Rain ahead and then way ahead as well

First of all, nice Cirrus-ee clouds yesterday…. Cirrus fibratus left, Cirrus uncinus (with tufts at the top, filaments of falling snow below) on the right.  What is remarkable to me is the finely stranded nature of these clouds.  With an aircraft, you would hit, in one of those strands, a burst of ice crystals just 10s of yards (meters) wide, some only about 10 yards (meters) wide.  Those crystals falling out in strands are the largest ones to have formed in these clouds, though they would likely be no more than a few human hair’s width in diameter (around 300-400 microns).  You would think that any turbulence up there would disrupt such delicate strands, but here, anyway, it doesn’t.   The snow falling out is more like a fall of dust, the crystals are so small and light, the kind of snowfall they get at, say, the top of Mt. Everest at 29,000 feet.








The weather ahead

Rain 1:

Begins later Thursday.  Agreed on by Canadians, NOAA, U of Washington’s  WRF-GFS more finely gridded model output,  etc.  Its “in the bag” I would say.  But, what exactly is “in the bag”?

Amounts: worst case, trace, best case, 0.25 inches by Friday morning, daybreak. OK, its not that much.  We’re struggling here to get a cloud here below 20,000 feet though, so we should be happy with any liquid that falls out of the sky.

NOAA model for the 500 millibar pressure level here

Surface maps with rain on them here.

Rain 2:  the big one maybe; getting more hopeful.  Complicated discussion below.

The models have shown a gigantic trough and low pressure system developing in the Great Basin every so often around the 20th of Oct, but then it has disappeared.  Looks a bit more reliable now since NOAA/NCEP spaghetti plots are showing something at that time reflecting this huge change in the jet stream.  The spaghetti plots had nothing earlier when this huge trough was forecast,  indicating it might be a huge bogus-outlier model run in which one could have little confidence in.  Now they got somethin’.  Below is an example of that “somethin'” in those plots, valid for 5 PM AST, October 20th.

The yellow line is the actual model run based on last evening’s data.  The blue lines are the “bad balloon” runs, where slight errors, or differences in data are used to see how robust the actual predicted pattern is.   If the blue lines and the yellow ones converge over one another in an area, like they do south of Japan-Kamchatka Peninsula in the western Pacific, then the forecast is reliable; “count on it.”

Is our forecast as reliable as the one for the western Pacific on the 20th-21st?

Nope, as you can see by how wide the spread of the blue lines is compared to that in the western Pacific.

BUT, the “bad balloon” blue lines ARE poking down to the south along the West Coast, “trending” toward the yellow line (last night’s actual prediction with the data as it came in).  Those blue lines are close to where the jet stream is going to be located with deliberate errors introduced.

So, there is a strong indication that the jet stream will at least dip southward along the West Coast because even with slight errors, it still does that, trend southward along the West Coast.  Now this is exciting, even if there is some remaining uncertainty on exactly how it will play out.  If nothing else, we’ll certainly be on the edge of some strong system about the 20th-21st.

The really picky eye will see that the yellow line (that reflecting a jet stream contour from the actual model run based on last evening’s global data) along the West Coast is still a bit of an outlier (that is, its outside the zone of the most blue lines).  Pretty much all of the blue lines are north of the yellow’s position.   So its very possible that we could end up with less of a trough along the West Coast than was predicted in the actual run using the global data as it was.   I guess this is confusing, but I am trying to make it less so, with little apparent success.

How much rain is foretold presently in “Rain 2”?

Below is a panel from last evening’s global data crunch for valid for 5 AM, October 21st.  You’re gonna like what you see:The colored regions are those in which rain is forecast to have occurred over the prior 12 h, meaning the rain likely started Saturday afternoon, the 20th here.

What’s fascinating is that in all these model runs, there has been a tropical storm/hurricane shown to the south of us.  Sometimes it fades away, or moves over New Mexico, but its always been there.  What’s fascinating #2 is that again, it is a hurricane that hasn’t even formed yet, but the model detects a “signal”,  a pattern, a combination of factors that will come together to form a tropical depression, then a tropical storm, and then a hurricane that is caught up in all the upper level goings on in the West. In the latest model run, one panel shown below, that fading hurricane is approaching Baja and its remnants are now drawn into Arizona.

So, there, in that hurricane remnant, is where the potential of a mighty rain lies on Saturday evening into Sunday, the 20th-21st of October.  Models are indicating 1-2 inches in portions of AZ over a 24 h period with this situation. Us?   About half an inch in 24 h, but stand by.

So, lots to hope for….

There is yet one more suggested rain (Rain3) after “Rain 2” so it would seem SOME rain will fall in the next two weeks!

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.