A weak system will move up from the south Friday and a secondary cold front will move through. Most of the rain is expected to be suppressed to our south. Very light sprinkles in the immediate PHL area at most.

NBM accumlated rain forecast Friday. Our area is in the 0.01″ range. (It should be noted that the Canadian models have more rain for our area, but their forecast is not matching up with current satellite imagery.) (Click on image for a larger view.)

The extended range model forecasts for the weekend and next week have changed somewhat over recent days with less of an upper air trough. Still very dry weather in the foreseeable future.

Thu 5:37 PM Forecast Review — The large spread in NBM forecast temperatures was a tip off that today’s forecast specifics were low confidence.

We finally got some sun west of the city, but there were more clouds than forecast in the afternoon. As a result, high temperatures were only 68º at KPHL and 64º at KLOM (Blue Bell Wings Field)

While some morning models showed the very light sprinkles that went through about 3 PM, they easily could have been interpreted as model noise and not all models showed these light sprinkles.

One of the value-adds that I try to offer on this web site is reporting the NBM high temperature standard deviation (sd) , which reflects the degree of spread in the models and gives an easy clue into confidence in the model forecast.

From basic statistics, remember that among a group of models (the NBM is based on a group of over 40 models) , about 66% of the model forecasts will fall within 1 standard deviation (±) if their forecasts are normally distributed.

When the standard deviation is larger, there’s a wider spread of model forecasts and therefore more uncertainty.

Based on the consistently higher than usual sd in today’s high temperature forecast, it was no surprise that today’s forecast was not spot on.

Thursday Update

Updated Thu 11/09 @ 10:16 AM — This morning’s model’s high temps are coming in 2º -3º lower than the NBM graphic posted below.

Updated Thu 11/09 @ 9:00 AM — Clouds should break for sunshine around noontime. High temperatures are somewhat lower than previously forecast, but still with low confidence, with a standard deviation of about 2.4º

Today’s 12z NBM high temperatures for Thursday. All have a standard deviation of about ± 2.4º which is high. Meaning that these highs have a good chance of be off by as much as ± 2.4º (Click on image for a larger view.)

There’s still a chance of very light showers Friday afternoon—

HRDPS total rain accumulation through 5 PM Friday afternoon. (Click on image for a larger view.)
Update

Update Wed 11/08 10:28 PM — Tonight’s models are leaning towards some light showers early Friday afternoon as a secondary cold front moves through. Somewhat heavier showers in southern Chester and Delaware counties.

Thursday

Updated Wed 11/08 @ 8:05 PM — A warm front still to our south will pass through about daybreak Thursday. Some residual cloudiness is expected during the morning hours Thursday before sun breaks out around noon.

Current ( 8 PM) satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. Warm front in RED. (Click on image for a larger view.)

A cold front comes through during the early afternoon, but cold air will take time to move in. High temperatures may reach 70º in Philadelphia and 68º in Blue Bell.

(Both high temperatures are ± 3.2º. That’s a high level of forecast spread. The Canadian models are forecasting 73º, but they tend to run high. )

It will become WINDY and gusty before and after the cold front, late morning and early afternoon.

Enjoy the warm weather if you can. By Sunday and Monday, we’ll be closer to the low 50s and even lower on Monday for highs.

Still no rain forecast.

Wednesday Thursday Update

Updated Wed 11/08 @ 9:36 AM — The warm temperatures for Thursday have been well-advertised. The warm front is to our southwest and will move through tonight. High temperatures near and above 70º are likely Thursday.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure (black contours) potential vorticity (fine white contours) and superimposed MRMS radar. Warm front position at 9:30 AM shown. (Click on image for a larger view.)

Not as well-advertised is a sharp drop in temperatures possible late Sunday into Monday.

Wednesday and Beyond

Updated Tue 11/07 @ 4:38 PM — We could really use some rain.

The front moved through as forecast and another cold front will move through Friday. It will cool down tonight and more so this weekend.

Wednesday will be cooler with highs in the low 50s.

Thursday will be warmer again with highs in the mid 60 to upper 60s!

A cold front moves through Friday.

It appears that our area will miss out on rainfall with Friday’s cold front as well. As mentioned yesterday, some models have as little as 0.05″ of rain later Friday. (The Canadian GEM is currently forecasting a bit more, about 0.15″) The latest NBM model has our area dry—

Today’s 18z NBM accumulated rainfall forecast has our region receiving no rain with the front on Friday. But other areas will receive some rain. (Click on image for a larger view.)

The weekend looks dry but a sharpening trough over us will bring some chilly weather by Sunday into Monday.

The NAEFS suggests that light wintry mix might fall far northwest of Philadelphia—

NAEFS shows freezing line (white contour) with some very light wintry precip at 4 AM Monday morning. Not currently forecast by any other model. (Click on image for a larger view.)
Tuesday

Updated Tue 11/07 @ 9:04 AM — A weak surface cold front will move through later today, but not before temperatures have risen to warm levels.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure (black contours) potential vorticity (fine white contours) and superimposed MRMS radar. I’ve drawn in the cold front position approaching central PA. (Click on image for a larger view.)

Clouds should thin and clear between 1 PM and 3 PM as winds become northwesterly behind the cold front.

Another Overview

Updated Mon 11/06 @ 7:51 PM — There’s not much happening and it’s expected to stay that way weather-wise. An anomalous ridge expected to develop in the west coast with a flat to slight ridge here will keep systems from moving towards us.

Today’s 18z GEFS shows an anomalous ridge in western Canada and flat to slight ridge over us by Thursday. This configuration keeps storms from developing in this area. (Click on image for a larger view.)

There remains a very low chance of light rain late Wednesday night and Friday. Total rainfall amounts are very low, on the order of less than 0.06″ total and likely less.

A chill down this coming weekend. but dry as surface high pressure takes over.

Our dry pattern continues into next week. What does this portend for the coming winter? Who knows!

Previously Posted Sun 10:14 PM —

Overview

We’ve moved into a dry pattern with several storms moving to our north and to our south. The same appears to be true for the coming week, as a low pressure system moves to our north on Tuesday and another disturbance approaches for Thursday into Friday with a cold front.

The models are unimpressive with rainfall for our area, especially the latest GFS.

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine white contours) and superimposed MRMS radar.
Disturbance 1 will move to our north on Tuesday. Disturbance 2 will move to our south on Wednesday night/Thursday morning. Disturbance 3 will drop south and drag a cold front through later on Friday. Any rain from these systems will be minimal here, with the best chance on Friday. (Click on image for a larger view.)

So mild temperatures, some cloudy days (Tuesday, possibly Thursday, Friday) Little in the way of rain except Friday.