Updated Sunday forecast Sun 8 AM highlighted below

This weekend’s weather will be marked by an upper air pattern change, where the cool upper air cyclonic flow will transition to a warmer upper air ridge by next week.


Prior to the upper ridge taking hold, an upper air low near our area will trigger some instability cloudiness and widely scattered showers Saturday afternoon. Forecasting the exact location of the showers is not possible. The GFS has us dry while the higher resolution “convection allowing” models show some light showers springing up.

Sunny in the morning, instability puffy cumulus partial cloudiness in the afternoon with widely scattered showers possible. There isn’t much “precipitable water”, so any scattered showers will be light. High temp 76.1º sd 1.1º (higher than usual confidence.)


A more developed warm front will be lifting through with most of the rain and dynamics far southwest of our area. Some models threaten our western suburbs with showers, but the model blend is keeping the immediate PHL area mostly cloudy and dry.

Despite impressive radar echos in western Pennsylvania Sunday morning, the HREF and NBM models keep the heavy showers and thunderstorms west of the immediate PHL area Sunday.

Some sun in the early morning, then mostly cloudy. A chance of widely scattered showers/thundershowers, mostly far western suburbs. Again, there isn’t much “precipitable water”, although more than Saturday. High 74º sd 2.8º



Recently, NOAA updated the global forecast weather model (GFS) to version 16. This version had numerous enhancements including an upgrade in cloud physics and an increase in the number of vertical levels of the atmosphere for which weather parameters are calculated.

Another fascinating upgrade is planned for next week.

The version 16.1 upgrade will incorporate a significant improvement in the accuracy of various three dimensional atmospheric measurements (such as humidity, temperature, pressure) that go into the “initialization” — the starting point for each GFS model run.

Incredibly, the new atmospheric data will come from the radio signal of GPS satellites!

Basically, the signal transmitted by GPS satellites will be picked up by other low orbit satellites as the GPS signal travels through the atmosphere.

The technique, known as radio occultation, detects changes in the GPS radio signal as it passes through different atmospheric thicknesses and angles. The changes in the signal reveal the physical measurements of temperatures, pressure, humidity to higher precision..

This improvement in data input is expected improve GFS model forecast accuracy.

More information about radio occultation can be found here.


Sun 07:55 PM Forecast Review — The HIRESW-ARW did the best with this forecast.


Considerable spread continues in the model forecasts today with the amount of rain/showers Sunday afternoon.

Many of the higher resolution models have only some sprinkles, mostly northwest of our area. The model blend has virtually nothing in the immediate PHL area.

This is reflected in the NBM radar/rain forecast:

 Model Blend  “NBM” 3 PM  -Virtually no rain in PHL  The outer white contour is 0.02″ rain


The GFS has more of a precipitation shield nosing into our immediate area between 12 and 3 PM

GFS Radar/ precip forecast 3 PM

The new HIRESW-ARW just available also has a hard time getting any measurable rain in Philadelphia with the 0.02 contour just northwest.

So a difficult call. I guess we’ll find out!

Weather… and other things "up in the air"

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