20160731

Previous Day
The WRFGFS was way too wet while the WRFNAM and WRFRR dev were very good including the moderate storms that moved through Tucson around sunset.  The WRFRR didn't have enough activity around Flagstaff.

WRFNAM

 Initializations
A complex situation this morning with an ongoing MCS over far northern Mexico, forced by the large IT to the SW.  Scattered to broken mid level clouds are slowly dissipating over southern Arizona so there is some question regarding the amount of heating we will receive.  I was pleasantly surprised to see all three of the 12Z initializations having a fair amount of clouds present.  Looking at the 2m temperature forecasts so far shows that the model runs are all a few degrees too warm, but not too bad. The WRFNAM was the most accurate.

The IT was initialized well by all as the Guaymas upper air data probably had something to do with that.  No other significant circulations were noted.  I noticed that the WRFRR is noticeably different with regards to the amount of RH at 700mb.  Strange.

WRFRR

WRFNAM

Minimal IPW errors in all three.  Overall, they are all good with no clear favorites except to say that the WRFNAM has been doing well recently thus that is my favorite.  Model confidence is high.

Day 1
A very wet atmosphere is in place across much of the state with IPW of 45mm at Tucson and 51mm at Phoenix.  By early afternoon, the surge continues to advect very wet air into the state.  850mb dewpoints are very high over much of the lower deserts, around 15C.  I'm a little worried that this may be a 'too wet to rain' situation in Tucson where convection fires over the mountains early thus clouds and cooling outflows don't allow the lower elevations to heat enough for deep convection.

'Too wet to rain' is certainly a concern when steering is as light as is forecast by the WRFGFS this afternoon.  The WRFRR and NAM have a bit high wind speeds; only 10 knots though.  The 500mb high has been suppressed slightly due to a strong trough over the NW US.    One center is in western NM and the other west of Vegas this afternoon.

CAPE again is forecast to be moderate to high, especially over the higher terrain.  With good heating, expect rapid development of storms in those locations.  Very heavy rain and wet microbursts are likely.

Deep convection forms in far southern Arizona and over the Rim/White Mountains.  The far southern Arizona activity moves slowly towards the Tucson area during the afternoon.  All the model runs are similar except the WRFGFS has quite a bit more activity near the border.

The Tucson afternoon forecast skew-t has high CAPE and a very good wind profile.  Mid-level speeds could be a bit higher, but the lower level inflow is around 15 knots while the upper-level flow is out of the SW thus blowing anvils away to the NE and not interfering with heating and vertical velocities.  It's been shown (CuPIDO 2006) that weak downward vertical velocities are present under anvils.  There was a nice example of that over Mt Lemmon recently.  The only issue is the fairly strong inversion at the top of the PBL thus a good outflow will be needed to lift to the LFC which is around 700mb.  Strong outflows are not forecast as the cloud bases are so low and IPW is so high.

The only model run to forecast even a moderate outflow into Tucson is the WRFGFS, which has been too aggressive with convection all summer so far.

The WRFGFS does develop some storms in the Tucson area, but they are only scattered.  The WRFNAM and WRFRR have only limited activity in the lower elevations with most remaining on the higher terrain.

WRFGFS

WRFNAM

The situation is a bit better (or a lot depending on the model run!) as the atmosphere is mixed nearly to the LFC during the late afternoon with moderate CAPE, and a good wind profile.  A few weak outflows are all that is needed to get deep convection started.  As the PBL is mixed higher in and around Phoenix, outflows are likely to be strong.  Also, with such a wet airmass, wet microbursts are likely with stronger storms.  Very heavy rain too.

 The WRFGFS and WRFRR both bring in moderate outflows from the south and northeast into the Phoenix area for a classic outflow intersection situation.  The WRFNAM as it has less activity in SE Arizona only has a moderate outflow from the NE.

WRFNAM

The WRFNAM is able to develop scattered deep convection with the single outflow.

On the other hand, if outflow intersections occur, like in the WRFRR and WRFGFS, stronger storms form over Maricopa and Pinal counties this evening.  The WRFGFS really goes nuts over Phoenix, but I believe it is overdoing things.

WRFRR

This just in from the 15Z WRFRR dev.  Not much activity around Tucson, but a strong/severe line of storms skirts the northern parts of Maricopa county.  I haven't had a chance to look at this run in detail so it's unknown how accurate it is.

I am going to skip Day 2 again due to the typical uncertainty with an active and complex Day 1.

20160730

Previous Day
My days of hyping yesterday's event worked out as is was one of the most active days in Arizona in a long time.  What was remarkable was the run to run consistency as shown by the Phoenix area temperature plot.  This event was actually predicted days in advance and I didn't mention it much until it got within 48 hours to be conservative.   The only issue was that WRF is typically unable to forecast the large temperature drops in active situations.

The strong to severe wind forecast verified well too as there were many severe wind reports and many power outages.  The call for very strong winds in SW Arizona also verified as there was a gust to 89mph at Gila Bend.

I was really sweating the situation around Phoenix by late afternoon as the IPW crashed to 30mm.  The various runs kept it much higher, in the upper 30mm range thus I was quite concerned that deep convection was going to be isolated or maybe nonexistent.  The 0Z sounding at Phoenix had only around 300 J/Kg!  In spite of that, moisture and surface convergence kicked in and severe storms formed mainly over the southwestern part of the city.  As IPW was much lower than predicted, the precipitation forecasts were too high.

 Initializations
Widespread activity over much of the state has left behind rain cooled air and out west, clouds.  Both Tucson and Phoenix model 2m temperatures are too warm, except for the first 12Z run, the WRFRR. However, the temperatures errors aren't that large so perhaps those errors won't have a large effect.  The Phoenix sounding has the 'green onion' shape meaning there is rain cooled air present near the surface.

The eastern 1/2 of the state is mainly clear so some recovery is possible here.  Active convection is still underway south of Yuma which is generating widespread clouds over most of southwestern Arizona.  Only the NAM was able to initialize clouds well.  The RR was especially bad as it has widespread clouds across southern Arizona.  The 6Z WRFNAM also looked pretty good so it might be of use today too.  A large inverted trough is located over the central GofC with ongoing clouds and storms.  It's almost looking like a tropical system.  The models seem to of initialized it a bit too far to the south and the Guaymas sounding is missing and could have been of some help better defining the location. Other than that, no obvious cyclonic circulations.  Of note on the 300mb plot at 12Z is the area of divergence and difluence over eastern Arizona.

The RR and NAM are a little too wet throughout northern Mexico.  Probably not enough to be corrected out.  As the morning situation is quite complex, model confidence is only medium today with the WRFNAM and WRFGFS being the favorites.

Day 1
Moisture has increased somewhat over the state due to the ongoing surge.  It's a bit hard to tell exactly what the status of the surge is as SW Arizona has ongoing activity and Puerto Penasco and Yuma IPW data is missing.  By afternoon, a moderate to strong surge continues to import moisture into Arizona with moderate to high IPW over much of the state.

The model runs have moderate to high CAPE over much of the state except for far eastern Arizona and around Phoenix.  Deep convection will likely initiate early over high terrain and be very strong.

Tucson's 12Z sounding was only slightly modified by previous activity and by late afternoon, the forecast skew-t indicates a high amount of CAPE, deeply mixed PBL, and very favorable winds.  Based just on this, I'd say a very active day in the Tucson area.  Both the WRFGFS and NAM are very similar.  The odd man out is the WRFRR as it has limited CAPE as it did not warm nearly as much.  (clouds?)

The situation looks very much like yesterday, as by late afternoon, organized convection is moving off the high terrain towards the lower deserts of Maricopa/Pinal/Pima.   The WRFGFS is especially active and I'll go with the middle of the road WRFNAM.  The WRFRR is nearly devoid of deep convection over southern Arizona.

WRFNAM

Thunderstorm outflows again converge over the deserts, driving additional convection.  Outflow wind speeds are still strong, but nowhere like what was seen yesterday.

The WRFGFS is much more active with severe storms around Tucson.

The WRF does forecast upper divergence for much of eastern Arizona and that could be partially responsible for the active WRFGFS.

The Phoenix evening skew-t forecast is capped with a shallow mixed layer.  Mid-level wind speeds are also weak. CAPE is quite high, but in spite of that, not much activity is expected unless strong outflows intersect to provide the mesoscale needed to lift to the LFC.

It's a different story in the WRFGFS as there is less of an inversion, a wetter airmass, and better NE steering. (and slight upper divergence?)

Outflows intersect over Pinal and Maricopa counties around 8-9pm and fire off deep convection. I think this solution is less likely than the WRFNAM.  However, if SE Arizona convection is strong and outflow make their way towards Phoenix, the below could happen.

I'm not bothering with a Day 2 forecast in this complex and uncertain environment.

20160729 Day 2

Some uncertainty surrounds tomorrow's forecast due to the impact of the active evening ahead today.     A gulf surge continues to import moisture into the state as southerly flow becomes established all across NW Mexico up to 850mb.   So calling this a 'gulf surge' is not quite right

as the southerly winds span more than just over the G of C.

CAPE over SC out to western Arizona decreases, as one would expect after a big day.  SE Arizona, which was left out of the action, has very high CAPE present in all the model runs.

Steering flow weakens somewhat as the 500mb high center meanders around NE Arizona into NW NM.  Temperatures are still around -7C thus quite cool.

As is so often the case with a big surge, the PBL is cooled and not deeply mixed.  The wind profile continues to be excellent.

This is even more pronounced at Phoenix as a strong cap is in place.  Excellent CAPE, moisture, and OK winds are the positives, but it's going to take a miracle to get that profile to go.

Once again, big storms line up over the Rim and White mountains with some deep convection going over far SE Arizona.  All the WRF runs are quite similar so there is some confidence in the models. 

Outflow winds are not nearly as strong due to the increased moisture and relatively shallow PBL.  Outflows converge over eastern Pima county during the late afternoon and early evening providing the local lift needed for deep convection.

Strong storms are in and around the Tucson area during the early evening with some developing into Pinal county later.  It's unlikely that Phoenix will see any significant activity.

20160729

Previous Day
Mostly a down day for the with a few strong storms to NE of Phoenix.  The WRFNAM predicted a strong outflow into the east side of Phoenix which did not materialize.  The WRFNAM had a bit too much activity while the WRFGFS didn't have quite enough.

WRFNAM

Initializations
There are scattered mid level clouds around western Arizona and eastern Arizona.  The eastern Arizona clouds seem to be forming due to mid level ascent perhaps from outflow from the large MCS in Sonora last night or more likely, on the nose of the 500mb jet across SE Arizona into NM.  In any case, the trend is for mostly clearing skies. The RR may actually be a bit too cloudy over the northern part of the domain.

No obvious cyclonic circulations present over or near Arizona. A closed cyclonic circulation is present over the south end of the GofC which was initialized too far to the north in the RR.

The WRFNAM has large wet bias from Hermosillo up into SE Arizona and then over into NM.  Large enough that I think it may impact the forecasts.  The 6Z NAM is much better as are the 12 GFS and RR.  The Guaymas sounding is not to blame for the NAM errors.  The 6z WRFNAM and 12Z WRFGFS/RR appear to be well initialized and any of these can be used with confidence today.  As all runs over the past day are quite similar, forecast confidence is high.

Day 1, July 29
A very unusual situation is present across much of the state where the combination of factors is about perfect for a major severe weather outbreak.  A surge has begun with dewpoints in the mid 70-degree range over SW Arizona and 80F at Puerto Penasco.  IPW has also responded as Yuma has increased to 45mm.  Normally the first day of a surge brings higher moisture and cooler PBL temperatures, capping the lower deserts.  This surge is weaker thus not cooling the PBL very much.

Moisture continues to be advected over the state during the day with IPW increasing slightly in most areas except for NE Arizona and far SE Arizona where there is moderate dry air advection occurring due to low-level easterly winds.  At this point, Tucson is located on the wet side of this line.  It is likely this boundary will be the initiation for deep convection over SE Arizona.

An impressive 500mb jet is present at 12Z from Tucson back towards west Texas as well as some cooling with temperatures around -7C over Arizona.  The wind speeds are highest over southern Arizona this afternoon with 20 to 25 knots.  Central Arizona has good ENE flow at 15 knots.  Very favorable speed and direction for moving storms off the rim towards Maricopa/Pinal/Pima counties.

What is truly impressive, and a bit scary is the amount of CAPE present this afternoon.  These are the largest CAPE values I've seen in a long time.  With the combination of heating and steering flow, this is about as good as it gets; a perfect storm.  Afternoon values over the lower deserts are from 1500 J/Kg to around 3000.  All of the 12Z runs are similar.  Looking at the 6Z runs as a sanity check, the WRFNAM is a bit drier and has somewhat less CAPE.  However, even here, values are as high as 2000 J/Kg during the afternoon.

WRFGFS

Deep convection takes it's time forming and doesn't really get going until late afternoon over the higher terrain.  Both the WRFRR and WRFNAM have strong/severe storms to the east of Phoenix and developing over eastern Pima county around 5pm while the WRFGFS holds off by another hour or two.

I took the middle of the road skew-t for the Tucson area.  Over 1500 J/Kg of CAPE, but the low-level easterly flow is close.  At this point, most likely the strong to severe storms will be over the west and northwest parts of the city. All of Tucson is at risk.  As in Phoenix, a deep mixed layer will be very favorable for strong/severe outflow winds.  Last night's soundings had around 2000 J/Kg of DCAPE which is probably similar to the amount today.  Excellent wind profile as 20 knot easterly winds in the mid level along with upper NW winds, blowing the anvils away from the storm motion.

Very, very strong outflows are produced by the storms with wind speeds about as high as I've ever seen.  I'm running one of the 12Z runs with an alternative PBL scheme to double check the wind speeds as the operational PBL scheme may be over predicting wind speeds.  Model wind speeds have been as high as 50 knots.  Using a conservative gust factor,  gusts could be above 70 knots.

This is a serious situation for Phoenix.  All runs have between 1500-3000 J/Kg of CAPE by late afternoon with the WRFGFS at the high end. Mixing ratio is from 11-12 g/kg.   Steering flow is a bit weaker, but still OK with very optimal NE flow.  Inflow is a bit weak but that isn't going to matter with such strong outflows and outflow intersections occurring in Pinal and Maricopa counties.

Strong to severe outflow winds blow through Phoenix around 6-9pm followed by strong to severe storms with very heavy rain, 2 inches/hour and continued high winds.

Models differ on the details of location and timing. In general it looks like a MCS forms and moves WSW.

Strong to severe winds are possible all the way to Yuma during the night.

While I have not shown any figures from the 0Z runs, they should be relatively good since yesterday was a very suppressed day. The agreement among the full time-lagged ensemble is extraordinary for severe winds, though the details of when and where, along with how much precipitation is of course harder to pin down.

Day 2 will follow later once I catch my breath.