20210814

20210814

It was one of the most active nights in years for Phoenix.   It started out with a line of thunderstorms that moved SW out of southern Coconino County, which then transformed into a Bow Echo, which I can't ever recall seeing before.  Doppler velocities were over 80mph for some time.  Many wind reports of 60-67mph were reported with the line.

Storms started to develop in Phoenix by this time and eventually were aided by the OFB from the Bow Echo.  The main impact was torrential rain and continuous lightning.

Other strong/severe storms moved across southeastern Arizona but died as they moved into the Tucson area.  They tried to form, but the OFB from the Cochise storms was weak, and there was only 1100 J/kg of CAPE, which wasn't enough to get things going.  However, showers and a few storms formed late and continued for much of the night, dumping more than 1" in many locations.  Somebody needs to get started on an Arc.

Model runs (excluding the NAM) were quite accurate around the Phoenix area.    Not so much around Tucson as they moved strong to severe storms into the valley during the evening, which didn't occur.  The WRFRR also missed the heavy precipitation east of Phoenix.

Discussion

A large anticyclone is centered over southern Utah up to Oregan.  Heights are up to 597dm again.  The situation is complicated over Arizona as a large MCV is moving west over southwestern Arizona, and another MCV/IT is from southeastern AZ back into NM.  Another trough over OK looks like it may cut under the ridge.  No upper-air data was received from Mexico this morning.

Both Tucson and Phoenix are worked over, with a cooled near-surface layer and little or no MLCAPE.  The 2m temperature in Phoenix was 73F.  I wonder when the last time it was that cool?  The wind profile is still very favorable, but it's going to be difficult to get deep convection to go, especially if it remains cloudy.  With a little sun?  Maybe, as with favorable winds and this much moisture, storms could go again, just about any time.  Also, there was another PW record for Tucson at 1.96"  inches.

Initializations

I'm almost afraid to look!  The situation is complicated with widespread clouds, showers, and MCVs over southern Arizona and NM.  The 6Z GFS cloud initialization is OK, but it's missing the western Arizona MCV.  It does have a second closed circulation over southeastern Arizona and an IT over much of southern NM.  The 9Z RR cloud initialization was also OK, plus it also has the SW Arizona MCV and the other features.  12Z RR is the same.  Once the monsoon season is over, I'm going to replace most NAM runs with the RR, as it's obvious that the WRFRR performs significantly better.  Speaking of the NAM, the 12Z isn't too bad as it has all circulations, and the clouds are initialized fairly well, except they may be too thick.  PW errors are minimal except for the 12Z NAM.  The WRFRR series is the clear favorite today, and considering the initialization challenges, they should perform quite well.

Day 1

The surge has petered out, but copious amounts of moisture remain.  Low-level easterlies are present over NM, and drier air has moved into NE Arizona.  

As would be expected, CAPE is much less today but still sufficient to support some activity.

I'm a little surprised to see this much CAPE at Tucson this afternoon.  I guess it's possible if the sun comes out, and it does appear that the morning clouds are dissipating.  There is minimal low-level shear as the lower troposphere is mainly easterly, but steering flow is still good.

Storms take their time developing, which would be expected due to all the clouds and cooler conditions.  By late afternoon, they are scattered about over the higher terrain of eastern and southeastern Arizona.  The storms east of Phoenix are of interest as steering flow is optimal for moving them that way.

Steering flow continues to be quite brisk over central and northern Arizona as one MCV remains over southeastern Arizona and the other south of Yuma.  The IT also remains in place over southern NM.

A repeat of storms is possible for Phoenix.  It's hard to believe I wrote that, as it's almost always the case that today would be a down day.  However, with the amount of moisture around, CAPE recovery, plus the optional vertical wind profile, it is starting to look like it might happen.  One issue is that the WRF runs are 2-3C too warm so far, and temperatures will need to catch up to the forecast.

Many runs do move storms into the lower elevations of central Arizona this evening.

But not all.  The 15Z WRFRR backs way off and has mainly showers this evening.  The difference is the amount of heating, as the 15Z is a few degrees cooler this afternoon.   We'll just have to wait and see.

Day 2

The 500mb anticyclone is centered over Nevada, while the elongated IT moves south and is along the US/Mexico border.  This results in favorable ENE flow over much of the state again.  

Steering flow may be good, but the E to NE flow in the lower troposphere is advecting dry air over much of northeastern Arizona and most of NM.  There is no gulf surge to counteract the drier air.

In the moist air, CAPE does recover and is moderate to high over much of the rest of the state by tomorrow afternoon.

Storms take their sweet time developing, and by late afternoon, they are scattered about over NE Arizona and far southern Arizona.  Frankly, the NE Arizona storms are a bit of a surprise as I thought dry air advection would keep them to a minimum.

Scattered strong storms move towards the Phoenix area by tomorrow night.

Phoenix certainly has enough CAPE, at around 1600 J/kg, despite PW decreasing to 38mm.  The mixed layer is shallow and will take a lot of work to lift to the LFC.

Some runs have storms again for central Arizona.

While others don't have much.