Monthly Archives: June 2021

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Updated Sat 8:10 AM highlighted below

Updated Fri 9:45 PM highlighted below

Updated Fri 7:45 PM highlighted below

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 showers early 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)

Winds
NBM Wind Meteogram for Blue Bell, PA
Summer Storms

THUNDERSTORMS: MONDAY

Forecast Updated 7:06 PM– Looking further at the latest GFS, some thunderstorms may fire up again at 11 PM to midnight.

Forecast Updated 6:15 PM– The latest GFS, just available, shows considerable convective inhibition (see update below) as the storms approach Philadelphia. As mentioned earlier, this may cause the storms to weaken as they approach. The exact point of weakening may be to our west, or just to our east, as the GFS is a relatively low resolution model. Much of the energy is being held back, as the main front will pass through on Tuesday with additional rain. We will see.

Forecast Updated 12:10 PM in highlighted text below:

A strong cold front will approach this evening. Ahead of the actual front, a prefrontal trough will trigger thunderstorms in the Philadelphia area this evening, likely starting between 5:30 6 PM and and 8 PM. Additional thunderstorms may move through up to around midnight.

Conditions are favorable for moderate to strong severe thunderstorms with CAPE values (convective available potential energy) in the 1500 J/Kg range and instability values (Lifted Index) around -5..5ºK. (More negative is stronger here.)

The latest HIRESW-ARW and HREF show higher CAPE values, in the 2000 + J/Kg range, especially western suburbs. Expect moderate to severe thunderstorms as early as 5:30 PM in far western suburbs, with heavy rain, moving into Philadelphia. They may weaken as they move further east.

All models show high CIN parameter inhibiting thunderstorm strength, just east of Philadelphia into NJ developing by 9 PM. So any thunderstorms making it into NJ may weaken.

Precipitable water values are high, in the 1.7-2” water range.

Last night’s models show considerable diminishment of the activity as it enters into NJ after 8 PM, as shear and storm motion vectors become opposed. CIN (“Convective Inhibition”) values become elevated in NJ after 8 PM. (Think of CIN as the anti-energy of CAPE.) Of course, this could change.

Last Monday, the GFS did the best with timing in a somewhat similar setup and I’ll be leaning on this for today.

I’ll likely update sometime during the afternoon.