Update Sun 05/07 @ 9:38 AM — Very light showers are forecast to move in between 6 and 8 PM. Any showers will be very light. Several models have the showers bypassing the immediate PHL area, going to the north and south of us. Highs today: 76º Blue Bell, 77º Philadelphia ( ± 1º)
Update Sat 5/06 @ 8:36 PM — Not much change in my forecast from Friday. Sunshine early, then sunshine through high clouds during the late morning and early afternoon. Clouds thicken between 3-5 PM. Chance of light showers moving in between 6 PM and 8 PM, earlier far west. High 74-76°.
The upper low associated with the blocked pattern for more than a week has finally moved off to our east.
An upper level high pressure ridge has moved in and it will provide more spring-like temperatures. Disturbances moving in from the west will rise over the ridge and move in from the northwest.
For the weekend, these disturbances are expected to stay west of us as they move across the upper ridge according to the NAM and GFS—
Today’s 12z NAM forecast for 2 PM Sunday. A disturbance in western PA will not get here until late Sunday evening, according to the NAM (Click on image for larger view.)
There isn’t high confidence in the above Sunday forecast. The ECMWF, ICON and CMC have showers moving in further north and as early as mid afternoon.
Previously Posted Fri 6:45 PM —
Saturday
Early morning cloudiness gives way to sunny skies.
High temperature 68.9º sd 1.4º ( NBM model location Blue Bell, PA)
Sunday
Sunny in the morning, increasing cloudiness in the afternoon. Light showers wait until late evening
High temperature 74.7º sd 1.6º ( NBM model location Blue Bell, PA)
The showers today will be scattered as in past days, but total rainfall (in some areas) may be greater. Showers start about 2-3 PM and continue through 8-9 PM.
Today’s 12z HIRESW 2.5KM model total rainfall forecast for later Friday (Click on image for a larger view.)
I’ll post the Weekend Weather Forecast this evening. (The weather looks to be much improved from past days, but some uncertainty about showers Sunday. )
Friday Forecast Update
Update Thu 5/04 10:59 PM — Tonight’s models continue with a forecast of sunshine Friday morning followed by clouds and pop-up showers during the afternoon, especially late afternoon and early evening.
The big change on Friday — the showers may be more substantial than past days and a rumble of thunder can’t be ruled out. As much as this weather will be the result of the influence of the upper low finally departing, a warm front type scenario will also contribute to the mix.
Thursday Forecast Update
Update Thu 05/04 @ 9:38 AM —
The early morning sun we saw might have been the most sun we’ll see today. Already, the cold upper air pool of air is causing instability cloudiness to form. Most models continue with the forecast of light scattered sprinkles and showers developing about 3-5 PM.
In some areas, the sprinkles might be negligible; in others, as much as 0.03″ of rain may fall. High temperature will be just shy of 60º (Blue Bell, PA)
NBM forecast—
This morning’s 12z NBM simulated radar forecast for 4 PM. ( Green contours are 1 hour accumulated rainfall ) (Click on image for a larger view.)
Incredibily, the NBM shows a similar forecast for Friday, perhaps even more showers!
For a greater insight into today, the upper air flow at 250 mb (~34000 ft) around the upper low is colliding with moisture circulating a mid level low at 400 mb (~23,000 ft).
Clouds and sprinkles will be the result. Lack of precision of measurements over the ocean will result in uncertainty regarding the exact placement of the sprinkles today—
Wind flow Thursday morning at 250 mb and 400 mb heights with supimposed water vapor satellite image. (Click on image for a larger view.)
Things do dry out for the weekend.
Previously Posted Wed 7:41 PM —
We have about two more days of an upper cyclonic flow that has given us the clouds, light showers and cool temperatures.
The big upcoming change will be the transition to an upper level ridge. Temperatures will move back towards the low 70s by the weekend—
NAEFS Max Temperature forecast for late Saturday. Also visible are the 250 mb (jet stream level) contours showing the ridge that will slowly move towards us. Temperatures near 70 on Saturday and above 70 on Sunday are expected.
Thursday will be similar to today, Wednesday, although there should be noticeably fewer scattered sprinkles and perhaps some more breaks of sun.
