Most of yesterday’s storms only affected the northern parts of our area, from central Montgomery county, Bucks county and northward.
A cold front moves through this evening. The models are again forecasting scattered thunderstorms, this time affecting more of the immediate Philadelphia area including areas not affected yesterday. The storms are expected to be scattered; again all areas won’t receive rain.
The weather parameters that are indicative of severe weather (Helicity, Shear, CAPE) are not as elevated as yesterday.
Sunny in the morning. Clouds increase about 3 PM
High temp 89.4º sd 1.8º Dew points 68-70º
Scattered thunderstorms 3-7 PM
Winds W 5-8 mph, increasing to 15 mph late afternoon.
[su_note note_color=”#ffffff”]Updated Sat 11:14 AM — After looking at this morning’s models, I’m trying to get out for a bike ride before the next batch of showers moves in. To my surprise, here’s the current radar—
I’m looking back at all the models. The Canadian High Resolution (HRDPS) model which isn’t bad but its timing is often not correct, has these rounds of showers moving through during the late morning and early afternoon. [/su_note]
What I meant by uncertainty in the forecast from last night is when we get showers moving through this morning not predicted by last night’s models.
(This morning’s showers WERE predicted by some of the late night models (2AM EDT runs) that became available around 4 AM.)
So what can we expect of the rest of the day? I’m going to approach as a probability forecast, not a deterministic forecast.
Here’s the latest model blend (NBM) 1 hour rain probability for 5 PM Saturday afternoon—
Note that these probabilities are 1 hour probabilities and generally anything greater than 20% is meaningful.
The NAM model just available this morning has showers and thunderstorms developing 2:30 PM northwestern suburbs and moving through between 3 and 6 PM. The NAM’s showers are more widespread.
Southern areas will be least affected today, according the the model blend, but the NAM has a wider area affected.
[su_note note_color=”#bceaed”]Forecast Review— The HIRESW models did the best with this forecast. The 11 AM -1 PM timeframe was very close. The RAP model picked up the early morning showers. The NAM and NAM NEST over-predicted the second line of showers during the afternoon.[/su_note]
Last night’s early models showed considerable spread with the timing of thunderstorms and showers today. I’ve had more time to look at those models as well as subsequent newer model runs from 2 AM (“06 Z model runs”).
So what’s happening?
The surface front moves through between 11 AM and 1 PM today with showers and thunderstorms as previously forecast. Prior to this time, expect windy and gusty conditions and possibly some scattered showers ahead of the front.
However, at the upper/mid levels of the atmosphere, additional disturbances (areas of “vorticity and vertical motion”) continue to move through in the upper level flow during the afternoon.
The actual upper air trough moves through slowly with these areas of vorticity triggering showers. Additional scattered showers and thunderstorms are possible during the afternoon until the upper air trough moves through. The NAM NEST shows this activity may be most enhanced at 2-4 PM.