Monday, August 31, 2020

Complicated Situation To End August

Very humid this morning after last night's light rainfall - cumulus are already forming on the Catalinas (above) before 8:30 am MST.

Plot of detected CG flashes (above for 24-hours ending at 01:30 am this morning from Atmo and Vaisala) shows that most thunderstorm activity remained well west of the metro area yesterday afternoon and evening. There were a couple of small cells near here though that produced some thunder and a light shower around 10:30 pm at house. Only 0.02" in the gauge this morning.

The ALERT network (northern portion below) data show that there were scattered reports and that two sites measured over half an inch. Amounts don't seem large enough, but there had been flow in the Rillito during the early morning hours. In the southern half of the network, Arivaca and Tubac gauges reported 0.04".

The 500 mb map above for this morning is from the Univ. of Wyoming weather page - the bad height at Chihuahua is messing up the analyses at other sites. There are several weak troughs over Mexico and the Southwest - one over western Arizona may be passing by today (note that in addition to the bad height at Phoenix, the wind speed seems a bit suspect too). The 500 mb temperature here is a warm -5 C, which is not too favorable for storms.

The morning TWC sounding (below) is wet with 1.65" of PW, and considerably more CAPE than yesterday. Steering level winds from 500 to 300 mb are west-southwesterly and fairly strong at 20 to 30 kt. The continuing west winds aloft are also not too favorable for storms, but the clear skies and unstable conditions may dominate.

The GEFS plumes for QPF at airport from the 06 UTC run (second below) are all consistent and forecast a distinct late night to early morning event tonight, and then little else during the week.

The early WRF forecasts that I looked at tended to keep most storm activity to our south and east. The NWS morning forecast indicates 50 to 60 percent chances of rain at the airport for today, tonight, Tuesday, and Tuesday night.

It seems a pretty mixed bag of signals this morning, and we'll just have to watch how all this plays out. However, I would hope to end the month, and also meteorological summer, with a nice amount of rain in the gauge tomorrow morning!

Sunday, August 30, 2020

Today's Outlook

View of Catalinas about 9:00 am MST this morning - note how much cleaner the air is today, after yesterday's cleansing rains. This post will contrast two different WRF model runs.

The 12 UTC 500 mb chart today (above) is quite different than recent days. The anticyclone has been suppressed far to south near the northern end of Sinaloa. This leaves us under the influence of the west coast trough over central California - certainly not a great summer pattern. However, moisture remains high across southern Arizona.

The morning sounding (below) has reduced CAPE that is centered around 500 mb. Lowest levels are cooled by yesterday's rains and will only heat out to a shallow BL today. However, there is CAPE in the 850 to 700 mb layer that could come into play late today.

The 06 UTC GEFS plumes for QPF at the airport (above) are very aggressive for late tonight and again tomorrow night. Totals over an inch are highest I can remember in quite a long while.

Forecast above is from the 06 UTC WRF-GFS and is for composite radar echoes valid at 3:00 am early tomorrow morning. Strong storms are forecast from central Pima County eastward across the Tucson area. The forecast total rainfall at the airport through noon tomorrow from this model run (below) is a very heavy 2.8 inches.

However, the 12 UTC morning forecast from the WRF-NAM (above, valid at 5:00 am) has shifted the heavy activity eastward, but is still impacting the southeast part of our area. Total rainfall (below, through noon tomorrow) at the airport from this more recent model run is only a tenth of an inch, and the heaviest amounts are shifted into Cochise County.

A blend of the two forecasts would make for a very interesting night here in eastern Pima County. Time will tell - but, I'm hopeful for a better event tonight, since last evening's rain will barely keep the ground here at house damp until mid-afternoon.

If we do get another event here, it would be one of these rare occurrence of widespread rains on two consecutive days.

Yesterday"s Rain Event

Middle clouds drifting over the eastern Catalinas just before sunrise this morning.

Thunderstorms yesterday left a donut hole over metro Tucson - CG flashes above for 24-hours ending at midnight last night (from Atmo and Vaisala) - however, showers did move across the metro area. Rain began here a bit before 7:00 pm MST - heard no thunder and there's 0.16" in the gauge this morning. Rains ended before midnight generally over the metro area.  The airport reported 0.11"; DM had 0.08"; and Atmo received 0.16".

Reports here from ALERT (north above and south below) are for 24-hours ending at 6:30 am MST this morning. Heaviest amounts over both the north and south parts of the network. Second below is from MesoWest for same period for southeast Arizona. I noted that Amado and Sells had 1.15" and 1.221" respectively. Very good event for some parts of the area, but just enough to wet the ground, or nothing at all, for other locations.

Saturday, August 29, 2020

High PW Today And Flash Flood Watch

Pre-sunrise color on altocumulus drifting overhead this morning. Down at bottom is view of another colorful sunrise today.

The NWS has issued a Flash Flood Watch for this afternoon and tonight, as per above. Some models forecast the heaviest rains tomorrow, but logic for focus on today and tonight is well explained in the forecast discussion. 

Thunderstorm activity yesterday was very isolated in southern Arizona (CG flash plot above for 24-hours ending at 7:30 am this morning - from Atmo and Vaisala), although there was heavy activity in Sonora during the night. Very limited rainfall measured in the ALERT network for 24-hours ending at same time (below),

The TWC sounding this morning (above) is very moist, with over an inch and a half of PW and much increased CAPE wrt to last few days. However, there are several inversions above 500 mb that could come into play, depending on depth of mixed layer this afternoon. Steering flow is from the southwest, which is often not favorable for storms here at the house.

The forecast sounding below (from the 06 UTC WRF-GFS run at Atmo) is valid for TWC at 6:00 pm MST - sounding indicates convection has already occurred in the area by this time.

Forecast above from the 06 UTC WRF-GFS indicates heavy storms in the east part of the metro area at 8:00 pm today, while forecast below from the 06 UTC WRF-NAM indicates storms approaching our area at midnight tonight.

Second below is the WRF-GFS forecast from same run as above valid for total rainfall through Sunday night at midnight. 

Looks to be an interesting weekend, although I did note that the 00 UTC run of the WRF-GFS forecast a distinct donut hole over the Tucson metro area.

Friday, August 28, 2020

Stormy Weekend Possible

Contrast the dirty skies here in the city with the sparkling skies over Summerhaven this morning.

The current 500 mb chart (above from SPC) shows that the anticyclone has shifted some and is now centered over southeastern New Mexico. There are southerly winds over Sonora and Arizona this morning. Note that there is a strong temperature gradient at 500 mb, with Empalme reporting only -2 C and Tucson having -9 C. So there may be some warm advection aloft. But the height of a mid-level inversion is at play also - inversion is above 500 here but right at 500 at Empalme.

The morning sounding from TWC (below, also from SPC) has some CAPE in middle levels, but amounts appear well below 1000 J/kg. Steering winds are a bit uncertain, with the southerlies turning toward the west above 500 mb - and neither direction is very favorable for storms at low elevations.


The presence of both tropical depression Hernan (currently decaying at the south end of the GoC), and also tropical storm Isella southwest of the end of Baja, will help to push higher moisture contents north into southern Arizona. These PW forecasts are from the 06 UTC WRF-GFS run at Atmo. Above is valid at 5:00 am this morning and below is valid at 5:00 pm tomorrow afternoon. Note the strong push of PW amounts over 2 inches into Arizona!

Forecast plumes for QPF at the airport (above from 06 UTC GEFS) indicates significant storms late tomorrow into Sunday morning. Average forecast amounts reach to nearly an inch. The ensembles also forecast a significant cool-down (below), with high temperatures down over 20 degrees - this would be some wonderful relief from our blistering August heat.

Forecasts of composite radar echoes here are from: 06 UTC WRF-GFS above, valid at 4:00 pm this afternoon, indicating another day with isolated storms for our area; but forecast below - from this morning's WRF-RR indicates much more thunderstorm activity over our area at 7:00 pm late tomorrow. 

Hopefully it will be an active and interesting weekend.

Isolated Showers Yesterday

Hazy and dirty skies continue this morning over Tucson, as per view of Catalinas from campus this morning at a bit after 7:30 am MST. Am going to do two posts this morning and just cover yesterday in this first one.

There were isolated thunderstorms over central and eastern Pima County yesterday afternoon, as per the composite radar chart below from 4:07 pm.

Plot of detected CG flashes (above for 24-hours ending at 3:00 am) shows storms near by, but I heard no thunder here at the house. We did not have even a sprinkle here, although there were a few ALERT sites that measured 0.04" or more - below. One site measured 0.35" near downtown.  

So, there we had a bit of an of an uptick, that hopefully will continue into the weekend.

Wednesday, August 26, 2020

Laura/More Of Same Here

Hurricane Laura (above) over the GoM is currently a Category 3 storm with winds over 110 mph as it heads toward a landfall near Lake Charles, Louisiana. The NHC forecasts (track shown below) that Laura could be a Cat 4 storm at landfall (winds over 130 mph).

Back here in southeastern Arizona the large-scale pattern continues with little change. Skies are very dirty, as per view south from Kitt Peak (above). We have been plagued by a Dog Day summer this year.

Yesterday remained very suppressed over all of southeastern Arizona, from Phoenix area all the way to the southern and eastern borders of the state. Below shows detected CG flashes for 24-hours ending at 1:00 am MST this morning (from Atmo and Vaisala). The no activity zone extends into southwestern New Mexico and a very large portion of northwestern Mexico. Seems to be about as bad as it can get.

At 500 mb (above at 12 UTC this morning) a huge anticyclone covers most of the country. The western lobe of the high is centered over northeastern Arizona, and windas are light easterly across southern half of state.

The 12 UTC sounding from TWC (below), has light winds and just a sliver of CAPE if the BL were to mix out to 600 mb (which is not likely). However, there may be enough heating and mixing over the mountains to get some slow moving storms going - if this were to happen the storms could produce locally heavy rains.

However, the 06 UTC WRF-GFS forecast (above, for precipitation through midnight tonight) indicates no activity except along the borderlands of southwestern Pima County.

Forecast below is for total precipitation through 6:00 am on September 1st - perhaps a sprinkle shower or two here at house before this miserable month ends.