Thursday, October 08, 2015

Couple More Photos FromTuesday

Photo above is by Mike Leuthold from Atmo, looking toward the east part of the Catalinas - he reported a funnel cloud with this storm a bit before he was able to get the photo.

Below is from twitter, forwarded by Mike, and shows a landspout-type tornado near Buckeye, Arizona. This one of a number during the summer that were not reported to SPC and so are essentially, officially "undocumented" severe storm events.

Thunderstorm activity was much reduced yesterday, especially after the morning activity had moved on - as was indicated by the WRF forecasts. Plot above shows CG flashes back through 7:00 am MST yesterday morning (Wednesday, October 7th). After noon, only 22 sites across the ALERT network reported rainfall, with these mostly in the Catalinas to Rincons - two sites in the Catalinas had just over half an inch. The morning showers produced 0.08" here at the house. October is off to a good start with just under half an inch here - note that 7 of 16 Octobers in my records here have had a total rainfall of 0.08" or less.

Wednesday, October 07, 2015

Overview Yesterday And Today

Two photos of one of the last strong storms yesterday afternoon. Above is view from campus just after 4:00 pm MST, as storm is moving north onto the Catalinas. Photo below is from Ron Holle who was looking toward the north end of the Catalinas at about 4:30 pm - the two photos illustrate how rapidly the storms were moving yesterday, since views are of the same storm.

The plot above (from Vaisala and weather,graphics) shows CG flash density for the 24-hours ending at 1230 UTC this morning. Active storms over Tucson area and to our north. There were several reports of severe hail here in Tucson and also around Casa Grande.

The system turned out to be a more prolific rain producer than was anticipated. Essentially 100% of the ALERT stations recorded measurable rain during the 24-hours ending at 7:00 am. Here at the house the rain for that period was 0.42". Across the network there were 40 sites with over half an inch and 6 sites with over an inch. At lower elevations the 1.24" at Craycroft and Golf Links was notable.

The 500 mb analysis for 12 UTC this morning (below from NCAR RAL) shows the 500 mb closed low just off to our northwest, with the morning storms in its cold core and to the east. Models emphasize the morning storms as the low moves southward across eastern Pima County.

However, the morning sounding (above for 12 UTC from SPC) is quite unstable below 750 mb. If there is some sunshine later today, the models could be conservative again for this afternoon. below is current NAM 500 mb forecast valid at 12 UTC tomorrow morning, with the low moving northwestern Mexico. As the large-scale ridge amplifies at higher latitudes along the West Coast of US and Canada, the low is being forced south and westward. If it ends up west of Baja, it may well visit Arizona again next week.

Showers At Sunrise

There were pre-dawn showers over parts of the metro area this morning - composite radar above is from 5:42 am MST and shows strong thunderstorm echoes over and west of the Catalinas. Lightning flashes were visible here at house to north and northwest, as shown by the CG flash density analysis below for the last six hours (from Vaisala and

Tuesday, October 06, 2015

Two Quick-Hitting Thunderstorms Here

There have been two quick-hitting thunderstorms here at house today - one at around 12:30 pm with wind gusts of about 40 to 50 mph and another about 2:00 pm with a brief downpour. Three shots of the storm north north of campus are shown below - times in upper-right corners. Quite a nasty looking storm that produced a lot of hail, but none at our location. Rainfall here was 0.22" from first storm, with another 0.11" coming from the second. However, much heavier amounts along the foothills to our north indicated by ALERT network data.

Brief Overview, Early AM October 6th

The WRF forecasts for yesterday from Atmo were again very good, with significant showers and thunderstorms staying north and east of Pima County, as the models indicated. Detected CG flashes through midnight last night are shown above - from Vaisala and Atmo. Almost no flashes indicated in Pima, Santa Cruz and Cochise Countries. There were light showers over parts of eastern Pima County, with 22 ALERT sites measuring 0.04" or more for the 24-hours ending at 5:00 am MST this morning. Amounts were light to very light and occurred from early am through early afternoon. Here at the house we had 0.02" in a shower around noon.

Across southern Arizona PW values are ranging from about three quarters of an inch to around an inch, with higher amounts out to the west, as shown by the 10 UTC blended PW analysis above from CIRA at Colorado State. High PW amounts remain fairly far south, across the lower GoC. The cold, 500 mb low is forecast by the models to move right across our area - forecast below from the 06 UTC WRF-GFS forecast is valid at 10:00 am tomorrow morning. The models forecast only light precipitation, except for at some high elevations, for southeastern Arizona as this system moves across our region. The closed low is forecast to dig south and then southwestward over the next several days, ending up west of Baja over the Pacific. Perhaps it will make a second pass through Arizona at some point.

From the 06 UTC WRF forecasts the GFS version is much wetter than is the NAM version. Forecast above is WRF-GFS composite radar echoes valid at noon today, while forecast below shows GFS precipitation through midnight tonight. So, the models continue to keep the more significant rains with this system to our north and over the Catalinas.

Monday, October 05, 2015

Light Showers At Sunrise

A fast moving area of light showers crossed parts of the Tucson area at sunrise. At 6:30 am 25 sites in the ALERT network had measured rainfall during past several hours - amounts were very light except for 0.31" at Mt. Lemmon and 0.24" at the Avra Valley Airpark out at northwest corner of the network. Just a trace here at the house. The showers intensified to include some thunderstorms as the area moved rapidly northward into Pinal County - above plot show CG flash density for last 6-hours ending at 6:00 am MST (from and Vaisala).

The sounding below is from TWC on campus - fairly moist and unstable conditions at that time with a very nice, Plains-like wind shear profile. However, the forecasts from both the WRF-NAM and the GFS version run at 06 UTC last night at Atmo are not nearly as favorable as the sounding would seem to indicate. Both versions of the model shut the shower activity down over our area for the rest of the day, as warmer air above 700 mb returns and PW decreases some. The model forecasts yesterday were quite good keeping activity well east and north of Tucson through midnight. Will they be as good today? 

This a hard call, given the CAPE available at low-levels that will combine with sunshine and heating this morning. Looks like yesterday's battle between low-level CAPE and warm, dry air inversion aloft will continue today. Current GPS PW estimates to our west are indicating values similar to what's forecast by the WRF models - so for now I'd say the morning rains were our weather event for the day, but of course will keep an eye out early this afternoon.

Sunday, October 04, 2015

Low-Level Moisture vs Warm Air Aloft Today

Very interesting situation today as cold, closed low at 500 mb over California continues to dig southward. Pattern over the states is generally blocked with closed lows over the southern corners, so California low will be slow moving and gradually weakening. This morning's TWC sounding (above from SPC) indicates increased low-level moisture, a strongly veering wind profile below 300 mb, BUT very warm and dry air aloft above 650 mb. So, a mixed bag. Continued moisture increases could perhaps overcome the warm inversion and allow strong thunderstorms to develop. 

The 12 UTC PW analysis from CIRA at Colorado State (above) shows higher PW just to south of the border - so the forecast issue for today is indeed whether low-level CAPE can increase and overcome the warmth aloft. Yet another day where sounding data from Guaymas could have been very useful. Perhaps a special sounding from TWC would help? - but the warm, dry air is within the southwesterly air stream - so there's little observational data and the models are on their own wrt the evolution of the warm air.

The 06 UTC runs of the WRF model at Atmo keep things fairly suppressed today over southern Arizona and the warm air aloft seems to prevail in those forecasts. The WRF-GFS forecast of precipitation through 5:00 am MST tomorrow morning is shown below. However, the 12 UTC run of the NAM model is much wetter than the WRF runs were, so a cross-check of the new WRF forecasts later this morning would be in order.

Friday, October 02, 2015

Quick Look Ahead To Next Week

While the weather excitement has reached very high levels along the East Coast, I'll just take a quick look at the model forecasts for the first part of next week here in the Southwest. The plot above shows detected CG flashes over Mexico for 12-hours ending at midnight last night (from Vaisala and Atmo). Very little thunderstorm activity indicated, due to strong anticyclone centered over Sonora, along with very warm middle-level temperatures.

The GFS model continues very similar to the ECMWF currently and digs the short wave currently over western Canada southward across California. The 500 mb forecast shown above is from the 00 UTC run of the GFS and is of 500 mb heights, winds, and temperatures valid at 12 UTC next Tuesday morning (October 6th). At that time model forecasts a closed low to be centered over Las Vegas with temperatures near -20 C, and a range across Arizona of nearly 10 C from southeast to northwest - quite a significant change. The closed low pulls higher PW northward from the GoC and northwestern Mexico, resulting in a significant precipitation event for all of the Southwest.

The 00 UTC GFS forecast of total precipitation through 12 UTC next Wednesday morning is shown above. The model forecasts quite some event for much of the west - but out east, even though model keeps Hurricane Joaquin off shore, the forecast  rainfall over the Carolinas is just huge - max contour on the forecast is for 15-20 inches!

The 00 UTC forecast from the WRF-GFS (below) for same period as above shows the small scale details, which indicate both a strong west to east gradient across southern Arizona and also a donut hole minimum over eastern Pima County. The WRF maximizes the event over New Mexico and from the White Mountains westward along the Rim Country. Time will tell.

Thursday, October 01, 2015

When Did Summer Monsoon Really End?

Russ Scott emailed me last week and asked when I thought the monsoon ended. I ended up replying that it didn't seem very clear to me. I was inspired to take a closer look when I noted one of the headlines in this morning's newspaper:

Dry, triple-digit day marks end of monsoon - The official monsoon “season” ended Wednesday with no rain for the day and a high temperature of 101 degrees.
I have just parsed through all of the skew-T plots for the upper-air soundings taken at TWC for June through September. Russ's question remains very difficult to answer. 

The only thing that ended yesterday was the arbitrary, calendar-based definition of the "monsoon season" that was adopted by the Arizona NWS Forecast Offices several years ago. The definition has nothing to do with the actual day-to-day, large-scale circulation patterns affecting our part of the state.

Although PW values remained high for much of the summer (thanks to the influences of several named tropical storms/hurricanes) the tropospheric wind profiles were highly variable through the summer. If one considers a wind profile that varies from east-southeast at lower levels to south-southwest at upper-levels typical for the summer monsoon circulation, then there were surprisingly few days with "typical" winds aloft.

Here are some sounding plots (from the University of Wyoming upper-air page) for the last part of the summer. The sounding above for August 16th has high PW but northerly to northwesterly winds through the troposphere. This was the first day of an extended period with westerly to northwesterly winds aloft and would be my candidate date for the end of the monsoon - based on circulation patterns. 

The sounding below, from near the end of August, shows a situation with almost no winds through the entire troposphere - a situation that seemed to plague us (especially at 500 mb and below) for an unusually large number of days.

The sounding at the bottom is from early September, when light winds below 500 mb were overlain by very strong, baroclinic southwesterly winds - a hybrid (or mixed-mode) situation of which there were quite a few - note the unusually strong upper-level jet with speeds of almost 100 kts. 

So, my subjective summary is that it was an usual summer with many days of hybrid circulation patterns accompanied by PW amounts that stayed high through much of the summer. Here at the house I noted the first summer thunderstorm on June 24 with the first measurable rain the next day during a brief, severe thunderstorm. (thanks to early moisture associated with Hurricane Blanca). The last rains and storms of September (21st to 23rd) were associated with the remnants of TD-16E, interacting with an upper-tropospheric trough in the westerlies.