This coming weekend will be influenced by an influx of colder air and a dip in the jet stream to form an upper air trough, pushing a cold front through. The disturbances rotating around an upper low will bring showers on Saturday.
GEFS upper air wind forecast (250 mb- jet stream level) for Saturday 8 PM. The dip in the jet winds and the upper air low in upper NY State will bring enough moisture and lift to create cloudiness and light showers on Saturday. The tropical jet (2) is just visible. (Click on image for a larger view.)
Saturday
Mostly cloudy with very light showers starting mid to late morning and lasting into early afternoon. Showers should be light (about 0.05 inches). Some models have some clearing in the mid afternoon, but most maintain the chance of light scattered showers until 3-4 PM. WINDY. High 54.2º ± 2.2º (NBM model Blue Bell)
Sunday
Mostly sunny, some periods of instability cloudiness. WINDY and chilly. High 46.3º ± 2.4º NBM model Blue Bell)
Winds
NBM wind forecast (Click on image for a larger view.)
Update Sun @ 9:32 AM — Clouds have moved in earlier than forecast today. The GFS forecasts clouds throughout the day, becoming WINDY in the afternoon. High temp 55.5º sd 1.1º
Sat 09:43 PM Forecast Review — So, how is it possible that the models showed the storms not getting into Philadelphia, yet we had a line of thunderstorms over the past hour?
Here’s tonight’s 8 PM HRRR showing the eastward extent of its forecast rain —
Tonight’s latest (8 PM) HRRR showing the eastern extent of the forecast precipitation. Clearly the model didn’t bother to look at the radar. (Click on image for a larger view.)
Here’s the much acclaimed ECMWF forecast, also showing the rain not making it into Philadelphia—
ECMWF 18z forecast showing eastern extent of rain tonight. (Click on image for a larger view.)
The weather models are often amazing. Not so much today.
Update Sat @ 5:52 PM — As forecast in the earlier HREF graphic, showers/embedded thunderstorms are expected to stay northwest of the city. Only light showers expected to make it into Philadelphia this evening—
Earlier HREF forecast 7 PM Saturday. Vertical velocity and CAPE elevated especially northwest of the city. Thunderstorms possible. (Click on image for a larger view.)
Sunday looks sunny in the morning with cloudiness by 2 PM as a secondary cold front moves through.
Update Sat @ 9:18 AM — Today’s forecast has been a perfect example of poor model performance intersecting with delayed evening model forecast availability (especially the GFS) due to Daylight Saving Time.
There’s still a range of forecasts and uncertainty regarding degree of cloudiness today. Last night’s GFS moved away from showers this morning, but has considerable cloudiness. Showers and thunderstorms develop as early as 4 PM , peak chance about 6 PM and continue through 8 PM. If we get more sunshine, expect stronger thunderstorms.
As late as it is this for this morning’s update, the new HRRR and other models have yet to come out. The HRRR will be available by about 9:33 AM.
Update Fri @ 9:54 PM — Significant Update (Models out later due to Daylight Savings Time)
A low pressure system and its associated warm front/cold front will affect our area on Saturday. A secondary cold front moves in on Sunday.
SREF forecast for Saturday at 5 PM. Warm front to our north with cold front moving through PA. (Click on image for a larger view.)
Saturday
Clouds and showers early will end mid morning Saturday. There’s still some question how much sun we’ll see on Saturday. My earlier view was much more optimistic about some bright spots and sun. But the majority of models have low clouds and damp conditions lingering throughout the day.
Friday night’s early models just becoming available . The HRRR shows breaks of sunshine mid day in many areas. Then clouds return mid afternoon with some light scattered showers. Then sunshine possible about 5-7 PM before thunderstorms move in.
High temp 72.1º ± 1.7º Blue Bell
A cold front moves through between 7 and 9 PM. Additional showers expected to move through during this time frame. Without sunshine, lower instability reduces the chances of thunderstorms.
Despite the lack of forecast sunshine, the HREF is still predicting some thunderstorm activity during the early evening Saturday from 7-10 PM, especially west of the city —
HREF forecast 7 PM Saturday. Vertical velocity and CAPE elevated especially northwest of the city. Thunderstorms possible. (Click on image for a larger view.)
Sunday
A second cold front moves through on Sunday. The current GFS is showing considerable cloudiness for much of the day. High temperatures of near 60 early morning fall into the 50s during the day. Windy. Right now, no showers expected with t
Early Edition
Update Fri 1:33PM— Several of this morning’s models maintain a low cloud deck for much of the day Saturday
A low pressure system and its associated warm front/cold front will affect our area on Saturday. A secondary cold front moves in on Sunday.
SREF forecast for Saturday at 5 PM. Warm front to our north with cold front moving through PA. (Click on image for a larger view.)
Saturday
Clouds and showers will end mid morning Saturday. There’s still some question how much sun we’ll see on Saturday, but I’m leaning towards an optimistic forecast of bright skies and sunny breaks. High temp 72.1º ± 1.7º Blue Bell (NBM forecast)
A cold front moves through between 5 and 8 PM. Significant instability and CAPE develops during the day, especially if we get some sunshine resulting in likely strong thunderstorms in some areas.
HREF forecast 7 PM Saturday. The white box shows areas most likely to have strong thunderstorms- Maximum Vertical Velocity and CAPE (shading) (Click on image for a larger view.)
Sunday
A second cold front moves through on Sunday. The current GFS is showing considerable cloudiness for much of the day. High temperatures of near 60 early morning fall into the 50s during the day. Windy. Right now, no showers expected with this front.
Of course, Sunday morning is the beginning of Daylight Saving Time. I love the extra daylight at the end of the day, but for “weather nerds”, Daylight Saving Time means the models come out an hour later. For those of us on EasternTime, the evening models become available inconveniently late for the next day’s forecasts!
I’ve written extensively about this in past years—
