20170731

Previous Day

A mostly down day for the Tucson and Phoenix areas while it was quite active over eastern Arizona where more heating occurred.

The WRFRRx was the best as it had the most activity over eastern Arizona.

Initializations

A weak large IT continues over far NW Mexico and into southern Arizona.  The 500mb high has become better established over Reno thus giving Arizona slightly more favorable steering flow.  500mb temperatures have also cooled a few degrees since last week and are around -6 to -7C.  All the 12Z initializations look good.

The southern ½ of the state is mainly clear except for far SE Arizona.  The northern ½ has scattered to broken clouds which seem to be increasing over time this morning.  The GFS and NAM do not have enough clouds over northern Arizona.  The RRx is better but it doesn’t have enough coverage either.  The RR does have clouds in far SE Arizona.  The WRFRR also has a better temperature forecast as the other two model runs are too warm at sites in SE Arizona. The NAM is again too wet at most stations while the GFS and RRx have minimal errors.  I would go with the WRFRR again today as it has the best initialization.  The WRFGFS also looks fairly good except for the lack of clouds.  This error is unlikely to impact southern Arizona.  In general, model confidence is a bit higher than over the past few days.

Day 1

SE Arizona has dried out, relatively speaking, as IPW is generally just under 40mm.  It’s still quite wet near the surface as 850mb dewpoints are still around 15C.

There is not much model consensus with regards to midday CAPE as the WRFGFS has the most over SW Arizona, the WRFNAM over south central Arizona, and the WRFRR (below) over  SE and south central Arizona.  I have no idea which is correct.  In any case, some parts of southern Arizona have very high CAPE which will support some strong storms.

It’s not the best 500mb pattern as winds are mainly southerly at 5 to 15 knots.  In spite of slightly cooler air at 500mb this morning, warmer air advects into far southern Arizona during the day.

Deep convection gets an early start over the typical high terrain locations.  As all the model runs are too clear in northern Arizona, storms will get a later start.

Not the best looking SkewT for Tucson as steering winds are southwesterly and not very strong while upper flow is quite strong and will blow anvils out in front of storms that form over the higher terrain south of Tucson.  The WRFRR also only forecasts marginal CAPE.

The WRFGFS (below) has quite a bit more CAPE at around 1300 J/kg and a cap to hold back convection until strong outflows can come through.   The winds situation is similar to the WRFRR.

By late afternoon, the WRFRR (below) has some weak storms in the Tucson area.  It appears most stay over the higher terrain.

The WRFGFS (below) is much more active and has severe storms in Cochise County during the late afternoon.  The WRFNAM is similar. As much as I want it to be so, that does not mean that the forecast below is more likely than the WRFRRx forecast.

The WRFGFS moves a strong outflow into the Tucson area during the evening. It’s strong enough to trigger deep convection as the WRFGFS is also forecasting decent westerly low-level winds.

Both the WRFGFS (below) and WRFNAM have scattered strong storms in Pinal and Pima Counties during the evening.   Very strong storms continue south of the border.

So, what about Phoenix?  The situation isn’t clear there either.  The WRFRR (below) mixes the PBL quite deeply by early evening and also has around 1200 J/kg of CAPE, which is enough to support storms.  

A few storms move south towards Phoenix and by early evening, a few weak storms are in the area.

The WRFGFS also has a moderate amount of CAPE which may be enough to support some deep convection.  

The strong storms in Pinal and Pima eject an outflow towards the Phoenix area later this evening. It probably isn’t enough to trigger widespread deep convection as there is no outflow intersections forecast.  A storm or two could pop up, though.

Day 2

The WRFGFS and WRFNAM spin up an MCV due to the large MCS they predict over northern Sonora tonight.  This leaves behind plenty of debris clouds as well as cool and more stable lower atmosphere.

I've always found it cool that the WRF can spin up MCVs. We'll see tomorrow if this actually happens.

Very wet air continues to be present tomorrow.

The WRFRRx 500mb again has weak steering flow with winds shifting to mainly southeasterly.

The WRFRRx is much more active around the Phoenix area during the evening and moves storms into the metro area from the high terrain to the east and north.  The other two model runs have almost no activity over southern Arizona tomorrow.   So it will all depend on what happens in northern Mexico tonight.