Category Archives: Weather Updates


The original forecast for Sunday can be found here.

I’m trying to decide if I’m going out on that bike ride today. The forecast for periods of clouds with light, scattered showers early afternoon still holds. The latest HRRR has the axis of showers more into NJ, while the RAP maintains the track through Philadelphia.

Current Radar, Water Vapor, and superimposed RAP model Wind Streams (at 250 and 500 mb heights) at 9:30 AM Saturday. Shows moisture moving north and upper disturbance moving down over a ridge from the northwest. The forecast for periods of clouds with showers early afternoon still holds.

Latest RAP (RApid Refresh) model simulated radar forecast with accumulated rain (white contours), 2PM

RAP forecast 2 PM


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.


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)

Updated Wind Forecast

NBM wind meteogram Blue Bell

Memorial Day Weekend Weather Update

Forecast Updated Thu 9:16 AM in highlighted text below—

As indicated in this morning’s update, the position of the closed low has changed from yesterday’s models and yesterday’s outlook. There continues to be quite a bit of “spread” in the statistical models regarding the position of the closed low.

So there’s a lower confidence in the forecast than usual. (I expect this forecast to be revised again; upper closed lows are very difficult to forecast accurately.)

I wanted to post this update to indicate the major shift in the optimistic forecast I posted yesterday. Additionally, it does not look like a good NJ shore forecast in particular with windy/cloudy and chilly conditions.

The general trend is for more clouds and chances of showers over the weekend, and a reduction in the amount of rain Friday.


As the upper air flow transitions from a ridge to a trough, a surface low, originally to track over us will now track to our south. Unfortunately, this is translating into lower rain amounts for Friday. Instead of 1 inch, it looks more like 0.5 inches of rain. The latest ICON model has well over 1 inch of rain for Friday into Saturday. The NBM has 0.6 inches

Memorial Day Weekend

An upper air trough develops over our area on Saturday and lifts out by Monday. The upper trough will pinch off into a “closed low”.

The entire scenario has changed in a day. The position of the closed low is now south of us and the GFS is forecasting a coastal storm to form!

GFS model forecast for Sunday 5 PM with upper closed low (blue) and surface low (black) (Click on image for a larger view.)

(Average seasonal high temp is 77º-78º)


Saturday will be cloudy with scattered showers. High 61º sd 3.5º Chilly. (very high spread – high uncertainty!) The latest NBM has high temps 59º with an extremely high spread of 6º! The ICON has us in the mid 50s!


Cloudy. Dry according to the model blend (NBM). The GFS has some showers, especially in NJ. High 62.5º sd 5.2º High 60.0º sd 5.7º Chilly and windy.

Updated GFS forecast Sunday afternoon showing closed low (blue), surface low and showers. The forecast position of the 500 mb low keeps shifting. (Click on image for a larger view.)

Cloudy (GFS) or partly cloudy (NBM) High 70.0º sd 3.5º Partly to mostly sunny (GFS) and (NBM). High 71.0º sd 3.5º

Thu 09:20 AM Forecast Commentary —The aim of this blog is to demonstrate both how good the numerical weather models can be, but also to show how fluid and changeable  forecasts are, with model forecasts shifts  and changes clearly marked. 

In contrast, many of the TV forecasters seem to live in the moment and seem to hope that you don’t remember what they said the previous day (or previous program).