20210722

 Previous Forecast

It was a very active day for areas west and northwest of Phoenix, much more so than what the models had predicted.  In addition, storms managed to develop over the eastern part of Phoenix late last night.

Model accuracy was bad, especially the normally well-performing WRFRR.  The WRFRR struggled as it kept developing anomalous deep convection shortly after the run started over western Arizona during the morning hours.  This stabilized the atmosphere resulting in little or no afternoon storms.  In addition, all the morning runs did not develop deep convection in the Phoenix area.  The lone exception was the 18Z WRFNAM.

The 18Z WRFNAM was also able to predict the western Arizona activity accurately.

Discussion

We shall soon see if the big storm/precipitation event lives up to the hype from yesterday.  The 500mb pattern is mostly unchanged as the anticyclone center is still over the Four Corners, and the inverted trough has only moved slightly westward over central Texas.  Arizona continues to have warm air aloft, as temperatures range from -5 to -6C.  Temperatures are as low as -9C with the IT.

The IT is creating an area of divergence over eastern NM.  As the trough shifts slowly to the WNW today, this favorable divergence moves over Arizona.

The past 24 hour PW trends are quite concerning as PW has decreased for much of the state, with the largest decrease over western Arizona and down to Puerto Penasco.  I expected to see a surge signal by this time.

Phoenix PW has decreased to around 40mm, which is still quite moist.  It just doesn't seem that way, as it's been so high recently.  This results in a marginal 855 J/kg of MLCAPE.  Low-level winds do not show any indications of a surge.

PW has also decreased at Tucson, but MLCAPE is a high 1300 J/kg.  Both soundings indicate an inversion continues around 500mb, but as the trough moves closer to Arizona, mid-levels will cool.  The IT has already made its appearance; note the 50-knot wind at 200mb.

Very potentially unstable air isn't far away as GOES CAPE values exceed 1000 J/kg just south of Arizona.  Rarely do I see GOES CAPE this high.  

The really wet air is also not far away, over the central Gulf of California, but it will take a big surge to move it into Arizona.

Initializations

There are a fair amount of high clouds over the state, but no ongoing showers or storms, so the initializations shouldn't have too much trouble.  Most initializations were accurate, except the 6Z NAM had too many lower/thicker clouds in some places.  The IT is right on the edge of the outer domain and looks to be accurately initialized, except by the NAM, which is too far west.  Unfortunately, most Suominet stations in Mexico are now inoperable, and only 3 are available, so it's not possible to easily identify PW initialization errors. At least for the SW US, PW was initialized accurately by both the GFS and RR, but as usual, the 6Z NAM was too moist.  To my surprise, the 12Z NAM actually has minimal errors!  It's again quite smokey over Arizona, and it does appear that the GEOS5 AOD forecast used by WRF is doing an OK job with solar irradiance.

Overall, the GFS and RR initialized well, so confidence in these WRF runs is high.  The 12Z NAM may also be of use today too.

Day 1

Is the surge going to kick in?  It may have just started as Yuma area wind speeds, and dew points have started to increase.  By early afternoon, the surge is well underway and an increase in PW over central and south-central Arizona.  

As was forecast yesterday, the surge is deep and reaches to at least 850mb.  850mb dew points are also high, around 14 to 15C, by early afternoon.

This Gulf Surge is impressive as it stretches all the way down the Gulf of California, even at 850mb.

The various CAPE forecasts for the early afternoon are quite wide-ranging.  The middle of the road is the 9Z WRFRR.  It has extreme CAPE over far south-central Arizona and moderate to high CAPE over much of the rest of the state.  

All runs are nearly identical in developing severe storms along the border of Pima and Santa Cruz Counties by mid-afternoon.  

The IT slowly makes its way into the Texas Panhandle this afternoon, resulting in southern Arizona being under difluence aloft.  A jet streak associated with the trough is over southern NM and into southern Arizona.  These features will help enhance and organize storms this afternoon and into the evening.

I'm not surprised, but the "all hell breaking loose" forecast is looking less likely.  The potential is certainly there as the Tucson afternoon Skew-T has considerable CAPE, and the low-level shear is impressive.  However, the PBL isn't mixed very deeply, and it will take strong outflows to allow it to convect.  

Strong outflows are forecast to move through much of eastern Pima County by late afternoon.   As DCAPE from the 12Z sounding was nearly 1700 J/kg, I would have thought outflows would be stronger. 6Z WRFGFS

The strength of the outflows seems to be the deciding factor regarding how widespread early evening storms will be.  The weaker outflows develop a few strong storms, but then they dissipate. 

Stronger outflows generate more widespread storms.  All runs agree on the development of strong storms over much of the eastern 1/4 of the state.  6Z WRFRR

The 9Z WRFRR and, to a lesser extent, the 6Z WRFGFS continue to develop storms on the outflow boundaries as they move north toward Phoenix, eventually resulting in severe storms in the Phoenix area.

The 12Z runs tell a different story.  Little or no evening activity for either Tucson or Phoenix.  However, strong to severe storms continue to expand and move to the west over eastern Arizona.

One thing that is certain is that the threat of severe weather is certainly there for both Tucson and Phoenix this evening.  How it actually works out is up in the air.  Phoenix has about 2000 J/kg of CAPE and a fairly deeply mixed PBL, and good low-level shear.  In fact, the surge signal in the wind profile extends all the way to 700mb.

It's the same story for Tucson, except the earlier afternoon activity has resulted in a cooling of the low levels and multiple inversions that may be difficult to overcome.

All runs move the eastern Arizona storms into the Tucson and Phoenix areas during the early morning hours.  

The biggest threat is going to be flash flooding.  The 4-panel 24 hour QPF has quite a bullseye on Gila County, which couldn't be a worse spot due to the recent big fires there.  WRF had large areas of greater than 1 inch, with some isolated areas over 3 inches.  WRF usually underestimates QPF, so it could even be more than depicted.

The soil moisture forecast at 5am tomorrow has saturated soils for much of eastern Pima, parts of Gila, and northern Graham Counties.  Flash flooding is likely.  

Most runs keep rain and some thunderstorms going until at least sunrise as a large MCV has spun up and is located over southern Arizona.

Day 2

Whew, I'm exhausted from writing about Day 1.    Day 2 confidence is low due to the previous widespread activity and the complexity of the overall situation.  Clouds and rain continue during the morning hours, forced by the large MCV.

By early afternoon, the MCV becomes an open wave and moves into northern Mexico.  The IT has moved little and is located in a favorable position in far southeastern NM.  If the atmosphere can recover, expect another round of rain and thunderstorms.

The 250mb forecast has much of Arizona in very favorable upper-level divergence/difluence.

The Gulf Surge continues to advect very moist air into the state, and PW has increased to extreme amounts over central Arizona.  OK, weather gods, turn off the hose. We've had enough.

Fortunately, much of the southern 1/2 of the state is worked over, and despite the increased moisture, clouds have kept heating to a minimum, and CAPE is limited for the lower elevations.  However, parts of the higher elevations along the Rim and the White Mountains to northwestern Arizona are quite high.  

Rain and clouds continue over south-central Arizona while strong storms form in northern and northwestern  Arizona and another area in far western NM.  

With favorable upper-level support, the NM storms move across Arizona during the evening hours.

CAPE appears to be limited over the lower elevations, so storms are likely to weaken as they approach the lower elevations.

72 hour QPF is extremely worrisome as many areas are forecast to receive over 3 inches of rain, and some locations, over 5 inches.