#Philadelphia #weather #PAwx #PhillyWx
Today is illustrative of the reason I include the the NBM standard deviations with high temperatures forecasts. When the temperature forecasts have an incredibly high standard deviation (as they had this weekend), it suggests major lack of agreement among the models and major uncertainty (and likely error) in the forecast.
- Saturday, we hit a high of 60º in Philadelphia. The NBM model showed 52º but with an incredibly high sd of ± 7.3º. Even the best performing model had a high of 56-57º.
- Sunday, the front came though before daybreak and the showers/flurries came through at and before daybreak. To give an idea of how far off the forecasts were, the latest NBM (13z) from this morning still shows showers at 10 AM, even though it’s sunny.
- Speaking of sunny, the forecast was for the sun to break out early afternoon, not mid-morning Totally off.
I can’t confidently account for the model forecast errors and high uncertainty. The “pattern” may truly be undergoing a change with greater ridging in the East. Since some models depend upon ‘continuity’ for their initial conditions; with a change occurring it threw off the forecasts(?)
We’ll have to see if the uncertainty (errors) continue with the Tuesday’s expected snow->sleet->rain storm forecast.
Sunday Light Showers-> Snow Flurries
Posted Saturday 02/28/26 @ 11:28 AM — As forecast yesterday, a sharp cold front will past through Sunday morning. Ahead of the front, light rain showers will develop during the morning. As temperatures chill rapidly in the lower and mid levels of the atmosphere, the rain showers will transition to snow showers and depending upon the dynamics, some snow squalls. Little to no accumulation near the city, but upper Montco and Bucks counties may see a coating, especially on grassy surfaces.
The latest GFS shows temperatures near the ground falling rapidly to 32º; the RRFS is slower with the temperature descent. Clouds break between 1 and 3 PM.
Originally Posted Fri 7:57 PM —A southwesterly flow of milder air will allow temperatures on Saturday to rise to levels not seen in awhile, although the trend today is for highs to be lower than previously advertised. A strong cold front moves through on Sunday morning .
Saturday Forecast
Some clouds in the early morning will break for sunshine for most of the day. Winds will be light. There remains extremely large spreads in the forecast high temperatures.
NBM high temperatures: Blue Bell, PA 49º Philadelphia, PA 51º
Unusually high uncertainty (based on standard deviation): ± 7º
Sunday Forecast
Cloudy in the morning. The RRFS and other models are forecasting showers moving through between 8 AM and noon, with the heaviest activity north of the immediate Philadelphia area. Clouds clear for increasing sunshine through the afternoon. Strong winds and gusts near 30 mph after the front.
Temperatures in the mid 40s during the morning will fall into the 30s by 5 PM.
An interesting system will affect our area late Monday through Wednesday. Updates over the weekend when things clarify.
