20200816

Previous Forecast

Activity picked up for southeastern Arizona.  There were some severe warned storms over western Cochise and eastern Santa Cruz Counties that produces quite a bit of rain, and probably some hail too.  KALK, near Sierra Vista, reported a wind gust of 69mph.

All the runs had the right idea of developing strong to severe storms over Cochise and Santa Cruz counties, even the WRFRR.  However, they were all a bit low with accumulated precipitation and didn't have quite enough areal coverage.  For example, there was quite a bit of activity to the northeast of Tucson that didn't show up in the model runs.

Discussion

The west coast ridge continues to increase in strength as LAS reported a 5980m height this morning.  Arizona is under generally NE to SE midlevel flow and temperatures continue around -7C, thus more favorable for storm formation and propagation.  It appears there was some sort of communication problem with Mexico as no upper-air data was received.

The Phoenix 12Z Skew-T does show an increase in moisture from the surface to just below 700mb, but it's still not enough to increase CAPE much.  It's not anywhere near what the model forecasts were saying yesterday.  What else is new.  The cap verified as it's quite strong.  Winds are favorable though.  We'll see later if the model forecasts increase moisture during the day.

Tucson is worse as there has been little or no moisture increase and hardly any MLCAPE.   Like Phoenix, winds are favorable.

GOES derived CAPE is present over much of central and southern Arizona, but there is none in northern Sonora in the wake of the giant MCS that moved across Sonora last night.  The MCV is located over the northern Gulf of California.  Also, note the more stable air over far eastern Arizona and into SW NM.  Easterly low-level winds are advecting this area of dry air into SE Arizona-not good!

Initializations

The 6Z NAM PW initialization in Sonora and into southern Arizona was terrible so I'm not even going to look at the WRF run.  The GFS was not as bad but had some 10mm wet errors in Sonora.   It looks like something went haywire with the presence of the MCS at 6Z.  The 12Z RR PW errors were minor, but the NAM struggled with some large errors continuing in Mexico.  I'm sure it didn't help to have all the upper-air data missing.  It was OK in Arizona so the NAM might be useful in the short term.  

Debris clouds in the proximity of the MCV were initialized pretty well.  However, the mid-level circulation was mainly missing.  Overall, the best initialization was the RR.  

Day 1

The key for today is where the dryline is going to end up during the mid to late afternoon.  The WRFRR has it from Santa Cruz, eastern Pima, and far eastern Pinal Counties by early afternoon.  The morning GOES PW loop shows dry air is moving to the west over southeastern Arizona.  Hopefully, once the typical afternoon up-valley flow is established, this westward progression will be temporarily halted and that is kind of what the model runs indicate.

The WRFNAM has the dryline a bit farther to the west, just west of Tucson.  This small difference is likely to result in a much different forecast for the Tucson area.

The afternoon CAPE forecast has moderate amounts of CAPE over central Pima and into Pinal Counties and is about 500 J/kg lower than was being forecast yesterday, but still sufficient to support strong storms.  In fact, some could still be severe as DCAPE from the morning soundings is around 1500-1700 J/kg.  

The animation of the Skew-T diagram for Tucson shows a general decrease in moisture/CAPE as the morning goes on but as the low-level weak westerlies kick in the afternoon, CAPE and PW gradually increase.  CAPE is a decent 700-1100 J/kg by then and the CBL is mixed fairly deeply.  NE flow dominated above the mixed layer so storms will be steered off of the higher terrain to the northeast, assuming any form!

I have my doubts as CAPE is minimal over the White Mountains.  However, the 12Z runs do develop scattered storms and move them to the southwest. 

The WRFRR develops stronger low-level flow which results in better moisture advection.  It's Tucson dewpoints are around 50F (below) while the WRFNAM dewpoints are around 45F.  

Most model runs are on the same page as they develop scattered strong storms over eastern Pima and Pinal Counties by late in the afternoon.  The WRFNAM is a bit more to the west due to its location of the dryline.

The WRFRR is a bit to the east and develops strong storms over Tucson.  Who knows which one is right?  Keep an eye on the afternoon observed dewpoints.

The 12Z WRFGFS just got out far enough to include here.  It has a bit more widespread activity, and like the WRFNAM, keeps the strongest storms just to the NW of Tucson.

The main threat from these storms is severe winds.  As seen on the Skew-T forecast plots, there is a deep inverted V profile, plus the large DCAPE observed on the 12Z soundings.  Winds could gust as high as 60 knots in some cases.

The situation for Phoenix is more clear.  In spite of a moderate amount of CAPE, the CBL is shallow and topped by a weak inversion.  Even the more optimistic WRFNAM doesn't look good as the LFC is far far away at around 600mb.

Strong to locally severe storms continue to move SW across far southern Pinal and central Pima Counties during the evening.

It still appears that Phoenix will see a dust storm this evening.  Some consolation prize.

Day 2

Dry air advection continues over eastern Arizona, while marginal moisture  is in place from about Phoenix and westward.

CAPE is moderate in the area of higher moisture so there is a chance of storms during the afternoon.  While CAPE is low over much of eastern Arizona, it is sufficient to support some activity over there higher terrain.

The higher CAPE to the west of Phoenix must be capped as no thunderstorms develop.