FROPA, cold just ahead

Summary:  Wildcat mod has the front with a rain-turning-to-maybe a bit of snow-in-rain (its NOT SLEET!) between 10 PM AST on the 10th and 2 AM on the 11th.

Duration of precip?

Just three hours or so, quite awful;  too short for any significant accumulation of snow in the cold air behind the front.  This forecast cribbed from the U of AZ Weather Dept model outputs found here from the 11 PM AST data of last night1.  Here is a map of the predicted amounts just after the precip ends here at 2 AM:annotated 0001Ep

Its interesting that on this website we have been talking about not chances of rain, but rather amounts of 0.10 inches as the least amount likely and 0.50 inches at the top amount, and then yesterday, due to fast movement, lowering the top amount possible to 0.40 inches, with a median amounts of 0.30 inches or so and this is what the Beowulf Cluster has come up with last night, the first of these runs I have looked at for this storm!  It seems like if I guess something, then it just seems to come true, except on all those other days…

Now for a discussion of some vexatious, tedious details about our rain prospects in the coming FROPA:

Here is the jet configuration as the rain is about to begin on the evening of the 10th, from IPS:annotated 2013010900_CON_GFS_500_HGT_WINDS_060
As you can see, this model run (from IPS), from the global data taken at 5 PM AST, has the jet core at this level (500 mb) close to right over us, but a bit to the north if you look closely.  Precip is almost always partitioned in a black and white manner by where the core is at 500 mb in the SW US in the cooler half of the year (Nov-Apr).

So, its of some concern that the jet has not gone past us here at 5 AM AST the morning of the 11th after precip has been predicted by the models! That jet core is nowhere near us when the Wildcat Beowulf Cluster has rain here at 11 PM AST on the 10th; its still upwind over western AZ.

So, of some interest to see whether the rule of thumb about the jet core and precip holds up in this situation.  If it does, we could have a real disappointing FROPA, just sprinkles maybe.   Because the deeper colder, moist air is on the left hand side looking downwind, it implies shallower moist air arrives here,  shallower clouds, or separated layering during the FROPA band.  On the right hand side looking downwind, is usually far drier air with clouds limited to shallow Cu-Sc, mid-level clouds, often Altocumulus lenticularis, and Cirrus.   I just now checked with Beowulf predicted soundings, sometimes forgetting that there are model predictions of moisture layers above us, and they check out with the above assessment during the day tomorrow when the jet is far away from us.

Now looking at the Tucson soundings from our Beowulf model during the time of the precip, 10 PM to 2 AM on the 10th to 11th, it IS true that the clouds WILL be pretty shallow!  In fact, the projected temperatures in the moist surge accompanying the rain band in that model run are only a little below our ice-forming temperature of -10 C (14 F).  If the model is off by a couple of degrees (too cold) we could get nothing, or traces, just harmless Stratocu.

Gee, this is so interesting for a Cloud Maven person now!  You, too, I am sure!

Here’s an additional thought.  Those Beowulf soundings are literally for over the mountain in Tucson.  Every mile to the north, because we are near the jet core, means everything will be that bit colder, cloud tops that bit higher.  Furthermore, with the southwest to west flow, they’ll bank up the best they can against our side of the Catalinas.If Beowulf looked that the vertical layering of air right over Catalina tomorrow night, it likely would see lower cloud top temperatures than those shown for Tucson, and greater chances for rain here compared with there.  Get it?

So, we’re really left with a more marginal rain situation than would appear to be the case looking at the Beowulf rain amounts in the first panel above.   Dang.   Seeing what really happens will be a joy in a sense because of all these factors.

After the front….

Waves of cold air, one after another.  Beginning on the 11th, it will be one of the coldest 7 or so days in years here. Hope you don’t live in wash, where temps in the teens are likely at some point.  The NWS of course, is all over this.  Its been well predicted for more than a week in advance.

After the cold

One of the more astonishing forecast series has come out concerning the flow patterns after our week-long spell of severe cold with a powerful jet stream shooting down at us from Canada.  All of the jet stream activity in the ENTIRE US is forecast to recede north into Canada!  That kind of pattern means the whole US has been “milded”, as that happens, to continue a language innovation theme here.  Take a look at this flow pattern over the US and compare it to the one in the second panel.  Its an amazing change, almost a summer-looking pattern when the jet stream recedes to Canada.


Not much going on here.  January thaw over the whole US?  Looks like it except for the extreme NE.

But this is NOT quite at the end of the model run.  What’s to happen next?  What would you guess?  No hints here on this map, that’s for sure.  Remember what to fall back on, the past.  Predict the future from the past as a first guess.  If you did, not seeing anything going on here, you would see in the last few panels, another powerful, cold trough beginning to head in our direction.   Of course, it may go away, but you have the past on your side; the atmosphere remembers what its been doing, and maybe it has not yet forgotten our cold spells after the mild one ahead that finishes out the month.

Brain, such as it is, exhausted, quitting here.

The End

Not quite:  Just peaked at the 11 PM AST US model run now at 7:50 AM, and the “past” comes back in a hurry in this latest run.  There is no substantial mild period to the end of the month.  Its MUCH shorter, maybe a week before things cave in again in the last week of Jan.  Hang on.


1If you do peak, you might also send some $$$ their way.  They are in dire straights these days.   Mr. Cloud-maven person has.