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MDT vs. CXY: Quantifying the Bias Between Harrisburg's Two ASOS Stations (and Why It Matters for the Pre-1991 Record) Something that doesn't get talked about much in discussions of Harrisburg climatology is that the "official" record changed stations in October 1991 — from Capital City Airport (CXY, Fairview Township, York County) to Harrisburg International (MDT, Lower Swatara Township, Dauphin County/Susquehanna floodplain) — and that switch was made with no homogeneity adjustment whatsoever. The ThreadEx composite that most people pull from ACIS just splices them together at that date and calls it a day. I decided to actually quantify the bias using the 2001–2025 overlap period, when both stations were running reliably as ASOS units. 25 years of IEM daily data, both stations. Here's what I found. --- PRECIPITATION MDT runs wetter than CXY every single month except April, with an annual median ratio of 1.106 — meaning MDT receives roughly 11% more precipitation per year (43.3" vs. 38.4" mean over the overlap). The ratio is most consistent and statistically tight in winter (DJF ratio 1.126, tight bootstrap CI), which makes sense — synoptic-scale systems produce more uniform precipitation and the floodplain position at MDT reliably enhances totals. The widest uncertainty is in the summer convective months (Jun–Sep CI spans nearly 0.3), reflecting the high year-to-year variance when a single tropical remnant or MCS can hit one site much harder than the other. September has the largest single-month ratio at 1.164. April is the odd one out at 0.988 — essentially no bias. Spring frontal/stratiform precipitation appears to be the most spatially uniform regime between these two sites. The implication for ThreadEx is that the unadjusted splice at October 1991 introduces approximately an 11% step-down in precipitation when you cross back into the pre-1991 CXY period. Any trend analysis using that composite without adjustment is going to be systematically affected. --- TEMPERATURE The temperature story is more interesting than the precipitation one. Max temp is straightforward — MDT runs cooler than CXY all 12 months, ranging from −0.19°F in summer to −0.97°F in January. River valley moderating daytime heating, nothing surprising. Min temp is where it gets physically interesting: the offset reverses sign seasonally. MDT runs cooler than CXY in winter (as low as −0.71°F in February) but warmer in summer (+1.09°F in July). The crossover is right around April, which is near zero (+0.03°F). What you're seeing is the competing effects of cold air drainage into the Susquehanna floodplain in winter (making MDT colder at night) versus the river's thermal mass keeping the MDT boundary layer warmer on summer nights. CXY sits on a ridge position in Fairview Township at ~106m and apparently drains cold air off efficiently in winter while losing the river's moderating influence in summer. It's a textbook valley-versus-upland nocturnal temperature signature and it comes through cleanly in 25 years of data. --- DEWPOINT AND RH MDT runs higher dewpoint and RH than CXY all 12 months — no sign reversal here. The Susquehanna just keeps MDT moister year-round. The RH offset is largest in winter (DJF +3.68%) and smallest in summer (JJA +1.69%), which probably reflects the relative importance of the river moisture source versus atmospheric moisture demand across seasons. --- PRACTICAL UPSHOT If you're doing any work with the Harrisburg long-period record — trend studies, climatological normals, CAD research, whatever — and you're pulling ThreadEx without thinking about this, you should be aware of the discontinuity. The pre-1991 CXY record can be adjusted to MDT-equivalent using monthly multiplicative ratios for precipitation and additive offsets for temperature/dewpoint, derived from the overlap period. The adjustment is stable enough (particularly in the cool season) that I'm reasonably confident applying it back through the CXY record to 1939. Happy to share the Excel adjustment factor table if anyone wants — monthly ratios with bootstrap CIs, plus seasonal summaries broken out by DJF/MAM/JJA/SON. Sent from my SM-S731U using Tapatalk

At a certain point you can't hand hold the public. They either start to get it or they experience the consequences of not. My take is they are going to do whatever they want and if it goes wrong in any way they look to first person other than themselves to blame. Whatever it is that introduced this rot into our cultures era specific mindset needs to be ruthlessly scrubbed out. Sent from my SM-S731U using Tapatalk

IDK, almost 40” of snow on the ground a few weeks ago and almost 2.5” of rain yesterday and I don’t see much in the way of runoff while driving around and not many basement pumping calls came in yesterday…..I think we are drier than you think

Really wouldn’t be that absurd if it did tbh, especially given how torch days in early spring tend to overperform. I’d say 75+ is certainly in play anyhow.

Man it's cold outside. That 80F weather was a straight up tease. Freezing me knickers off.

38 for today high after 39 yesterday. Low of 24 . Looks like lots of 25’s again tonight. Hard to do in mid almost late March

If we have a dry spring this will look a lot worse once last spring is wiped off the yearly total.

This circulation passed just west of the fire department I volunteer at. Video of this tornado was captured at the EOC. A lot of leg work went into this survey.

Most of us just love the GFS, though we won't admit it, or realize it. A dull day is spiked with fleeting outrageous denial of expectation. Palm Sunday, 8 inches of snow!!! April 7, 1971, I had 9 inches at 1325 ft and Afton Mt. at 1850 ft. had 17 inches.....................................................

I'll take it! It's my birthday weekend, so would be quite the gift. I have Friday off work as well and hope to spend some time outside

We’re in bad shape going into the spring up here. Most of the snow has melted and there has been no mud season or puddles in the backyard. Can’t remember the last time it’s been like that here. This little bit of water in the soil will be gone quickly when the plants start sucking it up. Keep the rain coming.

Once again, talks of a conus wide torch around the equinox screeched to a halt from the Great Lakes and northeast. As expected though, the southwest will fry.

River flood warnings, basement flooding, and moderate drought here. Ponds are full. Can we DOGE those maps please? Clearly, , they are weighting precipitation deficits way too much. I get maybe since 2024 we’re a little below normal precip wise…But when you get decent precipitation over the winter and there’s nothing to absorb it, It goes right into the ground reservoirs, etc. At least around here right now I don’t see any signs of any low level reservoirs or groundwater issues, etc. Those maps are absolutely horrific.

Could be!

Yea his is about half of what it was and that takes into account a few months early last year where one of (or some) of his 48 ground panels wasn’t working properly. 1.4mwh last March (to date) compared to 772.5kwh this month, to be exact. Sunny af today, though.

The strongest signal for something(such that it is) is here on the GEFS. Pretty meh beyond that. EPS and CMC ens don't suggest much of anything (of the frozen variety) in our region for the rest of the month.. if so that pretty much means through next Fall lol.

The going rate is one bottle cap per inch of bust.

It clearly wasn't from eating one Big Arch lol. This sort of thing is cumulative if the cause is from eating a bad diet. Hopefully he recovers.

Marquette

All chips in on Palm Sunday! ETA: I’m in negative chips currently. Anyone got a few I can borrow? I’ll pay you back next Monday with a 6 pack.

I bet someone scores just as the cold pattern finally relaxes.

Buckle up!!

I'd be all over that Velvet Magnum.