A Red Flag Warning for Redding, and the wind comes through twice
Critical fire weather from 11 AM Saturday to 11 AM Monday. The HRRR has Saturday evening gusting 45 to 55 mph where the official forecast says 31, and that gap is the open question in this period. Then a quiet but very dry Sunday, and a north wind back overnight into Monday.
Red Flag Warning from 11 AM Saturday to 11 AM Monday for South-Central Shasta County including the Redding metro, issued by NWS Sacramento. Gusts of 20 to 35 mph against relative humidity of 10 to 25 percent. The warning text is blunt about what that means: any fire that develops will spread rapidly, and outdoor burning is not recommended.
Hello all — fire weather update for Redding.
The ridge is breaking down, and what replaces it is worse for fire than the heat was. A strong upper low drops into northern California this weekend, and the wind on its edge is what the warning is about.
What has been issued
NWS Sacramento issued a Red Flag Warning Friday morning, in effect 11 AM Saturday through 11 AM Monday. It covers nine zones across the northern Sacramento Valley and foothills, and ours — the Redding metro — is the first one named. Criteria are gusts of 20 to 35 mph against afternoon humidity of 10 to 25 percent.
A Red Flag Warning is not a forecast that a fire will happen. It says that if one starts, the weather will not let it stay small.
Two pushes, not one
The warning is written for nine zones over 48 hours, and your site does not get it as one continuous event. The point forecast breaks into three pieces.
| When | Wind & gust | Dir | Temp | RH |
|---|---|---|---|---|
| Sat 9 AM | 2 g7 | S | 70° | 47% |
| Sat noon | 6 g13 | S | 80° | 38% |
| Sat 3 PM | 9 g22 | SW | 87° | 30% |
| Sat 6 PM | 13 g31 | W | 84° | 28% |
| Sat 9 PM | 10 g30 | W | 69° | 42% |
| Sun daytime | 3 g10 | N | 81° | 22% |
| Sun midnight | 9 g22 | N | 65° | 43% |
| Mon 6 AM | 7 g20 | N | 60° | 49% |
| Mon 9 AM | 10 g24 | N | 67° | 39% |
| Mon 3 PM | 5 g12 | NE | 91° | 14% |
NWS Sacramento gridded point forecast for Redding, updated 2 PM PDT 11 September. Speeds in mph, sustained then gust. Shaded rows are the two wind pushes; the italic row is the Sunday lull.
Saturday afternoon and evening is the first push, and it is the one with a real spread on it. The official point forecast peaks at 31 mph around 6 PM out of the west. The HRRR puts that peak at 45 to 55 mph between 6 and 8 PM. That is a 20 mph disagreement on the windiest hours of the weekend, and it is worth a section of its own below.
Sunday is the lull, and it is the deceptive part. Wind drops to almost nothing and the day will feel benign. Humidity goes the other way, down to 16 to 24 percent — the lowest reading of the warning period. Fuels dry out on a day like that even though nothing is blowing.
Sunday night into Monday morning is the second push. The wind turns north and gusts 20 to 24 mph from midnight through 9 AM. North is the offshore direction here: it comes downslope, so it warms and dries rather than cooling things off the way Saturday's sea breeze does. Overnight is also the hardest time to spot a new start.
What the models are doing
The driver is an upper low with a 65 to 75 knot mid-level jet moving into the Pacific Northwest Saturday. On its front side the gradient accelerates onshore flow up the valley — that is Saturday. As the low passes to our east, the flow swings around behind it to northerly — that is Sunday night. Guidance agrees on the sequence, which is why the office posted a 48-hour warning two days out.
Where they part company is how hard it blows Saturday evening. Every model puts the peak in the same three hours and from the same direction. Only one of them puts it above 40 mph.
| Model | What it is | Sat eve peak |
|---|---|---|
| HRRR | NOAA's 3 km hourly-updating short-range model — the only one here that resolves a frontal gust front | 45–55 |
| ICON | German global model | 36 |
| ECMWF | European global model | 34 |
| NBM | National Blend of Models — the starting point for the NWS grid above | 32 |
| ARPEGE | French global model | 32 |
| GFS | American global model, run on its own rather than blended | 27 |
Peak 10-metre wind gust in mph at the Redding grid point, 12 September, current runs as of Friday evening. All six place the peak between 6 and 9 PM from the west-northwest.
HRRR being the high outlier is not in itself a reason to discount it. It is the only model in that list with the resolution to see a gust front at all, and the coarse global models systematically under-do this kind of feature. It is also the model with the best-known habit of over-forecasting gusts in terrain. So the honest read is a wide one: plan on 30 to 35, and be able to handle 50.
What the HRRR spike actually is
Worth knowing what that model is showing, because it changes which department needs to care. In the HRRR the gust arrives with a wind shift from south to west-northwest, the temperature falls 19 degrees in three hours, and humidity climbs from 19 percent back up to 37 percent. That is a front coming through, not dry downslope wind.
Every other model agrees on that part — all of them have humidity recovering into the 29 to 40 percent range at the moment the wind peaks. So if HRRR is right about the magnitude, the 6 to 9 PM window is a wind problem rather than a fire-spread problem: limbs, anything loose outside, power lines, and the PSPS decision. The fire-weather concern stays where this briefing has it — Sunday's dry air and the north wind Monday morning.
One consequence worth naming. A 50 mph gust through a Red Flag Warning is squarely in PG&E's de-energization territory, and the Saturday evening window is when that call would land.
The gap at the end
The warning expires at 11 AM Monday because the wind relaxes, not because the air gets wet. Monday afternoon is forecast at 91 degrees with humidity of 14 to 15 percent — the hottest and driest reading of the entire period, hours after the product comes down. Winds are light by then, so it does not meet criteria and it should not. But fuels do not know that a product expired.
Saturday evening is the windiest few hours, and one model says considerably windier than the forecast — secure the outside for 50 mph, not 30, and expect a power outage to be live that evening. Sunday will feel like the event is over and it is not; that is the driest air of the weekend. Then the wind returns from the north overnight into Monday. Do not stand down at 11 AM Monday just because the warning does.
Checklist
Before 11 AM Saturday While it is still calm
Saturday afternoon through Monday While the warning runs
Good news for context: CAL FIRE lists no large active incidents in Shasta, Tehama, Trinity or Siskiyou counties. Nothing nearby for this wind to pick up, so the whole concern is new starts. Official sources are NWS Sacramento and CAL FIRE, with evacuation orders from Shasta County. Text OMG North any time this weekend — day or night.
Any questions, just ask.
Situational awareness, not an official weather product. Warning details are the NWS Sacramento Red Flag Warning of 11 September 10:02 AM PDT; site figures are the gridded point forecast for Redding updated 2 PM PDT and the Fire Weather Planning Forecast for zone CAZ124 issued 2:13 PM PDT; model discussion is from the NWS Sacramento Area Forecast Discussion and the Storm Prediction Center Day 2 Fire Weather Outlook, both 11 September; the model comparison is the current run of each model at the Redding grid point as of Friday evening, and model spread this far out is normal rather than a sign any one of them is wrong. For evacuation guidance, rely on the Shasta County Sheriff and CAL FIRE. If the banner timestamp turns red, that text has not been re-verified against the feed.