Arizona WRF Discussion: 20180708

Previous Forecast

It was quite active over NW, EC, and central/western Pima County. Tucson saw a few storms around 5-6pm too. The WRFNAM was the best run except it was not active enough over NW Arizona and not strong enough in western Pima County. NSSL MRMS precip below.

Initializations

The 500mb pattern continues to be mostly unchanged except heights have decreased to 5960m at the center. The mid/upper trough is quite weak but can be seen at 300mb stretching from NE NM to SW NM along with a weak upper jet nosing into SE Arizona. 300mb plot indicates there is quite a bit of divergence at the nose of the jet in SE Arizona and into Mexico. All upper air initializations look good.

It has cleared out over much of Arizona during the early morning hours except for western Arizona. There was some question yesterday if it would clear up as good insolation is required for an active convective day. The WRFRR has quite a serious error as it initialized the light showers in SW Arizona much too strong and has an area of deep convection there instead. This may cause the atmosphere to be too stable from Phoenix to Yuma later. The 6Z GFS and 12Z NAM are fine.

There has been trouble with the Suominet GPSIPW network and only a few sites were available overnight and it does look like a few more are available at 12Z. The RR initialized a little too dry over southern NM but was good elsewhere. The 6Z GFS was quite a bit too dry at the one reporting station in southern Arizona so this run might be a little suspect. The 12Z NAM initialized well. Overall, the 6Z WRFGFS and 12Z WRFNAM look the best.

Day 1

Moisture continues to increase over western and central Arizona due to weak flow from the Gulf of California and by midday, 40-45mm is present over the lower deserts. Easterly flow continues over far eastern Arizona. The easterly flow is quite weak now but it may continue to suppress convection there. The western boundary of the easterly flow could be a focal point for convective initiation today in far eastern Pima up to Gila County.

CAPE is moderate to high over most of the state and the best for supporting deep convection so far this season. Both the morning soundings from Tucson and Phoenix have some CAPE present along with significant DCAPE thus the threat for strong winds is high with storms. With increased moisture, very heavy rain is also becoming more of a threat too.

In spite of the overall synoptic situation being straightforward, the various model runs are all quite a bit different making it difficult to say exactly what will happen today. I have a problem with the WRFNAM (below) developing severe storms just south in Sonora so early. As seen above, CAPE is forecast to be very high there which may not be the case. Elsewhere, both the WRFRR and WRFNAM have storms getting a late start and I would of thought deep convection would initiate quicker today.

Tucson has from 500-800 J/kg of CAPE this afternoon which while not all that high, is sufficient to support storms. The wind profile is excellent with light low-level westerlies and ENE mid-level steering flow. Lighter easterlies aloft will prevent anvils from blowing out ahead of the storms.

The 6Z WRFGFS (below) and WRFNAM are much more active with very strong to severe storms moving SW towards the lower elevations of Maricopa and Pinal counties. CAPE is a bit higher in this run than the 12Z runs so that seems responsible. Looking at the few IPW stations that are available, all runs appear to have accurate IPW forecasts as of 15Z, so I don’t really know why the runs are so different. I would have expected a more active day like the 6z WRFGFS/NAM but maybe not. How’s that for a forecast!

The Phoenix late afternoon vertical profile has some CAPE and an excellent vertical wind profile. The problem is the PBL isn’t mixed deep enough and will take significant lifting to get it to convect.

By late afternoon/early evening, the 12Z WRFNAM (and WRFRR) have only a few areas of strong storms with some in and around Tucson.

And now for something completely different. The 6Z WRFNAM and WRFGFS (below) move very strong and some severe storms into central Arizona by late afternoon on steering flow around 30 knots. Typically, central Arizona lower elevation storms come in later in the evenings so this is quite unusual to see it so early. These storms continue rapidly across SW Arizona through the evening. The 12Z WRFGFS just became available and it is similar to the other 12Z runs, so perhaps the very active 6Z runs are less likely.

The 12Z runs do eventually move strong storms into the Phoenix area by later in the evening. So, it looks like storms at some point for much of the lower elevations of central and south-central Arizona but not nearly as strong as the 6Z runs.

The 12Z WRFGFS (below), like the other 12Z runs, moves convection into the Phoenix area during the late evening. Storms also threaten the Tucson area too.

Day 2

The slow-moving NM IT finally starts to get moving and progresses across southern Arizona tomorrow. 500 mb temperatures are around -8 C so lapse rates will be quite steep and winds will be very favorable for steering higher elevation storms into the lower deserts.

Upper air support is excellent with a large area of divergence/difluence over much of the state.

It is wet just about everywhere across Arizona and New Mexico. Weak easterly flow continues across SW NM and into SE AZ while weak flow from the Gulf of California continues to advect moisture into western Arizona.

CAPE is moderate to high over much of the state (and NM too) so the potential is there for a very active day.

There is no problem getting the storms going as long as it’s mostly clear. Very strong storms start early and by noon, widespread storms are underway over much of SE Arizona and into NM.

It looks like a round of strong/severe storms for Tucson by early afternoon. CAPE is excellent and it won’t take too much heating/lifting to get deep convection going. Also, some dynamics at play as there is a upper jet present. However, the negative is that anvils may blow over early and reduce heating.

Lines of strong to severe storms move rapidly to the west-southwest during the early afternoon across much of eastern and southeastern Arizona.

Strong to severe storms move into the lower elevations around Phoenix by late afternoon.

CAPE isn’t all that impressive at Phoenix tomorrow afternoon but should be enough to support deep convection. The PBL is mixed deeply and the wind profile is excellent.