Wednesday, September 30, 2009
Surface dewpoints and precipitable water have been increasing steadily for last day or so -
Tds into the low 50s out at Sells and Sasabe, and even at the airport, with PW amounts generally an inch or a bit more. The night brought some middle clouds and light breezes and increased moisture (note the hazy sky at sunrise in view above) - leading to a very mild low of 67F here at the house. The official low at the airport appears to have been 75F!
Looks like there may enough moisture for showers later today in at least far southeastern Arizona, as well as showers and thunderstorms with the S/W to the north. Unfortunately, the S/W in the westerlies and the subtropical disturbances to the south-southwest are a bit out of phase, or we would have been looking at a nice end of September precipitation event. Perhaps things will be in better sync by the weekend?
Tuesday, September 29, 2009
This morning a very large area of storms and disturbed weather continued from the south end of the GoC southward to 10 degrees North off the southwestern Mexican coast. There is now a very large circulation spinning away to the southwest of the end of Baja - very close examination of satellite loops indicates that there may well be two circulations embedded within the larger low pressure area. It is a very interesting situation and presumably the NHC will append some formal identification to the system(s) later this evening or tonight.
The issue of most immediate interest is how far north the push of subtropical moisture will come before the front from the northwest pushes through tomorrow. The satellite PW loops indicate that the moist subtropical air has now pushed up to about 30 degrees north - as indicated also per current Td readings of 79F at Guaymas (??) and 66F at Hermosillo. The models indicate that the moisture plume will push north mainly into New Mexico. However, with strong pressure falls to the north and the rapid northward progress of the moist air during the past 36 hours, it is possible that moisture could affect southeastern Arizona briefly tomorrow - it is an interesting situation worth watching.
Saturday, September 26, 2009
Thursday, September 24, 2009
In contrast to the dust down under, it is a crisp Fall morning here in Tucson. Temperature at house dropped to 54F this morniing for lowest minimum since sometime last Spring. This web cam view from the Computer Science building has not had a cloud in it since 5 pm last Monday afternoon!
There was a severe dust storm across northeastern Australia yesterday (23 September 2009). The satellite image above shows a huge, thick plume of dust curving around the edge of the continent and spreading south over the ocean. The dust storm was one of the worst in 70 years and even disrupted air travel into Sydney.
Tuesday, September 22, 2009
Sunday, September 20, 2009
Wednesday, September 16, 2009
The low that has been spinning away for a couple of days south of Baja has officially become Tropical Storm Marty this morning. The NHC predicts it to move slowly northward and have a rather brief life as a TS. It will apparently move north of 20N in the area where storms can trigger a Gulf Surge of low-level moisture. If Marty triggers a surge, it would most likely affect Arizona late Friday or Saturday. Thus, there is some possiblity that that the baroclinic cut-off that is sinking southward from the Great Basin into Arizona and Marty could interact, at least distantly. Given the weak character of Marty it is too early to guess exactly how things will evolve by the weekend - but at least there are a couple features of interest drifting around our part of the world.
Monday, September 14, 2009
The 6 am image above shows the Catalinas at sunrise this morning (Monday Sept. 14, 2009) and the mystery is: What are those low clouds doing there down on the mountain tops?
A look at the morning observations shows that there is southerly, moist flow over southeastern Arizona below 700 mb. This weak push of moisture has lead to an interesting situation. The thermodynamics could evolve today to produce a moderate amount of CAPE over much of the eastern third of the state. This would be happening in an environment that's atypical of the Southwest in summer. The morning sounding (which is a bit too wet) indicates a veering wind profile below 700 mb with moderately strong westerlies above - i.e., a wind profile more like the classic storms environment of the Plains, albeit weaker.
The 500 mb trough along the west coast has a strong negative tilt and is digging ashore all the way into northern Sonora. Ahead of this trough the upper-level flow is highly difluent, while the lower-level winds have become southerly. The NAM indicates that the southern end of the trough is going to swing rapidly across southern Arizona as it pulls inland and northeastward. This will bring subsidence and drying behind it.
Thus, there seem to be two possiblities today for southeastern Arizona - a rapid drying as the trough swings in before storms get going, or - storms occur in the east and northeast ahead of the trough. Storms that do occur would have some possiblity to be severe given the setting and a supercell or two would not that surprising.
Sunday turned out to be quite suppressed with only very isolated, terrain-locked storms - the exception being north in the Rim Country where there were some early and large storms. Only two stations in the Pima County ALERT gauge network mesaured 0.04" yesterday. The photo above shows a photogenic Cb north-northeast of the Catalinas at sunset last evening (Sunday 13 Sept. 09 about 6 pm). This storm was occurring on the west slopes of the Galiuro Mountains east of Mammoth, Arizona.
Sunday, September 13, 2009
Friday, September 11, 2009
Thursday, September 10, 2009
Yesterday was very suppressed over much of southeast arizona. Only 3 ALERT stations mesured precipitation and these were all up in the Catalina Mountains.
Pattern remains very complicated and with weak flow below 500 mb. Precipitable water continues a slow downward trend, as residual low-level moisture is gradually mixed away. CAPE today appears restricted to the mountains and also to New Mexico which has much more moisture and CAPE present and the potential for local heavy storms. Surface pressures continue to rise. At 500 mb a very extended trough lies within the broader anticyclone. This trough extends from southern California, across Arizona and then southwest new Mexico before dipping into south Texas, and then swinging northeastward across Arkansas and on eastward to the coast. Anticyclonic circulation centers are over: central California, central Baja, southern Wyoming, the Gulf of Mexico, and southeast Canada - what a mess!
The NAM forecasts that a very large, Rex-like block will develop over the US by Sunday afternoon (see above 500 mb forecast). The model continues to forecast showers over southeast Arizona for the next couple of days, but these will mostly be in the mountains. Here at the house I suspect that we'll not have much hope for rains unless another tropical system were to head north late next week, as the ECMWF seems to hint.
Wednesday, September 09, 2009
Tuesday, September 08, 2009
Remnants of the little orphan Cb less than an hour after photo below - very sad show.
This morning the Tucson sounding shows a troposphere that has very little flow - winds are essentially light and variable below 200 mb! There have been some subtle changes since yesterday:
Lapse rates are now very steep up to 600 mb; little surface heating is required to establish a very deep boundary layer; yesterday's hostile layer (see below) seems to have drifted away somewhere; precipitable water remains highest over south-central Arizona (note that we're in a cycle where the RRS sounding data are too moist in the morning and too dry in the evenings); there appears to be some CAPE present over low elevations, although best instability remains over the mountains; since there is little in the way of a defined steering flow, storms today will drift wherever the CAPE leads them.
About as exciting as the weather got yesterday (Labor Day 2009) - little orphan Cb north side of Catalina Mountains.
Was quite suppressed yesterday with storms able to form weakly only on the higher elevations. Apparently dry and warmer middle-level air advected in from the northeast between 700 and about 550 mb, limiting CAPE and increasing CIN and in general making the afternoon environment more hostile to convection.
Although the day-today changes were subtle, coverage of precipitation varied quite a bit day-to-day over the Holiday weekend. Rainfall coverage over Arizona 33 (Metro Tucson aka Eastern Pima County) was about: 20% on Friday; 30% on Saturday; 60% on Sunday; and only 13% on Monday.
Monday, September 07, 2009
Wet microburst over the Catalina Foothills yesterday evening produced by the small storm whose radar image is shown below - looking slightly east of north from house at about 6 pm MST.
There have only been slight changes in the overall situation since yesterday's discussion.
The weak 500 mb trough that is embedded within the larger 500 mb anticyclone has shifted slowly and now lies from northeast New Mexico southwestward across extreme northern Sonora, with quite a bit of residual cloudiness along it.
The shift of the trough has led to northeasterly winds in the layer from 500 mb to 300 mb -giving some hope of a defined steering flow, but the NAM forecast indicates weaking wind speeds by evening.
There seems to be a very slow down trend in precipitable water; dewpoints remain high across southern Arizona; and the lower half of troposphere continues to have low to moderate amounts of CAPE, but with a weak and chaotic wind regime holding forth. Thus, a somewhat similar day to yesterday should repeat with storms forming easily on mountains and then trying to propagate toward the south or southwest.
Sunday, September 06, 2009
I posted some photos on Facebook from yesterday's storms and also from Wednesday's storms. I think anyone should be able to look at them. Unfortunately the best photos eluded me.
As the air mass was drying from the west and early convection seemed to be capped, I thought that yesterday was going to be a down day in
I hopped in my car and drove east along I-8. When I got closer to the CBs they were spectacular: massive, with brilliantly lit hail streaks coming down their west side. The NWS said that they were topping out at 60,000 feet and radar was indicating 3" hail. I took some photos from the freeway exit at
An area of storms dissipating (note the translucent anvil cloud in center of the image above) over east-central Pima County has produced a nearly circular region of clear, subsiding outflow that covers almost the eastern half of the county. The dewpoint at Tucson has fallen to 48F at 2100 UTC. Will be interesting to see how this pronounced mesoscale feature plays out wrt convection during the rest of the day. (Note for perspective - Pima County covers approximately 30,000 square km.)
Saturday, September 05, 2009
No question today about whether storms will occur in the lower deserts - nice area of echoes currently northwest of Tucson - see vis image above. These are drifting a bit toward the south and southwest, reflecting the very chopped up pattern and its light winds in lower and mid-levels.
Moisture remains high in the lower half of troposphere and morning convection and heavy cloud is affecting much of Arizona, Utah and southern California. A weak middle-level trough will be moving slowly across Arizona today and tomorrow, and there appears to be nicely difluent flow at high levels with this feature. There also appears to be an inverted trough at 700 mb brushing westward along the Arizona borderlands. So, an active weekend on tap. Storms, given the small to moderate amounts of CAPE and light steering winds, will have potential to produce local heavy rains and perhaps wet microbursts.
Friday, September 04, 2009
Finally there was a somewhat decent rain event across southeast Arizona. All reporting stations and also RAWS sites in Pima, Santa Cruz, and Cochise Counties reported measurable rain with the exception of Douglas. Rainfall was also measured at 72 of the 93 Pima County ALERT gauges. So there was a quite high coverage of rainfall. Much greater coverage than I would have predicted (given the very weak wind fields present yesterday below 400 mb) and much greater than forecast officially. Nine stations in the Alert network reported a half inch or more - these were foothills and mountain stations, with the maximum amount being 0.91" at Empire Peak. The storms moved into town from the north, see above Computer Science web cam photo looking north at 6 pm. We had thunder, mostly in-cloud lightning and 0.24" of rain here at the house. Looks like, in addition to good mountain CAPE and some low-elevation CAPE, an upper-level jet nosed in from the Pacific to help provide a bit of large-scale vertical motion. There was perhaps a weak inverted trough across southeast Arizona but this feature was fairly stagnant - the sounding from Tucson was missing last evening so there's little hard observational data to examine.
Thursday, September 03, 2009
"Combined monsoon and Jimena fringe moisture contributed to very high precipitable water values over
Jimena has stalled and the remnant core may actually be moving a bit to the southeast over western Mexico. the NHC predicts the remnant low-level circulation to drift west back across Baja. Regardless, we will have to be satisfied with the nice increases in low-level moisture that have occurred over Arizona.
The overall large-scale pattern across all of soutwest North America is very chopped up, with numerous small features embedded here and there within the large 500 mb anticyclone. The Tucson sounding indicates a layer of easterly steering flow with upper-level winds reamaing westerly. There is some CAPE, which could be enhanced by local heating. Thus, it appears to me that things will be very driven by small scale details today. The NAM indicates a nice area of storms across southeast Arizona both today and tomorrow. I'm so beaten up that I've reached a state of "I'll believe it when I see it."