Truckee & Sierra Nevada Weather Intelligence

Yesterday this page called the front half of the week straight-ahead work weather. It mostly still is. But the 3 AM discussion put something on today that was not there yesterday. There is now a 15 to 40 percent chance of isolated showers along and south of I-80 between 2 and 10 PM, and inside that, a 5 to 10 percent shot at lightning. Small numbers. The reason they matter is the number sitting underneath them: DCAPE over 1000 J/kg this afternoon. That is the fuel for outflow gusts of 45 to 50 mph coming off a cell that was not over you twenty minutes earlier. You do not get a forecast for that. You get a look at the sky.

Right now it is 55 with humidity at 35 and the wind essentially off — 2 mph out of the north, clear overhead. Sunrise is 6:24. That is a clean first-light window and it is the best working air you are going to see today, so spend it on the work that wants cool and calm. The afternoon builds to 83 on our grid and 81 at the office. Two degrees apart. That is the second straight day those two books have closed on the same page, and after last week's seven-to-nine degree feud I will take it.

Humidity is the number to write down for today. It falls off after 9 AM and sits at 18 to 19 percent from noon straight through 3 o'clock. Anything under 20 gets a mention on this page every single time. What today does not have is the wind to go with it — our gusts cap around 20 mph and they do not show up until early evening. Dry, but not critical. The critical call today belongs to far northeast California and far northwest Nevada, where the office has 25 to 35 mph gusts riding on 8 to 20 percent humidity. That is not us. Nothing is posted for our zone and Reno's board reads clean for both states this morning. California is carrying three Red Flag Warnings and all three belong to somebody else — Modoc and the Klamath Basin up north, the Santa Susana and San Gabriel mountains eight hours south.

None of which changes the spark rule. Cured grass at 18 percent does not need a warning posted over it to catch. Grinder, chop saw, chain drag, hot exhaust parked in the weeds — it all goes up the same at 18 as it does at 12. The office said it plainly this morning: refrain from activities that may cause sparks today. Keep the extinguisher where you can reach it and not where it rides nicely.

Friday grew again, and more importantly it moved earlier. Yesterday this page said rain starts around five or six Friday morning and is mostly finished by early afternoon. Scratch the back half of that. The office now has showers likely tonight at 60 percent, likely again before 11 AM Friday, and then showers and thunderstorms likely through the afternoon. That is no longer a rain delay with a start time and an end time. That is a wet day.

The amount is a different argument entirely. GFS carries 0.16 inches, the European run 0.08, and the German run 0.56. The office puts it at a trace to half an inch in the highest Sierra with precipitable water over an inch and a marginal excessive-rainfall risk on top. One model is carrying seven times what another one is. They agree completely on whether. They do not agree at all on how much. Plan for wet ground and no compaction either way, and let the rain gauge settle the rest.

One good piece of news buried in there. Yesterday Friday's high was a nine-degree fight — our grid at 63, Reno's point forecast at 72. It closed overnight. Grid says 71, office says 69. When a nine-degree spread collapses to two inside a day, that is the models finally agreeing on how long the cloud deck holds after the front. Dress for 69 with water on it.

Now the weekend, which is where this note earns its keep. Once Friday's system clears we get the exact setup that drops Truckee nights through the floor: dry air behind the front, clear sky, no wind, and a valley for it all to drain into. The office and our grid are nowhere near each other on it.

SAT NIGHT  office 37°  ·  model grid 47°  ·  plan 34°
SUN NIGHT  office 37°  ·  model grid 46°  ·  plan 34°

A ten-degree disagreement is not a rounding error, and it is the same disagreement this page has been logging for two weeks. Cold air runs downhill and pools in the low spot exactly like water, and Martis Valley is the low spot. The models smooth that out because smoothing is what models do. The thermometer does not smooth anything. Our eleven-day verification has the overnight lows running 5.3 degrees warm every single night — on the 20th the day-ahead called 56 and we got 45. So the grid's 46 is not 46. Side with the office, then take a few more degrees off, and run the weekend mornings on mid 30s. If you have a late pour going down, water sitting in an exposed line, or anything parked open in a trench, cover it Friday night. Not Saturday morning. It is still August on the calendar and the calendar is not what freezes.

Wind stays reasonable through the weekend — low 20s in gusts Friday and Saturday, then 13 mph Sunday, the calmest day anywhere on this board. The day to circle is September 4th at 30 mph gusts, the highest number in sixteen days. If you have tarps, plastic, sheet goods or standing forms up on an exposed pad, that is the day they end up in the next drainage over. Weight them the night before.

Fire weather out at the far end, and I want to flag that the window moved. Yesterday this page put the driest air on September 2nd, 3rd and 4th at 11 percent and called the 5th the worst fire day on the board. Overnight the whole thing slid about three days right. The 2nd and 3rd now read 14 and 15 percent, the 4th came all the way up to 25, and the new bottom of the board is September 6th at 13 percent and the 7th at 11 percent. The saving grace on both of those is that they are nearly windless — gusts of 6 and 7 mph. Dry without wind is a fire you catch. Dry with wind is a fire you chase. The pairing that actually deserves the circle is September 1st: 15 percent humidity with 27 mph gusts. Five days out, and the only day on this board carrying both halves.

One line on the neighbors. Reno runs 89 today to our 83 and bottoms at 15 percent humidity to our 18. Six degrees and three points is what 3,000 feet of elevation buys you in late August. It charges you back this weekend — Reno's nights hold in the 60s while ours go to the 30s. Same air, different floor.

Confidence, graded like a line before it gets covered. Today's temperature is a locate — 81 against 83, both agencies, nothing left to argue about. This afternoon's isolated cell is nothing anybody can locate; a 5 to 10 percent lightning chance is not a forecast, it is a coin in your pocket, so watch the sky yourself between 2 and 10 and treat one distant rumble as the entire warning you are going to get. Friday's water is a locate. Friday's amount is a pothole, and a wide one. The weekend cold is a pothole too, but I am leaning on eleven nights of the same error in the same direction, and that is worth more than any single run. The September fire window is an old as-built, and it proved it overnight by walking three days to the right. Ask me again Monday.

Today is a work day — GO, sun up at 6:24, dry work knocked out before two and eyes up after. Tonight the rain starts. Friday is a write-off for anything that needs to stay dry or wants to compact, and it is the good kind of write-off because you can see it coming. Then the weekend hands us the first honest frost risk of the season, about two weeks ahead of schedule, and it is going to arrive quieter than the forecast says.