SLOweather

As I was reviewing SLOweather’s records for November, 2007, I did a little research on “average” or “mean” rainfall for San Luis Obispo for the month.

Average or mean rainfall entirely depends on the time frame for the measurement.

As far as “average” November rain goes:

For 1948-2005 the average was 2.47″.

For 1961-1990 the average was 2.75″.

For 1971-2000 the average was 2.17″.

“Average” weather conditions are usually measured over a 30 year period.

Today is the last day of November 2007, and, according to tonight’s SLOweather/Wxsim forecast, there’s only a 50% chance of rain overnight. And for the month, we’ve had… 0.06 inches of rain.

For the rain year to date (California’s rain year runs from July 1 to June 30), the 1948-2005 average for San Luis Obispo is 3.78″. Currently at SLOweather, the 2007-2008 rainfall total is 0.99″.

So, for the month of November, we are at 2.4% of the 59 year average for precipitation.

And, for the season, we are at 40% of the 59 year average. Any way you slice it, we are in a very dry period.

Even though there was a faint halo around the moon at 6AM Wednesday morning, followed by a reddish sunrise (both supposed harbingers of rain), there’s no rain in the forecast.

Overnight, SLOweather had gusty winds that sounded worse than they were. The peak gust was 26 mph, but the winds were 90 degrees off of the prevailing wind (NE instead of NW). The different direction always seems to make wind sound worse than it is.

At 8:15 AM, winds are still out of the NNE at 5 gusting to 19 MPH, and there are reports of electric service flickering around town. Humidities are still low, as those static shocks you’ve been experiencing the last few days will attest. Today, the RH is expecxted to drop into the low teens. That, coupled with forecast highs in the upper 70s, and the continued winds, makes for an extreme fire danger day in our area.

Look for lows tonight in SLO around the freezing mark.

This was taken at 6:30 this morning, just as the moon became full.

Tonight’s Full Beaver Moon rises locally at 4:49 PM, opposite the setting sun (4:53 PM). It sets tomorrow morning at 7:03 AM.

This full moon could also be the Full Travel Moon, or Full Dying Grass Moon, or Full Snow Moon.

www.stardate.com: “The Pleiades is to its upper right during evening. Aldebaran is to its lower right, and Capella shines farther to its left.”

It’s rare for SLO to truly be able to say that there is not a cloud in the sky. Usually, there’s something off on the horizon, or a little orographic puff above one of the Nine Sisters or one of the ridge lines.

But at 11:45 this morning, a scan of the sky showed nary a cloud in sight, not even the odd jet contrail.

Last night was the coldest of the season so far at SLOweather, dipping to 38 F at 6:25 AM.

The morning forecast run shows Thanksgiving Day today looking like a nice day in SLO:

Today: Sunny. High 68. Wind south around 7 mph, gusting to 17 mph, in the
morning, becoming west-southwest in the afternoon.

Tonight: Clear. Low 39. Wind northeast around 4 mph.

For a while, I’ve searched for some sort of quantitative way to relate current and recent weather conditions (such as those recorded by SLOweather) to operating a residential or commercial irrigation system. Since I haven’t yet found one, I decided to start developing my own.

Here’s the first cut:

http://wx.sloweather.com/irrindex.php

“The SLOweather 7 Day Irrigation Index (7DII) is a guideline for how much you might need to be watering turf or lawn in the San Luis Obispo area.

It is a running total of the last 7 days of rainfall subtracted from the running total of the last 7 days evapotranspiration or ET, as measured and calculated by the SLOweather weather station.

If the 7DII is negative or zero, then you should not need to water your lawn at all, as more rain has fallen than ET has removed from the soil.

If it is positive, then that value is how much irrigation water you might need to deliver over the next 7 days.”

There’s more at the link above, including the current 7DII.

SLoweather uses a Davis Vantage Pro 2 weather station. The indoor console communicates with the outdoor ISS via a buried cable.

The ISS holds the rain gauge, outdoor temperature and humidity sensors, and the UV and solar radiation sensors. When I originally placed it, it was pretty much in the clear, but now the cypress trees have grown up around it. This created a little microclimate that was probably slightly warmer than it should have been. Now that we are into late autumn and the sun is lower in the sky, the trees were shading the UV and solar sensors for a good portion of the day as well.

I have plans for a more permanent relocation next spring or winter. However, that requires installing a new post and a couple hundred feet of trenching, conduit and cable. For now, I’ve placed the ISS at the top of a light post about 10′ away from the old weather post.

The actual down time during the transition was only about 10 minutes, so I doubt anyone really noticed it on-line.

This evening’s NWS text forecast for SLO:

The NWS digital forecast, from the link on the above page, same date (for Sunday):

The first one says the high tomorrow will be 86 F, the second says it will be 67 F. That’s a 19 degree F difference in high temps for SLO for tomorrow. If today is any indication, the high will be even higher that the higher of the 2.

At least the lows match….

Tonight marks the end of Daylight Saving Time in the US. Actually, the official time locally is 2 AM PDT Sunday morning, when the time will become 1 AM PST. This marks the first fall-back of Congress’s seemingly ill advised attempt to save energy, or appease the outdoor products industry, or some dang thing. Let’s hope this change goes easier than “Spring Forward”, which rivaled Y2K in its hassle for anything computer-related. My Outlook calendar is still messed up.

Most everyone I know simply changes the clocks right before bed on Saturday night, rather than stay up until 2 in the morning to make it official. Things like cell phones (which get their time from the cell network), “atomic” clocks (which are actually radio controlled by NIST time station WWV in Boulder, CO), and computers should change automatically, but there is a plethora of other “time-sensitive” devices now that include clocks; things like regular clocks, VCRs, digital watches, set-back furnace thermostats, irrigation controllers, weather stations, home automation controllers, phone systems and more.

At least with “fall-back”, if you miss the change, you’ll be an hour early to church or to the big game tomorrow, instead of an hour late.