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Updated Sunday Forecast

Update Sat 02/25 @ 10:06 PM —Some early morning sunshine will fade behind some low cloudiness in the mid to late morning as a weak front approaches from the west.

Clouds break for sunshine in the early afternoon, but it will become quite windy, with gusts in the 30 mph range. High 53º, almost 20º warmer than today.

Update Sat 02/25 @ 10:35 AM — The morning models (12z model runs for EST ) become available between 8:15 AM and 11 AM. Accumulating snow is conspicuously missing. A model spin-up issue or our snow-less winter continues?

Update Sat 02/25 @ 9:12 AM — In a way, I’ve enjoyed this mild, snow-less winter.

But back last fall, I was spending considerable time improving the appearance of my model snow graphics in preparation of this winter. So far this winter, I haven’t had a chance to use these graphics and I must confess that today’s light snow showers are a thinly veiled attempt to put them into action.

Today we’re expecting to see some snow showers develop as low pressure moves both to our north and south.

The highest resolution models (2-4 km resolution) show virtually nothing accumulating, but the lower resolution models (13km NAM, and 10km Canadian RGEM) do show some snow accumulation, a coating to 1/2 inch in some locations.

Here’s current radar with superimposed RAP model graphics —

Here’s the latest NAM model snow accumulation, just became available—

Previously Posted Fri 6:00 PM —

Low pressure will move east and will be south of us as some moisture just brushes the Philadelphia area Saturday afternoon. The NAM, as mentioned in my earlier post had forecast a very light accumulation for us, but it appears that any light accumulation will be closer to the Delaware border and into South Jersey.

Saturday

It will be cloudy and cold Saturday with some snow flurries just reaching north of the city during the early to mid afternoon. To our south, near the Delaware border and into South Jersey, there’s a chance of a coating, up to 1/2 inch.

This simulated radar forecast from the NAM captures the extent of the northern extent of the flurries and also shows where a coating is more likely. This afternoon’s GFS has a fairly similar forecast—

This is the current NAM forecast for snow accumulation. It is likely over-forecasting snow accumulation—

High temperature only 34.0º sd 1.9º ( NBM model- location Blue Bell, PA)

Winds

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Saturday

Update Fri 2/24 8:51 AM — While the 00z NAM was an outlier last night with its prediction of very light snow Saturday afternoon, a few additional models (RAP, ECMWF) have joined the 06z NAM in predicting a coating up to 1 inch of snow for the immediate PHL area, especially south and west of the city. Conspicuously missing are the GFS, ICON and Canadian models in sharing that forecast.

Additional model runs today will clarify. Perhaps this will be another example of proving my mantra, “Never ignore the NAM”.

Previously Posted Thu 10:05 PM —

After a very mild day today, we’ll descend back to more winter-like temperatures, especially on Saturday.

Of interest is a storm that is currently forecast to move off the coast to our south on Saturday afternoon. Most models have had, at most, some snow flurries for Saturday afternoon as some moisture moves up from this storm.

Many models which had been showing snow flurries have even backed off on that forecast. The latest ECMWF, GFS and NAEFS have minimal flurries, or none at all.

Tonight’s NAM has caught my interest. It’s an outlier for now, but the NAM is forecasting much colder temperatures and much more moisture from this southern storm.

Tonight’s NAM is even forecasting measurable snow accumulation by Saturday evening—

Again, the NAM is currently an outlier with this forecast, so snow lovers, don’t get your hopes up too much. I just thought I’d pass this along in a winter which has shown no action.

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Update Thu 02/23 @ 8:40 AM — The latest 12z NBM captures the trend from last night— most models have trended a bit ‘lower’ for high temperatures today. But there’s still an extremely high spread in model predictions, as shown by a standard deviation (sd) of of almost 5º. This means whatever forecast temp you decide to hang your hat on has high uncertainty in either direction.

As mentioned yesterday, in these situations in the past, I would generally move towards the NBM mean plus the standard deviation, which for Philadelphia would be about 72-73º.

Last night’s ECMWF was in this ballpark. But the very latest HRRR has a high of only 65º for Philadelphia.

NBM high temps, to be reached between 3 and 4 PM. Then temps begin to fall into the 50s by evening—

Update Wed 2/22 9:36 PM — Tonight’s early models have the warm front barely making it beyond Philadelphia. High temperatures on Thursday are running a bit ‘cooler’ with expected highs in the mid to upper 60s. Still a large spread in the NBM forecast of almost 7 degrees, so for now, think 67°, as high as 74° or as low as 60°

Previously Posted Wed 5:51 PM —

It’s been well-advertised that we’re going to see some unusually high temperatures on Thursday. What hasn’t been well-advertised is that there is extremely high uncertainty in the model forecast high temperatures.

The latest NBM model (which statistically evaluates about 40 different model inputs and compares their forecasts to actual conditions six hours later) shows an extraordinarily high standard deviation (sd) of over 8 degrees.

With an sd of over 8 degrees, a forecast of an average high temperature of 69º means there are models forecasting a high of only 61º and other models forecasting a high of near 77º.

(Typical standard deviations for a next day forecast are about 1.8º, rarely more than 3º.)

Why? There is a considerable amount of model disagreement regarding how far north the warm front will move.

Here’s the latest Canadian RGEM model, forecasting 75-76º for Center City Philadelphia —

(The latest ECMWF is closer to 72º)

Here’s this afternoon’s NBM model mean forecast high temperatures—

I’m leaning towards the higher Canadian model forecast high temp because many times I’ve seen the NBM undershoot in situations where temps are far from normal.

However, keep in mind, it might just reach 69º or maybe as high as 80º. There will be a tight temperature gradient wherever the warm front stops. Location north to south will be a big factor.

But can we really complain about a high temp of only 68º in February?