Sat 9 AM Update: The 2AM model runs (06z UT) show sun with a bit more clouds and a slightly increased chance of scattered thunderstorms in the afternoon. Highs about 88-89.
from last night…
A typical August pattern— a weak, poorly defined, surface pressure pattern will be with us over the weekend. For most of both Saturday and Sunday, the wind flow will be from the SE, bringing warm and very humid air into our area. The southeasterly flow is the result of a weak cyclonic circulation off the coast from a poorly defined stalled frontal boundary to our south.
The models are in general agreement that Saturday will be mostly sunny, hot and humid. The NAM has more morning cloudiness than the GFS, but both show clouds breaking for sunshine. There is a large range in the temperatures predicted. Most of the models are in the 89-91 degree range, but the GFS is showing a high of 96 in the city, probably a modeling error of this new model (which is the FV3-GFS). (The Canadian HRDPS shows a high of 92.)
There is a low chance of widely scattered showers/thunderstorms in the late afternoon and evening, mostly north and west on Saturday.
A repeat performance on Sunday, with the models showing temperatures in the range of 90-92, again with the GFS model somewhat higher in the immediate PHL area. There’s a somewhat higher chance of thunderstorms late afternoon on Sunday.
The humidity will be noticeable with dew points in the uncomfortable range of 69-72 both days.
I’ll update over the weekend to narrow down the chance and location of any showers.
Fri 10 PM Update: Tonight’s RAP (Rapid Refresh) model shows an upper air disturbance moving through to our south on Saturday. Significant high, thin cloudiness is predicted, especially south of the city, late Saturday morning into the afternoon.
This weekend’s forecast is as straightforward as it gets.
High pressure builds in from the west. Sunny skies both Saturday and Sunday, near zero chance of rain. Some fair weather clouds at times in the afternoon.
Sat 7:30 AM Update: High temperatures Saturday updated to about 86-87. Scattered thunderstorms still expected mid to late afternoon as depicted in last night’s model data below.
Sunday looks to be considerably drier and somewhat warmer with a high of 88-89.
A typical summer weather pattern with a diffuse stalled frontal boundary to our south.
Summer daytime heating combined with the entrance of areas of vorticity and moderately high moisture (precipitable water values about 1.6 inches) will result in instability and mostly afternoon and early evening thundershowers.
The sky conditions will be similar to today (Friday) with most areas seeing considerable sunshine mixed with clouds. Highs around 85.
What everyone wants to know— when will these showers develop where? Unfortunately, the weather models are just not that capable in this situation, but I’ve looked at the models that did fairly well with the showers today and I’m providing a sampling of what I think might capture the answers. (Three high resolution models, including the new Canadian HRDPS.)
As shown below, the models are showing the period between 2-5 PM Saturday as having the highest areal precipitation coverage. These areas are now closer to Philadelphia than had been showing on my Thursday’s Outlook post.
For Sunday, the larger scale models (NAM, GFS) show things drier with a similar mix of sun and clouds. High 86.