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.



Sunday, July 16, 2017

Another Heavy Thunderstorm Day


We were away Friday and Saturday, but didn't miss much as it appeared to be mostly a down day in the metro area, although there was a weak tornado near Marana. Having trouble viewing model forecasts and other data online this am, perhaps due to storms and power surges between 9:00 pm and midnight MST last night. Photos at sunrise this morning - note the K-H waves west of Finger Rock (below).



Widespread thunderstorms over Arizona yesterday with a number of severe wind reports - especially west of Phoenix - which was in a donut hole. There were gusts to 66 mph at Douglas and to 58 mph at Luke AFB. The 24-h CG flash density above (from weather.graphics and Vaisala) ending at 6:30 am this morning shows that the center of action yesterday in the Southwest was clearly Arizona.


The ALERT rainfall data (above - for 24-hours ending at 6:30 am) shows 100% coverage for the third day of the past six. We had storms here in the late afternoon that produced thunder and spits of rain, but storms between 9:00 pm and midnight produced 1.28". This gives us measurable rain at the house on four of last six days. There were 13 ALERT sites with over an inch of rain and two sites with more than two inches.

Although there is a flash flood watch for all of southeastern Arizona, this afternoon could be another challenge. The 12 UTC TWC sounding plot below shows another "onion-like" sounding that has considerable low-level cooling. So even though there is an inverted trough approaching at 500 mb, it may be another day of heating fighting with wet ground today, so that best chance may be during the night tonight.



Friday, July 14, 2017


Only have time for quick post today. Three out of four days with storms now in metro area. Early morning storms moved southwestward off the Catalinas and were out to southwest at sunrise - above (view looking other way is at bottom).

Yesterday afternoon, the first storms that tried to come off the Catalinas dissipated quickly. Then strong outflows around 8:00 pm from storms to south (gusts from south of 40 to 50 mph here) really kicked off an active hour. Thunder and a bit more rain around 4:00 am ended up leaving 0.45" in gauge this morning.

Flash density plot below (from weather.graphics and Vaisala) is for 24-hours ending at 6:30 am. Most active thunderstorms were out in central Pima County and in Sonora. There were a number of severe thunderstorms around, including a gust to 58 mph at Mt. Hopkins RAWS.



The ALERT network had another day with nearly 100% coverage of rainfall - only 5 sites along northwestern edge had less than 0.04". The total here takes over 2 inches for July, making it already a good month.

Below is TWC sounding plot for 12 UTC. Good steering flow continues with considerable CAPE. Sounding seems to be of the "onion" type, perhaps due to the nearby early morning activity. Hard to know what thermodynamics will be by early afternoon. The 06 UTC WRF-GFS was very good with the early storms and then recovers things for another very active afternoon. Appears to be clearing from the north - looks like battle between sunshine and really wet soil will determine this afternoon's outcome at low elevations.





Thursday, July 13, 2017

More Storms Today?


Above is my subjective 500 mb analysis of the 12 UTC upper-air data. While the ridge seems full of weak troughs, the anticyclone of interest is northeast of Tucson. There is a pronounced, upper-tropospheric cyclone/inverted trough over the Big Bend country and north-central Mexico - this feature is moving slowly westward and will help keep things interesting in southeast Arizona into the weekend. There will be substantial difluence ahead of this feature affecting our part of the state.


The morning sounding is more unstable than yesterday's and it appears that BL heating can produce an afternoon sounding that needs little lift to trigger storms. There is decent steering flow from the northeast to east, so some organized lines are likely. The 9:00 am MST visible image shows mostly clear skies our part of state.



The forecast 06 UTC WRF-GFS sounding valid at 5:00 pm today (above) indicates some advection of a mountain layer  providing a bit of a cap over the moist, surface-based BL. Forecast steering flow is from the northeast. So that may delay storms. However, storms in low elevations should develop easily with some outflow kick and with cloud bases around 700 mb could produce severe, hybrid downbursts.

The composite radar forecast below from the WRF-GFS is valid at 6:00 pm and forecast at bottom is for total precipitation through 11:00 am next Sunday. So a number of interesting days appear to be on tap.





Quick Summary Last Evening


Sunrise this morning from along the Rillito. Two rain events in 3 days has brought out the flying ants and toads, plus a surprising number of walkers, runners, and riders along the Rillito around sunrise when it is very humid.


Thunderstorms rolled in a bit after 9:00 pm MST last evening, and the 06 UTC WRF-GFS forecast had almost perfect timing for this event. The CG flash density plot (above from weather.graphics and Vaisala) shows the thunderstorm activity was concentrated in eastern Pima and Santa Cruz Counties.

The ALERT rainfall for 24-hours ending at 6:30 am this morning is below. Heaviest rains of over an inch occurred up in north end of mountains and around Catalina. There was donut hole over the southeast metro area that apparently resulted when new storms developed well to west and storms heading into that part of City dissipated. There were 19 sites that had less than 0.04" or no rain - airport and DM had trace or nothing. Here at house we measured 0.35" in the gauge this morning.



These two photos were taken Tuesday morning the 11th when the Rillito was flowing and there was stratus fractus hanging on the Catalinas.