20190821

Initializations
Much of the southern part of the country is dominated by a large area of heights above 5920m.  One center is located south of Tucson over NW Mexico and into western NM while a weak trough is over west Texas and down into Chihuahua.  500mb temperatures continue to be around -5C but some cooler air is to the east of Arizona which may play an important role in our weather over the next few days. This air has to potential to advect into at least eastern Arizona and create steeper mid-level lapse rates.  Initializations look good.

Most Suominet sites are missing again.  The few that reported indicated the NAM had some significant wet errors in the southern Baja region.  The RR and GFS generally had minimal errors and are the favored initializations today.

Day 1
The tropics off the southwestern coast of Mexico have finally come alive after many weeks of almost no activity.  There is a tropical depression now and is forecast to move WNW and appears to be close enough to trigger a gulf surge. 

Moisture has increased over far southeastern Arizona, and by early afternoon, 850mb dewpoints are at or above 10C thus some activity is possible.  The rest of the state is bone dry.

I mentioned the slightly cooler 500mb air over New Mexico earlier, and the forecast is for some of that air to move into southeastern Arizona, resulting in good mid-level lapse rates.  With the moisture, there should be quite a bit of activity, but with weak steering, storms are most likely for the higher elevations.  It might be a good day for my new Paradise/Chiricahua cloud timelapse.

The combination does result in moderate to high CAPE in Cochise County. 

Thunderstorms get off to an early start and are scattered across Cochise County for much of the afternoon. 

Day 2
A gulf surge gets underway during the morning hours, and by early afternoon, PW has increased into the 30 to 40mm for the lower deserts.  This surge is relatively deep as 850mb Td are up to around 15C.

Unfortunately, the 500mb high is right over southeastern Arizona, resulting in light and variable steering flow.   It's still very warm too.

Wow, are there ever some impressive CAPE values! 12Z WRFRR is forecasting some areas approaching 4000 J./kg.

The top of the surge layer has a bit of an inversion on top, as expected, but at least in Tucson, it's not too strong.  With this amount of CAPE, the warm layer at 500mb isn't going to matter once convection initiates.  With the lack of steering, most storms will form and remain mainly over the higher terrain. 

Strong to severe storms develop during the mid-afternoon over the higher terrain of southeastern Arizona. Both the WRFNAM and WRFRR are similar, forecasting the strongest storms southwest of Tucson.

What about Phoenix?  The late afternoon Skew-T has a positive area only above the 500mb inversion.  Phoenix is deeply mixed, and perhaps with good thunderstorm outflow and a slight increase in PW, deep convection could develop.

The WRFRR storms get close but don't quite make it.  A lot of activity for Pinal County though, thus a big dust storm looks possible for Phoenix.