Wednesday, July 18, 2018


Previous Forecast
Activity was mainly restricted to the western ⅓ of the state and in some of those locations, the storms were strong to severe.  A large complex moved out of Yavapai County last night while other large storms roamed across central and western Pima County.
All three of the morning runs were generally accurate except none of them were able to forecast the activity that moved into La Paz County.  The 12Z HRRR was also unable to forecast these storms but the 15Z run was a bit better as it did have some activity.

The 500mb ridge axis stretches from off the central California coast to west Texas with a large inverted trough over much of NW Mexico. The center can be seen spinning of the southern Baja coast and is moving slowly westward. The RR and NAM had no obvious problems.

Widespread clouds, showers, and some storms were over far SW Arizona this morning and it was mostly clear elsewhere.  Both the RR and NAM were initialized well.

The NAM continues to have an overall wet bias but the magnitude of the errors are smaller today while the RR had only minor errors.  The WRFGFS error plot is available now and it has mainly small errors except in So Cal where it’s too dry. Overall, the WRFRR has minimal errors thus confidence in it is high but the other two runs should also perform OK.

Day 1
PW remains very high over southwestern Arizona while somewhat drier air is over eastern Arizona and into New Mexico.  GPSIPW plots indicate there has been a slight increase over eastern and central Arizona the last 12 hours as the easterly winds out of New Mexico weaken and moisture sloshes back to the east.  

CAPE is forecast to be high to extreme over parts of central and western Arizona today.  The morning soundings from both Tucson and Phoenix have a strong cap on top of the shallow mixed layer -- it looks like an Oklahoma summer sounding (except the winds of course).  So, like yesterday, the big question is if/when/where will the cap will break?

Steering flow is generally from the east southeast which is favorable for storms to propagate from the higher terrain of Cochise/Graham Counties into the Tucson area but much of this area is still in the somewhat drier and more stable air.

Deep convection struggles to initiate over the eastern Rim/White Mountains and SE Arizona and by mid-afternoon only isolated storms are underway.   This situation is giving me the willies as CAPE is so enormous plus all of the state is receiving full heating and if the cap busts, well, we know what the storms were like in western AZ and Pima County yesterday.

The WRFNAM has a bit more activity, probably because it’s a bit wetter. It has the same general idea of only a few storms around the Tucson area and in Yavapai County. Note the anvils extending the the northwest of the storms.

The WRFGFS is now available and it’s more similar to the WRFRR.

Well, Tucson is close to having a big day.  CAPE is huge with around 2000 J/kg, winds are favorable with low level westerlies and good mid-level steering.  What is needed is some sort of mesoscale lifting from outflow boundaries or local low level convergence. Normally, that would be provided by storms moving in from Cochise county but none in the model forecast today.  As the easterly low level winds have slackend, there is no low level convergence zone either.

Only a few storms are present during the late afternoon and early evening.  Maybe one or two will drift into the Tucson area.

It’s typical for the day of a surge that there is little activity over the lower deserts due to the cooling of the low levels but this is ridiculous!  The atmosphere must even be capped over most of the high terrain.

Just to rub salt into the wound….here’s Phoenix with enormous CAPE. They are a long way from the LFC.  The winds are excellent as anvils, if there were any, would be blown away from the direction of movement.

I also looked at the HRRR and it has a bit more activity than the WRF runs in and around Tucson during the late afternoon as deep convection develops over the higher terrain of Santa Cruz and far SE Arizona.

Day 2
Holy crap!  

Steering flow is still good with 10 to 20 knot southeasterly flow over much of the state.  No obvious mid or upper troughs were seen to help enhance storms.

The lower deserts are still strongly capped tomorrow.

Enough heating does mix the PBL a bit better but strong outflow boundaries are still going to be needed to lift to the LFC.

It appears that the heating will be sufficient to initiate deep convection over the high terrain including SE Arizona.  Both the WRFRR and WRFNAM are very similar.

Very strong and severe storms are scattered about SE Arizona and are moving towards the lower elevations.  All the 3 of the 12Z runs are quite similar so forecast confidence is high.

The PBL heights are quite low thus large scale outflow boundaries are not likely.  However, localized wet microburst winds will be strong to severe. Not having large scale outflows is a big negative as it’s going to be hard for storms to propagate far into the lower elevations.

Even with these severe storms and strong localized winds, storms are unable to move into the lower elevations!

Tuesday, July 17, 2018


Previous Forecast
Widespread activity over the state yesterday with some localized areas of very heavy rain.  Storms moved into the Tucson area around 4-5pm and promptly split and went around on the higher terrain.  Some activity continued over SW Arizona into the morning hours.
All three of the 12Z runs had the right idea with the WRFRR looking the best.  However, none of the runs forecast the late night/early morning activity over southwestern Arizona.

The 12Z 500mb map indicates the center of the ridge is over central California while a large inverted trough is located somewhere around southern Sonora.  There is no upper air data from Mexico this morning so we’ll see how well this feature gets initialized. It has a significant amount of deep convection associated with it over the Gulf of California and I would assume this is going to trigger a Gulf Surge at some point. Both the NAM and RR seem to have initialized the center a little too far south but who knows for sure.  In any case, the general circulation is broad so any minor error won’t matter much.

Clouds and showers are ongoing over southwestern Arizona but moving to the southwest with clearing behind. Another area of clouds is over northcentral Arizona.   The RR initialization of the cloud/showers was very good and also included the outer domain deep convection in Mexico. The NAM missed the Mexican activity but did an OK job with the clouds and showers over Arizona.

The string of bad NAM PW initializations continue with many areas again too wet with some errors as high as 10mm and looking back at the error plots, this issue began about 10 days ago.  Below is a plot of the errors with the shaded areas showing the amount of PW removed from the initialization and the point values at the GPSMET sensors.
Below is the error plot for the RR which indicates most stations have only minor errors this morning.  If anyone has a contact at NCEP that I can alert to this issue, please let me know.
And the 12Z GFS from today.

Overall, the WRFRR seems to be initialized the best.

Day 1
Moderate low-level southeasterly winds are advecting in dry air over much of New Mexico and eastern Arizona thus not much activity is expected there today.  What really catches my eye is the extremely wet air moving into SW Arizona. Morning GPSIPW plots already have 56mm at Buckeye and 54mm at Yuma so 60mm is certainly possible.

This dry air appears to stretch all the way back into Texas and all throughout NE Mexico.  Extremely wet air is all the way down the Gulf of California and I might have to increase my PW plot scale as 60mm may be too low!

There is a very impressive backdoor dryline that really stands out in the midday CAPE plot.  High to extreme CAPE exists west of the line which is around the Pima/Cochise County line. The NAM, in spite of its initialization faults, is quite similar to the WRFRR (below).  The 15Z Yuma sounding has a shallow surge underway with a strong cap so, in spite of all the CAPE, it’s going to be very difficult to get widespread deep convection over the low elevations.  However, the higher terrain of central Pima and far southern Maricopa look favorable as deep convection should initiate there at some point today/tonight.

Mid-level steering flow is good as it’s about 10 to 20 knots from the east thus storms that form will propagate towards lower elevations.  

Deep convection initiates over far south-central Arizona this afternoon along with some big storms over Yavapai county where CAPE is also very high.  However, these storms soon die off as they apparently move into a strongly capped environment.

Using Phoenix as a proxy for much of western Arizona, the PBL just isn’t mixed deeply enough and must still be capped. The existing higher terrain convection driven outflows apparently aren’t strong enough to break the cap.  Forecast surface temperatures are only in the 90’s for much of western Arizona and the convective temperature from the 15Z sounding at Yuma is 105F.

The potential is still there by late afternoon and early evening as CAPE is about as high as it gets in Arizona with values above 2500 J/kg.  Any storms that do get going will be monsters.

Both the WRFGFS and WRFNAM develop severe storms just across the border and move them into central and western Pima County this evening.

These continue over the higher terrain of western Pima county but once they move off the higher terrain, they die.  However, the WRFRR continues to redevelop storms over western/central Pima County well into the night.

The WRFNAM as a remarkably similar solution except it is more widespread which I attribute to the continued wet bias of the initialization.  A quick look at the 12Z WRFGFS is also very similar to the WRFNAM with respect to timing, coverage, and intensity. Looks like great storms for chasers but the rest of us are out of luck.

Day 2
Somewhat drier air moves into central Arizona with the wettest (and strongly capped?) air over far western Arizona.

Isolated mountain activity is all that is expected tomorrow except over far NW Arizona where some strong storms form by late afternoon.

It does look like this penetration of dry air from the east will be short lived as unstable air sloshes back to the east tomorrow evening.

Monday, July 16, 2018


Previous Forecast
WRF has really been struggling the past few days due to the complex nature of the initializations.  The WRFNAM and WRFRR (below) both did an OK job, but both had issues as they developed strong storms during the morning hours around Tucson and Phoenix.  In general, the runs have also been struggling to produce enough precipitation over NW Arizona the past few days for some unknown reason.

Here we go again.  The morning weather situation is again dominated by widespread clouds and a few showers but some clearing is occurring over the eastern ½ of the state.  The RR was fairly cloudy over the southwestern ⅓ of the state and did initialize some of the shower activity but in general, didn’t have enough coverage  The NAM falsely initialized a strong storm over Yuma county and generally, does not have enough clouds.

The 500mb map has the ridge centered over southern Nevada and a broad IT over NW Mexico.  500mb temperatures are slightly lower in NM with -6C compared to Arizona at around -5. Water vapor imagery shows a few weak waves moving SW over NM towards Arizona.  The RR has the Mexican IT initialized OK as does the NAM.

It continues to be very wet over the state with PW again ranging from the low 40’s to over 50mm.  Suominet went haywire yesterday with some stations reporting +200mm thus some sanity checks had to be implemented in the correction routine.   Data is back to normal today. RR PW initialization errors were small just about everywhere but problems continue with the NAM as large wet errors were present in Mexico and from California to northern New Mexico.  Central and southern AZ PW initialization was good. Overall, the WRFRR initialization looks the best and should perform fairly well as all errors are generally small. The 12Z WRFNAM looks quite poor so I took a quick look at 6Z WRFNAM and it had a decent amount of clouds over southern Arizona this morning so may also be useful.

Day 1
Very moist air continues to stream into the state from the Gulf of California and PW continue in the 45-50mm range today.  Also, there is a bit of low level westerly flow which should provide a bit of low level shear for storm organization.
CAPE continues to be minimal over southwestern Arizona due to cooling at the surface and warm air aloft.  Areas of SE Arizona have moderate CAPE while the high country has very high CAPE. Storms are likely to be strong with very heavy rain at those locations.

Steering flow is excellent for moving high elevation storms towards the lower deserts.  Some slightly cooler air is present in NM which may move into Arizona later and help steepen the lapse rate.  

There is also a little upper support as the flow is divergent/difluent over southern Arizona.

Strong storms initiate by early afternoon over the eastern Rim and White Mountains.

By late afternoon, Tucson has about 600 to 1000 J/kg of CAPE with a deeply mixed PBL and an excellent wind profile.  Just based on this, widespread storms are likely.

Strong storms propagate off of the high terrain and are moving into the lower elevations of Maricopa/Pinal/Pima by late afternoon.  

Outflow winds aren’t all that strong, around 30-40 knots, as the PBL isn’t that deep but isolated very strong winds are likely due to microbursts.

Storms also move towards the Phoenix area by early evening but the forecast CAPE is too low to support deep convection in the valley proper.  The PBL is mixed deeply and the wind profile is good so maybe a few weak storms may pop up as the outflow boundaries move through.

The WRFRR has nothing more than a few showers for the Phoenix area this evening.

Day 2
The 500mb ridge axis stretches from central California over to western New Mexico while a large IT is located over southern Baja resulting in moderate easterly flow across Arizona and unfavorable temperatures around -4C.

Drying is present over all of southern NM and into eastern Arizona.  SE Arizona will be especially devoid of deep convection due to downsloping winds.  The back door dryline boundary is about over the Tucson area thus just west of Tucson could be where convection initiates tomorrow.

CAPE is moderate to high west of this line especially over far southern Arizona.

Intense deep convection is present in central Pima County and down into Mexico during the afternoon.  There are a few strong storms over northwestern Arizona while much of the eastern ½ is quiet as drying continues.

Storms continue over parts of SW Arizona into the evening.  Both the WRFNAM and WRFGFS are quite similar so there is some confidence in this forecast.