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Saturday Forecast Update

Posted Saturday 12/09 @ 9:05 AM — The fog is expected to lift before noontime. There’s a range of model guidance regarding cloud cover in the afternoon. The best bet is for some breaks of sunshine through high cirrus clouds after noontime with an increase in cloudiness again around 4 PM.

High Temperatures: 51º Blue Bell 52º Philadelphia (uncertainty high due to cloud cover uncertainty ± 3.5-4.0º)

For Sunday: Rain moves in about 8 AM from the west. Some of the extreme wind speed forecasts have decreased in intensity to the 30-40 mph range. Still 2-3 inches of rain forecast, but with some shifts in the placement of the bands of heavier rain to NJ.

Update Fri 12/08 11:13 PM — Based on tonight’s model guidance, it’s unclear how much sun we’ll see on Saturday afternoon, after the fog mist and light drizzle lifts just before noon. Considerable cloudiness is more likely than sunshine.

Originally Posted Fri 6:43 PM —

Updated Friday 12/08 @ 8:46 PM — Minor changes below, highlighted.

A warm front will slowly move through early Saturday. Previous model runs just had clouds and fog with this front, but the latest NAM-NEST and HRDPS show light showers drizzle and fog Saturday morning.

An approaching cold front will have a low pressure wave develop along the front on Sunday afternoon. Heavy rain and strong wind gusts are expected with this front and low pressure system Sunday afternoon into the overnight hours of early Monday. The system is approaching faster than previously forecast, but may bog down over our area with several areas of low pressure developing.

Here’s the GFS forecast for early Sunday afternoon—

Sunday

The latest shows showers moving in around daybreak and rain heavy at times during the day, becoming heavier towards evening. Rain lasts through about 4-5 AM Monday morning. Rain will be heavy—

High temperature 62º ( NBM model— location Blue Bell, PA)
High temperature 64º ( NBM model— location Philadelphia, PA)
uncertainty ± 2.1º

High winds through much of Monday!

Stay tuned for updates.

#Philadelphia #weather #PAwx

Sunday Forecast Update

Posted Saturday 12/02 @ 5:50 PM — Rain moves in after midnight tonight and tapers off during the early afternoon. It will remain cloudy with light winds. It should no longer be raining in Philadelphia at the start of the Eagles game. Light winds and temperatures about 52º at 4:30 PM.

Sat 5:36 PM Forecast Review — Today’s high temperatures showed a wide range, as expected. The skies did clear about 11 AM to noon, except near the airport, which was reporting fog into early afternoon. PHL also had a relatively low max temperature.

RTMA measured and interpolated actual temperatures at 3 PM today—

Saturday Update

Posted Sat 12/02 @ 10:47 AM — Right after posting my update this morning, the 12z HRDPS became available. It’s showing 60º and mostly sunny in the afternoon. We’ll see.

Posted Saturday 12/02 @ 9:58 AM — This morning’s models still show low fog breaking for some sunshine through mid and high clouds between 11 AM and 1PM. (The latest HRDPS has clouds lingering until about 1:30-2 PM)

Here’s the HRDPS at 1:00 PM showing low clouds hanging tight in the low valleys near the rivers—

I don’t think many areas are going to see the 60º forecast by last night’s TV weather entertainers. This morning’s 12z NBM still shows a high of 57º with a very high standard deviation (high uncertainty) of ± 4º

So, there still remain a wide range of forecasts in temperature and cloud cover. The warm front is getting hung up near our area, causing the uncertainty.

Updates

Update Fri 12/01 11:22 PM — Tonight’s models suggest that high temperatures on Saturday are likely to be anywhere from 54° to 59°. Still high uncertainty. Clouds and fog may linger until 11 AM before skies brighten.

Posted Friday 12/01 @ 5:56 PM — The latest HREF and Canadian RGEM have become available and they capture the extreme uncertainty in tomorrow’s high temperature forecast—

18z HREF—

18z Canadian RGEM—

Previously Posted Fri 6:19 PM —

Two disturbances ahead of a cold front will affect our weather this weekend. One is bringing the rain we’re getting this afternoon (Friday). The next one will bring rain primarily Saturday night into Sunday.

As posted last night, there’s an usually large range of model forecasts for the next few days; the most significant differences (and uncertainties) are around the the cloud cover/high temperatures on Saturday and the location of the secondary coastal low formation on Sunday.

Let’s tackle each one individually.

Saturday

A disturbance and associated warm front is moving through Friday evening and it is expected to exit by Saturday morning. There’s a wide range in the cloud cover forecast. The ECMWF forecasting sunshine breaking through has our high temperatures in the low 60s! Most of our models show areas and periods of considerable cloudiness and are in the 56º to 59º range. The NBM shows an incredibly high standard deviation of ± 5.0º, which captures the very high uncertainty of the forecast.

Here’s the summary—

Clouds break for periods of sunshine through high clouds with periods that are considerably cloudy. I’m leaning towards a compromise on temperatures. A below average confidence forecast. Saturday will be dry. Rain begins again about 11 PM Saturday night

High temperature 57º ( — location Blue Bell, PA)
High temperature 59º ( — location Philadelphia , PA)

Sunday

As the low pressure in the Great Lakes and its associated cold front spawns a secondary coastal low, we’ll have periods of rain much of Sunday with a taper off during the afternoon. There are huge model differences in the placement of the low, the amount of rain, and when the rain ends. It appears that the cold front will move through Sunday evening. I’m going with a compromise.

Rain ends about 4-7 PM.

High temperature 54º ( NBM model— location Blue Bell, PA)
High temperature 56º ( NBM model— location Philadelphia, PA)
uncertainty ± 3.5º (still high uncertainty)

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