Monday, July 24, 2017


Previous Forecast
The weather around Arizona was very active as once again, strong storms with very heavy rain occurred from Mohave county down toward SE Arizona.  The west side of Tucson received heavy rains as well as parts of eastern Phoenix.

The WRFGFS (below) and the 15Z WRFRRx looked to be the most accurate.  

The 500mb pattern continues to be about the same as the high center remains to the north of Arizona, a trough off of the west coast, and a weak IT moving over western Arizona.  This last feature isn’t really seen very well in the upper air data but is obvious in the RADAR animation.  There is no upper air data over all of northern Mexico except for Mazatlan.  All the initializations look fine at 500mb.

Widespread heavy showers and thunderstorms have developed over central Arizona in response to the northward moving trough and the record setting wet air.  Yesterday’s model runs had the convection a bit farther to the SE around Tucson but this error is understandable due to the lack of upper air data upstream in Mexico.  Visible satellite imagery does show some clearing in far SE Arizona.   The RR initialized the morning activity and clouds very well and the WRFRR even had some clearing over SE Arizona this morning.  The NAM initialized the central AZ activity too far to the north and has the southern ⅓ of the state too clear.  The GFS is similar as it doesn’t have much morning activity over Pinal and Maricopa counties and it is somewhat too clear over southern Arizona.  Now, to the PW.  I’m sure this is a record for Phoenix as the most recent IPW is 60.4mm.  There has been moisture convergence there all night so no wonder there are widespread showers and storms. Note that the location label on the plot below is incorrect -- the sensor is in Tempe. Two other Arizona sites are reporting over 60mm: Roll (near Yuma) and Buckeye.  
The NAM has a slight wet bias over much of the area while the RR and GFS initialized IPW well.  In spite of a complex situation, the RR has initialized well which is what it was designed to do.
Day 1
Extremely wet air is present over most of the state today.

However, much drier air is moving into SW Arizona by the afternoon.

Mid-level flow becomes mainly from the SSW during the day as a trough near San Francisco moves slowly to the east.  Very warm air at 500mb continues and with weak negative vorticity advection behind the departing trough and as somewhat drier air advects into the state, activity is sure to decrease.

The CAPE forecast has moderate to high CAPE over Maricopa and Pinal Counties by late morning.  I find this to be a bit strange as those areas continue to have widespread showers and storms.  The WRFRR moves the morning activity to the north and allows some heating by late morning.

Showers and storms move north of Phoenix by late morning and by afternoon, widespread showers and storms are over northern Arizona with a few weak storms over southern Arizona.

Tucson has a moderate amount of CAPE, SE steering flow, and some low level shear thus a few showers and storms are possible this afternoon, mainly near the mountains.

In spite of an enormous amount of moisture and moderate to high CAPE, the afternoon should be mainly quiet in the Phoenix valley as there is a significant inversion on top of the cooled BL and it is unlikely any outflows will enter the valley to trigger deep convection.  Winds are also now unfavorable as they are mainly from the SSW.

A few storms continue around SE Arizona during the evening.

A few storms are possible northwest of Phoenix in the early morning hours as a short wave moves through.

Day 2
Considerable moisture remains over the state as IPW is nearly 50mm around the Phoenix area.  The model runs have a big difference with the amount of drying as the WRFRR is down to 40mm in the Phoenix area.  The WRFGFS (below) and the WRFNAM are a bit wetter.

At 850mb much drier air is present over western Arizona.  Much of the rest of the state remains very wet.

Moderate to high CAPE continues over much of Arizona so the potential is there for storms.  The WRFRR is the odd man out as it has much less CAPE.

Very warm air remains at 500mb with southerly to southwesterly flow over the state.  The west coast trough is stationary north of San Francisco.

Drying continues during the afternoon thus CAPE drops to around 4-500 J/kg over Tucson.  Winds are mainly unidirectional, from the south which is also not favorable.  All the model runs are similar.

Phoenix holds on to a bit more CAPE and moisture, but without any outflows, deep convection is unlikely.

Much less activity tomorrow so I probably won’t write a discussion.

Sunday, July 23, 2017


Previous Day
Most activity was in the southeastern ⅓ of the state with widespread heavy rain in far SE Arizona.  Tucson got into the action as parts of the city received more than an inch of rain.
In general, the WRFGFS (below) was the best run as it had heavy precipitation in SE Arizona.  The other two runs weren’t too bad either.  

The 500mb high center is somewhere over UT and a broad but weak IT is somewhere over NW Mexico with one wave over NW Sonora and maybe another south of El Paso.  All the initializations have some sort of IT in NW Mexico and it’s hard to say what is wrong and what is right.

Thick clouds and showers are present over SW Arizona with scattered to broken elsewhere across much of the southern ½ of the state.  The GFS and RRx look pretty good while the WRFNAM has too much storm activity in southern Arizona in the first few hours of its run.  This is a major problem and disqualifies the WRFNAM for today, at least for southern Arizona.  The GFS PW initialization is too dry over far eastern Arizona.  Otherwise, the RR and GFS PW errors are small.  Surface heating is a little warmer over far eastern Arizona but fairly close to observed elsewhere except the WRFNAM where it is much too cool in some areas due to incorrect showers and storms.  The WRFGFS and WRFRR initializations look fairly good and these are the model runs of choice today.

Day 1
Some of the highest morning PW so far this summer as Tucson has 48mm, Phoenix 52mm,  Lukeville, 52mm, and Puerto Penasco at 54mm.  A surge is underway (obviously) as surface wind speeds at Yuma are from the SSE at 15 knots.  IPW is forecast to remain at these extreme values during the day today thus storms that do get going will produce very heavy rain.

The 850mb dewpoint temperatures over SE Arizona are about the highest I’ve ever seen, excluding tropical system, with 17C in some areas.

500mb steering flow is mainly easterly or northeasterly at 10 to 15 knots which is enough to move storms off the mountains.  500mb temperatures from the upper air data are a very warm -5 to -4C and that continues throughout the day.

Both the WRFNAM and WRFGFS (below) have moderate to high CAPE over much of eastern Arizona while the WRFRR is a bit lower which has usually been the case recently.

Much of eastern Arizona is clearing up and getting some surface heating and deep convection is initiating over the highest terrain as of 17Z.  By early afternoon areas of strong storms are over the Rim and White Mountains and into SE Arizona.  Tucson is getting much more sun than was being predicted yesterday so some heavy showers and storms are possible today.

By early afternoon Tucson has a moderate amount of CAPE, some low level shear, and good steering winds.  The morning sounding says the LFC is around 750mb and Tucson is getting close to that so it looks like there will be some activity today.  Very heavy rain is again likely with storms.

Later in the afternoon, storms start moving off the high terrain towards central Arizona.  SE Arizona continues to have a few storms around.

The vertical profile looks good for Phoenix with good low level shear, some mid level easterly steering flow and a deeply mixed BL as well as 800-1500 J/kg of CAPE.  The LFC from the 12Z sounding was at 700mb and the mixed layer is close, so an outflow or two should form storms.  All this wet air around makes the warm air at the mid levels less of a negative.

The bad news is only the WRFGFS brings activity close to the Phoenix area by early evening and doesn’t actually develop storms in the metro area.  The other two models have no activity but the WRFNAM is suspect and the WRFRR has not been forecasting enough deep convection recently.  I think there is enough instability for at least a few storms in Phoenix early this evening.  There could be quite a bit more activity than forecast if the outflows from the east intersect with the outflows from the north.

As I’ve been taking my sweet time writing this discussion, the 15Z WRFRR is now available and it’s quite similar to the WRFGFS as big storms approach Phoenix and then die out as they move into the valley.  A few showers/storms form as the outflow moves through.

Some of the morning runs eject the Sonoran IT out over the state tonight resulting in some continuing activity mainly in southern Arizona.

Day 2
Day 2 forecast accuracy has been poor, as expected, due to the complexity and initialization uncertainty.  This continues for tomorrow as some runs continue activity during the morning hours as the IT is somewhere over southern Arizona.

There is not much more I can say with any confidence thus we’ll have to “wait and see” for tomorrow.  It does look like we are beginning to transition to SW flow as a trough drops down off the coast of California.

Saturday, July 22, 2017


Previous Forecast
Nearly a repeat from the previous day as there was quite a bit of activity over the higher terrain of the state and a few storms in and around Phoenix and Tucson.

The WRFRR was too dry while the WRFNAM and WRFGFS(below) were fairly good.  The WRFGFS had a bit too much precipitation around Tucson as the stronger storms were just east and south of the metro.

No major changes with the upper air pattern as a trough is located over far eastern Arizona and the 500mb high is centered north of Las Vegas putting Arizona in N to NE flow.  Slightly cooler air was seen in northern Arizona and perhaps that will filter down to southern Arizona giving a bit steeper lapse rate as recently 500mb air has been warm, around -5C.   Besides the trough over eastern Arizona, there is another over far eastern Sonora/western Chihuahua which has kept deep convection going over Sonora this morning.  The NAM and GFS are well initialized at 500mb while the RR doesn’t have the Sonoran IT as strong as it should be.  The RR, NAM, and GFS have initialized clouds and the Sonoran storms fairly well.  

The NAM is a little too wet in Mexico while errors are small elsewhere.  The GFS and RR have minimal IPW errors.  The WRFGFS is a little too warm in the deserts at 16Z.  Overall, the model initializations look good with no clear favorite except to say that the WRFNAM and WRFGFS have been the better runs recently.

Day 1
Very wet air continues over most of the state while some drying has occurred out in western Arizona.  Dewpoints have dropped to around 60 in Yuma.  Moist air continues to move into Arizona from the south during the day.   This is an amazing streak of over 40mm for the lower deserts, especially for July.

CAPE is moderate to high over much of eastern Arizona and along the rim so deep convection should form before noon over the high terrain in these areas.  At 16Z, RADAR already indicates storms over far SE Arizona.

The far western NM trough remains mainly stationary today thus keeping Arizona in light northerly flow.  Again, storms should form on the higher terrain and drift south with a few in the lower elevations in and around Tucson and Phoenix.

The 250mb high has moved northward and is over Arizona today thus providing weak upper divergence over the state.

Storms get an early start over eastern Arizona and by early afternoon, some storm storms are present over parts of eastern Arizona.

There is quite a bit of model spread as the WRFRR has more activity over NC Arizona while the WRFGFS(below) has very little there and is more active over far SE Arizona.

I’m not sure where all the CAPE at Tucson went today as the morning sounding had over 1300 MLCAPE.  Minimal CAPE is forecast by the afternoon as there is quite a bit of warm air above 500mb probably due to some subsidence as the upper high moves over plus slight drying.  Winds are OK as there is some mid level steering and low-level shear.  However, with no CAPE, not much activity is expected in the Tucson area during the day.

It’s the same situation in Phoenix.

The three 12Z runs more or less agree that there will be a few storms north and east of Phoenix later this afternoon.  They have different solutions in SE Arizona where the WRFGFS (below) is very active with big storms pushing to the SSW towards the lower deserts while the other two runs have much less activity and I can’t really say which may end up being right.

The WRFGFS storms mainly die as they move into the lower deserts as CAPE is forecast to be limited with only a few weak storms triggered as the outflow moves through.  

Day 2
Mid level flow increases tomorrow with 15 to 20 knots at 500mb as a IT is located south of Nogales and the high is to the north of Arizona.  

Moisture again increases as a moderate surge is forecast by tomorrow.

CAPE is forecast to be moderate to high over the higher terrain while it is low in the SW deserts.  Storms are again going to struggle as they move off of the high terrain in spite of all the moisture.

The surge advects in somewhat cooler air and with the warm air aloft, there is an inversion present at the top of the BL which will hold most lower elevation convection in check.

The forecast is uncertain for southern Arizona as the WRFRR keeps activity going overnight and into the morning hours forced by the Sonoran IT thus it could be cloudy and cool.  The WRFNAM also develops storms over southern Arizona during the morning hours.

My guess is afternoon storms again over and near the Rim to the White mountains while only a few showers over SE Arizona due to previous showers, storms, and clouds holding down heating.

Storms scoot around the Phoenix area during the evening with maybe one or two in the Valley.