20200820

 Previous Forecast

It was another fairly quiet for the state.  There wasn't even any precipitation along the border or in far northern Sonora.  The higher elevations had a bit of activity and there was some in NW Arizona, but the storms didn't make it very far west.  

A few weak storms developed this morning just south of Phoenix and in Yavapai County, similar to what was discussed yesterday.  The middle of the road WRFGFS had the best forecast.

Discussion

The big story for Arizona is hurricane Genevieve which is located just off the west coast of southern Baja, in a prime position to import moisture into Arizona.  Dr.Luis Farfan sent me a report of 280mm of rain in Cabo over the past 24 hours!  A trough is moving into the NW CONUS and is again suppressing the ridge to the south.  It's located over far NW Arizona this morning, which results in weaker mid-level flow.  Tucson still has an OK flow of ESE and 15 knots.  500mb temperatures continue to be moderately favorable at -7C.

PW has generally increased over Arizona and Sonora.    Arizona deserted are generally in the 30-35mm range and above 40mm in northern Sonora.  The large blank area is an area of thick smoke that moved down from western Colorado yesterday.

I got a pretty good shot of the smoke at sunset last night, as it was moving in.  This smoke may be thick enough to reduce heating over central Arizona today.

Both Tucson and Phoenix have had an increase in CAPE as Phoenix is up to 1000 J/kg of MLCAPE and Tucson is even better at 1800 J/kg of MLCAPE.  There are two inversions to worry about though, one on top of the moist layer and another strong one just above 500mb.  The Tucson wind profile is still OK as mentioned above.  Phoenix was mainly light and variable so storms will have to propagate by developing on outflow boundaries.

Initializations

The initializations did a good job getting the morning clouds and showers in the right place over central Arizona.  The 6Z NAM had a bit too much cloud cover though.  The 6Z GFS and NAM were a bit too moist for NM, while the 12Z RR and NAM had only minimal errors.  None are bad enough to be discounted.

Day 1

Finally, some decent amounts of moisture are advecting into the state.  A strong Gulf Surge continues today with 25-knot 10m winds over the northern Gulf of California and PW slowly increasing.

CAPE increases too and is moderate to high over much of the state.  Now the question is how capped is the moist layer?

As expected, it's very strong at Yuma.  Look at all that CAPE though!

There is only a weak inversion present and the mixed layer isn't too shallow at Phoenix so a good outflow could kick big storms off there.  CAPE is great at 1500 to over 2000 J/kg.  The wind profile isn't much good with only light easterly steering flow.  If storms can get going, they can propagate into the excellent low-level westerlies though.

The Tucson Skew-t diagram is a textbook example of a big storm outbreak as nearly everything is there.  A weak inversion at the top of the mixed layer, CAPE>1000 J/kg, and a fantastic wind profile with low-level inflow/shear, 15 knot NE mid-level flow and upper westerlies to blow anvils the opposite way of storm motion.  The only issue is that the CBL isn't mixed very deeply.  The LFC is around 650mb so it's going to take a bit of lift to get deep convection.   Outflow boundaries should be strong as DCAPE is quite high at 1400 J/kg so they could do it.

It takes quite a while for the initiation of deep convection over the high terrain as there must be quite a bit of CIN to overcome there too.  Strong storms don't really get going until the late afternoon.

A broken line of organized storms moves into the lower elevations of eastern Pima, Pinal, and Maricopa Counties after sunset.

So far so good, or is it?  Of course, the WRFRR and WRFGFS have to bring me down as they have a lot less activity, and storms dissipates before reaching the lower elevations!  The WRFRR doesn't worry me so much as it's always been on the low side, but the WRFGFS is disturbing.

Unfortunately, the 6Z WRFGFS is similar.

Sometimes you just have to go with what the observed and forecast soundings look like, thus I'm still optimistic it will be the WRFNAM and its forecast of an active evening for the lower elevations.

Strong to isolated severe winds are possible with some of the storms.

Here's another plot to worry about.  Only the 12Z WRFNAM is active for central and southcentral Arizona.

The operational HRRR 3 member time-lagged ensemble is more than the WRFGFS, as it hast activity in Pima and Pinal Counties.  It's less than the WRFNAM as it is a near-miss for Phoenix.

Day 2

The hurricane continues to move parallel up the coast of Baja and keeps excellent SE flow going up the Gulf of California, and moisture advection into Arizona.

CAPE continues to be moderate to high over much of western and southern Arizona.  Again, how capped is this unstable air?

The cap is strong at Phoenix, even by late afternoon.  The CBL is also shallow so it's not looking good there.

 

 

The situation is much the same in Tucson with a shallow mixed layer and a significant inversion at the top.  The wind profile is excellent and CAPE is high, so at least there is some potential if a very strong outflow moves though.

Even the WRFGFS which was not worked over like the WRFNAM has only isolated storms and moves/develops nothing for the lower elevations.  Sad.

The WRFRR does manage to move some storms into far SE Arizona tomorrow evening, but that's about it.

At least the heatwave is over, for now.