Category Archives: Weather Updates


Sun 6:30 PM Forecast Review — The forecast for Henri was a moving target for the past few days. Looking back, here’s what’s prominent: the GFS probably was the most consistent with the track. The Canadian and German ICON’s westward track was plain wrong and for a time, adversely influenced the official forecasts.  The areas of maximum rain Saturday night were close but still incorrectly placed. Last night’s NBM forecast for continuous rain here on Sunday was wrong.  Of the shorter range forecast models, the new HIRESW-FV3 probably did the best    

Hurricane Henri Forecast Updated Sat 11:15 PM here
The previously posted Weekend Weather Forecast is here

Forecast Update Sun 12:06 PM — The latest GFS just available shows the same lull in the precipitation for much of the rest of the daytime today as rain rotates around the upper low. The moisture source is an outflow band from Henri.

Water Vapor with superimposed GFS jet level wind streamlines showing moisture moving in north of us today. (Click on image for a larger view.)
Radar (MRMS- Multi-Radar Multi-Sensor from NOAA) showing the curvature of the precipitation will keep the precipitation chances low in Philadelphia for the balance of the daytime hours; the rain is rotating around our area. The western edge of this moisture plume will move back in about 3AM as the entire upper air low moves northeastward. The main precip shield of Henri is visible. (Click on image for a larger view.)

Rain moves in again about 3 AM Monday, similar to the NBM meteogram in the previous update.

Total additional rain is now about 0.8 inches, according to the GFS.

Forecast Update Sun 10:55 AM— With on and off rainfall now predicted through Monday morning, a NBM meteogram is the best way to convey the rainfree periods and most likely time for additional rainfall—

NBM 12z meteogram showing likely times of rain and thunderstorms (Blue Bell, PA) Anything greater than 17-18% (red line) is a good chance when working with 1 hour probabilities. (Click on image for a larger view.)

Forecast Update Sun 8:20 AM— Here is the current radar-based rain accumulation so far with this storm —

Radar-based rain accumulation totals as of 8:15 AM Sunday. There were areas of 3-6 “ of rain that has already fallen.

Note that the banding of the heaviest rain was north and east of where the models had predicted where the heaviest rain would fall. That’s probably as a good as they can do.

The latest NBM still shows considerable rain for today into early Monday, as the tropical moisture from Henri is expected to move in from the northeast. Areas north of Doylestown and near Allentown are predicted to have significantly more than 1 inch of rain additional.

NBM additional rain through early Monday morning.

Forecast Update Sat 10:38 PM — Tonight’s models just becoming available show a band of heavy rain to affect the Philadelphia area tonight. Current radar shows this moisture plume already working it’s way in.

With the exception of the NBM, most of tonight’s models have the heavy rain falling after midnight tonight and tapering off during Sunday morning.

Here are several of tonight’s models. Most have very heavy rain in the 3″ range except for the new HIRESW-FV3 which is based more on the GFS. (The actual GFS won’t be available until 11:44 PM, past my time on duty.)

It should be noted that the latest ICON model from this afternoon had backed down to 1.1″. While the amounts below are impressive, I’m not totally convinced that they’re not over-predicting rain.

NBM Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
HRRR Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
RAP Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
NAM Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
HIRESW-ARW MEM2 Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
HIRESW-FV3 Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)

While it is clear that Hurricane Henri will not directly affect our area, there remains much uncertainty about the moisture flow when the tropical system enters Long Island and becomes an “extratropical” system.

There is currently a HUGE range of predicted rainfall in the Philadelphia area for Sunday.

The GFS model has close to 1 inch of rain falling Sunday in our area. The latest model blend (NBM) has 3-5 inches of rain, while the HRRR and RAP are in the 2-3 inch range. The NAM-NEST has less than 1 inch. The ICON has 1.1 inches and the Canadian GEM has 1.1 inches.

With the recognition that today’s showers were over-forecast, I’m going with the 1 inch and below range of rain through Sunday. But things could change.

The new models become available about 9:35 PM through midnight. I’ll update sometime after 10:30 PM.


Updated Sun 7:25 AM highlighted below:

This update is really a cut and paste from my original forecast from Friday.

A weak warm front will move in before daybreak on Sunday. Some scattered showers before daybreak. Clouds early will break for sunshine by afternoon. 

As a result of this warm front, Sunday will have noticeably higher humidity (dew points) than Saturday. Dew points will move back into the uncomfortable 70º range.

Today’s models have the weak front moving through earlier on Sunday afternoon than previously forecast. Latest models have scattered showers and thunderstorms developing in western suburbs as early as 2 PM. High temp tomorrow 89.4º 88.3º± 1.2º Dew points uncomfortable, near 70º

Most models have the activity diminishing as it moves into NJ.

More rain later Sunday night, after midnight.

Meteogram timing still on-track

NBM precip forecast for Sunday, Blue Bell, PA (The peak probability is 2-6 PM and is not that impressive. ) Click on image for a larger view.
Canadian GEM regional model forecast 2 PM Sunday (Click on image for a larger view.)

Water Vapor Imagery and Advancing Warm front

I love looking at the clouds and seeing what they tell about the model weather forecasts. Today’s approaching warm front is visible on water vapor imagery. Latest water vapor imagery is can be seen at this link.

Unlike visible satellite imagery, the colors of water vapor imagery indicates the water vapor temperature and height. Here we see upper air disturbances spilling out ahead of the warm front just west of central PA about 2 PM Saturday.
The advancing upper air disturbances visible on water vapor to the north and west caused the development of some mixed clouds and cumulus clouds with vertical growth on the Pennsylvania side of the Delaware River at about 3 PM Saturday.