PHILLY SUNDAY FORECAST UPDATE

The latest models have heavy rain before daybreak Sunday, then moderate rain in the morning.

While skies will remain cloudy, the rain should be over about 1 pm on Sunday and much of the afternoon will be rain free (for a hedge, maybe some sprinkles).

Sunday 1 PM – Rain will be ending shortly. Temperatures never warmed to the near- 60 degrees predicted by the models.

 

 

PHILLY WEEKEND WEATHER FORECAST

The rainy, dreary weather forecast for the upcoming weekend has been well advertised.  What can I add to the information?

Well, it appears that the early part of Saturday morning will be [relatively] dry, but there is still a small chance of light sprinkles or drizzle in the morning with increasingly likelihood by noon and likely by the afternoon.  So, if you’re interested in outdoor activities, Saturday morning is the time.   Either way, any showers will be light until late in the afternoon. High 59.

By Saturday night, the chance of rain increases and moderate rain is likely as the evening progresses.

Sunday will be rainy, and rain could be moderate to heavy during the morning, decreasing in intensity during the afternoon.   High 60.

Sunday GFS forecast

Saturday 2 pm: Today’s rains moved in close to expected, but the initial burst was a lot heavier than I had predicted and a bit earlier. The models still have moderate to heavy rain for much of Sunday, especially during the morning.

 

FORECAST POST-MORTEM

We had about 2.5 inches snow just northwest of the city.  I had predicted zero to 1 around here, so what happened?

First, it was always what I called a boundary conditions forecast, since the forecast parameters for snow vs. sleet were so on the borderline of different outcomes.   I wrestled with the forecast for days.

Here’s what I got wrong- temperatures at two critical levels of the atmosphere below 7000 feet were forecast to stay at or below freezing.  I ignored these temperatures, since the other critical level of the atmosphere, “the thickness level 1000-500 mb”, (a measure of the average temperature below 18,000 feet) has always served me well and it was expected to warm up.  It didn’t.

It turns out the earlier NAM’s predicted 500 mb thickness level was wrong.   It had been forecast earlier in the day to rise to 5440 meters by 7 PM.  (Too warm for snow usually.)

However,  the 7 PM  NAM initial conditions showed the thickness level to be 5420 and not increasing.  (Snow vs rain occurs if  the thickness level is at or below 5400 in our area.)    So the mid levels of the atmosphere never warmed as expected.

This “critical thickness” level error was made by both the GFS and NAM models.   Since the “predicted thickness” level was so close to the “critical thickness”,  any error was going to blow the forecast.

Indeed, the NWS had to issue a Winter Storm Warning last night as late as 6:30 PM.  So they were also caught by surprise.

That wasn’t all that went wrong with the models forecast-  All models, including the National Blend of Models had the storm intensifying and lasting through at least midnight.  The NBM had the chance of precip of near 100%  at midnight in PHL.  But the storm was a faster mover, didn’t develop as expected, and was long out of here by 10 PM!

So that’s what happened — Parameters on the borderline weren’t accurately predicted and the storm speed and development wasn’t modeled properly!

Hey, bad modeling by engineers designing  bridges causes them collapse;  this bad forecast is just going to just melt away today!

WEATHER UPDATE 6PM

7 pm Saturday-Well, I wrestled with this storm and my usual approach led me to a somewhat poor forecast .  Snow has picked up in intensity and temperatures have dropped to 32. Temperatures in the lower levels of the  atmosphere prevailed  Maybe I’ll get another chance this season?

 

The forecast from last night and this morning remains on track-  “After an initial start as light snow between 4-6 pm, it appears to be a mostly sleet and rain storm in Philadelphia and its immediate suburbs before changing back briefly to snow before ending about 2 AM.”

While the lower levels (below 6000 feet) of the atmosphere are below freezing causing sleet and some snow to mix in,  the average temperature of the mid level of the atmosphere (below 18,000 feet)  is too warm to support snow formation.  Add to that the warm ground conditions from earlier today and little accumulation is expected in PHL and the immediate suburbs.

Below is the current radar at 5:51 PM with the area of mostly rain/sleet south of the red line.  (Sorry, difficult to see the county boundaries)

Storm Forecasts & Other Things "up in the air".