20200824

 Previous Day

It's the same old story.  Scattered storms over the higher terrain and nothing for Phoenix or Tucson.

The WRFGFS was the best run as it had quite a bit of precipitation for NW Arizona and not much for the Tucson area.  The RR had too much activity around Tucson.

Discussion

The 500mb pattern is mostly unchanged from yesterday as the anticyclone is centered over the Four Corners resulting in southerly or southeasterly winds over the state.  Temperatures are a moderate -6 to -7C.  Traditionally, this is a favored location for thunderstorms in Arizona-nothing has been traditional about this year.

Tucson is dry and CAPE is zero due to the low-level downsloping easterly flow from New Mexico.

Phoenix is looking a bit better as at least there is a little CAPE  The residual boundary layer is well mixed with no inversions, so if moisture and CAPE can increase, there could be some activity for Phoenix.  (same for Tucson)

Morning GOES derived CAPE does show some CAPE just to the west of both Tucson and Phoenix, so it looks like those areas will have the most activity later today.

Initializations

There are some clouds and light showers over southeastern Arizona.  The 6Z NAM, GFS, and 12Z NAM initialized these features fairly accurately.  The 12Z RR was missing these features.   The 6Z PW was too moist for large areas of NM and Arizona and won't be used today.  All others had only minor errors.  

Day 1

So far, it looks like another marginal day.  It's getting harder and harder to get the motivation to write these things as I put a lot of work into them each day,  And for what?  Just to say "another marginal day without much activity for either Tucson or Phoenix".  Monsoon put me out of my misery and bring back the westerlies.

The moisture situation is about the same as yesterday with marginally moist air over western and central Arizona while weak easterlies continue over southeastern Arizona, resulting in drying.

CAPE is mainly below 1000 J/kg for most areas of the state but is sufficient to support storm formation over the higher terrain.  Central and western Pima County has the best shot for strong thunderstorms as CAPE is moderate to high.

Like yesterday, scattered thunderstorms develop over the higher terrain of the Rim and White Mountains and also over NW Arizona.  Additional storms develop in eastern Pima County.  All the model runs are on the same page.

Some model runs are able to reverse the low-level winds to westerlies and increase CAPE and moisture for Tucson, which results in some thunderstorms in the area.  CAPE could be as high as 500-1000 J/kg and with the deep sub-cloud layer, thunderstorms could produce some intense microbursts.  

 

By early evening, very strong storms continue to move across Pima County and far southern Maricopa.

The WRFNAM has some weak activity developing in Pinal County during the evening.

It's not inconceivable that Phoenix might see a few weak storms as the CBL is mixed deeply and is near the LFC, at 600mb.  The mixed layer is very deep with a pronounced inverted V profile and DCAPE is ~2000 J/kg, so any storms that might form have the potential to produce very strong winds.

The 15Z WRFRR manages to move the southern Arizona storms into the lower elevations of Maricopa County this evening.

Lightning flash rates are forecast to be in the moderate range so there might be some good sunset lightning for the chasers as cloud bases will be high and precipitation rates won't be too high.

Day 2

Western and central Arizona continues to have marginal moisture for storms and far SE Arizona is still impacted by dry easterly flow from NM.  However, moderate amounts of moisture are sneaking into far southern Arizona, which may result in some strong storms.

CAPE is mostly low over much of the state, but sufficient to support storm development over the higher terrain.  

It looks just like what is forecast for today!

While Tucson only has 150-500 J/kg of CAPE available, it may be enough as by early afternoon, the CBL is mixed deeply and nearly to the assumed LFC of around 600mb.  Steering is a bit weak, but it is from a favorable northeasterly direction.  Mid and upper flow is unidirectional and upper winds are stronger thus anvil shading may impact storm development for lower elevations. Storms that do manage to form won't have much precipitation but will have a lot of wind.

CAPE is marginal for any activity in Phoenix.  At best, probably showers and a few weak storms tomorrow evening.