On schedule for major AZ storm beginning Monday

Now that the models have reconciled, “come together”,  to show a large storm affecting Arizona and us here in Catland beginning later Monday, it seemed interesting to ME to show you how this one gets here.  This is where our numerical models do things that in the olden days before them we could never anticipate.

Here is a loop from last night’s global data on how a large, vigorous low center forms over San Diego within about 84 from right now.  This loop of the high and low pressures in the middle troposphere (around 15,000-20,000 feet above sea level) from the University of Washington’s “WRF-GFS” model for the West Coast and environs:

Watch what happens in the Gulf of Alaska.  Everything is rolling along FAR to our north, going toward the east, looks like it’ll stay up there with those waves moving one after the other into Canada.  There is no hint of anything moving this way.  Then suddenly, the wave rippling along in the Gulf turns south bound as it hits the West Coast, part of it ripped off and becoming a spinning center just off California and the rest continuing on into central Canada!  Really, its amazing to see such a drastic change like that shown in this sequence.

This type of sequence has been very much like this winter’s past storms have gone through, rippling along in the northern Pacific Ocean, and then suddenly part of those waves shearing off from the main jet,  making a right turn toward the SW US, becoming an isolated, spinning, wobbling center away from the steady strong westerlies of the jet stream.  Some years have had lots of these “cut off” lows, and it seems we are headed in that direction this winter, at least through the first 6-7 weeks.

One of the keys to this happening is how vigorous is the storm immediately upwind of the one that plunges south, oddly.  This influence of upwind storms in perturbing the jet stream downwind was discovered WAY back in the 1950s.  In the current situation, an extremely intense low developing in the central western Pacific sends huge amounts of heat and clouds northeastward behind, upwind, of “our storm” and that heat and moisture (see here protruding into the Aleutians in this loop) helps amplify  the jet stream downwind where our storm wave is by causing the winds shift more to the north downwind of the warm air plume at the backside of our wave).

What it is doing is building,  for a time,  a high pressure ridge aloft within and just ahead of it.  In response to that “ridge building” plume of heat and moisture caused by that intense storm upwind of “our wave”, the jet stream downwind begins to head more to the south, IN our wave.  The intense storm behind our wave is building “amplitude” in the jet stream.  Amplitude in “synoptic” meteorology generally  means the jet stream winds go more to the north and south rather than just east and west. And that’s what you see happen for a time in this loop, and its this greater amplitude that causes a part of the once steadily progressing wave across the Gulf of Alaska to go, “Oops, must go south now” as the winds from the north increase tremendously on its backside.

The assymetry of the winds in these waves tell you where its going next.  If they are stronger on the backside, it will go south or southeast.  If the winds are strongest on the front side, or east side, it will go north or northeast.  If you can see the winds in the above loop, you’ll see that they reach 120 mph from the north on the backside as it plunges S, but the winds on the east side are only about half or less of that.

Hope this is somewhat intelligible.  Still rewriting…!

Personal predictions?  I think we’ll get at least 0.50 inches, probably see some more snow mixed in with the rain by Tuesday morning.  Think about a great wildflower bloom this spring.  In the meantime, enjoy the warmth and cirrus of the next couple of days.  One thing that might help is that disturbance SE of the Hawaiian Islands feeding moisture into the jet stream (seen here in this loop again).  I thought at first that bunch of clouds might be related to the MJO (not a coffee brand, but the “Madden Julian Oscillation” which can have a profound effect on US weather).  But it wasn’t.  The MJO is in the area of the Maritime Continent now.   Go here for the latest NOAA details on this subtle wind regime that travels around the globe toward the east.

Last evening’s “cirrus-ee”, (ice crystal clouds),  sunset.

The End.