20210918

 Discussion

A western CONUS trough has finally pushed the 500mb anticyclone center to the east of Arizona, resulting in quite a strong southerly flow.  Temperatures have decreased to -6 to -8C.

The 12Z soundings from Phoenix and Tucson are unimpressive, even sad.  While moisture has slowly increased over the state, there is still limited CAPE available.  Worse, there is a mountain of an inversion to overcome above 500mb.  Why the model runs are going crazy with convection later today is a mystery to me, at least so far.

Model Initializations

There has been quite a bit of trouble over the past two days as there were problems with missing initialization files from NCEP.  Those were solved last night, so most runs succeeded.  As mentioned recently, the NAM is slowly being phased and replaced by a mix of the RR and HRRR.  This morning, clouds are scattered about the state, making for a bit of a challenge for the initializations.  Also, GOES-derived CAPE is present over parts of southwestern Arizona, so that is a positive development.  The RR and HRRR do not have any clouds over far western NM but are OK elsewhere, including the far NW Arizona convection.  The 6z GFS did have thick clouds in far eastern Arizona, which isn't correct either as clouds are further east and not as thick.  The WRFGFS also had scattered showers over eastern Arizona this morning, in error.  CAPE was initialized accurately by all.  There is no clear favorite, and the errors don't seem significant enough to impact accuracy.

Day 1

Something has to change for storms to form today as moisture and CAPE are lacking, and the strong inversion above 500mb isn't going to allow much activity.  A band of high 850mb dewpoints is present this afternoon over central and western Arizona. Typically, this would be enough to support widespread deep convection, especially over the higher terrain.

Moderate CAPE is associated with the area of high moisture.

Now the important bits-the model forecast Skew-T plots.  This afternoon it's not looking very favorable for deep convection at Tucson despite moisture increasing to nearly 40mm.  The inversion above 500mb is very strong.  Another big question with the forecast is the increase of moisture to almost 40mm in Tucson.  It's only 32mm now, so it has a long way to go to make it to 40mm, which is probably the minimum needed for deep convection.

The story is better at Phoenix as the inversion is weaker as the mid/upper trough moves closer to Arizona.  This results in quite a bit more CAPE.

The OLR plot is interesting as it has widespread shallow convection below the inversion with scattered areas where the inversion is overcome, resulting in deep convection.

Most of the WRF runs forecast storms in and around the Tucson area by late afternoon despite the marginal-looking Tucson Skew-T.  Again, I have my doubts that moisture can increase by another ~8mm by this afternoon.

Phoenix is looking good by late afternoon as the PBL is mixed well, and CAPE is around 1000-1500 J/kg.  The inversion also weakens.

Most runs develop strong storms in and around the Phoenix area tonight.

The 4-panel plot shows precipitation for both Tucson and Phoenix is likely.  As the increase in moisture at Tucson to 40mm looks unlikely, the less active runs are most likely.

Day 2

Much warmer air is advected into the state behind the departing trough.  This is likely to shut down any deep convection, except for the far southeastern part of the state.

20210905

 Discussion

Moisture has gradually increased for much of the state over the past day or two.  Tucson is up to around 40mm this morning, as well as a considerable MLCAPE of 1300 J/kg.  The vertical wind profile is also moderately favorable to steer higher elevation storms into the Tucson area.  There is a significant subsidence inversion at 500mb, but it shouldn't be a problem with the amount of CAPE available.  DCAPE is moderately high at 1300 J/kg, so storms will likely produce strong outflow boundaries, assisting in propagation. 

The 500mb map shows the anticyclone is centered over California, generally north to northeasterly flow over the state.  The inversion is obvious with warm air at around -3 to -5C.  Cooler air is upstream, though.  There is a weak IT located over the far northern Chihuahua/Big Bend area, which is responsible for the ongoing showers and storms over far southern NM. This feature is likely to enhance thunderstorm activity in southern Arizona later today/tonight.  Another IT/MCV is located over far northern Sinaloa and is responsible for the ongoing activity there.

Initializations

Remarkably, all initializations did well with both the clouds and ongoing showers/storms over southern NM.  Most did well initializing the cyclonic circulations except for the 12Z HRRR, which had the IT a bit too far to the east.  The HRRR and 12Z RR also didn't have enough clouds over far southeastern Arizona.  Yes, you read that right; I've replaced the 12Z NAM with the HRRR and used the 0Z GFS for lateral boundary conditions to extend the run to 84 hours.  From the first few runs, the WRFHRRR is usually quite similar to the HRRR, at least for the first 6-12 hours.  After that, they start to diverge.  Model accuracy is expected to be good, with the favored runs being the GFS and RR.

Day 1

Most, if not all, model runs over the past day, or two have been quite consistent in increasing storm activity for southeastern Arizona today.  A surge is underway but is shallow and weak.  The 850mb shows the wet/dry boundary quite well, with most of southeastern Arizona having dew points at or above 10C.  Very dry air is still in place from about Phoenix and westward.  

 

CAPE is low to moderate throughout much of SE Arizona.  CAPE is minimal west of El Paso due to the ongoing activity there this morning.

Scattered storms develop during the early afternoon hours, mainly over the higher terrain of SW NM and southeastern Arizona.  This may be a little too fast as the runs cleared out too quickly.

It looks good for the Tucson area as CAPE is from 900 to 1400 J/kg, the PBL is mixed deeply, and the vertical wind profile is excellent.  There is low-level shear, mid-level northeastern 10-15 knot steering flow, and NW winds aloft keep anvils from advancing ahead of storms.

The Chihuahua IT moves a bit to the west, bringing cooler air at 500mb and NE steering flow over much of the state.

Later in the afternoon, storms move off of the higher terrain into the Tucson area, while other storms move off of the higher terrain of SW NM into eastern Cochise county.

Generally, outflow winds aren't too strong, at around 30-35 knots.  A few locations could see stronger winds due to microbursts.

The runs have been quite consistent over the past 36 hours, and the most recent four runs continue that way.  Some runs have precipitation amounts exceeding 2" for a few locations.

Storm intensity decreases into the evening.  Storms cannot propagate down I-10 even with a favorable OFB due to the lack of CAPE.  

 

Day 2

PW decreases over southern Arizona, resulting in mostly low CAPE, but maybe enough for a few storms.

At Tucson, it's not looking good for storms for multiple reasons.  The first is the old rule that the next day after a big day is usually down.  The other issues are a strengthening of the 500mb inversion and very dry air aloft.  The wind profile is mainly unidirectional.

Little or no activity is forecast.