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Central PA - Spring 2021


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2 minutes ago, Bubbler86 said:

12Z EC still pretty gung ho though Friday further south.  In defense of this it is suggesting over 1/2 of the qpf would be from a disturbance that slides under PA and sort of dies out to our S/W.  So not all pop up broad-brushing. 

image.thumb.png.35cd9c2598dcca6e00e858e26a6217a9.png

 

In all honesty, i would take that .7 at this point ya know

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3 minutes ago, TimB84 said:

Not that it absolves the forecast of any incorrectness, because the percentages still don’t work out in the favor of 60%, but isn’t it (percentage chance that precip will form)*(percentage of the area that will see measurable precip)? So if the met is 80% confident that precip will form, and if it does, it will cover 50% of the area, 80%*50% = 40% chance of rain?

You may be right and I may have simpled it down but some people take the percentages more literally in that if the 40 happens, no one should see rain, if the 60 happens then everyone.  I know that is nonsense but it is how some read their local forecast. 

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53 minutes ago, TimB84 said:

Not that it absolves the forecast of any incorrectness, because the percentages still don’t work out in the favor of 60%, but isn’t it (percentage chance that precip will form)*(percentage of the area that will see measurable precip)? So if the met is 80% confident that precip will form, and if it does, it will cover 50% of the area, 80%*50% = 40% chance of rain?

Here is the official explanation which sounds similar to yours though they go about explaining it in a convoluted way in my opinion.  But what it does do is make the 60% chances over the last two days look even worse as to a forecast.  It would take some pretty high confidence and coverage to get to 60%.   A better forecast maybe should have been 80% confidence and 30% coverage or 25%. 

https://www.weather.gov/media/pah/WeatherEducation/pop.pdf

 

 

 

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5 minutes ago, Bubbler86 said:

Here is the official explanation which sounds similar to yours though they go about explaining it in a convoluted way in my opinion.  But what it does do is make the 60% chances over the last two days look even worse as to a forecast.  It would take some pretty high confidence and coverage to get to 60%.   A better forecast maybe should have been 80% confidence and 30% coverage or 25%. 

https://www.weather.gov/media/pah/WeatherEducation/pop.pdf

 

 

 

Yes, to get a 60% chance of rain by that formula, you’re implying that at least 60% of the area gets measurable precip.

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3 minutes ago, TimB84 said:

Yes, to get a 60% chance of rain by that formula, you’re implying that at least 60% of the area gets measurable precip.

This was enlightening as I did not realize there was a confidence factor in it though as you alluded to there are only so many ways to get to a given percentage.  A forecast could be 30% confident that 100% of the area sees .01 or more but that still comes down to 30%.  100% confident that 40% of the area sees .01?  Only 40%.  

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I have always taken the POPs to mean the probability of seeing measurable precipitation at any given location within the forecast area, over the forecasted time period.  Likewise, I believe you can interpret this to mean that roughly the same percentage of total surface area within the forecast zone will see measurable precipitation, over the forecasted time period.

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3 minutes ago, Bubbler86 said:

This was enlightening as I did not realize there was a confidence factor in it though as you alluded to there are only so many ways to get to a given percentage.  A forecast could be 30% confident that 100% of the area sees .01 or more but that still comes down to 30%.  100% confident that 40% of the area sees .01?  Only 40%.  

I second Mount Joy’s explanation. I couldn’t have wordsmithed it that well.

2 minutes ago, Mount Joy Snowman said:

I have always taken the POPs to mean the probability of seeing measurable precipitation at any given location within the forecast area, over the forecasted time period.  Likewise, I believe you can interpret this to mean that roughly the same percentage of total surface area within the forecast zone will see measurable precipitation, over the forecasted time period.

This is well articulated as to why this works. So by “the area” or “the forecast zone”, does the NWS view this as by county?

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1 hour ago, TimB84 said:

There seems to be a lot of broad-brushing, especially on the forecasting side, and especially with the NWS (and that’s probably understandable given their wider audience). That said, I would guess that the issue on the model side has something to do with resolution, but like @Bubbler86said, even the hi-res models only seem to handle it marginally better.

Yep, it's a tale as old as time, models just don't have the resolution or competency to handle the finer details and miniscule size of convective activity.  I'm sure we'll get better over time but don't know that we'll ever be fully there, just too much randomness and unpredictability with convection, particularly isolated diurnally-sun-driven pop-up storms.

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2 minutes ago, TimB84 said:

 

This is well articulated as to why this works. So by “the area” or “the forecast zone”, does the NWS view this as by county?

I would assume it is specific forecast zones vs using county boundaries.  RE: Parts of Franklin, Adams and Chester counties are in the LSV while parts are not.  

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Just now, Bubbler86 said:

I would assume it is specific forecast zones vs using county boundaries.  RE: Parts of Franklin, Adams and Chester counties are in the LSV while parts are not.  

I would assume the same. I would imagine there are certainly times when one location in a county has a 40% chance of rain and another has 70%. They really can’t use a rigid definition of that formula for practical reasons.

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3 minutes ago, TimB84 said:

I second Mount Joy’s explanation. I couldn’t have wordsmithed it that well.

This is well articulated as to why this works. So by “the area” or “the forecast zone”, does the NWS view this as by county?

Thank you kind sir, and I'm not entirely sure how they parse their gridded forecasts.  Perhaps others would know?

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Just now, Mount Joy Snowman said:

Yep, it's a tale as old as time, models just don't have the resolution or competency to handle the finer details and miniscule size of convective activity.  I'm sure we'll get better over time but don't know that we'll ever be fully there, just too much randomness and unpredictability with convection, particularly isolated diurnally-sun-driven pop-up storms.

I would have less issue with it if the qpf totals were not broad-brushed on what is supposed to be a decent resolution 12K Nam.  I would realize that the popcorn nature of the qpf output would be very suspect but that is better than the blob of precip it has shown too often the last few days.  

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1 minute ago, Bubbler86 said:

I would have less issue with it if the qpf totals were not broad-brushed on what is supposed to be a decent resolution 12K Nam.  I would realize that the popcorn nature of the qpf output would be very suspect but that is better than the blob of precip it has shown too often the last few days.  

Agreed, and the GFS has always been notorious for overdoing the light precipitation, was one of its known biases for many years.  But you're right some of the high-res models should be better.  Who knows though.  I mean, think about one of those random days where no precipitation is in the forecast but come mid-afternoon the radar shows a couple kernels of popcorn, driven by some uneven heating of surfaces due to pockets of sunshine and perhaps enhanced by some terrain elements, how the heck do we ever get to a place where those are predicted with any type of accuracy?  Hard to envision.

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8 minutes ago, Bubbler86 said:

I would have less issue with it if the qpf totals were not broad-brushed on what is supposed to be a decent resolution 12K Nam.  I would realize that the popcorn nature of the qpf output would be very suspect but that is better than the blob of precip it has shown too often the last few days.  

There is a lot of precip currently falling in western CTP, it appears. 5 SPS currently issued.

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8 minutes ago, Mount Joy Snowman said:

Agreed, and the GFS has always been notorious for overdoing the light precipitation, was one of its known biases for many years.  But you're right some of the high-res models should be better.  Who knows though.  I mean, think about one of those random days where no precipitation is in the forecast but come mid-afternoon the radar shows a couple kernels of popcorn, driven by some uneven heating of surfaces due to pockets of sunshine and perhaps enhanced by some terrain elements, how the heck do we ever get to a place where those are predicted with any type of accuracy?  Hard to envision.

Yea, not expecting perfection.  Some of it is forecasting ability and using other forecasted obs to know what has happened in the past that models have missed.  

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8 minutes ago, TimB84 said:

There is a lot of precip currently falling in western CTP, it appears. 5 SPS currently issued.

Taking a look at the far out view would make one thing its a super rainy through much of the eastern 1/3 of the country.  Yellows and reds all over the place. 

image.png.839da591625aa2f675017cd9d5d2b947.png


 

 

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Can anyone familiar with the State College area offer advice on a good brewery to check out? Playing golf at Mountain View with the in-laws tomorrow and looking to try some beer while in the area before heading back. Thanks!

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Finally!  I finally got a thunderstorm to move over me slowly from north to south.  Decent amount of thunder preceded its arrival along with during the storm.  Picked up 0.53" in about 20 minutes.  This is the first precip of this week (starting last Sunday).  Hopefully more to come.  Temp dropped 10 degrees from 83 to 73.  Always like that feature of thunderstorms although the high humidity it leaves behind isn't so pleasant.

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