Rain and when today

For those in a hurry:  no rain before 5 PM AST, rain beginning between 5 PM and 8:42 PM; most of rain after midnight into mid-day.   Since people forget, I’ve added a musical reminder there: “After midnight, we’re gonna let it all hang down.”  Could be that incoming front talkin’  to us.

It will also be a fantastic day for clouds, probably some nice lenticulars today downwind of the Catalinas and elsewhere as the wind strengthens above us.  Keep your camera handy.

For retirees, or others who have a LOT of time….

The long story line about how to forecast when

Its ALWAYS fun and a challenge to try to foretell when the rain will start to fall by the clock.  Did some radio work like that in Seattle, i.e., “rain beginning between 11 AM and 2 PM”, “no rain before noon”, and also for the Washington Husky baseball and softball teams, when to take the tarp off, resume play, when put the tarp on, etc.

Those radio forecasts were made early in the morning on weekdays (KZAM-FM, 1982) and weekends (the latter during Weekend Edition on KUOW-FM, an NPR affiliate, 1987-1992).  It was thought that such a forecast, given early in the day,  would be the most useful one for a listener planning his/her day if it could be done well enough. This would be ESPECIALLY true on weekends when the listener could do whatever with his/her time rather than be in a building all day at work.  But, could you do it well enough using the surface obs, weather maps, satellite imagery, and model forecasts generated the evening before, such as they were in those days.  It was deemed a forecasting experiment in those days.  It wasn’t being done elsewhere in SEA.

BTW, there was no weather radar in Seattle during those days if you can imagine such a thing.

However, I will also add that radar is really not that useful in SEA anyway since forecasts for the next 8-12 h is a window of time in which radar can’t be used, for example if the incoming rain is still off the coast of Washington!  Reading the sky was much more important in determining how close the rain was, integrating that interpretation with the satellite imagery.

Or those days when showers would form that aren’t present in the early morning when you’re giving your forecast.  The passage of a front in Seattle, BTW, is usually followed by a brief clearing followed by passing showers, unlike say, back East where once a band of rain goes by, that’s it for the rest of the day.

So, here we are in Catalina facing a great storm, our best of the winter.  How close can you come to getting the start time here in Catalina?  That’s the game.  Today, the models are SO MUCH better, you have to look at them carefully, and know whether they run a bit fast in bringing in rain, and if there are any significant errors built in.

OK, here goes the first look, based on the passage of the core of the jet stream at the 500 millibar level, around 18,000 feet.  The criteria that rain starts when its overhead and after it passes was developed during the early 1970s whilst the author, a forecaster, was working for a HUGE randomized cloud seeding experiment in Durango, Colorado.  It was found that almost no precip fell in Durango before the 500 mb jet passed (95% of the wintertime precip there fell only after this happened).  That study was later expanded to the ENTIRE US and it was found that wintertime (November through April) precip in the interior of the Southwest also followed that Durango criteria very closely; it was almost a black and white predictor.

So, let’s look at when the jet passes over us here (using IPS MeteoStar’s great forecast renderings again) and see what time it goes by (of course, in this era, the models also “know this” relationship; not so well in the early 1970s).

Valid for 5 PM AST today! Note the coloring. It shows where the velocity of the wind is higher and lower.  Yellow, browns and reds are highest.

Valid for 11 PM AST tonight. Should be raining real good by now; jet has passed overhead and is to the south and east of us.

Like Seattle in some ways, this early onset of rain is from clouds for the most part, are ones that haven’t formed yet!  They’ll be forming and filling in to the southwest of us and here, deepening as this big trough works inland toward us.

Next, if you were to measure the movement of the frontal rain band now in southern California over the past 12 h or so with a ruler, or piece of paper, you would find that at its present rate of movement that frontal band would be here around 10 AM tomorrow.  So, you would have rain starting in the evening, peak rain in the morning.  Done.

Now after this simple exercise, let’s see what the BEST model has in mind for the rain start, that from the U of A, processed by their intimidating “Beowulf cluster” (you can call it up here) based on data from 11 PM AST last night.  This should be a very accurate forecast, and we hope resembles what was said above; no rain before 5 PM, but raining soon after that.  (BTW, I have not looked yet; part of the “game” today;  can you do as well as the model with a simple approach?

OK, here is the coverage of rain expected by 5 PM AST in the U of A model:  not much, but its upwind.

Coverage of rain by 5 PM AST. Not much.
Rain coverage by 9 PM tonight. Should have rained some to be redundant.

In sum, that simple technique was not so bad.

The main frontal band is still to the west at this point, 9 PM AST,  and it doesn’t pass over until after midnight. Then the rain lingers into the mid-morning to early afternoon.  Amounts still look VERY substantial. This model projects around a half an inch or more here in Catalina by tomorrow night.  The range of values, given the various uncertainties in models and clouds and weather, bottom amount 0.25 inches (only 10% chance of less), top amount, an inch (only 10% chance of more), these percentages generated from this keyboard, BTW.

Here is the Beowulf total storm rain ending at midnight tomorrow night:


What’s after this?

Mods still showing spate of storms over the next couple of weeks, this one not a one-hit wonder.  And there’s support for these storms in our venerable ensembles of spaghetti.  Main brunt of storms: California.  Expect to read about them.

But are our rains too late for our spring flowers?  Dunno.

The End, at last.