The large cutoff low at 500 mb that has developed off of southern California (see top water vapor image) has pulled more moist, low-level air into southwestern Arizona and southern California during the past 48-hours. Precipitable water amounts over southwestern Arizona range from around an inch at Tohono O'odham CC to an inch and a half out at Yuma. The San Diego sounding this morning (middle image) indicates some elevated CAPE (although PW is 6 mm too wet when compared to GPS observations). The radar data from Yuma this morning are detecting some rainshowers from around Ajo westward across extreme southern California. So, a bit of weather in the Southwest to keep an eye on. In the longer term the ECMWF is very aggressive in digging a strong trough into Arizona at 6 to 7 days - the ECMWF has been very much more accurate than the NWS GFS model in its predictions of digging troughs along the west coast the last several winters.
There were a couple of very isolated, light showers in far southeast Arizona yesterday and 0ne scrawny Cb was visible to the southeast of Tucson at sunset. This morning, a backdoor front from the southern Plains has moved into southeast Arizona. Depoints remain around 50 to 60F right along the border. The morning (12Z Sunday September 26th) 500 mb analysis (top) shows that a well-defined, inverted trough is poised to brush westward across southern Arizona today and that 500 mb temperatures have cooled to around -8 to -9C in the deep easterly flow associated with this feature. The morning sounding at Tucson (middle) indicates little CAPE at low elevations. So, with the downslope winds and not much instability, expect at most some isolated buildups along the Borderlands.
Very nice morning with a cool low in the upper 50s here at house. The deep mT moisture has been pushed south again to around Guaymas. However, the model forecasts are interesting from Sunday afternoon through Tues-Wed next week. The NAM 500 mb forecast valid 5 pm Sunday afternoon is shown in top panel and the ECMWF four-panel forecast valid 5 pm Tuesday afternoon is shown at bottom. Both models have a substantial inverted trough/closed cyclone moving westward across western Mexico and brushing along the Borderlands. The models are basically dry, but it's hard not to speculate that such a pattern, if it materializes, will bring moisture and showers back north. The pattern is complicated by what would be a significant period of easterly winds with a backdoor front associated with the strong central US system - but, of course, all this too far out to worry about details yet.
Cyrus Jones (Dept. of Atmospheric Sciences) has sent along time series from the Atmo weather station for the rain burst - produced by warm-rain, tropical-like showers over lunch hour on Wednesday, September 22nd. The intense rain rate is shown in the top series; the trace for T and Td is shown in middle; and the SLP trace is shown at bottom. The amounts along the track of the rain band, plus the short duration of the rains, indicates rain rates of 2.5 +/- approx. 0.25 inches/hour. There was brief pressure spike with the event, and winds were gusting over 40 mph at the roof-top anemometer. This was a quite interesting and most unusual event for here.
First - NHC kept providing a very strange forecast for Georgette late into the day and early night yesterday - the top image is what one found at NHC at 6 pm last evening. They seemed to ignore the strong trough that was sweeping through the Southwest. The trough quickly sucked up the remnants of Georgette and related heavy rains are now off to the east in New Mexico and Texas Panhandle.
The IR image (top) shows that Georgette is over the south-central GoC this morning and that a large plume of cloudiness extends northward into southwestern Colorado. The morning sounding from Tucson (middle) indicates deep moisture and mostly southerly flow. The sounding has more than an inch and three quarters of precipitable water, but very slight CAPE. Thus, unless we would get some heating (which doesn't appear likely until drier air sweeps in from west), rainfall at low elevations will be dependent mostly upon vertical motion ahead of the Pacfic trough that's approaching. The trough approaches Arizona too quickly to allow Georgette remanants north into the state, and shunts the vorticity maximum of the old TS over toward El Paso by tomorrow morning (bottom panel is 500 mb NAM forecast from this morning for midnight tonight). It appears that forcing for vertical motion and heavy rainfall amounts will continue to focus over the RIM country northeast of Phoenix. The midnight run of the Atmo WRF-GFS does forecast a heavy swath of rainfall to extend from near sasabe north toward Payson - but this is hard to evaluate because of the "too wet" nature of that forecast run.
So far, the big intrusion of mT air from south has not led to much precipitation, except probably in the Rim Country northeast of Phoenix. Here at house we had 0.20" during the night - this turns out to be one of the higher amounts at low elevations in the Tucson metro area. The ALERT network data indicate 32 of 93 stations with rainfall during past 24-hours, or about 34% areal coverage, (9 stations had had rainfall at 5 pm yesterday afternoon). Only one amount was greater than half an inch - Florida Canyon had 1.06" (with 1.02" of that in a downpour yesterday afternoon), which is far and away the highest amount I can find reported in a gauge early this morning. The RAWS observations indicated that 13 of the 15 working stations in southeast Arizona had measurable rainfall yesterday but amounts were light. The RAWS high elevation observations do indicate that, as radar loops tended to show, most afternoon storms yesterday tended to stay on the mountains.
Top image is a heavy storm south of Kitt Peak around 4 pm today (21 Setember 2010), while middle image shows a storm building over the Catalinas about the same time.
How much rainfall many locations, such as here at the house, in Arizona have for September will depend on how things evolve between Tropical Storm Georgette and the approaching Pacific trough. Georgette (top IR image) was spinning away yesterday, and was officially named early this morning. She is forecast to move north up the GoC while weakening into a remnant.
Yesterday (Monday 20 September 2010) thunderstorms stayed mostly on higher terraing from the southwest to south to northeast of the Tucson metro area. Nogales had a strong outflow from the south and then thunderstorms with 0.32". Two stations in the ALERT network had rain down toward Nogales, with Tubac reporting 0.67".
Very nice sunrise this morning (top) with light showers over the eastern Catalinas - couple of stations up there have had one to two tenths of an inch of rain this morning. Focus in this post will be on today - although, both the Atmo WRF runs and NAM indicate a substantial rainfall event by Wednesday evening - the WRF is more widespread in its forecast; whereas, the NAM focus is on the northern mountains of Arizona.
Edited to add additional info on the WRF forecast from Mike Leuthold.
This post is a continuation of the previous one.
Low-level moisture is slowly on the increase over southeastern Arizona, as forecast by yesterday's Atmo runs of the WRF model. The top image shows this morning's Tucson sounding with PW of almost an inch, and perhaps even a tiny bit of CAPE by afternoon. Winds are easterly from the surface to above 500 mb, with some speeds near 20 kt. Thus, a nice vertical wind- shear profile.
It is extremely dry and clear over Arizona this morning - see top visible satellite image from this morning. But there is a Pacific trough off to the northwest and a large, subtropical inverted trough (to the far southeast) to the north of what was Hurricane Karl. These two features will clash a bit over southeast Arizona tomorrow afternoon and Monday (September 19th and 20th).
Hurricane Karl intensified to a Cat 3 storm and is apparently going ashore near Vera Cruz, Mexico at this time. Karl seems to have turned toward the southwest and it remains to be seen how and where its remnants will come out into the Pacific. It's not clear whether the remnants will be far enough north to trigger a moisture push up the GoC. We'll just have to watch.
Not much changing. Last three days Tucson has had a high temperature of 100F. Low level moisture has increased very slightly and lows have stayed up in 60s here at house. This morning's (Thursday September 16) Tucson sounding has come in with data that are too moist by 4 to 5 mm when compared to the GPS PW. The data plot seems to indicate that the humidity sensor was responding slowly to changing RH and damping down the actual trends.