Friday, September 8, 2017


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.

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.  

Thursday, September 7, 2017


Previous Forecast
Not much activity except for far western Arizona.  Sonora was quite active and is responsible for producing outflow surges into SW AZ where dew points are now in the 70’s.
The WRFGFS (below) and the WRFNAM forecast performance was good.  Both runs of the WRFRRx were way too active.

Mostly the same story as yesterday with a big trough in the east, a ridge in the west, and a weak trough along the California coast.  A weak IT is located somewhere around far southern Baja.   Skies are mainly clear except for far western Arizona where a few showers and storms are still ongoing.   I didn’t mention it yesterday but both the WRFGFS and WRFNAM kept activity going into this morning over western Arizona.  All model initializations look good except for none of them had enough western Arizona activity.  That shouldn’t be a problem as the activity is decreasing and in a location that won’t impact Tucson or Phoenix.  IPW initialization errors were fairly high in the NAM as much of NW Mexico initialized too wet as well as much of CA and NM.  The GFS and NAM were better but also too wet in NM Mexico.  The correction routine should account for this bias, though.  In general, initializations are OK and model performance should be good except for maybe the WRFRRx as it has been much too active the past few days.

Day 1
Moisture has increased overnight over much of southern Arizona as IPW is now 25mm at Tucson and 30mm at Phoenix.  The Gulf Surge is quite shallow on the Yuma sounding but better than nothing.  The surge continues and advects moisture into the western ⅓ of the state during the day increasing IPW by another 5mm or so.  Dry SE winds continue over the eastern ⅓ of the state and again. There is a dry line like feature from around Tucson to the east of Phoenix which may act as a focus for convective development today.

All model runs agree that CAPE will be moderate to high over much of SW Arizona and as far east as Tucson and Phoenix.  This will support some very strong to severe deep convection today.

Upper-level difluence and divergence due to a weak west coast upper-trough will help with thunderstorm organization and intensity this afternoon and evening.

Tucson IPW is forecast to be around 30mm this afternoon which results in moderate to high CAPE.  Low-level westerlies are advecting moisture into the area while steering flow is favorable from the SE and 15 to 25 knots.  There is also some low-level shear present.  The only negative issue is that anvils may blow across the Tucson area and restrict deep convection.  Other than that, it is likely to be an active day in the region especially just west of Tucson.

All runs are quite similar as they develop strong to possibly severe storms over Pima County by afternoon.  

Tucson is right on the edge as the runs disagree on how much activity there will be there.  The middle of the road (WRFGFS) predicts a strong outflow coming through during the afternoon with a few weak storms developing.  Could be more, could be less.

Pima County storms send strong outflows toward the Phoenix area during the late afternoon.  It sure looks like there is going to be a big haboob in Phoenix at a minimum.

Wow!  Both the WRFNAM and WRFGFS have from around 1400 to 2000 J/kg of CAPE for Phoenix by late afternoon which is some of the highest of the summer.  The WRFGFS (below) also has decent low-level shear with westerly low-level winds and southeasterly mid-level winds.  

All three model runs develop strong to severe storms on the outflow boundary as it moves north into Pinal and Maricopa counties during the late afternoon and early this evening.  (WRFNAM and WRFGFS)  The two are nearly identical except for the timing, so confidence is high.

Big storms continue over much of central Arizona into the evening.

It looks like parts of Phoenix will see some strong to severe weather this evening.

The strong outflow boundaries continue to move to the NW later in the evening and trigger additional deep convection and possible severe weather in western Arizona.

Day 2
The Gulf Surge is forecast to continue tomorrow thus it’s likely to be another active day.

There may be problems with debris clouds restricting heating and also with the cooling in the lower troposphere due to the surge and the previous day’s activity.  CAPE is also forecast to be lower but still sufficient to support storm development.

Storms initiate again over the higher terrain of Pima county and are steered towards central Arizona due to SSE mid-level flow.

Strong to severe storms are again possible tomorrow evening in central and western Arizona.

Wednesday, September 6, 2017


Previous Forecast
A fairly active day over NW and NC Arizona and far SC Arizona.  The WRFRRx runs were too active around the Phoenix area as only a few showers formed when the outflows moved through.  The WRFGFS and WRFNAM (below) were the best runs but still had a bit too much activity west of Phoenix.

The long wave pattern over North America has become highly amplified with a big ridge in the west and trough in the east.  The leftover mid-level circulation of Lidia continues to drift slowly to the southeast off the coast of CA and will be a big player in our weather in a day or two.  A weak inverted trough is located over the southern Gulf of California.  It is clear in Arizona/NM and into NW Mexico.   All of the 12Z model initializations have the main features initialized well and IPW initializations have only minor errors.  

Day 1
For the most part, yesterday's model runs dried out much of central and eastern Arizona and this is the case as IPW is down to 19mm at Tucson and 25mm at Phoenix.  It is still fairly wet out along the Colorado River Valley as Yuma is 36mm and Blyth is 32mm.  This boundary remains approximately in the same position during the day as dry easterly winds continue to dominate the eastern ½ of the state.

Moderate to high CAPE is present this afternoon along the CO River Valley and this dryline-like feature is likely to again act as an initiation point for convection over the higher terrain of the area.  

There is some upper difluence/divergence over the state provided by the west coast trough thus some of the storms that form in western Arizona may exhibit some organization.

Model runs mostly agree on developing a few storms over NW Arizona and SW Arizona by later in the afternoon.

A few strong storms continue into the evening.

Day 2
The west coast trough continues to remain about where it is which puts Arizona under moderate mid-level southeasterly flow.  The Mexican IT moves off the coast thus the winds in the lower gulf are also southeasterly.  

Winds at 850mb are also generally from the SE along the Gulf of California and up into Arizona thus some moisture returns to most of the state.

Moderate CAPE is forecast to be over the SW ½ of the state which should support a more active day.

Storms develop south and west of Tucson and Phoenix during the late afternoon.

In spite of moisture increasing at Tucson, CAPE isn’t sufficient to support much activity.  The vertical profile does exhibit a deep inverted V signature thus storms west of Tucson are likely to produce strong outflow winds.

Phoenix looks a bit better as the WRFGFS has some CAPE present during the late afternoon/early evening as well as a deeply mixed layer with a deep inverted V profile and a bit of shear.

The WRFNAM has very strong/severe storms during the late afternoon mainly in central and western Pima county.

Both runs have strong outflow boundaries moving toward Phoenix during the evening with the WRFNAM being especially strong.

As this outflow moves north, a few storms manage to form in and around the Phoenix area.