Updated Sun 8:35 AM — The latest HREF forecast for clouds—
Sunday morning (06z) HREF showing percentage clouds (dark/grey) and cloud cover spread (white and pink contours) at 2 PM. Greatest uncertainty remains over the Philadelphia area with clearing northwest of Pottstown (KPTW). No rain west of the Delaware river, although it will be close by. An edge or boundary area forecast resulting in high uncertainty. (Click on image for larger view)
Updated Sun 7:25 AM — It’s raining this morning. I was unclear about my clearing forecasts. My forecasts for clearing were to start late morning to 2PM.
Updated Sat 11:03 PM — Tonight’s HiRESW follows the NAM-NEST with a forecast of sun breaking out as early as late morning.
Update Sat @ 10:05 PM —A low confidence forecast for Sunday; the immediate Philadelphia area is an “edge or boundary” condition for the weather model’s cloud and precip forecasts. The individual model’s resolution and forecast biases are also in play.
The latest NBM model (Model Blend) captures this quite nicely regarding precip chances and cloud cover, with conditions quite different far northwest (Green Lane) and Jersey Shore. In between is Philadelphia—
NBM Probability of 0.01″ Rain for 2 PM. The edge condition is right over Philadelphia. Basically, the NBM has a dry forecast for Philadelphia and western suburbs. (Anything more than 18% chance in a 1 hour period is a fair chance of rain I have the color scale set as such.) Rain is likely at the Jersey shore, and unlikely northwest of the city. (Click on image for a larger view.)NBM % total cloud cover and rain accumulation 2 PM Sunday. The Red contour lines are areas of higher uncertainty cloud cover; it may be less or more cloudy Basically, the NBM is showing light rain/drizzle at the shore and a fair amount of cloudiness over the immediate PHL area. Areas northwest of Green Lane and even Doylestown may see some sunny breaks and/or bright skies. (Click on image for a larger view.)
The latest NAM just available has some showers in areas where the NBM is dry. The earlier HREF model was essentially dry for Philadelphia. The Canadian RGEM had showers and the ECMWF was dry, but had showers move in from the east after 4 PM. So a very tough forecast situation.
Based on the latest NAM-NEST just available, I am leaning towards dry conditions, periods of clouds and breaks of sun from Philadelphia and north and west.
NAM-NEST Clouds and simulated Radar 2 PM Sunday Areas northwest have considerable sunshine
Update Sat @ 8:29 AM —
Latest MRMS rainfall totals (24 hours beginning 7 AM) Numbers are millimeters (25.4 mm = 1″) North, northwest and western suburbs have received 1-1.8 inches of rain over the past 24 hours. (Click on image for a larger view.)
Update Sat @ 7:14 AM — Last night’s models continue the uncertainty around lingering showers and cloud cover for Sunday. The NBM shows some clearing and dry conditions just north and west of Blue Bell (KLOM) —
Today’s 07z NBM showing showers in Philadelphia at 11 AM, with some thin clouds/sunshine towards Allentown. A 25 mile difference in model precision leads to a very different forecast for Sunday. Additionally, the latest GFS and HRRR show additional showers rotating in around a coastal low in the late afternoon Sunday (4 PM and later) in approximately the same areas.
Update Fri @ 11:20 PM — Tonight’s HIRESW actually has sunshine after noon time northwest of the city on Sunday, then clearing in Philadelphia during the afternoon!
Update Fri @ 8:28 PM — Little change in the forecast posted below. The heaviest rain will occur this evening and overnight into early Saturday. Total rainfall in the 2-2.5 inch range, with 2 inches of it occurring before 11 AM Saturday.
The notable feature for Saturday will be windy conditions. Following morning steadier rain, light intermittent rain expected throughout the remainder of the day.
Little/no rain expected Sunday, but cloudy conditions will persist for most of the day. Reverting to original Sunday forecast below: Little/no rain Sunday morning, with some bright spots, possibly some sunny breaks early afternoon before spotty precipitation moves back in late afternoon/evening.
I’ve updated the Saturday and Sunday high temps below with the latest NBM model run.
Previously Posted Fri 12:07 PM —
A highly amplified jet stream level trough will spawn a closed low in the mid level of the atmosphere and low pressure at the surface. Clouds, wind and rain for Friday through early Sunday.
Current position (Friday AM) of upper trough, as revealed by satellite mid level water vapor imagery—
Water Vapor image Friday at 9 AM revealing deep upper level trough in the center of the US. This will move eastward and be over us by Saturday night as shown in the GEFS model forecast immediately below. (Click on image for a larger view.)
Forecast position of upper trough Saturday 9 PM—
GEFS showing 250 mb winds (jet level ~33,000 feet) for Saturday at 9 PM. A highly amplified trough with low pressure over the eastern US. (Click on image for a larger view.)
Saturday
Rain is expected much of the day, although it will lighten and become a bit more spotty late in the day. Very WINDY and gusty; a strong northeasterly wind will keep high temps near 51.6º± 2.1º (NBM model, Blue Bell) (Average seasonal high is 71º!)
There’s remains uncertainty regarding the timing of the rain ending. The latest NBM and ECMWF have clouds, but no rain here on Sunday. The higher resolution models (HIRESW-FVS, NAM, NAM-NEST continue with light showers into the early afternoon, then mostly cloudy, with some bright spots, possibly some sunny breaks. High temp near 53.0º± 3.9º (High temperature spread = high uncertainty) NBM model, Blue Bell)
Some additional scattered showers possibly spring up Sunday evening.
