20210730

 Previous Forecast

It was an active day for most of the state except for the areas that had morning clouds and showers; Tucson to Phoenix.  Some areas had heavy rain and flash flooding, like Miami, Arizona.  Also, a squall line moved across northern Arizona accompanied by a haboob, again!

Model accuracy was good concerning timing and locations, but precipitation amounts weren't high enough.  It pains me to write this, but the WRFNAM had one of the better runs as it had a fairly accurate initialization.

Discussion

The 500mb anticyclone has shifted to the southeast and is now over Oklahoma, resulting in favorable east to southeasterly steering for much of the state.  Temperatures remain favorable at around -6 to -8C.  The IT is located over Chihuahua, moving slowly to the northwest, and is likely responsible for the ongoing deep convection in western Sonora.

At 300mb, the IT location has put much of Arizona and Sonora under favorable divergence/difluence areas.  

Moisture has increased over central and western Arizona as Phoenix now has 45mm of PW.  Phoenix also has a better-looking vertical profile as MLCAPE is 564 J/kg and has no significant inversions.  The wind profile could be better as there is no low-level shear, but mid-level steering is good.

Tucson is somewhat worked over in the lower atmosphere, but MLCAPE is OK, at 435 J/kg.  The wind profile is unidirectional, and it appears, some drier air is advecting in on low-level southeasterly flow, similar to what the forecasts were calling for yesterday.  So, a mixed bag here.

Initializations

It's more straightforward than yesterday, fortunately, as it's mostly clear over the eastern 2/3rds of the state.  

Humm, is it going to be 2 in a row for the NAM?  It had the best initialization of clouds and the MCS in Sonora, both at 6 and 12Z.  They all had a mid-level IT/MCV associated with the Sonoran storms.  The RR has also initialized clouds well but missed the MCS in Sonora.  The 6Z NAM PW initialization was not very good, but the rest were mainly accurate.   The 6Z GFS is missing.  I'd bet on the 12Z WRFNAM and WRFRR today.

Day 1

The IT gets sheared out and moves northward during the day but continues to provide favorable weak upper divergence over the state.

It is a little hard to decipher what is going on at 500mb, but it appears the MCV is absorbed into the circulation of the IT.  The old center can be seen as cooler temperatures, and the new center is over the northern Gulf of California by this afternoon.

As noted from the Tucson upper-air data, low-level easterlies have made it as far west as Tucson at 12Z.  This dividing line is still north-south of a line centered in eastern Pima/Pinal Counties by early afternoon.  Dry air has also moved into the rest of eastern Arizona too.  A weak surge is underway this morning but weakens by afternoon.  Very moist air is present in western Arizona by this time.

CAPE is minimal except for a band that goes through central Arizona.  I'd expect limited activity on either side of the line.

Between the drying over eastern Arizona and the old rule that a down day followed an active day, not much is happening by mid-afternoon despite the somewhat favorable upper-air situation.

Activity does begin to pick up by late afternoon to the west of the dryline, and most runs agree that southcentral Arizona and areas around Flagstaff will have storms.

Phoenix looks favorable for storms this evening as CAPE is over 1000 J/kg, low-level shear, and moderate mid-level southeasterly steering flow is present.  The PBL is also mixed well, and nearly to the LFC, around 700mb from the morning sounding.

It does not look favorable for much activity in the immediate Tucson area due to the deep low-level easterly flow drying out the mixed layer, resulting in no CAPE.  This situation has become more and more common throughout the last few years.  Also, I hope another old weatherman's forecast rule doesn't come true, which is "a good July is followed by a bad August."  This was true in 2017 when Tucson set the previous July precipitation record.  It was followed by almost no precipitation in August!

Most runs develop storms for the Phoenix area this evening, with a few being potentially severe due to winds from wet microbursts.  Activity decreases into the late-night hours.

Day 2

I don't have many model runs to rely on as there were cascading failures that caused many to run short.  It continues to be quite moist for central Arizona, and moisture increases over southeastern Arizona despite weak easterlies, as moisture has also increased over southwestern NM.

CAPE has also increased and is moderate to high for most of the state except for far eastern Arizona.

Storms get off to a slow start and are mainly restricted to the higher elevations.

Uh oh.  As the IT moves away, mid and upper winds become westerly, and temperatures increase.  This results in a nasty inversion at around 500mb.

This results in little or no activity for the lower deserts.

20210729

 Previous Forecast

It was a quiet day for the lower elevations.

Almost all the runs had precipitation too far to the west in southeastern Arizona.  However, they accurately predicted the second wave of storms there during the overnight hours.

Discussion

Not much change in the synoptic-scale pattern.  Cooler temperatures are still over the state, at -6 to -8C.  The Mexican IT is slowly inching its way closer.  Too bad both Guaymas and Chihuahua upper-air data is missing.  Guaymas has been missing for a while now.  Maybe they ran out of consumables? 

 The 300mb shows southeastern Arizona under divergent flow.  Perhaps that is the reason for the early morning activity in southeastern Arizona.

Both the Tucson and Phoenix Skew-T diagrams are 'meh,' as there isn't much CAPE available.  Steering winds aren't bad at 10-15 knots, but there isn't any low-level shear.  Moisture has increased quite a bit at Tucson as it's now 37mm.  Nothing jumps out at me and says this will be a big day, except for the approaching IT.

Initializations

Getting an accurate initialization will be a challenge as widespread cloud and some showers are ongoing this morning.  A weak cyclonic rotation was observed over southern Arizona, perhaps an MCV?  

The 6Z GFS and 12Z RR did a good job initializing clouds and the weak activity.  The 9Z RR didn't have enough of either.  The 6Z GFS also had a weak inverted trough over southern Arizona, as did the 12Z RR.  Those are the favored initializations today, but the overall confidence is low.  Suominet PW data was missing at 12Z.

Day 1

The ongoing clouds and showers will throw a monkey wrench into the previous forecast of a big day/evening, as they are going to restrict heating, at least this morning, for much of the Tucson and Phoenix areas.  850mb dew point temperatures are around 10C, which should be enough to support some activity.  Drying easterly downslope flow is present over far eastern and southeastern Arizona, which is likely to keep deep convection to a minimum out there.  The boundary is again over the Pima/Cochise line, which may help trigger and organize deep convection.

The 12Z WRFRR seems to have CAPE better forecast, at least compared to the morning soundings.  The 6Z WRFGFS has quite a bit more, which appears to be in error.  I don't see much happening for Tucson and Phoenix with marginal CAPE.

CAPE is sufficient to support some strong storms over the higher terrain, though.

There haven't been even two runs that are similar with their locations and timing of thunderstorm activity.  It's possible for a few storms this afternoon around Tucson as there is just enough CAPE.  It's more likely they'll remain over the higher terrain.

Phoenix looks a little better as CAPE could be as high as 1300 J/kg, which would support some storms, as long as there are some outflow boundaries.

The 12Z WRFRR keeps storms out of the Phoenix area and just scattered around Tucson.

The 6Z WRFGFS moves a strong line of storms towards the Phoenix area by early evening, but it weakens as it moves into the valley.

A large outflow boundary does move across Phoenix, but little or no deep convection forms.

Activity continues into the evening over western, southern, and northwestern Arizona.

The upper support is there over Arizona as the IT moves to around Chihuahua by this evening, providing divergence/difluence.  This should help scattered activity to continue for much of the night.

Day 2

The IT shears out, and what is left of it moves towards Arizona.  

At 500mb, the IT is located just south of Nogales, with some cooler air over southern Arizona.  

A weak surge begins overnight, which results in slightly increased moisture and more favorable 850mb dew points of 10 to 12Z.  Dry air continues over far southeastern Arizona and into NM.

CAPE is a bit higher by tomorrow, and hopefully, will result in more activity.

Most runs have a surprising lack of activity tomorrow.

Tucson is under the influence of the southeasterly low-level flow, resulting in low to moderate CAPE and unidirectional wind profile.  Maybe a storm or two in the area, but overall, not much activity.

It's the same story at Phoenix, with somewhat drier air moving in from the east.  CAPE is limited, but the PBL is mixed deeply, so some late afternoon or evening storms are possible.  What a waste of a good IT.

 

20210728

Previous Forecast

The model runs were generally accurate, with precipitation mainly in southern Arizona.  However, no runs predicted the evening activity in Yuma county last night.  The 9Z (not shown) and 12Z WRFRR were the most accurate.

Discussion

The 500mb anticyclone is still center quite far from Arizona, over South Dakota, resulting in mainly southeasterly flow over the state.  Temperatures remain fairly cool at -6 to -8C.  A weak IT is over southern Arizona as Tucson winds are from the SSW.  Another closed mid to upper-level low is located over southern Texas, which is forecast to move towards Arizona over the next few days.  The TWC office calls it an "easterly wave," which isn't correct, as Bob Maddox mentioned in his blog yesterday.

"Bit of commentary - forecast discussions from NWS Tucson have been referring to this feature as an "easterly wave." Such waves have their strongest winds in low levels - certainly not the situation with this upper low. Brownsville sounding this morning shows the strongest winds with the low are up around 250 mb, so it's definitely not an easterly wave.

There is a departing mid-level wave/MCV over far SW Arizona.

The 300mb plot shows the IT (or TUTT?) dominating the winds over northern Mexico.

Moisture has continued to decrease over the state as Tucson is down to 32mm.  The air mass is considerably worked over with dry air present in the residual boundary layer above the modified near-surface layer.  MLCAPE is minimal at around 200 J/kg.  

Phoenix looks a lot better as there are minimal inversions, 39mm of PW, and 660 J/kg of MLCAPE.  The vertical wind profile isn't all that great as mid-level winds are weak, and upper winds are quite strong and will move anvils way out ahead of storms, resulting in shading and weak downward vertical velocities.

Initializations

It's mostly clear across much of the state.  The GOES PW shows the drier air has moved into far eastern Arizona.  Cloud initialization was fairly accurate by most runs except the 6Z NAM, which was too cloudy.  PW was also initialized fairly well, except for the 6Z NAM.   

Day 1

Moisture continues to slowly decrease today.  The 850mb dew points are a marginal 8-11C.

CAPE isn't very favorable, but it's sufficient to support some afternoon storms for the higher terrain of the state.

Let's check-in at 250mb to see how the IT is progressing.  It's still not in the outer domain, but its

influence can be seen as a weak area of influence is over NW Mexico, along with a weak upper jet near the Big Bend.

Scattered thunderstorms develop over eastern Arizona during the afternoon hours and generally move to the WNW.  Storms are knocking on Tucson's door by late afternoon.

Maybe, maybe not.  Tucson does recover somewhat by late afternoon as CAPE may be as high as 700 J/kg.  The low-level shear is also pretty good, as is steering flow.  Anvil shading may still be a problem, though.

Most runs do not move storms into Tucson.  Instead, which is typical, they stay over the higher terrain of eastern Pima County.  Scattered showers and storms continue in far SE Arizona (for some reason) into the evening hours.

While Phoenix has some moisture and CAPE, multiple inversions prevent any deep convection unless significant outflow boundaries move through, which is very unlikely.

Day 2

The IT moves to a location over southern Chihuahua by tomorrow, placing much of eastern Arizona in favorable upper-level divergence/difluence.  A weak jet streak is over southern NM.

There is an open wave at 500mb, along with cool air, around -10C.  Steering flow in Arizona is mainly easterly.

Moisture is mostly the same, with 35-40mm of PW over the lower elevations.  Low-level easterly flow out of NM is again impacting far southeastern Arizona.

The other improvement over today, besides the upper-level support, is that CAPE is better with 500 to as much as 1500 J/kg.

By afternoon, there is a lot more activity compared to today.  

It looks like Phoenix has a good chance of early evening activity.  Steering and shear are favorable, along with a deeply mixed PBL. CAPE isn't great, but 1000 J/kg should be sufficient.

There isn't much agreement between runs for Phoenix as some have a lot of activity, and some have only a little.  The middle of the road is the WRFGFS.

The more active 9Z WRFRR has a nice haboob forecast for central Arizona and severe winds.