It’s not just snowstorm accumulations that are difficult to predict….

Today’s models are a perfect example of where model prediction of QPF (total accumulated precipitation) is so difficult.

Had this been snowfall, we would have been predicting 1-2 feet of snow for today.  (QPF values were 1-2 inches of water).  Instead, last night’s late models revised that down to 0. 50 inches of water.  So a forecast of 2 foot  snowstorm would have had to be revised down to 5 inches!

(A spectacular change- the evening NAM was predicting 1.29 inches for PHL during the afternoon alone.  The 1 AM model run reduced that to about 0.30!)

Significant errors in forecast QPF  is very a frequent occurrence.   When it rains, no one really notices or cares when the QPF is not forecast properly.  But if it’s going snow, the models let you down in a way that is much more obvious.

It’s often not about “changes in the track of the storm” as the TV people like to talk about.  It’s more about the three dimensional distribution of energy and moisture in a storm that can’t be accurately measured and then modeled.

Today’s forecast is a case in point.

10 AM Update – Yet another turnaround….this morning’s NAM shows a QPF of 2.32 inches for PHL with a band of the heaviest rain right though the Delaware Valley.  A totally different precipitation coverage map compared to last night.   Even last night’s experimental “National Blend of Models” had decreased the precipitation forecast for today in PHL.

Good thing this isn’t falling as snow.

In addition to these large swings in forecast QPF for certain areas, snowfall amounts can be anywhere from QPFx10 or QPF x20!  So errors in QPF are multiplied by a factor of ten or more when we have snow.

While we’re at it, I get a real kick out of the TV “Future Tracker” where they depict the locations of the heavy rain in advance.  I want you all to know those forecasts are not to be ever taken literally.



2 thoughts on “It’s not just snowstorm accumulations that are difficult to predict….”

  1. Wow, it’s amazing with all the research and the advances in technology that it’s still so difficult to accurately predict the QPF. It will be interesting to see if machine learning or newer models will fare any better, or if it will always be “all wet”.
    Also, I wanted to thank you for your blog. I enjoy the explanations you provide on the data/models and you are more consistent than those folks on TV.

  2. Thanks for the compliment about the blog. There’s been incredible improvements in the models over the years and hopefully things will continue to improve. I’m not sure that the measured data input will ever be complete enough for any model to nail it. The NWS seems to be pursuing statistical models and statistical post-processing to improve accuracy.

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