Updated Sunday forecast, Sat 10:13 PM highlighted below
The weak front that moved through Friday evening will allow some drier air to move in for Saturday.
An mid-level disturbance may bring some clouds in the morning and even some sprinkles around noontime on Saturday, mostly far northwestern suburbs, but most of the day and the area will be partly sunny. High temp 84.7º ± 1.3º NBM Blue Bell.
For Sunday, the boundary that went through on Friday will return as a warm front. Considerable cloudiness in the morning with very some light showers possible, especiallyin far northern and western suburbs.
Clouds break for hazy sunshine Sunday afternoon with an increasing chance of showers and thunderstorms in the evening, especially western suburbs. The showers/thundershowers look less likely in Philadelphia and immediate suburbs.
Increasingly hot and humid. Dew points return to the uncomfortable upper 60s to near 70º. High 86.0º ± 2.6º NBM Blue Bell.
High 86.9º ± 1.7º NBM Blue Bell.
There is some uncertainty about the timing of showers on Sunday with the Canadian GEM having a somewhat different forecast than the GFS. Stay tuned.
Sun 06:50 PM Forecast Review — Sunday’s showers is a perfect example of why I began this blog post from Friday with
“exact forecasts with an upper closed low are fraught with difficulties, inaccuracies and uncertainties.”
A mid-level upper air disturbance rotating around the upper low, unpredicted by most of the earlier models today, caused the scattered showers Sunday afternoon. I don’t think weather models, as good as they can be, can do any better with forecasts and upper air closed lows.
Looking back at this morning’s models, the HIRESW-ARW2 did predict the showers, but it didn’t become available until after 10:20 AM. Had I actually looked at the HIRESW-ARW2, I don’t think I would have updated my forecast; there were too many models not showing showers.
Our weather early this weekend will be affected by a cyclonic flow of air around an upper closed low pressure system. This flow and the upper closed low pressure system’s cold air aloft results in instability clouds and showers.
As my regular readers know, exact forecasts with an upper closed low are fraught with difficulties, inaccuracies and uncertainties. Such may be the case this Saturday. The upper low’s effects are expected to diminish by Sunday.
(Today, Friday, is a perfect example. Some areas are having showers and some areas have sun breaking through clouds. This morning’s models have been unable to pinpoint the proper timing and placement. )
I’ve compared current radar with six different models run this morning. The HREF has had the highest skill in the forecast location of the current rain this afternoon. So I’m going to lean on that model for Saturday.
Here’s the current HREF forecast for Saturday at 1 PM (there’s a chance of showers in many areas) —
HREF forecast for 1 PM Saturday showing rain (bright green contours) and clouds (blue shading) Upper low center (L) Arrows show upper air cyclonic counterclockwise circulation around upper low. (Click on image for a larger view.)
So, it looks like considerable clouds and showers develop Saturday afternoon. It won’t be raining all afternoon.
The upper low moves away on Sunday and Monday, but still a low chance of showers Sunday late afternoon.
NBM – 1 Hour Rain Probability meteogram, Blue Bell. Anything over 15% probability for a 1 hour period is a good chance. (Click on image for a larger view.)
Considerable cloudiness, perhaps some sunny breaks at times with scattered showers, especially in the afternoon. Breezy. High 73.2 ± 3.2º (NBM Blue Bell) Update:Tonight’s models have reduced the showers Saturday afternoon. Following morning clouds, look for considerable sunshine in the afternoon, except far western suburbs.
Sat Morning Update: Despite my optimistic update from last night, the early morning model runs (06z) have reverted back to the original forecast of plenty of showers developing throughout the day, especially late afternoon.
NBM 1 hour rain forecast Saturday 3 PM—
NBM Saturday 3 PM – 1 hour rain and cloud (dark shading) (Click on image for a larger view.)
The upper low moves off. Some cloudiness in the morning, breaking way to a mix of sun and occasional clouds. A chance of a shower late afternoon. . High temp 78.4 ± 2.1º NBM Blue Bell
Sunny and warmer. Some models show widely scattered showers between noon and 3 PM, although this may be model noise. High 86.0º ± 3.1º (high spread!).High 87.6º ± 1.6º NBM Blue Bell.
The setup—a quasi-stationary front northwest of Pennsylvania will have a wave of energy move along it, causing the front to temporarily sag closer to us early Saturday along with a disturbance (1) moving through from the northwest in the upper levels.
At the same time, moisture from coastal disturbance (2) moves up from the south at the mid levels of the atmosphere. High pressure follows with warm/hot and humid conditions later Saturday and Sunday.
Current satellite water vapor image at 5 PM Friday with superimposed RAP model windstreams. Upper air disturbance orange arrows (1) moves down over us on Saturday from the northwest, as moisture (blue arrows -2 )moves up the coast in the mid levels of the atmosphere. (Click on image for a larger view.)
A few days ago, Saturday looked like it would be sunny. Less so now. A mid-level area of moisture associated with this coastal disturbance will affect us on Saturday as an upper air disturbance moves through. This disturbance is weak and the forecast for showers may be over-done but several models currently show showers.
Periods of clouds and possibly a light, widely scattered shower late morning or early afternoon, especially from Philadelphia westward. Sunshine increases after 2 PM. Windy/Breezy as the disturbance moves through. Increasingly humid. High 86.5º sd 1.7º (NBM Blue Bell)
Updated Fri 7:45 PM: Friday afternoon’s GFS, just available, maintains a possibility of light showersearly afternoon Saturday. It is slower with the sun reappearing, so the 1-2 PM forecast of sunshine above may be unrealistic.
Updated Fri 9:45 PM: Tonight’s early models, the RAP, HRRR and NBM, all show showers mid-day on Saturday, moving from south to north. Highest probability is from about noon to 2 PM (± 1 hour.)
Updated Sat 8:10 AM: Despite the early sunshine, the models continue to to predict widely scattered showers, developing in the noon to 2 PM timeframe today. I’m not so sure, but I generally won’t go against the forecast of several models.
NBM 1 hour precip forecast at 2 PM Saturday, along with cloud forecast (darker = more clouds) Note that the Jersey shore remains mostly unaffected. (Click on image for a larger view.)
Following. this wave, high pressure will build back, pushing the frontal boundary northward again. A flow of increasingly hot and humid air, typical for the end of June and beginning of July, will begin to affect us on Sunday and beyond.
Mostly sunny , very warm and increasingly humid. (Dew points in the uncomfortable upper 60s) High 90.2.º sd 1.2º (NBM Blue Bell)
