Tuesday, August 31, 2021

Flash Flood Watch

Nice pre-sunrise color over the eastern Catalinas this morning.

ALERT rainfall observations (above and below for 24 hours ending at 8:00 am MST this morning.  Looks like about 60 to 70 percent areal coverage. Here at house we had a light shower around 6:30 pm that produced 0.01". Note heavy amounts up around Mt. Lemmon.

The 500 mb level (above for 12 UTC this morning) is still dominated by light winds and weak height gradients over the Southwest. There is some hint of a shortwave across northern Sonora that will affect southeast Arizona later today.

The morning TUS/TWC sounding (below) remains moist with 1.64" of PW - Empalme to our south on the GoC has 2.40"! Map of TPW for 14 UTC (second below) shows very high PW has pushed northward all the way to Las Vegas area. Here in southeast Arizona there is still a noticeable gradient  between moist air to our west and drier air to our east - however winds are light and strongest convergence will again be along outflows. There is still an inversion way up at 300 mb, but model forecasts weaken this feature by afternoon.

The 06 UTC plumes for QPF at the airport (above) forecast a significant event for later today through tomorrow - although range of amounts is substantial (around two inches), and the operational GFS is a dry outlier for this period.

Forecast below is for composite radar echoes valid at 7:30 pm this evening, with strong echoes covering southeast Arizona. The NWS morning forecast is shown at bottom. Should be an interesting day!

Monday, August 30, 2021

Nora Dissipates

An isolated storm at west end of Catalinas last evening (above) produced considerable thunder here at the house. There was a bit of wind also, but no rain.

Most of the ALERT network was also dry (above and below) for the 24 hours ending at 8:30 am MST this morning. Only 17 sites had 0.04" or more, and those were mostly in and near the Catalinas. It was basically a down day for lower elevations.

The 500 mb chart (above) remains chaotic over the Southwest, with the most distinct feature being the circulation of now Tropical Storm Ida, whose impacts were very severe as per reports from Louisiana on national news this morning.

The TUS/TWC sounding is quite a mess this morning (below). There is little residual BL, winds are light and variable below 350 mb, and there is just a sliver of CAPE for the layer below 700 mb. I would expect storms at higher elevations again today and that they will have trouble moving into lower elevations.

Although Nora has fizzled, her remnant circulation at 500 mb appears to continue moving in our direction, as a trough develops along the west coast. These features interact and produce significant precipitation across Arizona through mid-week - forecast above from 06 UTC GFS shows accumulated precipitation through 11:00 pm Wednesday night. The Tucson NWS has issued a Flash Flood Watch for tomorrow into Wednesday night - details below.

Sunday, August 29, 2021

Near Severe Here

Thunderstorms approaching the house at 4:00 pm MST yesterday and seen pouring down rain north of campus at 4:36 pm - below. Radar at about the same time - second below.

Storm produced wind gusts E 50 to 60 mph here, along with pea-sized hail, and 0.52" of rainfall.

ALERT 24-hour rainfall ending at 6:00 am this morning - several sites on north side of Catalinas reported more than an inch. Two sites in south part of network also had rain: 0.47" at Florida Canyon, and 0.20" at Tubac. Atmo reported 0.37", but the airport and DM did not even have a Trace.

IR and visible images here show Ida about to make landfall south of New Orleans - NHC rated her at Cat 4 with 130 to 150 mph winds.

Saturday, August 28, 2021

Few Storms In Metro Past 24 Hours

View of Catalinas this morning from campus at a bit after 8:00 am MST.

Plot of detected CG flashes for 24 hours through 1503 UTC (8:03 am MST) shows flashes over much of Pima County (from Atmo and Vaisala). Some early flashes were near here - there had been pre-dawn showers around Campbell and River and I suspect that there was bit of thunder here. There were a few reports of rain across the ALERT network (below - for 24 hours ending at 8:00 am).

IR satellite image above shows Hurricane Ida in the GoM and a large MCS at north end of GoC at about 4:00 am today. The GoC convection has left behind at least one large MCV. Outflow winds gusted from the south to over 30 mph at Yuma and jumped the dewpoint up to an ugly 78 F at 6:00 am this morning.

The morning sounding from TUS/TWC continues with a fair amount of high-based CAPE - above about 600 mb with light east-northeast steering winds. The 500 mb pattern (below from Univ. of Wyoming) continues to be very chaotic with height gradients and light winds. There appears to be an anticyclonic center between here and Phoenix.

The 12 UTC WRF-RR forecast below is for composite radar echoes at 7:30 pm this evening - storms again over much of Pima County including the northern parts of the metro area.

The current NHC forecast for Huricane Nora (above) continues to head the storm our way, with it dissipating south of here. The visible satellite image below shows both Nora and Ida over in the GoM. The NHC forecast for Ida (second below) increases the storm to a Cat 3 or stronger hurricane and takes it northwest toward southern Louisiana and New Orleans. 

Looks like an interesting and stormy week ahead.

Friday, August 27, 2021

Ida And Nora

Thunderstorm over the Catalinas yesterday afternoon about 3:00 pm MST.

Tropical Storm Ida (NHC outlook above) is forecast to hit western Cuba today as a hurricane and then to threaten southern Louisiana as a moderate hurricane by Monday night. The 1530 UTC visible satellite image below shows Ida, as well as Tropical Storm Nora off the southwest coast of Mexico.

The NHC outlook for Nora (below) currently forecasts the storm to become a hurricane and track right up the GoC. This track results in a pressure gradient pointing south, down the GoC - thus, strong moisture advection into southern Arizona may be delayed until the remnants of the storm arrive. This a much more difficult situation to accurately forecast than when storms move southwest of Baja.

Also of interest is Hurricane Nora of 1997 - this storm crossed Baja and came into southwestern Arizona, but produced no rain here in the Tucson (ended up being called NO Rain At all)! 

There were afternoon storms yesterday, as per plot of detected CG flashes - above for 24 hours ending at about 2:00 am this early morning (from Atmo and Vaisala). Eight stations in the ALERT network had more than 0.04" as per map below for 24 hours ending at 8:00 am this morning.

This morning's sounding for TUS/TWC (above) has a deep residual BL as well as CAPE above about 650 mb. The light and variable wind profile has no real steering winds. Storms on the mountains could end up drifting toward lower elevations.

Precipitation forecasts from the WRF runs at Atmo are: above from 12z WRF-RR this morning showing rainfall forecast through 5:00 am tomorrow morning, and below from 00z WRF-GFS showing model's forecast of rain amounts through 5:00 am September 1st. So there continues to be some chance of a bit more rain across Tucson region before the end of August.

Wednesday, August 25, 2021

Hazy And Hot

Hazy skies this morning, as heat returns for next few days. The NWS here has issued an Excessive Heat Warning from now through Friday, with max temperatures expected to hoover around 105 F. Images down at bottom show thick smoke from the Caldor fire, near Lake Tahoe, yesterday afternoon.

Plot above shows detected CG flashes yesterday afternoon - from Atmo and Vaisala. The thunderstorm activity was isolated, but expanded along the Borderlands from Santa Cruz County out to western Pima County. The IR satellite image (below from 7:45 am MST) shows clear skies over almost all of Arizona.

The 300 mb analysis above shows that the large anticyclone in the upper troposphere is centered near the southeast corner of New Mexico. The morning sounding for TUS/TWC (below) shows that PW was 1.41 inches, but with only a sliver of CAPE at most. Second below is the 12 UTC WRF-RR forecast for precipitation through 5:00 am tomorrow morning. The models forecast PW to fall some today, with only a few sprinkle showers around this afternoon. 

Tuesday, August 24, 2021

More Rain Before End Of August?

View of the Catalinas and clear skies from mid-town at 7:55 am MST this morning. 

Plot of detected CG flashes (below) for 24-hours ending about 2:00 am this morning indicates quite a bit of thunderstorm activity yesterday over the southeastern corner of Arizona. No storms in Pima County, but observations from the airport indicate that an outflow from the activity to our southeast moved across the metro area before 9:00 pm last evening, jumping the dewpoints back into the 60s. Even with the moisture increase there is little CAPE this morning.

At 500 mb this morning (above) a huge ridge and anticyclone dominates much of the US, GoM, and northern Mexico.

Plumes for CAPE (from the 06 UTC GEFS runs - above) show amounts increasing by the weekend - but the spread becomes large with very noisy results from the ensemble models. Same is true for QPF at the airport - note that operational GFS (blue) is a very wet outlier wrt to most of the runs. Precipitation amounts by the end of August range from zero to an inch and a quarter - so the question of more August rain has no clear answer at this time. However, the GFS (the blue outlier) forecasts significant rain (bottom and plumes above).