Updates Tues 7:15 PM & Wed 8:30AM are highlighted

The forecast for the weekend continues to evolve regarding timing of the rain. Based on the latest GFS, we may luck out and have most of the rain fall Friday evening and Saturday evening, leaving much of the daytime hours cloudy, but rain-free.

A blocked weather pattern has been in place for the past several days with low pressure stuck in the middle of the country as well as low pressure system stuck in the Atlantic. This blocking pattern will evolve into a classic “omega block” (the jet stream flow resembles the Greek letter omega “ Ω “) by Friday.

GEFS Jet forecast for Friday.  (Click on image for a larger view.)

The latest GEFS model shows this Atlantic low pressure system remaining even more entrenched as the low pressure in the central US tries to move east.  High pressure caught in the middle will bring a moist, easterly wind flow and cloudiness to our area from Friday through Sunday. 

Periods of showers and rain are most likely Friday and again Sunday.  

Additionally, the Central US  low pressure may induce secondary low pressure formation off the Delaware coast.  This coastal secondary low may make the rain on Sunday moderately heavy (the ICON model cranks out almost 1.5 inches of rain Sunday.)  

GFS Forecast Sunday 8 AM  (Click on image for a larger view.)

The models have backed away from heavy rain. The rain is looking less continuous over the weekend. Indeed there may be periods of cloudy, but dry weather. The exact timing of the rain over the weekend has shifted. Sunday’s rain may occur mostly before daybreak.

Blocked weather patterns result in long stretches of persistent weather.  We never really know when the block will break down until we see some kicker system appear.  So the block will be in place…until it isn’t.  The models aren’t all that good at showing that moment in advance.  As a result, long range forecasts sometimes become totally undone.  

According to the GEFS  that persistent low in the Atlantic and its effects on our weather may be with us much of next week!



Sun Forecast Review — The clouds broke about 9-10 AM.   Some additional minor cloudiness moved in this afternoon, less than forecast on Friday.

The high temperatures near 64-65º were close to the NBM prediction only when one added the standard deviation (3.5º). 

As mentioned several weeks ago, mean high temps of the NBM forecast tend to run low (a recently documented NBM problem) and adding the standard deviation seems to get us closer to where we end up. 

Updates Sat 9:20 AM & Sat 6:55 PM highlighted

High pressure and cold air moved in Friday with a highly amplified upper trough.

The upper air trough is expected to move eastward on Saturday and the edge of an upper air ridge moves in. Sunny skies with somewhat closer to seasonable temperatures are expected Saturday afternoon. Saturday’s expected high temperature 52º±1.2º (NBM model- Blue Bell) (average high is near 59º)

However, a weak upper air disturbance will approach, embedded in the upper air flow. This will bring mid level cloudiness later Saturday afternoon, probably by 4 PM. 5 PM

HIREF model showing mid-level clouds moving in from the west about 3-4 PM Saturday (Click on image for a larger view.)

This disturbance moves through Saturday night and Sunday morning.
Several models do have some light sprinkles moving through late Saturday evening and very early Sunday morning.

Sunday starts cloudy. Some models crank out some light rain sprinkles early morning Sunday.

SREF showing upper air vorticity moving through Sunday morning. This disturbance is circulating around low pressure stuck off the coast of Nova Scotia.   (Click on image for a larger view.)

Clouds break for sunshine about noontime 10-11 AM but some clouds may move back in later Sunday afternoon. High temps 61.8º± 3.5º (NBM model- Blue Bell)

Winds for the weekend—

NBM wind meteogram Blue Bell PA   (Click on image for a larger view.)
What’s new with the weather models.

The GFS version 16 went live last week and is now the GFS operational model. 

Additional new weather model versions are being prepared for operational release in May: several HIRESW Models (High Resolution Window) which forecast out 48 hours as well as a 60 hour version.  

Also coming is a new version of the HIREF Model (High Resolution Forecast)  which will extend to 48 hours.    The HIREF model is looking interesting— I’ve been looking at its cloud cover forecasts and they have been  pretty good lately; they appear better than the new RAP and HRRR models which seem to over-forecast cloudiness. 

The cloud cover forecast above is from the new pre-release HIREF.  We’ll see how it does tomorrow.  


Wed 05:43 PM Update — It’s not just snowstorm forecasts that change. It’s just more obvious when a snow forecast has gone astray with a 10:1 ratio making the error more obvious.   

This afternoon’s models have moved the heavy rain south and eastward into N.J.  Here’s the latest NAM-NEST model—

NAM NEST model accumulated precip  (Click on image for a larger view.)


Wed 08:40 AM Update — The models have moved the axis of heavy rain today and tonight  just to the south of the immediate PHL area.  Current RAP has the axis of heavy rain as shown—

RAP Precip forecast  (Click on image for a larger view.)

The ICON and early morning Canadian models have the axis even further to our south.     So there remains uncertainty about total rainfall  from PHL and north.   Likely closer to 0.8-1.0″ 

from last night…

The models have begun predicting an increasingly heavy rain event for Philadelphia, from Wednesday afternoon into daybreak Thursday. Rain starts about noon on Wednesday and tapers early Thursday morning. The heaviest rain will occur Wednesday evening, towards midnight.

Previous model runs had an axis of heavy rain to the south of our area, through southern Delaware.

Today’s model runs, specifically the the ICON model and the Canadian High Resolution model (HRDPS) have the axis of heavy rain through the Philadelphia area.

(The ICON model did particularly well predicting the heavy rain in advance of  last week’s storm.)

ICON Model (German) accumulated rain forecast. (Click on image for a larger view.)

This afternoon’s HRRR and NAM models are moving in the same direction with total rainfall in the 1-1.8″ range for Philadelphia.

NAM accumulated rain forecast for Philadelphia by Thursday morning. (Click on image for a larger view.)

The heaviest rain will occur Wednesday evening, towards midnight. There remains some spread as to where the rain axis will be heaviest. The NAM is slightly south of Philadelphia, while the Canadian models are slightly north.

This event is still more than 24 hours away, but the trend towards heavy rain has been consistent.

Weather… and other things "up in the air"

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