Upcoming weekend forecast features extreme uncertainty

September 24, 2012, 10:35 pm
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Monday afternoon, I posted on my Facebook Page about the uncertainty associated with the forecast for the upcoming weekend:

My forecasts are always honest, but sometimes that's not what we want to hear. For example, while Friday holds 50-60% agreement among all possible forecast solutions (essentially 50-60% predictability...and the day looks decent, BTW), Saturday AND Sunday hold less than 10% predictability.  In other words, of all possible solutions, the most agreement you'll find is 10%. That's terrible, and shows that all of this weekend's forecasts, at this juncture, are luck.  So, the most honest forecast is a chance for showers both days, but too early to know which day better, or if we can squeeze out a dry day. I'll keep you posted...


So, how does a forecast become so uncertain?  How is it possible that the solution that seems "most likely" has only 10% agreement?  In this case, the jet stream winds aloft - the fast river of air high in the sky that steers our storms and separates cool and from warm in the atmosphere - are the root of the uncertainty.  Consider the forecast weather pattern for Friday, which holds nearly 60% certainty:

WEEKEND_PRECIP

Though the weather pattern at this time may have moderate confidence, there are some important features that promote uncertainty: 1) An amplified pattern, meaning a sharp ridge/trough, or bump up and dip down in the jet stream.  This introduces difficulty with timing, speed and intensity of energetic disturbances being steered by the jet stream wind. 2) A strong upper level storm, full of energy and carrying cold air with it.  Questions often arise over how each spoke of energy rotating around the upper level storm will interact with it, and in turn, how that will shape the jet stream.  3) Available tropical and Atlantic moisture that can contribute to thunderstorm and heavy rain band development, resulting in the release of latent heat into the atmosphere, and changing the weather pattern.

By Sunday, the atmospheric pattern looks like this:

WEEKEND_PRECIP2

The above representation is, essentially, an average of the possible solutions - in question as of this early week post is position of the upper level low, strength of the upper level low, position and speed of the jet stream, and therefore, placement/intensity of warm and cold air either side of the jet stream.  All of this will contribute to placement, timing and intensity of possible rain bands this weekend, and is the reason there is no greater than 10% predictability.  At this stage, the most honest forecast is to acknowledge a chance of showers both weekend days, but also realize that either day's forecast should turn wetter, or drier, as we near the weekend.

-Matt

Tags: New England, NECN, showers , weekend weather
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