Thursday, August 21, 2008

Response to AJ-NWS

Response to AJ of NWS - see AJ's comment on the previous post.

AJ makes a very good point here and one that serves to emphasize how difficult the forecast challenge is for the coming 84 hours. The three principal models each have different solutions, and we have little in the way of observational data to help evaluate the model initializations, since the key features analyzed by the models are within a great data void.

First, if one looks at the NAM, the GFS, and the ECMWF model runs from this morning – which I hadn’t when I wrote the original blurb – it is clear that each of the three models has somewhat different solutions for the initial conditions.

As per at 500mb:

The GFS analyzes an IT from south of GoC into north central MX where it connects with a trailing trough from the S/W over the central Missouri River Basin.

The ECMWF has this feature analyzed further north, extending from the south central GoC to SE Kansas.

The NAM is similar to the ECMWF except that it has several additional vorticity maxima further south and southeast which neither of the global models have.

We do know that such a feature is present and that at 12Z this morning it extended from Nebraska south-southwestward entering Mexico in the western Big Bend country.

Each model goes it own way in the forecast fields. The most important issue relates to whether or not there is a feature in the actual atmosphere that is going to spinup into a tropical system off Baja. Only the NAM indicates this evolution. The NAM solution would result in a very deep intrusion of subtropical moisture into the Southwest. At this time we mostly know that there has been ongoing convection in the regions where the NAM forecasts tropical development.

Both the GFS and ECMWF strengthen the inverted trough over north central Mexico and forecast this as the main feature that affects southern Arizona on Saturday and Sunday. The GFS stalls this feature over north central Mexico, while the ECMWF is more aggressive and keeps it further north and moving westward. The wind shear profile forecast on Sunday by the GFS is not nearly as favorable for organized storms (assuming that the key ingredients of moisture and CAPE have moved/developed north of the border). The ECMWF is probably in between the two other models wrt wind shear.

Current IR and water vapor satellite imagery indicate that the feature over north central Mexico has a clear circulation now and that it is moving westward. Significant convection is developing ahead of this feature over the Sierra Madre Occidental.

So where does all this lead? The forecast situation appears even more complicated than what I originally wrote. The most likely scenario for the next 84 hours is not at all clear at this time, and the evolving situation merits careful monitoring.

Challenging Forecast for the Weekend - Aug 23 and 24 2008

It appears that the large-scale setting here in the Southwest will be changing rapidly during the coming three days as very warm air in middle levels pushes into the Great Basin. Note the very warm 500 mb temperatures of only -4C at Medford OR and -3C at Desert Rock NV this morning. This warm intrusion builds the anticyclone back to our north and helps setup a more typical late summer flow regime.

Pressures have fallen and are quite low over the interior Southwest and may drop another couple of mb over the next 36 hours. There is also a weak inverted trough over the lower end of the GoC that has widespread convection associated with it. As this first disturbance shifts westward, there should be a low-level surge of moisture back into the lower Colorado River Basin. This moisture surge could begin as early as morning tomorrow but should be quite pronounced by mid-day Saturday.

The NAM forecasts that a tropical storm will develop just southwest of the tip of Baja and this feature, if it materializes, would deepen the moisture inflow to Arizona on Sunday and Monday. The NAM also forecasts very favorable wind shear profiles for organized convective storms in southeastern AZ both on Saturday and Sunday afternoons. Thus, both days could be very active if there is CAPE available.

It appears that the crucial aspect of the weekend synoptic setting will the temperatures in the middle levels. If the very warm air to the north and northwest spreads over southern Arizona and northern Mexico there would be little CAPE to realize, and it would likely be fighting an inversion aloft. However, if the easterly flow that develops at 500 mb has a fetch from the central and southern Plains by Saturday afternoon, the 500 mb temperatures would be much cooler and supportive of storms.

I think that the more favorable situation will evolve and, depending on the details, either Saturday or Sunday afternoon and evening should be quite active with strong and organized storms.

Saturday, August 16, 2008

Slide shows re Severe Storm of 13 August 2008

Still bringing the new PC online, so none of the photos I took of damage produced by the early nigttime storm of Wednesday August 13th have been posted in the gallery. However, each of the Tucson newspapers have slide shows attached to their online articles about the storm.

Thursday, August 07, 2008

Computer Problems

I have been struggling with a failing PC here at the house and have been just able to get online every once in awhile. Thus, everything I usually do is to some extent on hold. We are going to replace this PC, but I may be out of touch for much of the next week or two, until a new computer is up and functional here. Bob

"When it rains it usually pours, but sometimes it just spits."

Rex Block
Private Investigator
Forensic Meteorologist

Sunday, August 03, 2008

Sunday Morning Postmortem

Well my guess for yesterday afternoon was about as bad as I can do. Obviously the negatives dominated the local area around Tucson - only 7 of the 91 Alert gauges had rain yesterday, and the highest amount was 0.59 way down south at Tubac.

So what went wrong - I think that it was mainly related to the dry air in the upper-troposphere curling across southeast Arizona and keeping there from being much activity to the east - as per the WRF forecast. Although there were more storms to the east than the WRF indicated, they were not widespread and didn't produce any significant, organized outflows. With the GoC surge and increasing low-level moisture, the Tucson sounding at 00Z yesterday afternoon had substantial CAPE, but needed mesoscale convergence to kick it loose. That didn't happen.

As for today - moisture is way up as expected and there is substantial CAPE at both TUS and PHX this morning. El Paso is fairly stable so we have to assume that the air over the southeastern mountains is somewhat like the Tucson sounding. Morning wind shear from middle to upper levels is unidirectional but the NAM indicates that the wind profiles may improve by evening. Mainly a shear zone affecting southeast Arizona today and with no data from the south, it is hard to have much confidence in the model predicted winds aloft. So, there will be a lot of small and mesoscale interactions driving the evolution of storms today - good potential for more severe across all of southern Arizona and Phoenix could get hit by a strong, severe storm event. The chances are probably better for here at house today, but, even though it's not a binary call like yesterday, the shear profile could throw anvils over this part of town. So it's wait and see, as per many to most storm days down here.

The other wild card at play here is that there is a lot of strong morning storm activity out to the west of Tucson. If these storms continue to build produce early, cool outflows, that would complicate things greatly. I got a couple photos as sunrise showers moved by, producing an early morning rainbow. I'll post these to the blog later today.

Saturday, August 02, 2008

Storm Outlook for Today

Visually spectacular storms yesterday evening - if anyone has photos for pasting on the blog, let me know.

Hot boundary layer temps and pocket of cooler 500 mb air, plus slight increase in moisture allowed storms, particularly at higher elevations, to overcome the strong inversion near 400 mb. Rain reported at 23 of the 91 Alert gauges in eastern Pima County - again mostly in mountains and foothills - the 1.50" at Agua Calienta Park seems an outlier, but there was spectacular lightning off that way. One wind damage report out at Pinal Airpark. Afternoon cells did not seem to have much in the way of precip shafts but then things obviously changed before dark.

Here at house - lightning, thunder and brief spit of rain shortly after 9 pm.

Situation today is very difficult with a number of competing plus and minuses.

Negatives first:

Very dry air to east in NM

Morning soundings continue with marginal IPW and boundary layer moisture.

Best difluence ahead of approaching upper-level, inverted trough appears to remain south of border.

Upper-level winds weak as we remain near the center of upper anticyclone.

Cool pocket at 500 mb has moved on to our west and northwest.


Nice push of subtropical moisture occurring into lower Colorado Basin, with some signature in PHX am sounding and Yuma VAD showing the influx to be about 4,000 ft deep. GPS IPW shows nice increases at the stations around the northern GoC.

Outflows from MCS over Mexico last night have pushed Td up across all of southern AZ and helped the GoC surge along.

Tropical low spinning off Baja in good location to keep low-level moisture flow going for a couple of days.

Very nice steering winds for storms that will tend to leave anvils to rear of strong cells.

Approaching inverted trough that extends down to 700 mb.

Summary - Looks good for strong storms with likely tropical squall line organization and somesevere storms and heavier rains, but today or tomorrow? I think things will come together today better than one might expect and that afternoon and evening should have strong storms - tomorrow will of course be highly affected by what does or does not happen during the next 12 hours. Chance of rain here at house probably at least 50% this evening.

Friday, August 01, 2008

Art Douglas Photos

Art has sent some digital photos from southeast Arizona - four of these have been posted on the Blog Gallery of weather images at

Art's photos can be viewed directly at