20200724

Previous Forecast
It was an active past 24 hours over Arizona and surrounding areas.  Most of the low elevation activity occurred yesterday morning, which resulted in outflows moving into Phoenix. This resulted in cooling and a more stable atmosphere.  Storms continued into the night over NW Arizona, and NW Mexico was also very active. 

Some locations around the Tucson area ended up getting more than 3 inches of rain.  There were some areas of very heavy rain north of Phoenix too.

Considering the complexity of the situation, the WRF runs performed pretty well.  The various WRFRR runs were the best as they did well initializing the initial cloud and storms.

Discussion
The 500mb pattern is mostly unchanged from yesterday as a weak trough continues over much of the western CONUS. The 500mb anti-cyclone is centered far to the east.  Arizona has a mainly southerly flow of around 15 knots, so steering flow is a bit better today.  A large and diffuse inverted trough is located over NW Mexico with several smaller cyclonic circulations embedded in the flow. 

In the wake of yesterday's morning activity, PW generally decreased over southern Arizona, but that has now reversed, and PW has increased overnight back into the low 40mm range.  PW is still very high in Sonora as Hermosillo maxed out at 61mm last night!   Both Tucson and Phoenix have 2-300 J/kg of MLCAPE at 12Z, which isn't all that great, but better than zero, I guess.  Steering flow is generally weak southerly at 5-15 knots.  Also, not that great.  Tucson's profile is nearly saturated and very tropical looking.  If the clouds can burn off, which doesn't look that likely, there could be some big storms with very heavy rain, as they aren't going to move much.

Initializations
Clouds, showers, and even a storm or two are present over much of eastern Arizona due to the weak cyclonic flow over northern Sonora.  There is an MCV over the Bootheel of NM.  The 6Z WRFNAM was way too strong with the activity in eastern Arizona shortly after initialization, but it might be OK elsewhere.  The 6Z GFS was pretty good, but it appears the IT might be too far north, over Tucson.  It was able to resolve the MCV.  The 12Z RR was pretty good for clouds and showers.  The IT and MCV are also well initialized.  The 12Z NAM also looks quite good.   The RR had only minimal PW errors, while the GFS was a bit worse.  The 12Z NAM was also OK except for too wet in western Arizona.  IMO, the RR looks to be the better of the initialization, but the 12Z NAM and 6Z GFS should be OK too.

Day 1
One of the vital ingredients for today is heating.  As of 16Z, satellite imagery does show some dissipation of clouds in the thick band that stretches over most of eastern Arizona.  A few showers and storms have formed at the periphery of the MCV, now located in SE Arizona.   This is a problematic forecast situation for the WRF as storms will develop quickly in areas that clear out, and who knows where that will be exactly?

Moisture isn't a problem as 850mb dew-points are a very wet 14-16C, so heavy precipitation is possible with storms.  Very dry air as moved into western Arizona.

Low to moderate CAPE is seemingly randomly distributed over Arizona.  It's quite high in SE Arizona, so it's most likely that the area will be active this afternoon.  Central Pima and into southern Maricopa may be another active area, especially as skies are clear.  Storms could pop up just about anywhere, though, except for far western Arizona.

Showers and storm develop over much of eastern Arizona by mid-day, which is a bit late as there is activity already.

Tucson afternoon CAPE is only around 5-800 J/kg, but with a PBL mixed to near the LFC, it won't take much to trigger storms.  Steering winds continue to be poor, so storms won't move much, so flash flooding with be an issue.

Model runs generally agree on moving storms into the Tucson area by later in the afternoon and scattered thunderstorms moving slowly to the southwest, towards Phoenix.

It still looks likely for storms in Phoenix this evening.   CAPE is pretty good at 700-1300 J/kg, and with a decent outflow, thunderstorms will develop.  Steering is poor, so like Tucson, storms won't move much once they form.

The model runs do develop scattered storms in the Phoenix area this evening. 

Lots of rain for eastern Arizona!

Day 2
The moisture doesn't go anywhere as PW is still around 40mm for most lower elevation areas of central and western Arizona.

CAPE is quite limited but will be enough to support some afternoon and evening activity.  It will probably depend on how much heating is realized.

One anti-cyclone is centered over northern Arizona, resulting in weak easterly to northeasterly flow over the state, providing OK steering.  It would be better if it was 15 to 20 knots, but it's better than nothing.  There continue to be occasional weak inverted troughs rotating out of Mexico, and another is located over southeastern Arizona tomorrow.

Only scattered to isolated activity during the afternoon hours.

Limited or no activity for the lower deserts.