Westerlies Dominate Today & TS Kay Develops South of Baja

Thunderstorms occurred yesterday mostly over western half of Arizona. There were some thunderstorms off to the southwest of the metro area before sunrise this morning, and cool outflows prevailed during my early am walk. Plot of detected CG flashes above is for 24-hours ending at 6:00 am MST this morning (from Atmo and Vaisala).

The larger-scale pattern is very complex this morning. The 500 mb analysis above is from the 12 UTC NAM model. There are two weak troughs affecting Arizona today: one extends southwestward from northern Colorado into the Pacific west of southern Baja; while the other extends westward from the Four Corners across California and well out into the Pacific. Regardless, all of the Southwest is in the westerlies, with little left of a monsoon pattern, except for residual low-level moisture. The short wave across eastern Arizona rapidly weakens and vanishes, while the California trough persists. 

The morning sounding plot below from SPC shows the westerlies now prevailing through the entire troposphere, in a sounding with minimal CAPE. The situation would be very unstable with an increase in PW from the west. Clearly a Fall-type transition situation. The 06 UTC WRF forecasts differ, with the NAM version again keeping afternoon storms out to the west, while the GFS version forecasts more widespread storms affecting all of Pima County. 

Meanwhile, Tropical Storm Kay has developed south of Baja, as per the morning graphic above from NHC. Kay is forecast (below from NHC) to track southwest of southern Baja, along a path that would be good for GoC surge initiation, if the storm were larger or stronger. So, it appears to be a situation that's unclear, but worth monitoring.

I looked at the 06 UTC WRF forecasts of PW on the 5.4 km grid and found that the two models have quite different forecasts for the track of Kay. Both forecasts shown are valid at 11:00 am on August 22nd - WRF-NAM top and GFS version below.The storm is located much closer to Baja in the NAM version forecast, while the GFS version is more similar to the NHC forecast. A puzzling difference, since the NAM is essentially embedded within the GFS. My guess is that the NAM version is reflecting the TS location within the 00 UTC GFS forecast, while the GFS version reflects the new 06 UTC GFS forecast. The difference leads the NAM version to forecast a significant GoC surge, while the moisture intrusion is not nearly so strong on the GFS version.

Quick Look At Today

During the late night and early morning hours there was considerable thunderstorm activity from western Pima County northward to regions well west of Phoenix - plot of detected CG flashes above from Atmo and Vaisala for 24-hours ending at 6:00 am MST this morning. These storms were well predicted by yesterday morning's WRF forecasts - see Mike Leuthold's early afternoon discussion at:

http://arizonawrf.blogspot.com/2016/08/20160817.html

This morning low-level moisture has increased back to around 1.25 inches, as higher PW to the west has inched eastward. The blended PW analysis above is from CIRA at Colorado State University and is for 11 UTC. Driest air continues over far northwest and southeast Arizona.

This morning's skewT plot of the TWC sounding data (above from SPC) is somewhat similar to Tuesday morning's. The upper half of the troposphere remains in a westerly flow regime, with features of interest residing in the lower half, where the wind field remains weak and chopped up. There are three elevated mixed layers (EMLs) present from just above 500 mb down to just above the surface. The lowest of these has considerable CAPE, but the two old boundary layers above are hostile and stable, meaning that considerable forcing would be needed to produce storms in eastern Pima County.

The forecast of PW below is from the 06 UTC WRF-NAM and is valid at 5:00 pm this afternoon, when the model predicts a strong east-to-west moisture gradient across Pima County. The evolution of storms this afternoon will depend very much on where storms first develop and how their outflows evolve. My first guess, based mostly on the morning sounding, is that storms may be most likely in our area during the night and early morning hours. Best strategy is to check the 12 UTC WRF forecasts and watch for Mike's discussion later today.

Drying In Low-Levels Overnight

The blended PW analysis above (for 12 UTC from CIRA at Colorado State University) indicates PW has fallen to around an inch over southeast Arizona, as drier air encroached from the east to southeast during the night. The morning skewT plot below for TWC (from SPC) fits with the CIRA analysis, as PW is at 1.02 inches and sounding is quite stable. Both WRF models run at 06 UTC forecast a down day for southeast Arizona, with no thunderstorm activity.

Storms In Northwest Metro Yesterday Evening

Even though the situation was marginal yesterday, the nasty capping inversion moved away and some storms developed at low elevations late in the day. View above at 5:52 pm MST shows Cbs and developing towers to the north. My photo below at 7:17 pm shows rain shaft to the north-northwest of here.

Radar at 8:04 pm and CG flash density for 12-hours ending at 7:00 am this morning both show the storm activity focused over Catalinas and northwest metro area.

The ALERT rainfall for 24-hours ending 7:00 am this morning (above) indicates 24 sites with measurable amounts (0.04" or greater). The airport had a 0.01" and DM had a Trace. Radar indicates that heaviest rainfall may have occurred in the data void north of downtown toward Limberlost, since heavy radar echoes were nearly stationary over that area. Here at the house we had several periods after 6:45 pm with gusty winds of 30 to 40 mph, a bright lightning show, and then a brief period of heavy rain just after dark - total here was 0.32".

Finally, the observed 00 UTC sounding from TWC is shown below - compare to the forecast sounding from the midnight WRF-NAM shown in previous post, which was quite accurate.

Thunderstorms Return To Southeast Arizona Today

Edited to add: Forgot to mention below - At 7:00 am MST this morning the observed dewpoint temperature in Yuma was 72F, but north up the river in Las Vegas dewpoint was 17F. A slight difference of only 55F.

Thunderstorm activity continued to be very suppressed over southeast Arizona yesterday - above shows detected CGs through 24-hours ending 5:00  am MST this morning (from Vaisala and Atmo). Northern Mexico was also suppressed to what seems an amazing extent. Below shows CGs for 12-hours ending at midnight last night emphasizing the near total lack of thunderstorm activity across all of northern Mexico.

The time-series of GPS PW at the university for past seven days (above) indicates steadily decreasing amounts from the 12th through the 14th, but with a slow and slight increasing trend now apparent. Values however continue around 30 mm, which is more representative of pre-monsoon conditions.

The morning sounding from TWC on campus (above skewT plot from SPC) shows several aspects of interest. The troposphere continues to be essentially a two-layer atmosphere above and below 500 mb. Temperatures at top of the lower layer have decreased considerably as moisture has slowly increased and the boundary layer has become very deep. However, there is a very nasty inversion centered at 500 mb, and this feature will be the important player today, as it limits available CAPE considerably and caps convection rooted at lower elevations. This inversion is, however, maximized around Tucson, when other soundings are considered across the region. So its evolution during the day will be important.

The 06 UTC run of the WRF-NAM at Atmo forecasts some storms in Pima County at 7:00 pm (above), and more so in Pinal County. The sounding forecast for TWC at 6:00 pm (below), however, has almost no inversion present and what there is has been lifted up to around 400 mb - quite a difference from the morning observation. There's no real way to know how the inversion might be changing today, other than to keep an eye on the character of any convection that occurs - development of Cbs to the north would indicate that the model was on track.Any storms that might develop would have the potential to produce strong, local downbursts due to the very deep boundary layer.

Storms Last Saturday - Extended Period Of Little Activity For Third Week Of August

We've been away the last two days at Santa Rita Abbey (~5 north-northwest of Sonoita) and missed the Saturday evening storms and lightning show in Tucson - there was 0.27" in gauge when we returned late yesterday.

Storms set in where we were beginning about 4:30 pm Saturday and continued until well after dark - my guess is a half inch or so there. Photo above shows a downburst along the west flanks of the Whetstone Mountains from storm producing light rain. The sun kept shining under the clouds until dusk, producing a spectacular show of rainbow after rainbow. Nice double rainbow shown below.

Both photos show how very green the landscape is out there right now.

The GEFS plumes for QPF at TUS (below from 06 UTC last night) forecast little to no chance of rainfall at the airport through the 23rd - so a nice chance to catch up with the weeds, which have been growing wildly.

Suppressed Again Yesterday - Chance Of Storms Today

Middle clouds around this morning led to beautiful sunrise. Thunderstorm activity was quite suppressed again in our local area yesterday. Plot of detected CG flashes below (from Atmo and Vaisala) is for 24-hours ending at 6:00 am MST this morning. Two ALERT sites in Catalinas had rain of less than 1/10 inch during past day.

The morning sounding from TWC (skewT plot above from SPC) remains very unstable, even though PW has continued to slowly decrease. Middle-level temperatures have cooled substantially. Afternoon storm bases would be quite high, producing gusty outflows that could locally be severe. Steering level winds are muddled, with no organized flow regime established yet in lower troposphere. Winds around 500 mb will be trending toward north to north-northeasterly during the afternoon. Both the WRF NAM and GFS versions forecast late afternoon storms for eastern Pima County that propagate rapidly southward. The composite radar forecast below (from 06 UTC WRF-NAM) is valid at 6:00 pm this evening. The GFS version forecasts considerably more activity, but was also too active at sunrise. Regardless, could be an interesting evening.

Morning Storms Yesterday Followed By Severe Clear

Edited to add: The NWS PHX Office was interested enough in the strange "subsidence bomb" yesterday that they posted the following loop to twitter - see (thanks to Mike Leuthold for alerting me):

https://twitter.com/NWSPhoenix/status/763834595989000193

It was a very strange day yesterday, with the thunderstorms that produced the morning rains over the Catalinas and parts of Pinal County dissipating by midmorning. ALERT rainfall shown above occurred mostly before or a few hours after sunrise. Plot of detected CGs below (from Atmo and Vaisala) ends at 7:00 am MST this morning, but the blue tones indicate that the storms of interest occurred 21 to 24 hours ago. Outflows from these storms were weak and couldn't move south up the Santa Cruz Valley, and folks in the metro area were left looking at dark skies to the northwest during the morning.

When the thunderstorms dissipated, a huge circular area of subsidence and clearing expanded rapidly outward, covering almost the southeastern half of Arizona. The satellite image above is from 12:45 pm MST yesterday. It was intriguing to watch this "subsidence bomb" expand hour-by-hour in the visible imagery. I went back and looked at the 12 UTC WRF-NAM forecast of vertical velocity at 600 mb, and it actually seemed to forecast something like this happening. 

This morning the atmosphere over eastern Pima County remains moist and unstable below about 600 mb, but the winds below 400 mb remain very chopped up and disorganized (12 UTC TWC skewT above from SPC). Probably another strange day on tap with storms trying to develop mostly on the mountains. The 06 UTC WRF-NAM forecasts subsidence to dominate over much of Pima County this afternoon, with larger storms staying off in Cochise County and along the Borderlands. Both versions of the model suggest an uptick tomorrow, and then a serious dry-out coming in from the east on Sunday and Monday. Forecast below is from WRF-NAM for PW at 11:00 am on Monday morning.

A few mornings with lower humidity and cooler temperatures wuld actually be most welcome after the past week or so.

Heavy Morning Rains Again?

Heavy clouds hanging on the Catalinas at 7:00 am MST this morning, as PW in lower half of troposphere remains very high. Could see some lightning flashes on north side of mountains while I was walking.

Yesterday provided the classic example of how difficult it is to predict convective events two and three days out - while the forecast POPs for yesterday (5:00 am through 5:00 pm) had been 80% for a number of forecast periods, the main event for the metro area occurred on Tuesday morning. Yesterday was mostly suppressed in eastern Pima County, with only 9 ALERT sites reporting light rains, mostly in the Catalinas. Plot of detected CG flashes for 24-hours ending at midnight last night shows fairly widespread thunderstorm activity for the state - there were some severe storms along the Utah border and also in Maricopa County.

This morning thunderstorms developed within the past few hours, over and north of the Catalinas. See Mike Leuthold's Day Two discussion from yesterday afternoon, since the WRF models were forecasting early am storms.Composite radar image above is for 6:40 am. ALERT rainfall below is for 3-hours ending at 7:00 am - with some heavy amounts on north side of Catalinas.

The 12 UTC morning sounding plot for Tucson (above, from SPC) is very much as forecast by the WRF models yesterday, with a very unstable situation with little lift needed for storms - there appears to have been a MAUL layer at sounding time between 700 and 850 mb. Will storms be triggered over the metro area this morning? Tough call, since the storms have generally been shifting slowly northward. However, there appears to be a weak outflow ( see MesoWest surface plot for 7:00 am below) moving southward toward western parts of the City - probably a coin toss or a bit better right now for rain at the house.

Unusual Large-Scale Setting Today

The large-scale setting today is somewhat unusual and is similar to transition situations often seen at the end of the summer. Graphics above are 12 UTC analyses from NCAR RAP of 300 mb (top) and 700 mb (just above). At 300 mb a fairly strong trough is located out west near the Colorado River. However, the 700 mb trough has lagged behind, or been overrun, by the features in the upper-half of the troposphere. The 700 mb flow is much weaker and with a fetch from the subtropics and high moisture content.

Below are morning sounding plots (skewTs from SPC) for Tucson (top) and Yuma (bottom). The Tucson sounding indicates a two layer troposphere - moist and tropical airmass below 500 mb and a dry, westerly trough above. The Phoenix sounding is similar. However, the Yuma sounding is more typical of a Plains severe thunderstorm environment, as is the Flagstaff sounding.

This meteorological split personality results in a situation today where the main threat for the southeastern third of the state remains heavy rain storms, while the northwest third could experience severe thunderstorms. The middle third has a poissiblity for a bit of both.

A very interesting day on tap.

Yesterday Morning's Heavy Rain Event

First - regarding the post just below. I have taken a close look at the webcam views to the north. The fast-running loop from Atmo made it appear that a construction crane collapsed. However, the crane is back this morning, so it was apparently rapidly lowered by folks at construction site as the storm front approached.

The heavy rain event unfolded yesterday morning as storms developed northward over the metro area. The ALERT Network map above is for the 24-hour period ending at 7:00 am MST this morning. Areal coverage is 87 sites of 93 with rainfall.I count 22 sites with greater than an inch.

Here at the house we received 2.84 inches of rain, most of which fell from 8:30 to 9:30 am. This was the heaviest rainfall in a day or less since I started records here in 1999. There have been 5 other days during the years with amounts of 2.00 inches or more - heaviest of those was 2.32" on August 13th 2008.

There were six high-water rescues in the City but apparently no injuries or deaths.

The new version of Pima County's ALERT Network precipitation maps appears not to allow the user to examine the past 12-hours, nor to select specific periods of data. I show below the metro west sector of the network for the period 7:00 pm through 7:00 am this morning. The event was well over by evening. This is also illustrated by the plot of detected CG flashes at bottom (from Atmo and Vaisala) for 24-hour period ending at 1:00 am this morning. The green tones indicate flashes that occurred during the morning yesterday.

I am hopeful that some of the key aspects of the new ALERT display can be upgraded so that the data base can be easily accessed for past periods in support of research efforts. If I have missed these capabilities in the new system, someone please send me a heads up.