Category Archives: Weather Outlook


Last Updated Thu 7:45 PM     Forecast updates are highlighted.

Update Thu @ 7:40 PM— The models are still trying to come together with this weekend’s forecast. Today’s GFS and NAM have returned to the earlier forecast that our area will be in an area of jet stream level lift and upper air disturbances on Saturday, causing cloudiness and possibly some light sprinkles until after noontime. Greatest chances of sprinkles areas north of the city. Then breaks of sun and clouds for the rest of the day.

Sunday looks to have some sun early morning, then significant periods of cloudiness and some bright spots.

Here’s a meteogram based on the latest GFS model cloud and simulated radar forecast for Blue Bell, PA (remember that meteograms are only for a single location, but in this case, most of the PHL area forecast will have somewhat similar conditions) —

GFS forecast Sat and Sunday total cloud cover and simulated radar for Blue Bell PA. Cloudiness Saturday morning gives way to some bright spots and sunny breaks. Arrows suggest possible sprinkles of rain. (Click on image for a larger view.)

Update Wed @ 8:17 PM Friday and the weekend continue to look better.

Two cold fronts move through and the expected drop in high temperatures to more seasonable levels is apparent from the NBM (model blend) forecast meteogram for Blue Bell—

(Click on image for a larger view.)

On Saturday, the upper air disturbance moves to our north and west giving us clouds in the morning but breaking for partly sunny skies in the afternoon. Any showers on Saturday will be mostly to our north and west and will be light.

Sunday starts cloudy with breaks of sunshine by afternoon.

Update Wed @ 9:51 AM—This morning’s NBM (model blend) has fewer clouds both Friday and Saturday. (Clouds move in later Friday afternoon and dissipate Saturday morning.) Last night’s GFS also moved in that direction. Will keep an eye on it.

Tues 8:39 PM:

We’re having some fantastic sunny, mild autumnal weather and it will continue through Thursday. Temperatures are well above seasonal averages (Avg Hi 62º Low 43º Blue Bell, PA)

A cold front will slowly move through on Friday with cloudiness and possibly some very light showers. The front will stall to our south. Upper air disturbances will move through on Saturday with cloudiness. It looks like we’ll be in the “left exit region” of the jet streak, assisting the development of light showers on Saturday from these upper air disturbances.

A secondary cold front will be pulled in behind the disturbance giving us more seasonable (read colder) temperatures for Sunday.


Update Fri @ 9:30 AM—

Incredibly our high temperatures today, Friday, and Saturday will be at or above 80°F.

Regarding Saturday, last night’s models continue their forecast of timing the surface frontal passage, based on wind shift and lowest barometric pressure, about 2 PM on Saturday.

Meteorologically, this will be an anafront and the models are showing that most of the rain and embedded thunderstorms will occur after the frontal passage. The model trend has been for the precipitation to occur somewhat later, now starting about 3:30 to 6 PM.

More information on this with my regular Weekend Weather Forecast this Friday evening.

Update Thu @ 7:25 PM—A cold front on will move across our area between 2 and 5 PM on Saturday. Today’s NBM model shows the possibility of some thunderstorm activity with and after the passage of the front.

As mentioned last night the main issue will be gusty winds. Total rain accumulation is not expected to be heavy with the front, although a brief heavy downpour might occur.

Meteorologically, this front is interesting in that it is what is known as an ‘anafront‘, where most of precipitation occurs behind the frontal passage. So the actual front moves through between 2 and 4 PM but much of the rain and possible thundershowers will move through it as late as 5 to 7 PM.

NBM peak rain and thunderstorm probability (Click on image for a larger view.)

Update Wed @ 8:14 PM— We will finally see some sun Thursday and Friday.

The models are in good agreement with a cold front moving through Saturday afternoon. The strongest dynamics with this front will be far north of our area, but the models are still forecasting some rain in our area.

The front will lower our temperatures closer to seasonal averages Saturday night into Monday. The main issue will be gusty winds on Saturday, peaking with the frontal passage. Current timing for the frontal passage is about 2 -4 PM Saturday. Windy/breezy conditions into Sunday.

GFS Forecast for 2 PM Saturday. Front will be just to our west. (Click on image for a larger view.)

Update Tue @ 5:15 PM— Mild days through much of Saturday. A rather strong cold front moves through mid to late Saturday afternoon with showers and thunderstorms expected. Windy conditions on Saturday and into Sunday. Temperatures chill down on Sunday with low temperatures Sunday night as low as the upper 40s.

The low pressure system that affected our weather on Sunday will continue to linger off the coast. More clouds expected on Tuesday.

The coastal system will gradually move off by Wednesday, allowing an upper air ridge to bring in warm air to the Philadelphia area.

Unseasonably warm weather Wednesday through early Saturday.

GFS forecast for Wednesday 2 PM. An upper air ridge builds over our area as low pressure off the coast gradually moves away. Warm temperatures expected through Saturday. (Click on image for a larger view.)

The GFS, Canadian GEM and ICON all show a cold front moving through. The current timing is Saturday afternoon. The models differ with the amount of showers/storms in our area. Here’s the GFS forecast for 2 PM Saturday—

GFS forecast Saturday 2 PM. Shower develop ahead of the cold front. Red line nosing down from Canada is the 540 thickness line, the general boundary between rain and snow where precipitation falls. That line stays well north of us, even after the cold front moves through. Blue lines are the “500 mb heights”. (Click on image for a larger view.)

Despite a cold front passage, temperatures rebound by next week

GFS jet stream position next Monday. Another ridge is poised to move in—

GFS 250 mb winds and heights. Monday 2 PM (Click on image for a larger view.)


Forecast Updated Fri @ 9:09 AM — Some changes to Saturday’s forecast. An the easterly flow expected to give us mostly low clouds for Saturday may bring in some sporadic light drizzle at times as well. The GFS began showing this last night.

As for Sunday, there’s still a dichotomy between the cloudy but dry GFS forecast for Philadelphia on Sunday and somewhat wetter forecasts from the other models. This forecast dilemma has persisted for days and still remains, although there are signs that the NAM is moving towards the GFS forecast. The ICON has already joined the drier GFS forecast.

The ensemble/statistical version of the GFS, called the GEFS, which averages 31 variations (referred to as ‘pertubations’) of the GFS model still has a wetter forecast.

I’m staying with the forecast for cloudy but dry for Philadelphia, north and west, for Sunday, but I think waiting for tonight’s models might be required.

GFS cloud/rain/wind forecast for Sunday 1 PM. The blue/green shading is rain. In the Philadelphia area, on the order of 0.01 inches water. (Click on image for a larger view.)

Forecast Updated Thu @ 8:04 PM —Friday will likely be the best day of the next three. Considerable cloudiness will break for a mix of sunshine and clouds.

Saturday looks to be mostly cloudy. Most areas dry. A few isolated light ‘spits’ in some areas possible.

Sunday remains a forecast dilemma regarding any rain. We know it will be cloudy.

The GFS continues to keep Philadelphia and areas north and west dry due to blocking high pressure. The latest ICON has joined that camp.

The NBM (model blend) shows some rain for most of the area Sunday, reflecting the fact that most models are forecasting some rain. (The latest GEFS also shows some rain. )

NBM accumulated 3 hour rain forecast 2 PM Sunday. (Click on image for a larger view.)

So the dry GFS (for Philadelphia north and west) is still the outlier. But I’m leaning towards the GFS. I’ll try to nail it down with my regular Weekend Weather Forecast later on Friday.

Forecast Updated Thu @ 8:36 AM — The forecast for blocking high pressure to prevent northern movement of the coastal low, as consistently predicted by the GFS, continues to be the likely scenario for Sunday. The basic depiction from last night’s GEFS still holds. This means that Philadelphia, north and west, will get little, if any rain, on Sunday. Mostly a mix of low clouds, widely scattered sprinkles possible. (Greater chance of rain in South Jersey.)

Moving backwards in time to Saturday, the upper trough will bring mostly cloudy skies with widely scattered sprinkles possible.

The Canadian GEM and the NAM continue to have wetter forecasts for Sunday. I don’t think they’re correct.

Stay tuned.

Forecast Updated Wed @ 7:17 PM— Things haven’t changed much over the past day (see original post below).

For the remainder of the week, we’ll continue to be under the influence of a closed low pressure system that finally opens up into an upper trough, (as described in the original post below).

We may have some decreased cloudiness on Thursday and Friday.

There’s major forecast uncertainty for the weekend, especially Sunday. A significant degree of cloudiness may still be with us on Saturday, the result of an upper trough over the area.

For Sunday, a semi-tropical system is expected to form along the southeastern coast. The GFS has high pressure blocking this system from much northern movement on Sunday, sparing the Philadelphia area from heavy rain (in fact from just about all the rain)—

GFS 2 PM Sunday forecast. High pressure off the coast of Maine will block the semi-tropical low from moving northward, sparing us from heavy rain. (Click on image for a larger view.)

Most models have a bit more rain/showers for Sunday. especially east into NJ.

The statistical “ensemble” version of the GFS (the GEFS) has a chance of light showers on Sunday extending into Philadelphia—

GEFS Precipitation chance: YES/NO (GREEN=YES) forecast 2 PM Sunday. (Click on image for a larger view.)

The latest ICON model shows accumulated rain for Sunday for NJ—

ICON model total rain by 8 PM Sunday. (Click on image for a larger view.)

While the GFS is somewhat of an outlier with its forecast compared to the few other current models that forecast out beyond 84 hours, I currently favor the GFS. So I’m leaning towards a much drier forecast for Sunday.

(The GFS did better overall with several storms last winter and its track of IDA was overall better as well.)

Stay tuned.

Original post from Tuesday—

As forecast last week, a closed upper low formed in the mid section of the country and we’ve been under the influence of a cyclonic upper air flow. Moisture is being brought up from the south around this upper low—

GEFS jet level winds (250mb) for Wednesday. White arrows upper flow of moisture. (Click on image for a larger view.)

The models are forecasting this closed upper low to lift out into an upper trough by the weekend.

Unsettled weather will continue here until the the upper low lifts out and the residual trough disappears.

GEFS jet level forecast for Saturday afternoon. Blue line represents the upper trough (dip in the wind flow) The jet streak energy in Nebraska along with the energy in Oregon will bring a pattern change. (Click on image for a larger view.)

The upper low lifting out represents another short term pattern change. The speed that the upper low lifts out and the timing of the trough will determine the weather for the weekend.

Saturday still looks a bit unsettled with this upper trough. Sunday looks better. But even that general forecast is lower than average confidence.

Updated Tues 9:30 PM— To be watched is a tropical type system that may move partially up the coast over the weekend, brought northward along the upper trough.