20170908

Previous Forecast

There was a big increase in activity as strong to severe storms developed over much of western Arizona.  Phoenix did get a dust storm and even had a brief severe warned storm while Tucson only had a few brief showers.  

Both the WRFGFS and WRFNAM (below) were quite good while the WRFRRx runs were too active, especially the 12Z WRFRRx.  This specific run has been very poor recently and it shouldn’t be trusted until comparing it to the other two 12Z runs.

Initializations

The long wave pattern has finally changed a little as the big NW CONUS ridge has moved east into MT and now a trough is in position along the west coast.  Southern winds are present over much of the west now so hopefully, that will be the end of the smoke for awhile as it goes up into Canada.  A couple of MCVs were noted on the satellite imagery.  One is over NW Arizona and the other is just SW of Nogales.  Widespread clouds are associated with these features.  NW Mexico also has widespread clouds perhaps associated with an IT that can be seen on the 500mb map thanks to the Empalme sounding.  The RRx has the main features initialized well as well as the clouds that are present over central Arizona.  The NAM has also initialized the large scale well but is missing most of the clouds that are over Arizona.  The WRFNAM did eventually develop some mid-level clouds across western Arizona by 17Z but those weren’t in the right location.  The GFS has the large scale initialized well but is missing most clouds which could be a major problem.  Also, no model resolved the little MCV SW of Nogales.  The NAM and RRx were a little too wet in NW Mexico while the GFS had only minimal errors.  Model forecast confidence is low to moderate due to lack of initialized clouds.  A quick look at 16Z 2m temperature forecasts indicates that the WRFGFS is a couple of degrees too warm most locations while the other two are pretty close.  It appears the WRFNAM will be the most accurate for today.

Day 1

A strong surge is underway as Yuma has SSE winds at 20 knots.  Looking at the 14Z NYL sounding indicates that it’s quite shallow and that there is only minimal MLCAPE.  That is the case at both Tucson and Phoenix as they only have 1-200 MLCAPE so it’s not looking too good so far.  IPW has increased somewhat since yesterday as most lower elevation desert locations are from 30 to 36mm of IPW.  A surge continues during the day which keeps IPW at or slightly above current readings while dry air continues over far eastern Arizona.

CAPE is forecast to be moderate over much of the state and appears to be sufficient for storm development and quite high over far western Arizona thus big storms are possible there.

There is an upper-level shortwave moving through Arizona during the day which may act to enhance deep convection.  .

Deep convection gets an early start in areas that are mainly clear.  Storms are likely overdone in NW Arizona due to the morning clouds and showers that are present this morning.

There is some CAPE present at Tucson this afternoon which is sufficient for a threat of scattered afternoon storms.  What stands out to me is the good directional shear along with some low-level shear thus some storms will exhibit rotation and organization.  Wind speeds are a bit low though and it’s too bad CAPE isn’t a little higher.

As is so often the case, most storms stay on the higher terrain around Tucson and mainly stay out of the valley.

The WRFGFS has quite a bit more activity as it has strong storms over Pima and into Pinal and southern Maricopa this afternoon.  These storms send out strong outflows towards Phoenix.  My guess is this may be too active as there is too much heating.  In fact thick clouds continue over parts of the state with little decrease noted thus even the less active WRFNAM may also have too many storms over central Arizona this afternoon.

Moderate CAPE is forecast for the Phoenix area by late afternoon thus if some strong outflow boundaries can move into the Phoenix area, there could be some additional storm development.  Like Tucson, directional shear is good but it would be better if wind speeds were higher.

Like Tucson, storms don’t make it into the Phoenix area.  Still, a few may pop up as the outflow boundaries move though as there may be sufficient CAPE.

The WRFGFS has the same idea around Phoenix as storms move near and then die.  It has strong convection over the CO River Valley.

Showers and a few storms may continue over southern Arizona into the evening but the main action is mainly over the western ⅓ of the state where moisture/CAPE and better dynamics are present.  The WRFNAM and WRFGFS has strong storms continuing over NW Arizona.

Some areas in NW Arizona receive a lot of rain thus flash flooding is going to be a problem.

Day 2

A cutoff low is present off the west coast which keeps Arizona i a mainly southerly flow.  Mid-level temperatures have been quite cool recently but are forecast to increase somewhat thus CAPE may be less without an increase in moisture.

Moisture does continue to increase and is around 35 to 40mm by midday.  

CAPE is moderate over much of the state so with sufficient heating, storms are possible.

There is again some directional shear tomorrow but winds continue to be fairly light.  

It looks like the main problem for tomorrow will be widespread clouds and reduced heating.  

In spite of moderate CAPE only a few storms form and are restricted to the eastern ⅓ of Arizona as there is just not enough heating.  Both the WRFNAM and WRFGFS are similar thus confidence in the Day 2 forecast is higher than it would normally be.