Category Archives: Philadelphia Weather Outlook

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.

Philadelphia Area Weather Forecasts, Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Phillywx #Philadelphia #weather #PAwx

Forecast Change for Saturday Update

Updated Thu 11/02 @ 8:33 PM — It looks like a weak upper trough will affect our area on Saturday with more cloudiness than previously forecast, especially during the afternoon hours.

So while Saturday will be milder than Friday (Friday’s highs are expected to be upper 50s, just short of 60º in most places), the highs on Saturday will only reach about 61º-(62)º in some places, somewhat less than previously forecast due to the increased cloud cover. These highs may be different by Saturday.

NBM high temps for Saturday (Click on image for a larger view.)
Forecast Change for Saturday

Updated Thu 11/02 @ 6:21 PM — A quick update; I haven’t reviewed today’s models in detail. It appears on the latest GFS that a weak cold front tries to approach on Saturday with an increase in clouds. This was not previously forecast. I’ll update later with some new models.

Canadian RGEM shows cloudiness on Saturday. (Click on image for a larger view.)
Thursday through Sunday Outlook

Updated Wed 11/01 @ 8:27 PM — The cold weather expected tonight is being somewhat hyped on TV/radio, in my opinion. It’s going to be cold, but it’s already November and guess what, it sometimes gets pretty cold in November.

In case you missed it, here’s the low temperatures expected tonight (with an uncertainty of ± 2º) —

Today’s 18z NBM low temperatures forecast ( 2º increment contours are labeled)

There isn’t much else to talk about regarding the weather. A sprawling area of cold high pressure will affect our weather for the next several days, with moderating temperatures over the weekend.

Next chance for rain is Monday into Tuesday.

Wednesday Outlook

Updated Tue 10/31 @ 7:53 PM — Some showers may move in later tonight (Tuesday), especially from the city eastward.

The cold temperatures expected for tonight have been well-advertised. Here’s the 18z NBM model forecast lows—

NBM low mean temperatures for Tuesday night into Wednesday morning. Some uncertainty with the above standard deviation around ± 2ºF (Click on image for a larger view.)

Clouds linger in the morning. There’s uncertainty with the degree of clearing we get on Wednesday. The GFS, HRRR, HREF all show low clouds until noon to 2 PM. The Canadian models have clearing with partly to mostly sunny skies. I’m going with the Canadian models with more sun than clouds mid day into the afternoon.

Tuesday Wednesday Outlook Update

Updated Tue 10/31 @ 9:47 AM — Perhaps you’ve noticed that the forecasts have been less than consistent in recent days? There have been larger than average spreads in the model forecasts since the weekend. Luckily, the model forecasts have begun to come closer together.

The models guidance forecasts low pressure development in the western Atlantic and the position and track are close enough for some moisture and clouds to be thrown back towards us later today, Tuesday and early Wednesday.

I get a kick out of everyone thinking the European model (ECMWF) is so great. The ECMWF runs on Sunday and Monday had the low pressure system too far away to affect us. Kudos to the Canadian model (and to some extent our GFS ) that had forecast the low consistently closer to the coastline.

Sometimes I wonder if the TV forecasters put too much stock in the ECMWF forecasts. (They’re paying thousands of dollars a year to get the ECMWF; maybe they figure they have to use it?)

The ECMWF model data has been available for free as a reduced resolution and limited parameter format for about a year and a half. It’s been very nice to have, but so far, I haven’t been that impressed with its forecasts. Our GFS, GEFS combined with the Canadian models have provided better guidance for these forecasts, in my opinion.

For longer term forecasts, the NAEFS (which combines the GEFS and Canadian Ensembles) has been more useful for these forecasts.

Tuesday: Some cloudiness moves back in from the east about 4 PM today and some showers are possible late Tuesday night into Wednesday morning.

Wednesday: Some cloudiness and even some widely scattered showers linger from Philadelphia and eastward into mid afternoon Wednesday as a weak surface low develops near us.

Today’s latest 06z NAEFS forecast for Wednesday 2 PM shows a low pressure in the western Atlantic with a weaker low over our area Wednesday. Slow clearing Wednesday, especially from Philadelphia and eastward.
Forecast Update

Updated Mon 10/30 @ 5:23 PM — The question for this week is how much will the developing low pressure system in the western Atlantic affect our area?

It appears now that the answer is that some clouds may spill up into our area later on Tuesday and some shower activity is possible in eastern areas on Wednesday, especially early Wednesday morning. Here’s the latest NAEFS—

Today’s 12z NAEFS model has some showers possibly brushing the Philadelphia area early Wednesday. Not all models are on board with this forecast. The ECMWF has the system further eastward. The latest GFS is close to the NAEFS.
Monday Update

Update Mon 10/30 8:30 AM — The fog many areas are encountering this morning was forecast by last night’s models, but not yesterday afternoon’s models. It appears that we’ll move from fog to mostly cloudy today. The front is moving slowly, even more slowly than forecast yesterday, with the actual front now expected to move through early to mid afternoon (instead of this morning.)

Due to the slow movement, another weak low pressure system is forecast to develop along the front and affect us with rain showers starting about 4-5 PM.

Monday

Previously Posted Sun 8:09 PM —

A cold front is to our northwest and its energy and associated rain is sliding up past us to our northwest—

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. Most of the energy will move up to our northwest missing us.
The upper trough visible over North Dakota will move eastward and be over us by Wednesday with much colder temperatures. (Click on image for a larger view.)

The actual front moves through our immediate area late morning on Monday. Ahead of the front, some additional showers are forecast to break out from the city and southeastward before daybreak Monday, but most of the energy with this front is moving up to New England.

Early morning sunshine will be replaced by cloudy conditions later in the morning and windy and gusty conditions as the front moves slowly through our immediate area.

By Wednesday, the trough will move moving over us, as low pressure develops in the western Atlantic.

GEFS forecast for Wednesday 5 AM shows the trough moving closer to us as low pressure develop too far off the coast to directly affect us. Temperatures in the 30s expected Wednesday night. (Click on image for a larger view.)