Sunday, July 23, 2017

Brief Overview - Yesterday's Heavy Storms

Outflows from storms to south and line of storms moving into Catalinas from the northeast, collided over or near the mountains, and new storms were developing by 5:00 pm MST - above. By 7:30 pm mountains were becoming visible as serious street flooding was affecting north-central Tucson. We were on the east side near Kolb and 22nd and headed home about 6:50 pm - this is about a 25 minute drive usually. We were stranded at the intersection of Tucson and Glenn about 7:30 pm, when I took the photo below. Tucson was flowing as if it were a wash and cars were stalled in all directions in the high water. It took us more than an hour and a half before we zigged back and forth to avoid the worst flooding and got home. Quite a trip.

The map below shows ALERT network rainfall for 24-hours ending at 6:00 am this morning - essentially 100% coverage again - with three sites along west side of network having 0.04" or less. There were 13 sites reporting more than an inch. Here at the house I found 1.40" in the gauge once we got back.

This gives us 5.46" for July - second highest July amount since we've been here. Wettest July was in 1999 with 6.63".

Found three RAWS sites in southeast Arizona with more than an inch and also noted gusts to 68 mph up at Prescott and 52 mph at Safford. Quite an afternoon and evening!

Saturday, July 22, 2017

Reduced Activity Yesterday - Pattern Remains Stagnant

Storm developed along the foothills north of here a bit before 7:00 pm MST (above), but could not come into this part of town. The storm seemed to shift southeastward and dissipate. Here at house we had thunder and a couple of spits of rain.

Plot below is of detected CG flashes for 24-hours ending at 1:00 am today, and shows considerably less activity over eastern Pima County than occurred yesterday (from Atmo and Vaisala).

These two plots show 24-hour precipitation ending at 7:00 am this morning. Basically a metro donut hole, but with storms coming off the Rincons and developing into a line that moved southward away from the City. Plot above is from ALERT network and plot below is from MesoWest.

The 12 UTC 500 mb analysis above is from NCAR and shows several things of interest: western lobe of anticyclone is again over the Great Basin; height gradients over much of western and central U.S. are extremely weak; I count five inverted troughs from Louisiana to southern California; Huge data void remains over all of northern half of Mexico. The troughs are very weak and moving slowly, if at all. Their main role seems to be in determining local steering flows for storms that do develop.

The TWC sounding (below) this morning is little changed from yesterday and thus it looks like another wait and watch day.

I did look briefly at the 06 UTC forecasts from the WRF model run at Atmo. Both versions had a significant area of storms forecast to our northeast around sunrise and so I didn't look further. Perhaps the 12 UTC runs will start off better - check the new runs and Mike"s morning discussion.

Friday, July 21, 2017

Widespread Storms And Rain Yesterday Evening

Kitt Peak view to the north at 4:41 pm MST yesterday afternoon. View from campus to the north at 5:00 pm shows rain shower over the mountains with its outflow apparently producing  a nearly circular arc of new updraft clouds. The storm moved into the foothills producing heavy rains and there were several more redevelopments as the outflows spread south across the metro area. Only 0.24" here at house with this event.

ALERT network precipitation through 6:30 am this morning (below) shows 100% over the metro area with 11 sites measuring over an inch. Quite some event given the light and variable wind profile that prevailed yesterday.

This morning's sounding plot (above) continues to show light winds aloft but with a more systematic direction profile backing from north-northeast at 700 mb to westerly at 200 mb. Moisture and CAPE continue high, but some drying is occurring in upper troposphere. The visible image below seems to indicate sunshine this morning. The 500 analysis for 12 UTC at bottom indicates weak IT and cyclone at 500 mb approaching slowly from the east. Storms again on mountains today but with  light steering toward the south. Perhaps a repeat, but we'll have to watch how things evolve today.

Thursday, July 20, 2017

Uncertain Day Today - Downburst Near University Yesterday

Edited to add: Art Rangno reported 3.36 inches of rain yesterday at his place in Catalina. His approximate location is shown in map below.

Plot above shows detected CG flashes for past 24-hours ending at 6:00 am MST - from Atmo and Vaisala. Cluster of green flashes Tucson area was with the down burst storm that hit university area a bit before my afternoon post yesterday. Note the time bar that indicates the mid-afternoon storms were all that impacted metro area. Widespread rains with heaviest near downtown and in the Catalinas ALERT below is for 24-hours ending at 7:00 am. Quite a few reports of trees down with this storm - reports for yesterday from SPC is second below.

The upper-level IT that was near El Paso yesterday has elongated northwestward and is being pulled apart as it circles around the central US anticyclone - water vapor image above from 5:15 am this morning.

The morning TWC sounding (above) remains wet and unstable, particularly for the mountains. However, winds through the troposphere are essentially light and variable. Mountain storms may drift anywhere - following outflows and most unstable air. Widespread debris cloudiness over Arizona and northern Mexico at 6:45 am (visible image below) means uncertain heating. This seems to be a day where we just sit and watch what evolves.

Wednesday, July 19, 2017

Moderate To Heavy Storms Across Parts of Metro

Moderate to heavy thunderstorms moved across portions of the metro area during the past two hours or so. View from campus shows the situation  at about 3:00 pm MST above and below at about 3:30 pm.

ALERT rainfall for past three hours, ending at 4:40, pm is shown below - rainfall occurred over most of metro area, except for  the east and south sides. There were three amounts over an inch (plus Atmo which had 1.35" and gusts to 45 mph) and a site with over two inches in the Catalinas. We had 0.34" here with gusts of about 30 to 40 mph. Nice storm and I sat out on porch and enjoyed the cool air and moderate rain.

Mike L asked in his discussion about impacts of upper-tropospheric cyclone/inverted troughs such as the one near El Paso around 1:00 pm today (as per water vapor image above). That's a tough question. When such systems move westward across northern Mexico large MCSs often occur - these can trigger GoM surges if conditions are dry across southern Arizona or cause increased low-level moisture advection into the state, along with increased storm activity.

However, the current IT is blocked by the intensifying 250 mb anticyclone over Sonora and is heading toward the Four Corners. It appears that the dry air ahead of the circulation will basically shift north-northwestward, impacting mostly New Mexico and northeastern Arizona. As this happens it appears that upper difluence may weaken - compared to today - and perhaps even become confluent flow. 

My basic feel is that storms may be more isolated tomorrow, BUT that question will have to await the rest of today's weather and the morning observations.

More Of Same?

Heavy thunderstorm over the Catalinas early yesterday afternoon shown above from campus at a bit before 2:00 pm MST. The anvil from this storm spread out over much of metro area, suppressing low-elevation storms. There was a nice display of mammatus overhead as this happened (below). This storm may have produced thunder here at house but I didn't hear any; if there were some spits of rain, I also missed those - so zip for rainfall here.

The plot above shows detected CG flashes for 24-hours ending at 6:00 am this morning. Note the elongated suppressed zone from metro Tucson to Phoenix - with storms staying mostly over higher terrain. The ALERT rainfall (below), for same period, shows that the NWS mountain forecast zone was very active, while reverse was true for the low-elevation zone, which was basically skunked. The storm shown above produced some rain amounts of over 2 inches.

The morning sounding data from TWC remains similar to last several days, except that winds above 700 mb have increased substantially relative to yesterday. The surface cool layer is also deeper, which damps the SPC CAPE analysis above. The winds are nearly uni-directional from the east-southeast with little speed shear through 150 mb. This gives some hope that storms my move faster than the anvils, providing more favorable conditions for lower elevations. The 11:00 am forecast TWC sounding (below - from the 06 UTC run of the WRF-GFS at Atmo) indicates decreasing wind speeds at upper-levels, which would be good, if that actually happens. Mixed layer CAPE in the forecast sounding is over 1500 J/kg.

The WRF-GFS initiates storms very early, and composite radar forecast below is valid at 1:00 pm. These storms run off rapidly to the northwest impacting Pinal County and the Phoenix area later in afternoon. The shear profile is better than yesterday, indicating the chance for organized storms and severe winds is higher today. Finally, the WRF-NAM forecast is similar, although winds above 500 mb bring in drier air and storm developments are about three hours later than in the GFS version. Regardless, looks like today could be considerably more interesting than yesterday.

Tuesday, July 18, 2017

Soggy Pattern Continues

Above view is of Thimble Rock a bit before sunrise.

Above is 24-hour CG flash density ending at 6:15 am MST this morning. Active storm region shifted northward some, although there was much thunder here at house during early afternoon. A very light shower produced only 0.03".

Alert rainfall below for 24-hours ending at 6:30 am indicates heaviest amounts mostly over the mountains and little for metro area, except for the nearly stationary storm west of downtown. This storm produced 2.48" at one ALERT site, producing flooding and several rescue situations.

This morning's TWC sounding is similar to yesterday's (skewT plot below) with almost 2 inches of PW and substantial CAPE, but winds remain very light below 300 mb. Strongest winds are above 200 mb and from the northeast, and these will likely produce considerable anvil shading for parts of the metro as storms develop again very early in the day. Threat continues for local heavy rains, wet microbursts, and some flooding due to slow moving storms.

Monday, July 17, 2017

Very Active Period Continues

News was just breaking nationally yesterday evening: a flash flood produced by storms along the Rim swept away a family late Saturday afternoon. The family was swimming at a spot along the East Verde River near Payson when the flood swept them away, apparently resulting in 10 deaths.

Finger Rock peeking above stratus fractus this morning.

Storms did affect most of the metro area late yesterday, after dark and early am. Lightning and thunder here produced only 0.16", and amounts across the ALERT network were generally light over the City. There were 16 sites with no precip or less than 0.04" - these were across the east-central metro into the Rincons. Four ALERT stations had more than an inch of rain.

The heavy-hitting action was in the Phoenix metro with many severe thunderstorms and the first (I think) signifcant rain of the summer. PHX had 0.41", along with gusts to 62 mph. Detected CG flashes for period ending at 6:00 am MST shown below (from Atmo and Vaisala).

The morning sounding from TWC (plot above) is very moist with little CAPE and weak winds. The visible satellite image below is from 8:00 am MST. A mesoscale convective vortex has really helped spin up a circulation (off to our west-northwest) in the first inverted trough that moved by yesterday. It appears that we have clearing from the east and will get some significant sunshine.

Took a quick look at the 06 UTC WRF-GFS forecasts from Atmo. The model did quite well with the early morning activity and the sounding forecast for 12 UTC was quite good. By mid-afternoon the model forecasts significant storms again in eastern Pima County - composite radar forecast above is valid at 4:00 pm.

Rainfall amounts (below through midnight tonight) are quite large, with forecasts below indicating heavy rains from Santa Cruz County north to eastern Maricopa County. I noted that the forecast TWC sounding for 3:00 pm indicated that easterly steering winds had returned and that mixed-layer CAPE exceeded 2000 J/kg. Quite a forecast recovery. The model did get the 500 mb circulation this morning fairly accurately. So it may be on track again. With the thermodynamics that are in place, there would continue to be a threat for severe, wet microbursts.

Finally, the eastern Pacific continues very active - IR image below is from 14 UTC. Cat 3 Hurricane Fernanda is furthest west; looks like next system to east has already become TS Greg; and the third system near the Mexican coast is also forecast to strengthen.