Category Archives: Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST- FRI EARLY EDITION

I’ve been reviewing the latest runs of the NAM and GFS models.  Both models have a warm front moving up from the south and through Philadelphia  between 8 AM and 10 AM Saturday morning.

There are large differences between the models regarding the precipitation.  Specifically, the NAM cuts off the precipitation following the frontal passage, allowing things to dry out after 10AM until late afternoon, when additional thunderstorms may form.   QPF values are less than 0.30 inches water.

However, the GFS maintains enough dynamics for showers and thunderstorms throughout Saturday, especially early afternoon Saturday.   QPF values are over 1.20 inches water!

I’ve been looking for clues as to which model forecast is correct.  For some reason, the non-operational NBM and EKDMOS blended models have not been available for the past day. Other models (CMC, ICON) support the wetter GFS.

My own experience with warm fronts in summer is that things tend to clear out following a warm frontal passage.  So I’m inclined to go with a more optimistic forecast.

So… after early morning showers, expect precipitation to end by 10 AM with mostly cloudy conditions, but skies may brighten considerably.   Expect scattered thunderstorms to re-develop late afternoon into evening.  High 81.   (A lower confidence forecast than usual.)

For Sunday, a mix of sun and clouds.  Warm and humid.  High 88.  Chance of thunderstorms late afternoon.

I’ll update Friday evening, but it may be late.

[su_note note_color=”#d9f2da”]Fri 10:30 pm update: Tonight’s NAM continues with a break in the showers between 10 am and about 5pm Saturday.  So staying with the optimistic forecast.[/su_note]

WEEKEND WEATHER FORECAST

This weekend’s weather will be the nicest we’ve had in many weeks and the forecast  will be easier as well —  a  far cry from the boundary conditions with a stalled front  we’ve had to deal with over past weekends.

Both Saturday and Sunday will be sunny.  High on Saturday 84 and high on Sunday 90!

[su_note note_color=”#d9f2da”]Sat AM Update: High temperatures have shifted a bit higher for Saturday 86 and Sunday 91.[/su_note]

We’re finally entering a summer-like weather stretch. Monday’s high is expected to be 96-98 with increasing dew points.


By the way, the NWS announced today an expected major update in the GFS model. The new GFS model, to be referred to as the FV3-GFS,  will become operational in mid January 2019, in time to improve snow forecasts.

From what I could infer from reading about the new model, its improvement is in part due to changes in the geometry of how the globe’s atmosphere  is broken down into a 3 dimensional mathematical grid.

The new geometry, a Finite Volume Cubed Sphere, (hence FV3) allows for better computation at the edges of each cubed grid, and reduces approximations where some of the physics would ordinarily result in infinite mathematical series or unsolvable equations.  This should result in better forecasts .

The pre-release version of this new model was used for this forecast’s high temperatures. We’ll see if the highs on Monday really reach 96+.