ChescoWx

Hello all. I have spent the last couple of months digging into one question and would welcome feedback and pushback. The question is not whether raw or adjusted data is more accurate. It is simpler than that... based on the data any of us can access, can we prove that NCEI's adjustments to our local temperature record are necessary, and sized correctly? What the adjustments do here Using every Chester County station, 1895 to 2025, the raw record warms about +0.05°F per decade, which is not statistically significant. NCEI's adjusted county series warms +0.25°F per decade. The gap between the two grows at about +0.2°F per decade and that gap is significant. Most of it comes from cooling the early decades, which NCEI runs 1.4 to 2.0°F below what the thermometers recorded. I also compared the three nearest long-record airports, all outside the county, against their own NCEI adjusted series: PHL (1941 to 2025): raw +0.44, adjusted +0.40. NCEI barely touches it. ABE (1948 to 2025): raw +0.31, adjusted +0.60. NCEI cools the early record by up to about 2.5°F, which roughly doubles Allentown's warming trend. RDG: NCEI only has an adjusted series from 2000 on, and it cuts Reading's trend from +1.36 to +0.77. Over the same 1948 to 2025 window, NCEI's adjustments add roughly +0.2 to +0.3°F per decade of warming both at Allentown (+0.29) and in Chester County (+0.17 against all county stations, +0.33 against our two longest continuous stations, West Chester 2NW and Phoenixville 1E). So Allentown and Chester County get about the same boost. PHL is the one that gets almost none. Against our most rural long-record stations, PHL warms roughly 0.2 to 0.3°F per decade faster since 1948. That looks like urban and airport growth that the adjusted PHL series keeps in, although the uncertainty range on it runs from about zero to +0.5. What we can prove Phoenixville shows a step of almost 2°F in 1987. Raw data has real artifacts in it. What we cannot prove We cannot show the adjustments are the right size, or that every one is needed. Coatesville 1SW is the clearest example. It had a documented move in January 1946 and NCEI lists a 1.4°F step for it, then lowers everything before 1946 to match. I compared Coatesville against every neighbor I could find with data on both sides of 1946... Phoenixville, Ephrata, Holtwood and Lancaster, all between 21 and 28 miles away. Each of those tracks Coatesville closely enough on its own to catch a step of about 0.4 to 0.7°F, roughly a third to a half of the size NCEI applies. Against those stations averaged together the step is only -0.25 to +0.07°F and not significant. So these neighbors were sensitive enough to see a step the size NCEI applies, and none of the ones without their own 1948 changes consistently show it. Phoenixville and Ephrata cooled right along with Coatesville. The only neighbors that show a bigger step are West Chester and Lancaster, and both of those had their own location changes in 1948. Coatesville also carries 12 NCEI breakpoints and only 4 are documented. West Chester carries 15 with 4 documented. NOAA's own station history for observation times conflicts with the reading-time schedules commonly assumed for these stations. To be fair, we also cannot show NCEI got it totally wrong. As NCEI compares against a much wider network of neighbors than we can see. So my conclusion: adjustments are necessary in principle, but from public data we can neither verify nor disprove their size here. What would close the gap NCEI's pairwise log for each station, showing which neighbors, which years and what weights were used for each break. I reached out to NCEI who told me this is not something they publish. The time-of-observation step on its own for each station, separate from the other adjustments. The original observer forms (B-91s) for the years where NOAA's records conflict or are blank. Dated site move and instrument change details, including the MMTS changes in the 1980s and ASOS at the airports in the 1990s. Overlapping readings at the old and new sites when a station moved. The Climate Reference Network only began in the 2000s, so it is too short to help with the early record. A re-run of NCEI's published code on the same inputs, to see whether we get the same breaks. Reading's adjusted series before 2000. Let me know if any questions or if you see anything that does not hold up!

Hello all. Announcing something new on chescowx.com...ChescoWx Studies! This is an ongoing series looking at what makes Chester County's climate unique, this will include deep analysis of climate patterns and cycles, weather monitoring equipment and tools and past storm write ups and details. Each study takes one question, tests it against our station records and gives you the answer with the numbers behind it. I would welcome any feedback or pushback on the methods or the results. The series lives here and will grow as new studies are finished: https://chescowx.com/chescowx-studies/ Study 1 - Equipment & Siting. Three stations at my place in East Nantmeal. A ground-level fan-aspirated Davis, a rooftop Tempest and a ground-level Ambient with a passive shield. The Tempest highs ran about +3°F in July through September and +3.3°F on sunny, calm days, with 45 days of 90°F vs 8 for the Davis. To separate the roof from the shield, I looked at the Ambient. From 2021 to 2023 its summer highs matched the Davis within a few tenths while the Tempest was +2.8°F...so it is the roof, not the shield. Once the Ambient shield was damaged, though, it ran almost 7°F warm on sunny days. Location first, then upkeep. Study 2 - Elevation & Terrain. Most of our hilltop stations came along decades after the valley ones, and the weather varies from one decade to the next, so I compared them over the same years (2012 to 2025) using USGS 3DEP elevations. 19 stations with at least 8 complete years, by band: 100 to 300 ft (4 stations): high 64.4, low 43.6, 90 degree days 19, freezing nights 111, growing season 182 days 300 to 600 ft (8 stations): high 63.3, low 44.5, 90 degree days 11, freezing nights 101, growing season 192 days 600 ft and up (7 stations): high 62.3, low 45.3, 90 degree days 9, freezing nights 94, growing season 204 days Days follow elevation and nights follow terrain position. Highs drop about 1.7°F for every 300 ft, but lows track how far a station sits above its immediate surroundings, which is why they actually rise as you go up. For snow I used a same-storm comparison (521 storms, 60+ human-observed sites) so a snowy decade at one station does not skew it. Overall the hills get about 8% more snow for every 300 ft (29% per 1,000 ft), but the time of season matters a lot: Oct-Nov (24 storms) about 25% per 300 ft Dec-Jan (251 storms) about 6% Feb (151 storms) about 5% Mar-Apr (95 storms) about 18% March 2004 is a good example: East Nantmeal 11.0, Coatesville 2W 8.8 vs Exton 3.0, Chadds Ford 3.4, West Chester 2.2. There are always a few sites that do not fit, and the southern uplands around West Grove and New London run less than their elevation would suggest. The north also gets a bit more than the south, while the western part of the county gets more only because it is higher. Precipitation showed no elevation effect at all. Let me know if any questions, if you see anything that does not hold up, or if there is a question you would like to see tested next!

Hello all. Announcing something new on chescowx.com...ChescoWx Studies! This is an ongoing series looking at what makes Chester County's climate unique, this will include deep analysis of climate patterns and cycles, weather monitoring equipment and tools and past storm write ups and details. Each study takes one question, tests it against our station records and gives you the answer with the numbers behind it. I would welcome any feedback or pushback on the methods or the results. The series lives here and will grow as new studies are finished: https://chescowx.com/chescowx-studies/ Study 1 - Equipment & Siting. Three stations at my place in East Nantmeal. A ground-level fan-aspirated Davis, a rooftop Tempest and a ground-level Ambient with a passive shield. The Tempest highs ran about +3°F in July through September and +3.3°F on sunny, calm days, with 45 days of 90°F vs 8 for the Davis. To separate the roof from the shield, I looked at the Ambient. From 2021 to 2023 its summer highs matched the Davis within a few tenths while the Tempest was +2.8°F...so it is the roof, not the shield. Once the Ambient shield was damaged, though, it ran almost 7°F warm on sunny days. Location first, then upkeep. Study 2 - Elevation & Terrain. Most of our hilltop stations came along decades after the valley ones, and the weather varies from one decade to the next, so I compared them over the same years (2012 to 2025) using USGS 3DEP elevations. 19 stations with at least 8 complete years, by band: 100 to 300 ft (4 stations): high 64.4, low 43.6, 90 degree days 19, freezing nights 111, growing season 182 days 300 to 600 ft (8 stations): high 63.3, low 44.5, 90 degree days 11, freezing nights 101, growing season 192 days 600 ft and up (7 stations): high 62.3, low 45.3, 90 degree days 9, freezing nights 94, growing season 204 days Days follow elevation and nights follow terrain position. Highs drop about 1.7°F for every 300 ft, but lows track how far a station sits above its immediate surroundings, which is why they actually rise as you go up. For snow I used a same-storm comparison (521 storms, 60+ human-observed sites) so a snowy decade at one station does not skew it. Overall the hills get about 8% more snow for every 300 ft (29% per 1,000 ft), but the time of season matters a lot: Oct-Nov (24 storms) about 25% per 300 ft Dec-Jan (251 storms) about 6% Feb (151 storms) about 5% Mar-Apr (95 storms) about 18% March 2004 is a good example: East Nantmeal 11.0, Coatesville 2W 8.8 vs Exton 3.0, Chadds Ford 3.4, West Chester 2.2. There are always a few sites that do not fit, and the southern uplands around West Grove and New London run less than their elevation would suggest. The north also gets a bit more than the south, while the western part of the county gets more only because it is higher. Precipitation showed no elevation effect at all. Let me know if any questions, if you see anything that does not hold up, or if there is a question you would like to see tested next!

Hello all. Announcing something new on chescowx.com...ChescoWx Studies! This is an ongoing series looking at what makes Chester County's climate unique, this will include deep analysis of climate patterns and cycles, weather monitoring equipment and tools and past storm write ups and details. Each study takes one question, tests it against our station records and gives you the answer with the numbers behind it. I would welcome any feedback or pushback on the methods or the results. The series lives here and will grow as new studies are finished: https://chescowx.com/chescowx-studies/ Study 1 - Equipment & Siting. Three stations at my place in East Nantmeal. A ground-level fan-aspirated Davis, a rooftop Tempest and a ground-level Ambient with a passive shield. The Tempest highs ran about +3°F in July through September and +3.3°F on sunny, calm days, with 45 days of 90°F vs 8 for the Davis. To separate the roof from the shield, I looked at the Ambient. From 2021 to 2023 its summer highs matched the Davis within a few tenths while the Tempest was +2.8°F...so it is the roof, not the shield. Once the Ambient shield was damaged, though, it ran almost 7°F warm on sunny days. Location first, then upkeep. Study 2 - Elevation & Terrain. Most of our hilltop stations came along decades after the valley ones, and the weather varies from one decade to the next, so I compared them over the same years (2012 to 2025) using USGS 3DEP elevations. 19 stations with at least 8 complete years, by band: 100 to 300 ft (4 stations): high 64.4, low 43.6, 90 degree days 19, freezing nights 111, growing season 182 days 300 to 600 ft (8 stations): high 63.3, low 44.5, 90 degree days 11, freezing nights 101, growing season 192 days 600 ft and up (7 stations): high 62.3, low 45.3, 90 degree days 9, freezing nights 94, growing season 204 days Days follow elevation and nights follow terrain position. Highs drop about 1.7°F for every 300 ft, but lows track how far a station sits above its immediate surroundings, which is why they actually rise as you go up. For snow I used a same-storm comparison (521 storms, 60+ human-observed sites) so a snowy decade at one station does not skew it. Overall the hills get about 8% more snow for every 300 ft (29% per 1,000 ft), but the time of season matters a lot: Oct-Nov (24 storms) about 25% per 300 ft Dec-Jan (251 storms) about 6% Feb (151 storms) about 5% Mar-Apr (95 storms) about 18% March 2004 is a good example: East Nantmeal 11.0, Coatesville 2W 8.8 vs Exton 3.0, Chadds Ford 3.4, West Chester 2.2. There are always a few sites that do not fit, and the southern uplands around West Grove and New London run less than their elevation would suggest. The north also gets a bit more than the south, while the western part of the county gets more only because it is higher. Precipitation showed no elevation effect at all. Let me know if any questions, if you see anything that does not hold up, or if there is a question you would like to see tested next!

Of interest the NWS COOP site Phoenixville 1E recorded a low of 32 degrees on Tuesday morning the 6th this is the 20th earliest freeze since records began in 1890. Their average 1st freeze is October 23rd. We should see a beautiful fall day today with highs a few degrees above normal near 70. We turn a bit cooler for the weekend with rain holding off till later Sunday afternoon with the remnants of the Hurricane in the Gulf. We look to turn much chillier again by the middle of next week.

Of interest the NWS COOP site Phoenixville 1E recorded a low of 32 degrees on Tuesday morning the 6th this is the 20th earliest freeze since records began in 1890. Their average 1st freeze is October 23rd. We should see a beautiful fall day today with highs a few degrees above normal near 70. We turn a bit cooler for the weekend with rain holding off till later Sunday afternoon with the remnants of the Hurricane in the Gulf. We look to turn much chillier again by the middle of next week.

Our great stretch of autumnal weather continues through Saturday. Temperatures will stay below normal today before rising to near to slightly above normal tomorrow and Saturday. Rain chances ramp up with the tropical storm remnants by Sunday into Monday. We look to turn sharply cooler around the middle of the month.

Our great stretch of autumnal weather continues through Saturday. Temperatures will stay below normal today before rising to near to slightly above normal tomorrow and Saturday. Rain chances ramp up with the tropical storm remnants by Sunday into Monday. We look to turn sharply cooler around the middle of the month.

This morning was the coldest morning across the area since last spring. Below are the low temperatures and the date we were last this chilly.

This morning was the coldest morning across the area since last spring. Below are the low temperatures and the date we were last this chilly.

The GFS ensemble thinks a below normal temperature month for October is underway...

My ChescoWx forecast has beaten the NWS point and click in 36 of the last 40 days!

My ChescoWx forecast has beaten the NWS point and click in 36 of the last 40 days!

Some light rain and below normal temperatures today before we clear out for most of the rest of the week. We start the week with below normal temperatures with some lower valley locations likely to see some 30's for the first time this year by Tuesday or Wednesday morning. We start a slow warming trend through the week before we turn a bit cooler by next weekend.