Category Archives: Commentary

Philly Winter Weather Update- 6:30 PM

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There’s a reason the TV weather people are essentially saying it will snow tomorrow and accumulations will range anywhere from 1-6 inches.  (That’s quite a range when you think about it!)

But the inconvenient truth is that even in established storms, the models have trouble with the exact placement of moisture plumes, the convergence of winds and the dynamics of lift that leads to precipitation.  As an example, the ‘blizzard’ a few weeks ago was expected to have its heaviest snow south and west of the city.   But the heaviest snow turned out to be north and west.  That error occurred with an existing storm system that was moving and strengthening in time, a relatively ‘simpler’ situation.

In the situation tomorrow, we are dealing with something totally different.  Tomorrow,  the models are ‘smart enough’ to predict the dynamic development of a new storm off the coast, but that’s where the limits of current atmospherics modeling end.   The models have already been inconsistent with the exact placement of this development and they simply can’t do the exact math with moisture, precipitation etc. because the exact initial state of the atmosphere where it will develop can’t be measured right this moment.

Anyone that get’s the exact snow amounts correct tomorrow in any specific location will do it only by luck.  And that includes me.

The best we can do tomorrow is talk about what the models are saying.  It’s likely not going to accurate enough.    For instance, the GFS has already backed off of it’s high QPF, while the NAM now shows another low pressure system forming towards evening at the coast, and its QPF is now very high, over 0.68 inches water.  So 1-6  inches is a good guess of a range.  It could be less and it could be more.

Sometimes, the most accurate forecast is being honest about the fact that we just can’t know precisely this time around. 

Interesting Weather Forecast

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There are many past winter storms where there have been large differences between the GFS and NAM models regarding QPF (amount of precip).

By definition ( if not by other measures) tomorrow’s rain is not a winter storm, but the models differ significantly with the amount of precipitation forecast.  The NAM has been advertising 2 inches of rain for PHL, while the GFS has a light rain, less than 0.50 inches total.  That’s a giant difference.

When dealing with snowfall, the differences are obvious when a heavy precip forecast goes bust.   It will be interesting to see if the NAM or the GFS model verifies tomorrow.   Something to consider for later in the season when we likely see divergence in forecasts.




About the upcoming rain

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An interesting scenario coming up, with moisture remnants of previous hurricane Patricia move up as a extratropical  everyday low pressure system.

There are big differences in the models here, with the GFS cranking out an inch or more of (much needed) rain, but with the NAM model having  the moisture and heavy rain missing us and moving to our west.   The NAM has high pressure blocking the rain and keeping the low pressure to our west.

With the NWS and TV forecasters predicting heavy rain for Wednesday, this will be a good test of the NAM vs GFS models for precipitation in the upcoming winter.    In such similar discrepancies last winter, the drier NAM model was the winner.   We’ll see.