Updated Sun and Mon Forecast on Sun 11:08 AM highlighted below
There are some changes and modifications in the forecast that are more clearly expressed with new post, rather than my usual edit of yesterday’s weekend forecast. See that post for more background info.
Current Sat 3:30 Water Vapor/Radar/RAP jet-level wind streamlines showing upper low—
Water Vapor image Sat 3:30PM depicting upper low. Also shows current radar and jet-level wind direction (Click on image for a larger view.)
The weekend forecast is mostly on-track. (The hiatus in the rain in Philadelphia this afternoon was predicted by some but not all of the models.) More rain will be moving in later this evening and late night Saturday into Sunday.
The upper low will swing out of here somewhat faster on Monday than yesterday’s models were showing—
Monday's Upper Low Position from Friday Models
Monday's Upper Low Position from Saturday's Models
It will be cooler than previously forecast on Sunday. Dry on Monday.
Cloudy with periods of rain throughout the day. Difficult to identify a dry period, but mid to late morning until about noon, there may be a pause. (clearly not!) High 52.7º sd 4.7º (NBM Blue Bell)
Rain ends Sunday evening from south to north about 6 -8 PM 8-11 PM according to new HREF model.
Monday
Becoming mostly sunny, a few periods of clouds at times, especially during the mid afternoon.Cooler than previously predicted High 71.2º sd 1.6º High 72.1º sd 1.3º(NBM Blue Bell)
The forecast trend towards a cloudy, rainy and somewhat chilly Memorial Day Weekend continues. The only bright spot, literally and figuratively, will be Monday.
The current forecast weather setup, first took shape with Wednesday morning’s models. The ICON model was one of the first to pick up on the heavy rain.
An upper air trough develops over our area Friday through Saturday and lifts out by Monday. The upper trough will pinch into a closed low. Unfortunately, this upper air closed low will be far to our west, putting us in what’s called the “right entrance region” of a strong jet streak on Saturday.
Here’s the current (Friday morning) radar/water-vapor and RAP model jet stream flow showing the developing upper low—
By Saturday, this is the GFS forecast showing the upper low and jet streak entrance region—
GFS Model 250 mb level jet forecast 2 PM Saturday. Note the position of the cold upper air low puts us directly in what’s called the “right entrance region of the jet streak” (The red circles in the entrance region box are areas of strong vertical motion (vorticity) at the jet level.) The southerly flow and orientation of the jet brings in moisture. The vertical motion causes the rain. (Click on image for a larger view.)
A strong moisture flow aloft will result in heavy rain. The latest GFS and NBM have joined the heavy rain forecast of the ICON model. Anywhere from 1.5 – 2 inches of rain now forecast from Friday afternoon through Saturday afternoon.
By Sunday morning, the same upper air low will induce the formation of a coastal low resulting in additional light rain, especially Sunday afternoon and evening and especially towards the coast.
GFS surface and 500 mb forecast showing upper low and coastal surface low. More clouds and rain.(Click on image for a larger view.)
Memorial Day Weekend Forecast Details
Friday
Showers may start as early as noon- 2PM Friday, but the heavier rain moves in 3-6 PM Friday.
Saturday
Cloudy with rain early, then intermittent showers. There may be some breaks in the rain at times. It will be chilly with a northeasterly wind. High 54.6º sd 4.2º (NBM Blue Bell)
Sunday
Cloudy with rain. The models are suggesting more rain over the entire area for much of the day on Sunday with the coastal low closer to the coast. An increasing chance of lightRain into the afternoon and evening, especially east into NJ. High 59.2º sd 4.6º (NBM Blue Bell)
Monday
The upper low moves out along with the coastal low Sunday, after midnight, allowing clearing. Partly sunny, some clouds . Milder, but still cool for May. High 72.9º sd 1.8º (NBM Blue Bell)
(The Canadian and ICON models forecast slower clearing with considerable cloudiness lingering on Monday )
Wind Forecast
NBM model Wind/Gust forecast (Click on image for a larger view.)
Monday's Upper Low Position from Friday Models
