April to have measurable rain in Catalina!

In the meantime, step aside;  a cold front is upon us, a dry one, unfortunately.  Should arrive by noon, bringing some small Cu here and there, some Stratocu piling up against the Catalinas, and maybe some lingering Altocumulus lenticular clouds which we got right now (4 AM) downwind of the Catalinas.

From Intellicast, this nice map:

As of 4 AM AST, the 24 h temperature change. The blue blob shows the encroaching cold air.
As of 4 AM AST, the 24 h temperature change. The blue blob shows the encroaching cold air.

Barometer will rise, too, as the cooler, denser air piles on top of it.  There’ll likely be a brief windshift to the NW, followed by backing to the SW again.

Over the next couple of days, the deep cold air in the  interior of a lingering, massive trough will settle over us, dry up top, but enough moisture in the lower layers below to produce eventually deeper Cumulus, though not today, ones likely to reach up to the “glaciation” level, which will be close to -12° C to -15° C in this situation beginning later tomorrow through the April 1st.   The bases of the clouds will be near the freezing level.

Glaciation means that ice will form in those Cumulus clouds, and some (snow) virga will drop out the bottom.  So, some snow showers or just light rain showers are likely on the Catalinas, maybe a trace or hundredth here, too,  beginning later tomorrow  through April 1st.

Should be some really pretty deep blue skies, too, cloud shadows producing quilt-like patterns on the mountains, that sort of thing we are so lucky to enjoy here.

As you know, this end of month March “lion” (at least in wind, anyway) was long foretold in the NOAA spaghetti.  Remember how we could laugh at model outputs that didn’t have a big trough here at the end of the month?

But now we wait and see if we can drain a cloud or two of a hundredth.  Overall rain chances not looking so “strong” now out of this whole several day situation.  Dang.

Clouds will be around today, especially after the cold front goes by, but its unlikely they’ll have anything drop out the bottom.


“2warm4ice”,  to be that bit textual.

Model says today’s cloud tops won’t reach -10° Ç, our magic temperature where we can usually start to thinking about ice forming in AZ clouds, those with our usual cool  bottoms.

Of course, if you’re really sophisticated, you know that the temperature at which onsets in “continental” Cumulus clouds like we have here in old Arizony,  is related to cloud base temperature:

The warmer the cloud bottom, the higher the onset temperature for ice1, “strangely believe it”, as we like to say here after Jimmy Hatlo the cartooonist thought of it first when he was making fun of RIpley’s “Believe It Or Not.”

Now onto the forecasting frontier, forecasting weather patterns way ahead, to far in advance and too specific to be truly professional

Let’s start with something easy.  Its gonna warm up real good after this big trough goes by– see spaghetti below, where a big ridge moves over us for a couple of days.  It won’t last.

Valid at 5 PM AST, April 3rd.
Valid at 5 PM AST, April 3rd.  Notice, too, that unlike most of the spaghetti pe degree of chaos introduced by the deliberate errors input at the start, have little effect (as usual).  The  blue and red contours are bunched really well.  So the positions of the ridges and troughs are normally well predicted out to this time.

Then,  uh-oh, as Robert Ellis Orrall used to sing, in 192 h, predictability begins to fall apart, but not real bad, and it shows a trough is moving in over us.

Valid at 5 PM AST, April 5th. Red contours are bunched enough so that a nice sized trough in the SW is pretty guaranteed.
Valid at 5 PM AST, April 5th. Red contours are bunched enough so that a nice sized trough in the SW is pretty guaranteed.

Finally, at the extreme end of the medium range forecasting frontier, this:

Valid at 5 PM AST, April 12th. Stormfest Southwest!
Valid at 5 PM AST, April 12th. Stormfest Southwest!


Hence, the conclusion that we share that April, will in fact, have measurable rain.  Of course, we only average about half an inch in April, as the overall climatology begins a serious a battle against rain heading into the ovenly days of May and June.

The End


1The old English cloud scientist, Frank Ludlow (1952, Quart J. Roy. (haha, “Royal”, oh my) Meteor. Soc.) noticed this first, then that great Soviet Communist cloud scientist, A. M. Borovikov and his companions did (1961, Israeli Translations).  Finally, Rangno and Hobbs woke up and noticed this tendency in 1988, (Atmos. Res.) and then again in 1995 (J. Appl. Meteor.–you’ll have to go quite a ways to find the relevant diagram)  in their cloud studies and in comparisons with other ice onset reports.




4 thoughts on “April to have measurable rain in Catalina!”

  1. Ok.. 1 to 2 more weeks, and then.. RAIN!

    I’m ready, I’m excited. This is it, man, I feel it.

    1. Yep, I think FINALLY some decent rains will occur here at least once during the next two weeks. But, then I have been WRONG on this quite a few times this winter having been loaded up with rain bias due to the Big Niño out there. What a flop its been for the late winter and spring for SW so far!


  2. Nope, not necessarily this week, but things are looking up for rain over the next two weeks, and likely beyond that.


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