Arizona WRF Discussion: 20180802

Previous Forecast

Activity mostly remained over the higher terrain of the state with some areas of very heavy rain. The exception was around the Tucson area where storms developed during the evening.

All of the 12Z runs performed fairly well as they all had the general idea of most activity remaining over the higher terrain. The WRFRR seemed to be the best as it was more active over the western Rim.

Initializations

The 500mb high center is right over central Arizona thus steering is mainly light and variable. There is a weak trough over west Texas but this feature is likely to be no factor in Arizona as it should move off to the SW and over northern Mexico. Scattered to broken mid-level clouds are present from the Tucson area up towards the Rim and the White Mountains and may be associated with a possible weak MCV over the Flagstaff area. The NAM and RR have the 500mb situation initialized well but neither has near enough clouds. Generally, these sorts of clouds burn off during the morning hours and are not a problem but they seem to be taking their sweet time today. The RR PW initialization had minimal errors. The NAM was a bit too wet in NM and into Mexico but much better than it has been over the past few weeks. I’m a bit concerned about the lack of clouds in the model runs and this could result in too early and widespread convection in those areas where it’s partly cloudy.

Day 1

Part of the story today is where the 500mb high center ends up as that will define the steering flow. By early afternoon, the WRFNAM moves it into west central NM placing southern Arizona in generally weak SE flow. The WRFRR does not have a well-defined center but the axis stretches from south of Phoenix to western NM thus it has little or no steering over the state.

PW has remained mainly steady and is wettest over far SW Arizona as Yuma is around 50mm. Elsewhere in the lower elevations, it’s mainly from 35-45mm. Unfortunately, Puerto Penasco is down and this has resulted in the two runs having a 10mm difference in PW in that area. I’m going to go with the drier WRFRR because looking at other stations, it seems to have the best forecast of PW so far. PW remains about the same into early afternoon along with weak flow from the GofC and some moisture has pushed westward into far SW NM.

CAPE is moderate over eastern Arizona while very high over western Arizona and the higher terrain of central Pima County. That may be a hotspot for storms today.

Both runs develop storms over the high terrain from west central NM to Yavapai County and over the high terrain of Santa Cruz county during the afternoon.

By late afternoon, strong storms are underway over central Pima County.

Tucson late afternoon CAPE is only around 600 J/kg, mid-mid level steering is weak, and the PBL isn’t mixed very deeply thus it’s looking unlikely for any significant storms. NW upper winds may also spread anvils over the Tucson area.

The storms over central Pima County eject a strong outflow into eastern Pima and Pinal counties tonight, so it looks like a big dust storm or even a haboob is likely for Pinal and southern Maricopa Counties.

In spite of marginal CAPE, this outflow manages to trigger scattered convection around the Tucson area this evening. The probability of this happening isn’t high as this only occurs in the WRFRR run but as we saw last night, in spite of marginal CAPE, Tucson had quite a bit of activity.

As the outflow moves towards Phoenix, it weakens and only triggers a few storms. However, the PBL is mixed nearly to the LFC plus there is around 1400 J/kg of CAPE. So in spite of neither run developing much activity in and around Phoenix, it is certainly possible that there could be some storms. The 15Z WRFRR will be interesting to see (yesterday it forecast a big outbreak in Phoenix for today). Also, all of the overnight runs also had at least some storms in Phoenix this evening.

The WRFGFS just became available and it has some strong storms in the Phoenix area this evening.

Another possible scenario for the Tucson area is round of late night elevated CAPE storms. While the WRFNAM didn’t do much around Tucson during the early evening, it does develop scattered storms early tomorrow morning. This is unlikely to happen if Tucson gets storms earlier.

Day 2

Here comes the beginning of the end of the monsoon (at least for a little while) as a strong trough over the NW CONUS begins to suppress the ridge. The 500mb anticyclone is centered just off of San Diego thus placing the northern ½ of the state in NW flow while southern Arizona is mainly light and variable.

A weak upper trough moves over eastern Arizona during the afternoon thus providing a bit of upper divergence and enhancement to eastern Arizona afternoon storms.