20180801

Previous Forecast

It was a down day for most of the state except for the higher elevations of the NW part of the state.  The model runs, for the most part, were accurate. The 12z WRFRR developed some strong storms along the border and the 15Z went even further and developed some deep convection over the Tucson area and both were completely wrong.  

Initializations

The long wave pattern remains mostly the same as the 500mb high center is located over southern Utah/northern Arizona while a deep trough, for this time of year, is located over the Mississippi River.  A broad weak IT is located over northern Baja along with a weak MCV over the far northern GofC. Both the NAM and RR have the MCV initialized too far south but this error shouldn’t matter. Skies are clear over all of the SW which was initialized correctly.   IPW was initialized much better by the NAM as most errors were small but there still appears to be an overall bias (below). Perhaps the email I sent to NCEP yesterday describing this problem helped? The RR PW was well initialized.

Day 1

PW has increased slightly across the state with lower elevations reporting mainly 35-42mm.  However, Yuma is up to 50mm as a surge continues. It’s weak but enough to keep it wet over much of the state today/tonight.

CAPE is higher today and is very high at some locations like Tucson and Phoenix. Both the morning Tucson and Phoenix soundings have 1300-1500 J/kg of MLCAPE so this high forecast Max CAPE seems realistic.  Again, it’s more stable over far SE Arizona as low-level SE flow is again causing drying and stabilization, so storms are likely to struggle there.

The 500mb anticyclone is located over northern Arizona which provides only light E to SE flow over the southern ½ of the state, so storms won’t move much but at least it’s easterly.

It’s quite active over the high terrain during the afternoon and a few storms over the higher terrain west of Tucson.  As CAPE is low in Cochise county, there isn’t much activity there (none in the WRFRR).

By early evening storms have moved off of the higher terrain into SE Arizona.  Both the WRFRR and WRFNAM are similar so there is some confidence in this forecast.

The mixed layer looks a bit strange as there seems to be a fairly shallow surface-based mixed layer and an elevated layer.  In any case, mid-level steering is around 10-15 knots so is sufficient in moving storms from the east into the Tucson area this evening.  Upper winds are northerly so as long as there isn’t too much activity to the north, anvil shading shouldn’t be a problem. CAPE is sufficient to support storms as it’s forecast to be around 900 J/kg.  

Both the WRFNAM and WRFRR have scattered storms in and near Tucson this evening.  Other storms are north and east of Phoenix.

Based on the forecast skew-t, I’d say deep convection in the Phoenix valley is unlikely.  The inversion at the top of the PBL is strong plus the PBL isn’t very deeply mixed.

Both runs eject outflows from the nearby deep convection into the valley during the evening.  WRFRR outflows are quite strong but are probably insufficient to trigger deep convection. It’s not impossible, though!

Some activity continues in Pinal and west of Tucson into the evening.

The 15Z WRFRR just became available and it does develop a few storms in the Phoenix area as strong outflows move across the area this evening.

Day 2

Moisture generally increases over the state, including southeastern Arizona, as low-level winds shift back to westerly and a weak surge continues to advect moisture into the state.  It’s depressing that we’ve had such high moisture values but so little to show for it over most of the lower elevations.

The 500mb high center is near the Flagstaff area keeping southern Arizona in the light easterly flow.

CAPE is again moderate to high over the lower elevations but pretty low over far eastern Arizona.  Of interest is the low-level moist and unstable air moving northward up the valleys of NE Sonora into Cochise county.

The WRFNAM and WRFRR have significantly different forecasts for SE Arizona as the WRFNAM is much more active during the late afternoon.

The WRFRR becomes quite active over SE/SC AZ during the evening hours instead, so it looks like the Tucson area is likely to see activity at some point.

It’s the same story around the Phoenix area as the PBL is not well mixed but at least there doesn’t seem to be an inversion present thus a strong outflow or two may be enough to trigger deep convection.  

The WRFNAM does manage to fire up a few storms during the evening as a strong outflow boundary moves into Phoenix from the south.  There is a chance of a dust storm and/or a haboob.