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.
Update Fri @ 10:24 AM — A big feature of this system will be the windy conditions. Sustained winds near 20 mph with gusts in the 35+ mph range for much of Saturday.
Friday’s 12z NBM Wind Meteogram for Blue Bell, Pennsylvania (Click on image for a larger view.)
Update Fri @ 7:50 AM — The rain is moving in now from the west, on schedule. The latest GFS continues with the forecast for the heaviest rain to fall today, Friday through Friday night and into early Saturday. Then light rain to intermittent rain through early Sunday morning.
Total rainfall will be 2-2.5 inches with locally higher amounts. About 2 inches will fall during this earlier period.
Update Thu @ 10:46 PM — Tonight’s NBM, GEFS and NAM are showing the rain ending by Sunday late morning. Most show cloudiness lingering through the afternoon, especially areas south and east.
Update Thu @ 7:44 PM — 2-3 inches of rain expected Friday through early Sunday afternoon, with the greatest accumulations late Friday into early Saturday morning, according to the latest GFS.
Update Thu @ 5:53 PM — Rain is expected to move in from the west mid morning Friday (about 9 AM ± 2 hours). The ECMWF has rain starting as early as 6 AM and the Canadian GEM about noon.
Rain continues through Sunday morning. The models are coming together with clouds lingering but rain tapering and ending Sunday around noon or so. Some rain may re-develop Sunday evening.
I’ll have a better handle on this forecast tomorrow with my regular Weekend Weather Forecast.
Update Thu @ 9:36 AM — A quick update. The latest ECMWF just available continues the rain into Sunday early afternoon, especially south and east of Philadelphia. (The GFS has the rain ending early Sunday morning.) So there’s uncertainty whether Sunday will be a nicer day.
Latest ECMWF model just available shows rain lingering into much of Sunday, especially south and east. (Click on image for a larger view.)
Update Thu @ 9:04 AM — The expectation for clouds and rain for Friday and Saturday is still in the forecast. An upper level trough will slowly move eastward. Surface low pressure just to our south will bring an easterly flow of chilly air off of the ocean and rain. The rain looks to start as early as 7-9 AM Friday.
GFS Upper air wind (jet stream level 250 mb) forecast for Saturday at 2 PM. Deep trough over the eastern US with upper lows centers will spawn surface low pressure systems to our immediate south. (Click on image for a larger view.)
It still looks like we’ll have clearing on Sunday
Update Tue @ 8:30 PM — Not much change since yesterday’s posting. Very light precipitation amounts for Wednesday expected: 0.10-0.25 inches of rain, mostly coming in the morning hours .
The European and German models are predicting considerable rain for Friday through Saturday, on the order of 2″ of rain. The upper low looks to be just slow moving rather than quasi-stationary. As a result, Sunday should be dry with decreasing cloud cover.
It’s been announced that the NWS-NOAA will be upgrading their “Weather and Climate Operational Supercomputer System” referred to as “WCOSS” in a major way on or about June 27th.
This system does the actual data ingestion and weather model computations.
New Cray Supercomputers will almost triple processing power from 16 petaflops to 40 petaflops. This may result in weather models being available faster and in the case of evening models, earlier. It’s a giant upgrade and hopefully the transition goes smoothly.
(They are asking for forecasters to be alert for any systemic weather model errors.)
This week will feature a typical springtime weather pattern with plenty of showers and a quasi-stationary upper closed low in the forecast for the Philadelphia area.
The first low pressure system will move in for Wednesday—
GEFS ” statistical mode” forecast for Wednesday showing low pressure (1) directly over us with rain/showers. A second low pressure system (2) takes shape in northern Texas, expected to move in Friday through Saturday. (Click on image for a larger view.)
The second system moves in for Friday through Saturday, as a closed upper low becomes quasi-stationary just to our south.
Jet stream (250 mb winds) for late Friday/Saturday morning—
GEFS mode forecast 250 mb (jet stream level) winds showing closed low over Kentucky for Saturday 2AM. (Click on image for a larger view.)
An easterly flow make it chillier with this system. As is often the case with closed lows, the exact position of the upper low (likely to change by Saturday) will make all the difference. Right now, it looks like rain.
GEFS (statistical mode) forecast for Saturday at 2 PM. Upper low and surface low will combine for rain. Southerly track means chilly easterly wind off the ocean for us. (Click on image for a larger view.)
The inclement weather may linger into early Sunday with slow clearing.
The exact position of upper closed low forecasts are difficult to forecast and I expect things might change by the weekend.
