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Questions on errors in weather models
I just looked at the weather.com information for Fitchburg, MA.
The report shows the current temperature (actually, the 7:52 PM temperature) as 81 degrees. Looking at the hourly forecast, the 9 PM temperature is predicted to be 71 degrees, 68 at 10 PM, and so on. I've checked the weather reports for a few local towns several times today, and the situation was always the same: The current temperature has been much higher than the series of temperature predictions in the hourly forecast. And today isn't unique -- I've seen that happen many times with errors in both directions. My question is: How can this happen? I can understand how distant forecasts can be off, but shouldn't the current temperature be a strong factor in short-term temperature prediction? Thanks for any insight. B.J. -- B.J. Herbison / / http://www.herbison.com/herbison/bj.html The Next Asylum / 203 Long Hill Road / Bolton, MA 01740-1421 / +1 978 634-1061 -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
B.J. Herbison wrote:
I just looked at the weather.com information for Fitchburg, MA. The report shows the current temperature (actually, the 7:52 PM temperature) as 81 degrees. Looking at the hourly forecast, the 9 PM temperature is predicted to be 71 degrees, 68 at 10 PM, and so on. I've checked the weather reports for a few local towns several times today, and the situation was always the same: The current temperature has been much higher than the series of temperature predictions in the hourly forecast. And today isn't unique -- I've seen that happen many times with errors in both directions. My question is: How can this happen? I can understand how distant forecasts can be off, but shouldn't the current temperature be a strong factor in short-term temperature prediction? You should probably ask them - but I don't know if you'd get a response. I thought automated forecasts of that sort adjusted to some extent to current conditions - at least for the short term. Maybe they don't. Programming something like that would not be difficult - some weather providers actually have their forecasts updated automatically based on trends of radar & satellite images - though I think we are still a long way from that being as good as a human for that. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
I checked the forecast for today for my zip code (18344), and after a few
clicks & pop-up ads, I got the following : Thursday, May 13 Temp °F Feels Like Dew Point Precip. Humid. Wind 11 AM Partly Cloudy 76° 78° 60° 20% 59% From the South at 5 mph 12 PM Partly Cloudy 78° 80° 60° 20% 54% From the South at 6 mph 1 PM Isolated T-Storms 79° 80° 60° 30% 52% From the South at 7 mph 2 PM Isolated T-Storms 79° 80° 60° 30% 52% From the South at 8 mph 3 PM Isolated T-Storms 79° 80° 60° 30% 51% From the South at 8 mph 4 PM Isolated T-Storms 78° 79° 59° 30% 51% From the South at 8 mph 5 PM Isolated T-Storms 77° 79° 58° 30% 52% From the South at 8 mph 6 PM Isolated T-Storms 76° 78° 58° 30% 54% From the South at 7 mph 7 PM Isolated T-Storms 74° 74° 58° 30% 57% From the South Southeast at 6 mph 8 PM Isolated T-Storms 72° 72° 58° 30% 61% From the South Southeast at 6 mph 9 PM Isolated T-Storms 69° 69° 58° 30% 66% From the Southeast at 6 mph 10 PM Partly Cloudy 67° 67° 58° 20% 72% From the Southeast at 6 mph 11 PM Partly Cloudy 65° 65° 57° 20% 77% From the Southeast at 6 mph Friday, May 14 12 AM Partly Cloudy 63° 63° 57° 20% 81% From the East Southeast at 6 mph 1 AM Partly Cloudy 61° 61° 57° 20% 85% From the East Southeast at 4 mph 2 AM Partly Cloudy 60° 60° 56° 20% 87% From the East Southeast at 4 mph 3 AM Partly Cloudy 59° 59° 56° 20% 88% From the Southeast at 4 mph 4 AM Mostly Cloudy 59° 59° 55° 20% 88% From the Southeast at 4 mph 5 AM Mostly Cloudy 58° 58° 54° 20% 88% From the Southeast at 4 mph The temperatures are good - 77 at MPO (which I don't think is in zip 18344, actually) and my location. That was updated at 10:30 AM though. Their day/night forecast is for a high/low of 80/57 and precip probabilities of 30/30 %. The thing I find most peculiar is the precipitation probabilities. If they are hourly probabilities, this would imply a very large likelihood of precip during both day and nighttime periods. If the probabilities are considered independent, the likelihood of precip from 11 AM to 8 PM is 1 - (.2^2 * .7^7) = .947 = 94.7 %. They are not close to being independent events though - if thunderstorms do form, it is likely precip will occur a few hours of the day. Yet if the probability each hour is 30 %, the daily probability should be much greater - because the storms won't persist all day. So the correct interpretation of the forecasts shown above is that there's a 30 % chance of precip today, and if it does occur, it'll persist all day (i.e., occur each of those hours the probability is 30 %). That is obviously not what they mean, but if the daily probability is 30 %, those for each hour should be much lower. Note that the nighttime probabilities are almost the same as the daytime ones - 20 % every hour between 10 PM & 5 AM. I can't believe precip is nearly as likely those times as it is between 2 & 7 PM. I am thinking the probability is about 70 % for the daytime period now and 20 % for the nighttime - hourly probability would be about 5 % ending at 12 PM, peaking at about 25-30 % ending at 5 PM, then decreasing to about 10 % ending at 10 PM and to near 0 % ending at 5 AM. Saying that is one thing, programming a computer to think like that another. Maybe I am overestimating a bit, though storms are common over the elevated locations during warm & humid days. We had some hail as large as 3/8 inch diameter yesterday. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
Joseph Bartlo wrote:
I checked the forecast for today for my zip code (18344), and after a few clicks & pop-up ads, I got the following : Thursday, May 13 Temp °F Feels Like Dew Point Precip. Humid. Wind 11 AM Partly Cloudy 76° 78° 60° 20% 59% From the South at 5 mph 12 PM Partly Cloudy 78° 80° 60° 20% 54% From the South at 6 mph 1 PM Isolated T-Storms 79° 80° 60° 30% 52% From the South at 7 mph 2 PM Isolated T-Storms 79° 80° 60° 30% 52% From the South at 8 mph 3 PM Isolated T-Storms 79° 80° 60° 30% 51% From the South at 8 mph 4 PM Isolated T-Storms 78° 79° 59° 30% 51% From the South at 8 mph 5 PM Isolated T-Storms 77° 79° 58° 30% 52% From the South at 8 mph 6 PM Isolated T-Storms 76° 78° 58° 30% 54% From the South at 7 mph 7 PM Isolated T-Storms 74° 74° 58° 30% 57% From the South Southeast at 6 mph 8 PM Isolated T-Storms 72° 72° 58° 30% 61% From the South Southeast at 6 mph 9 PM Isolated T-Storms 69° 69° 58° 30% 66% From the Southeast at 6 mph 10 PM Partly Cloudy 67° 67° 58° 20% 72% From the Southeast at 6 mph 11 PM Partly Cloudy 65° 65° 57° 20% 77% From the Southeast at 6 mph Friday, May 14 12 AM Partly Cloudy 63° 63° 57° 20% 81% From the East Southeast at 6 mph 1 AM Partly Cloudy 61° 61° 57° 20% 85% From the East Southeast at 4 mph 2 AM Partly Cloudy 60° 60° 56° 20% 87% From the East Southeast at 4 mph 3 AM Partly Cloudy 59° 59° 56° 20% 88% From the Southeast at 4 mph 4 AM Mostly Cloudy 59° 59° 55° 20% 88% From the Southeast at 4 mph 5 AM Mostly Cloudy 58° 58° 54° 20% 88% From the Southeast at 4 mph The temperatures are good - 77 at MPO (which I don't think is in zip 18344, actually) and my location. That was updated at 10:30 AM though. Their day/night forecast is for a high/low of 80/57 and precip probabilities of 30/30 %. The thing I find most peculiar is the precipitation probabilities. If they are hourly probabilities, this would imply a very large likelihood of precip during both day and nighttime periods. If the probabilities are considered independent, the likelihood of precip from 11 AM to 8 PM is 1 - (.2^2 * .7^7) = .947 = 94.7 %. They are not close to being independent events though - if thunderstorms do form, it is likely precip will occur a few hours of the day. Yet if the probability each hour is 30 %, the daily probability should be much greater - because the storms won't persist all day. So the correct interpretation of the forecasts shown above is that there's a 30 % chance of precip today, and if it does occur, it'll persist all day (i.e., occur each of those hours the probability is 30 %). That is obviously not what they mean, but if the daily probability is 30 %, those for each hour should be much lower. Note that the nighttime probabilities are almost the same as the daytime ones - 20 % every hour between 10 PM & 5 AM. I can't believe precip is nearly as likely those times as it is between 2 & 7 PM. I am thinking the probability is about 70 % for the daytime period now and 20 % for the nighttime - hourly probability would be about 5 % ending at 12 PM, peaking at about 25-30 % ending at 5 PM, then decreasing to about 10 % ending at 10 PM and to near 0 % ending at 5 AM. Saying that is one thing, programming a computer to think like that another. Maybe I am overestimating a bit, though storms are common over the elevated locations during warm & humid days. We had some hail as large as 3/8 inch diameter yesterday. Joseph, this is off topic, but it should still make it through the spam block. I found your assessment of the song "American Pie" by Don McLean very interesting and illuminating. -- Heck is where people go who don't believe in Gosh. http://tinyurl.com/36p7k I use Usenet Monster to post on usenet, it's anonymous (which means stalkers have a HELLUVA time finding you), and it's inexpensive (as low as $6.00 per month). Oh, and your gigs roll over. Here's a link: http://tinyurl.com/2sr4w Visit my new website: http://www.rbwaters.com In this message m I literally *DESTROY* Alex "Dink" Cain (AKA Cainman) In this message m Robert "I'm So Queer I Can't Even Pee Straight" Buchanan gets smacked HARD! -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
http://www.rbwaters.com wrote:
Joseph, this is off topic, but it should still make it through the spam block. I found your assessment of the song "American Pie" by Don McLean very interesting and illuminating. Thank you. Yes - it is off-topic and you could simply e-mail me. Maybe you are trying to tell me I should write about pop music instead of weather ? Yes, I overpredicted the thunderstorm potential, but American Pie almost certainly includes nothing about Kennedy also. So one incorrect comment about weather is no worse IMO than one incorrect comment about that. My post about the previous day's weather was basically correct. Regardless, my primary purpose for that post is to note how the hourly precipitation probabilities in TWC forecasts are misleading. If you look at them, you'll see that the highest probability for any hour of the daytime period corresponds with the total daytime probability on their daily forecasts. That is fundamentally incorrect from a statistical view. Even during a day when showers are likely, the probability for any particular hour is quite low. It doesn't rain all day. My post about the Gettysburg trip is a good example of this. As I mentioned, TWC forecast maps would lead you to believe we'd have basically a showery day - but it was a beautiful day, perhaps with a brief period of late day or evening showers. That was long after we left, so I don't know what they saw. I.e., the probability of rain while we were touring the battlefield, etc. most of the day was very low - maybe 10 % for any hour - but I bet TWC forecast had something like 60 % for each of those hours. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
TWC hourly forecast today for Mount Pocono :
Friday, May 14 Temp °F Feels Like Dew Point Precip. Humid. Wind 12 PM Mostly Cloudy 65° 65° 59° 20% 69% From the South at 8 mph 1 PM Mostly Cloudy 67° 67° 60° 20% 66% From the South at 8 mph 2 PM Mostly Cloudy 70° 70° 60° 20% 64% From the South at 9 mph 3 PM Partly Cloudy 72° 72° 61° 20% 61% From the South at 9 mph 4 PM Partly Cloudy 74° 74° 59° 30% 59% From the South at 11 mph 5 PM Partly Cloudy 75° 75° 59° 30% 59% From the South Southwest at 11 mph 6 PM Partly Cloudy 74° 74° 59° 30% 60% From the South Southwest at 10 mph 7 PM Showers 72° 72° 59° 40% 63% From the South Southwest at 8 mph The daily forecast : High/Low (°F) Precip. % May 14 Partly Cloudy 76°/60° 20 % Now some of the hourly probabilities are greater than the daytime ones, which is impossible :( Someone in charge probably graduated with a D- in probability & statistics. I would direct these concerns to them, but I've never seen an indication they consider them. We were discussing this here. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
On 14 May 2004 10:17:58 -0400, Joseph Bartlo
wrote: Regardless, my primary purpose for that post is to note how the hourly precipitation probabilities in TWC forecasts are misleading. If you look at them, you'll see that the highest probability for any hour of the daytime period corresponds with the total daytime probability on their daily forecasts. That is fundamentally incorrect from a statistical view. Even during a day when showers are likely, the probability for any particular hour is quite low. It doesn't rain all day. My post about the Gettysburg trip is a good example of this. As I mentioned, TWC forecast maps would lead you to believe we'd have basically a showery day - but it was a beautiful day, perhaps with a brief period of late day or evening showers. That was long after we left, so I don't know what they saw. I.e., the probability of rain while we were touring the battlefield, etc. most of the day was very low - maybe 10 % for any hour - but I bet TWC forecast had something like 60 % for each of those hours. You should write TWC about this and see what they say (they'd probably email you with an ad). Heck, you may even win a date with one of their single female OCM's, but finding a non pregnant one is pretty difficult these days. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
The Artist Formerly Known As Your Highness wrote:
You should write TWC about this and see what they say (they'd probably email you with an ad). Heck, you may even win a date with one of their single female OCM's, but finding a non pregnant one is pretty difficult these days. My response may have me banished from this group, so see ne.weather if you want my response. I don't think there's anything wrong if I don't cross-post it. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
TerriTickle wrote:
Joseph Bartlo wrote: Well, on the trip to Gettysburg last weekend, my mother asked me if I wanted to see an Amish community on the way back. I replied that I could probably find plenty of ignorant people in Mount Pocono :) Same thing for single women who were given jobs they don't deserve. Hey now, some single women have very respectable jobs, you know. Take a look at me : I have a highly respectable career distributing short duration filmed entertainment. There are lots of Amish people in Pennsylvania. And lots of single women. My question is, though, are there lots of Amish Single Women? I'll bet there are, and in Pennsylvania no less. -- Heck is where people go who don't believe in Gosh. http://tinyurl.com/36p7k I use Usenet Monster to post on usenet, it's anonymous (which means stalkers have a HELLUVA time finding you), and it's inexpensive (as low as $6.00 per month). Oh, and your gigs roll over. Here's a link: http://tinyurl.com/2sr4w Visit my new website: http://www.rbwaters.com In this message m I literally *DESTROY* Alex "Dink" Cain (AKA Cainman) In this message m Robert "I'm So Queer I Can't Even Pee Straight" Buchanan gets smacked HARD! |
Questions on errors in weather models
Stephen Stein wrote:
I won't comment about the rest of the Artist's post, but I actually would be curious to see what TWC would say about this. Then e-mail them. I e-mailed them regarding a couple things and as I recall only received an automated message acknowledging it. This is their problem, not mine. I understand how to present probabilities - though am not always good at guessing them. But Joseph, I wonder when you question the accuracy of hourly forecasts. I'd expect them to be better about things like precip prob at a well-known location like Gettysburg. But if you're talking about the hourly forecasts given for a specific zipcode, I wouldn't expect much in the way of accuracy. Especially for a place like the Poconos, where I'm sure there's a large variability throughout the area. For the case I was mentioning, the specific location was not an issue. For that, they clearly overestimated the potential for rain - no different than my 70 % for Thursday. The thing I am criticizing is forecasting HOURLY probabilities which are as great or sometimes greater than DAILY ones. That is mathematically IMPOSSIBLE. For example, suppose they forecast a 40 % chance of rain for each hour between 12 & 7 PM. If the events were independent (i.e., showers were random, began and ended immediately, and had that probability any hour), then the chance of a shower during that entire period is 1 - .6^7 = .972, or 97.2 %. I.e., the chance of not getting a shower one hour is .6 - 2 hours is .6*.6, etc. For 7 hours, the chance of not getting a shower is thus (.6*.6*.6*.6*.6*.6*.6) = ...6^7 = .028 . So the chance a shower does occur is 1 -.028 = .972 . Showers are not independent events, nor are they instant. They linger, and often occur in general areas. So the estimation of probabilities is to some extent an issue of forecasting where those areas of greatest activity will be, but it is also to some extent random as described above. So if the hourly probabilities were 40 %, 97.2 % would be an upper bound on the probability during the entire 7-hour period - it would actually be lower. 40 % would of course be a lower bound. If the probability of any hour is 40 %, the probability during the entire period must be at least that (some of TWC forecasts don't obey this simple rule). For synoptic rain or snow events, the hourly probabilities can often be close to the daily ones because an entire storm can either hit or miss a location. So in that case, you are essentially saying there's a 40 % chance the storm does affect your region (and if it does so, you'll probably get precipitation most of those hours). But that is not the case for summertime showers. For them, each hourly probability should generally be much less than the daily one. So to summarize : for the above example, if there's a 40 % chance of a shower each hour, there must be between a 40 - 97.2 % chance of at least one during the entire 7-hour period. 80-85 % would probably be typical. If the daily probability is 40 %, the hourly ones should generally be about 10-20 % at most - some hours as low as 5 %. Experience supports this, because one brief shower does not ruin a whole day. If you are on the golf course, you duck under a shelter for 20 minutes and then continue your play. 40 % chance of a shower - it happens - and the rest of the day is fine. That's what those low probabilities represent, and why it is misleading to place probabilities of 30-40 % each hour if the daily one is 40 %. I live in Acton MA, but as far as I know, the TWC forecasts are all based on Bedford, which is about 10 miles closer to the coast. I don't know about other weather stats, but I know that snow amounts can vary tremendously between those locations. I can't say much about the 10 mile zone, but something I mentioned previously on ne.weather is that snow cannot blow or drift from the ocean to land, but does across land. Noreasters often have strong inland winds, so it makes sense that snow will not collect well right along the coast, and generally advances inland (along the ground). I don't know to what extent this is responsible for the observed differences of amounts. To me, these forecasts are like extra decimal digits beyond the limits of significance in a physics problem answer, when you really don't have enough data to make a prediction beyond regional daily conditions. In that way, I think that publishing hourly predictions is misleading. Perhaps TWC should limit itself to what it can reasonably predict. There are local effects which are predictable, but I don't think they are included in the model(s) they use. Nor do I always get them right here in Mount Pocono. We had a dark cloud overhead which lingered almost all day yesterday - we were the last spot to clear, and probably the coldest spot in the state. I had a maximum of 72°, which was a ° lower than MPO on the plateau (we are usually a bit warmer this time of year, though late in the year when vegetation is more abundant, we are often a bit cooler during day). Yet during a warm, sunny morning with warm air aloft, we here near the top of the slope facing ESE in Mount Pocono can be the warmest spot in the state around 10-11 AM. In that situation, we are often warmer than PHL & ABE. We are east, so heat a little sooner than western locations of the state also. The hourly forecast from TWC and the NWS graphics don't show this at all - but you certainly feel it. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
On 15 May 2004 03:52:25 -0400, Joseph Bartlo
wrote: For the case I was mentioning, the specific location was not an issue. For that, they clearly overestimated the potential for rain - no different than my 70 % for Thursday. The thing I am criticizing is forecasting HOURLY probabilities which are as great or sometimes greater than DAILY ones. That is mathematically IMPOSSIBLE. snip The NWS gets around this in their tabular forecasts by placing a qualitative description in the precip block. Their tabular forecasts are experimental and aren't used in all regions. BWI is using it, however: http://tinyurl.com/yqsu8 Note that they differentiate between rain and thunder. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
On 15 May 2004 13:22:37 -0400, The Artist Formerly Known As Your
Highness wrote: On 15 May 2004 03:52:25 -0400, Joseph Bartlo wrote: For the case I was mentioning, the specific location was not an issue. For that, they clearly overestimated the potential for rain - no different than my 70 % for Thursday. The thing I am criticizing is forecasting HOURLY probabilities which are as great or sometimes greater than DAILY ones. That is mathematically IMPOSSIBLE. snip The NWS gets around this in their tabular forecasts by placing a qualitative description in the precip block. Their tabular forecasts are experimental and aren't used in all regions. BWI is using it, however: http://tinyurl.com/yqsu8 Note that they differentiate between rain and thunder. Whoops, it's not BWI, it's Sterling, VA's NWSFO that's issuing this product. -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
Questions on errors in weather models
The Artist Formerly Known As Your Highness wrote:
The NWS gets around this in their tabular forecasts by placing a qualitative description in the precip block. Their tabular forecasts are experimental and aren't used in all regions. BWI is using it, however: http://tinyurl.com/yqsu8 Note that they differentiate between rain and thunder. I think they'd find when verifying these forecasts that those hourly PoP's are much too high also. Most summertime rain is showers - how many hours of a day does that verify correctly? -- This article was auto-posted by the ne.weather.moderated Weatherbot program. The author is solely responsible for its content. ne.weather.moderated FAQ/Charter: http://www.panix.com/~newm/faq.txt ne.weather.moderated moderators e-mail: (Please put "wx" or "weather" in the subject line to avoid the spam block.) |
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