20210724

Previous Forecast

 It was another busy day and night for parts of the state.  Morning storms moved through eastern Phoenix, resulting in flash flooding.  Widespread storms were present throughout northern Arizona, and finally, a large area of thunderstorms moved out of NM into northeastern Arizona last night.  Fortunately, the precipitation over most of Gila county was minimal.

The model runs from yesterday were all quite good, even the WRFNAM.  Timings, locations, and amounts were all generally accurate.  The exception was around Phoenix, as some runs didn't have strong enough activity during the morning hours.

The HRRR also did well.

No mas!  Below is the 48 hour QPE and QPF from the model runs.  As is typical, the WRF underpredicted precipitation but generally got the right idea of when and where.  Even the WRFNAM did OK and was one of the better runs around Tucson.

Discussion

Areas of showers and a few thunderstorms are moving through south-central Arizona this morning.  These are the remnants of the squall line that moved out of NM into Arizona last evening.  The IT that is responsible for all this activity is somewhere near Albuquerque.  The forecast from 2-3 days ago had it over far SE Arizona by this time, so that didn't turn out.  There is an MCV located over the White Mountains and is moving slowly to the SW.   It's still quite cool at the center of the low, at -9C.  Arizona is under mainly unfavorable NW flow, and temperatures are a warm -5C.

At 300mb, the eastern part of the state continues to be under difluent/divergent flow, but it's considerably smaller/weaker than yesterday.

Both Phoenix and Tucson have a nearly saturated vertical profile and have 52 and 47mm of PW.  Phoenix has no CAPE to speak of, and Tucson has around 500 J/kg of MLCAPE.  Both have unidirectional NW flow, which is not typically favorable.  Based on the upper air data, little or no deep convection is expected for Phoenix and only limited amounts for Tucson.

Initializations

The NCEP model initializations have quite a challenge this morning.  Practically the whole state is covered by clouds, and southern Arizona has widespread showers and a few storms.  The 6Z GFS and 9 and 12Z RR did a reasonable job initializing all the activity.  The NAM was too active as the WRFNAM had thunderstorms over southern Arizona.  The MCV was more or less initialized by all initializations.  PW was initialized well by all, even the NAM.   There are large gaps in the GPSIPW sensor network, especially around Phoenix.  Perhaps some Phoenix agency should invest in a Suominet Suominet GPSIPW sensor?    It appears that the RR has the best initialization, but the WRFGFS should also perform well.

Day 1

Speaking of PW, it appears the 12Z RR is too wet in the Phoenix area as forecast PW is around 55-56mm.   Perhaps this discrepancy would have been identified and corrected if there was an operational sensor there.  In any case, PW remains very high into the afternoon as a weak surge continues to advect moisture into the state.

850mb dew points are still in the heavy precipitation category as they are generally 14-16C.

CAPE is minimal over the lower deserts but sufficient to support deep convection, once again, along the Rim to the White Mountains.  Satellite imagery indicates that skies are clearing in NE Arizona, so it looks favorable for storms in those areas today and tonight.

All the runs look different with regards to the area of precipitation moving through southern Arizona.  They all look a bit too active.  The least active is the 12Z WRFRR.

Moderate precipitation continues in southeastern Arizona into the afternoon hours.  Stronger storms form over northeastern Arizona in the more favorable heating/CAPE region.

By early evening, most runs have widespread storms over the high country of northern Arizona.  Steering moves is to the SW towards the Phoenix area.

Most runs manage to move activity into the Phoenix area during the evening hours.  However, it weakens as it moves into the lower elevations.

The 14Z run of the HRRR is similar.

There is a little CAPE available around Tucson, and some runs develop activity there in the early morning hours.

Precipitation amounts are again expected to be heavy for some areas.  The areas of concern look to be northern Arizona, mainly over the higher terrain, and for southeastern Arizona.  The WRFNAM is likely to be way too active.

Day 2

The IT finally moves a fair distance as it is located over western Arizona during the early afternoon, resulting in steering flow mainly from the southeast.  It's lost most of its cooler air by this time, and temperatures are again quite warm at around -6C.

The Surge has weakened, but impressive amounts of moisture continue over the state.

CAPE recovers a little over the lower elevations but is generally less than 1000 J/kg.

Both Tucson and Phoenix have a little CAPE, so the potential is there for a few storms.  The main threat will be heavy rain.  No organized activity is expected as the winds are mainly unidirectional with no low-level shear and upper support.