Wednesday, July 29, 2015
Light showers continue this morning (composite radar above is for 7:15 am MST), with most echoes to the west and south of Tucson, moving toward the northwest. Heavy clouds cover much of Arizona, New Mexico, and northern Mexico this morning (visible satellite image below is from 7:00 am), meaning much reduced heating today is likely today.
The morning skewT plot of the TWC upper-air data (below, for 12 UTC from SPC) presents another onion shaped sounding, with cool, worked-over air below 800 or so mb. Winds aloft have increased from the south-southeast above 700 mb, providing a bit more steering flow for today. The directional shear profile remains fairly poor, but anvil shading may not be very important today, given stable low-levels and significant clouds. Typically, this would be a totally down day, however, the situation is a bit more complicated today. While the low-levels have been stabilized, a couple of layers aloft from 800 to 600 mb have considerable CAPE, if lifted a bit, thus elevated showers and thunderstorms are possible, if this situation continues through the day.
The forecast models are all over the place for today and I'll just discuss them very briefly. The 06 UTC runs of the WRF model at Atmo are quite different for this afternoon. The NAM version has mostly light showers, with some thunderstorms on the mountains. The GFS version forecasts afternoon thunderstorms across the metro area. The new NWS NAM forecasts a very wet day for most all of southern Arizona, and keeps the weak, inverted trough at 500 mb nearly stationary out to our west. The 12 UTC run of the NWS HRRR forecasts rather anemic storm activity mostly to east and north of the Tucson area. It's best to wait for Mike's discussion of the latest morning runs of the WRF model
Posted by Bob Maddox at 7:50 AM
Yesterday afternoon produced the most active thunderstorm day so far during the summer. Photos here are from house - top, looking south at 5:29 pm MST and below at very bottom, looking north at 5:45 pm.
The composite radar chart above is from 4:20 pm and shows numerous heavy thunderstorm cells. The storms (as indicated by a number of radar loops) were slowly propagating in a wide variety of directions, building into most unstable, nearby air. Thus, there were a number of heavy rainfalls reported. The surface observations below from TUS show several gust front passages and a wet microburst (or downburst) that produced gusts to 64 mph (also reported were poles down just to the northwest of the NWS observation station). The time series of T and Td from Atmo (second below) seems to indicate three distinct outflows moved across the campus - Atmo had just over 0.40" of rain, while the airport measured over an inch during a very short period, with a storm total of 1.36".
The plot of CG flashes for 24-hours ending at 11:00 pm MST last night (above from Vaisala and Atmo) shows the clusters of heavy storms over parts of metro area, and also indicates the lackadaisical movement of the cores. There were some CGs very close to here and I shut down the electronics for about half an hour. Rainfall mostly stayed south, with only 0.08" during the thunderstorm and then a period of debris cloud rain after dark produced another 0.10" - there was just enough rain yesterday at house to keep this from being driest ever July since I began the records here.
Plot for the Metro-west sector of the ALERT network below illustrates the hit-and-miss nature of the storms. Across the entire network only 9 stations did not measure rainfall, making yesterday very high POPs day.
Posted by Bob Maddox at 5:55 AM
Tuesday, July 28, 2015
There were several isolated thunderstorms in southeastern Arizona yesterday afternoon as per detected CG flashes above through 1:00 am MST this morning (from Vaisala and Atmo). Thunderstorm activity increased more markedly across northern Mexico - thus, a bit of an uptick from preceding day. There was one report of 0.04" of rain across all of the Pima County ALERT network and basically the break in summer storm activity this side of border continued for yet another day.
The larger-scale pattern this morning continues to be ill-defined and has some pro indications for increased storm activity, but also carries considerable cons with it. The water vapor image below is from 6:30 am this morning. The most distinct circulation feature in the upper-troposphere is a cyclonic circulation centered just west of Baja at about 25 N. There is a skinny snake of dry air air along the east side of this circulation. The circulation appears to be moving westward, or even a bit west-southwestward this morning. Over Texas a broad, diffuse, inverted trough continues in the upper-levels. The most distinct circulation center embedded in the broad trough appears (via loops) to be centered south of the Big Bend. The latest NAM forecasts weaken this feature and move it west-southwestward.
The skewT plot above is from SPC for the 12 UTC data from TWC. Pros - PW is up and over 1.60 inches; CAPE is moderate and considerably up from recent days. Cons - Indistinct steering flow this morning due to L/V winds from 500 to 300 mb; upper-level flow is from south, which is quite negative since anvils spread ahead ahead of storm cores and stabilize the low-levels.
SkewT below is the same plot but for PHX upper-air data. Conditions at Phoenix are much different than those here at Tucson. Pros - none that I see (note that PW is considerably lower due to the dry air just above the surface). Cons - No CAPE, plus all those listed for Tucson.
Last evening there were only forecast runs for the WRF-NAM at Atmo, with the GFS versions missing. The 06z forecasts from WRF are forecasting a big storm day (at least in comparison to past past week or so) for eastern Pima County. The forecast sounding above is for TWC at 2:00 pm MST - the CAPE is indicated to be over 1600 J/Kg (appears to be considerably too high for the mixed layer conditions), even though PW is down some (~1.50 inches) due to the northwest winds in the BL. The model forecasts a much more organized southerly flow to be in place above the BL this afternoon - but will this actually happen? The new NAM forecast seems to indicate continued L/V middle-level winds this afternoon. The model forecasts storms to move fairly quickly toward the north; however, if the steering flow remains weak, storms will hug the higher elevations and only come off if there are organized outflows. There should definitely be an increase in thunderstorm activity today, but many locales will probably be "anviled out" due to the southerly winds at upper-levels.
The two forecasts below are from the 06Z WRF-NAM. The first is of composite radar echoes valid at 4:00 pm this afternoon and the bottom is for accumulated rainfall though midnight tonight. The 00 UTC version of the WRF-NAM forecasts kept most thunderstorms south of the Tucson metro area from southeast to southwest, and with much less rainfall. All-in-all, a very difficult day for precise forecasting.
Posted by Bob Maddox at 8:29 AM
Monday, July 27, 2015
Yesterday was almost totally suppressed and quiet north of the border, except for the far southeast corner of Cochise County. Northern Mexico was also very much down, after stabilizing impacts of the MCSs on Saturday night - Mike Hardiman mentioned this in his comment yesterday.
This morning we continue to be under the cloudiness wrapped around the west side of the 500 mb anticyclone, even though the center of that feature has shifted far to east this morning and is now over Arkansas. There are two features of interest within the easterly flow on south side of anticyclone - a weak cyclone is located over southern end of Baja and a large, somewhat diffuse, inverted trough is over east Texas. The inverted trough is forecast to move slowly in our direction, while the feature over southern Baja shifts northward and mostly westward.
The TWC morning sounding (skewT for 12 UTC data below from SPC) is considerably more moist this morning with PW up by almost 4/10s of an inch and surface dewpoints at airport up more than 10 F from 24-hours ago. But, even with this moistening, I estimate only a sliver of CAPE for low-elevations this afternoon, although mountains should be more unstable. The troposphere overhead seems devoid of any meaningful, organized wind field - a really miserable situation that continues into our new week.
Forecasts from the various Atmo WRF runs overnight indicate only isolated showers and thunderstorms along the border and some of the higher elevations of southeastern Arizona today. Things will likely pick up by mid-week as the feature from Texas approaches.
Posted by Bob Maddox at 7:33 AM
Sunday, July 26, 2015
First, a review of yesterday afternoon. There were several thunderstorms over far southeastern Cochise County yesterday evening. Plot of detected CG flashes above (from Vaisala and Atmo) is for 24-hours ending at 6:00 am MST this morning. There was a substantial increase in thunderstorm activity over northern Mexico during past 12-18 hours. The 09 UTC IR image below shows a large MCS over northeastern Sonora, with a new MCS developing to its southwest. the second MCS is now active over southern Sonora.
Outflows from the decayed, nearer MCS may be helping keep low level dewpoints south and east of Tucson in the 60s F this morning; however, high elevation RAWS stations indicate that dry air aloft continues, except perhaps right along the southeastern Borderlands. The WRF models forecast storm activity to continue today mostly in northern Mexico, with perhaps some light, middle-level sprinkles over southeastern Arizona.
Plot above shows the 12 UTC skewT plot (from SPC) for the NWS TWC upper-air data.. Low-levels continue very dry, with no CAPE. There is a layer of seriously bad data aloft from about 650 to 550 mb, where the lapse rate is strongly super-adiabatic. Middle and upper-level moisture continues as debris cloudiness and moisture advects northward around the west side of the anticyclone, which is now over far eastern New Mexico.
The sounding data should not have been transmitted given that it was obviously bad. It has now gone into several upper-air data bases to perhaps confound future users of the observations. The sounding below from NWS/SRP in Phoenix, presents a better picture of how pathetic conditions in low-levels continue today.
Posted by Bob Maddox at 7:54 AM
Saturday, July 25, 2015
Almost a completely down day yesterday, as per the early WRF model forecasts. The plot of detected CG flashes (above, from Vaisala and Atmo for 24-hours ending at midnight last night) shows a few flashes in the east end of Catalinas and also down in the Santa Ritas, with most activity in Cochise County. Across the ALERT network only 4 sites had very light rainfall during past 24-hours, so yesterday and last night were essentially zero POPs periods for almost all of Pima County.
There is abundant debris cloudiness from Mexico at middle and high levels across all of southeast Arizona this morning (visible image below from 6:45 am MST). A quick look at this image would lead one to believe that we should be in thick of the action today.
However, the 12 UTC sounding from TWC (skewT plot above from NCAR RAL) shows dry air within most of old boundary layer, i.e., up to 700 mb - only moisture is holding on near surface. The morning Phoenix sounding (below for 12 UTC) is even more dismal with very dry air below 700 mb. Note the northwest winds at 10 to 20 mph up at Phoenix, which will be pushing this drier air our way. So, it is not surprising that the model forecasts this morning indicate only isolated thunderstorms in Santa Cruz and Cochise Counties today.
While June was interesting, July has proven to be very depressing here in the local area.
Posted by Bob Maddox at 7:36 AM
Friday, July 24, 2015
First, a review of yesterday. As is typical, the morning showers put a severe damper on storm development in eastern Pima County for the remainder of the day. The composite radar chart from 5:00 pm MST yesterday afternoon (above) shows weak echo across the entire local region from Nogales to Oracle, with thunderstorms far removed to the east and west of Tucson. Plot of detected CG flashes (from Vaisala and Atmo below, through 11 pm last evening) shows the same pattern, with a large void of thunderstorm activity reflecting the path of the morning showers.
Not surprisingly, rainfall did not live up to expectations, given the extremely moist atmosphere. From 10:00 am (after the showers had mostly exited to the northwest) through 5:00 pm only 17 ALERT sites measured rainfall amounts that were very light (exception was 0.59" at Dan Saddle in the Catalinas). Then for the period from 5:00 pm through 5:00 am this morning 31 sites had rainfall from cloud-debris showers moving in from the south. The nighttime amounts were very light, with the heaviest outlier being 0.28" on Redington Pass. Here at the house we had 0.08" from the morning showers and then 0.03" as the evening showers passed by. All-in-all, quite a dismal day for late July.
This morning has arrived with some sunshine. The visible satellite image - above for 6:00 am MST - shows a broad swath of cloudiness that extends across all of Sonora, eastern Arizona and into Colorado, but with large breaks over southeastern Arizona. Presently skies are mostly clear over the Catalinas as per campus webcam view below. We can start the day with better hopes than yesterday.
This morning's skewT plot of the 12 UTC TWC sounding data (above from SPC) remains quite similar to those of the past couple of mornings - very wet with CAPE available given heating, but with a hostile wind profile for the metro area. The light/variable winds in the lower troposphere now are at 700 mb and below, with light southerly steering winds for thunderstorms. Matt Bunker's PW analysis for the 12 UTC soundings this morning (below - links are available in an earlier post) shows the TWC sounding to be by far the wettest in the Southwest.
Possible problems for this afternoon, even with sunshine, include: the heating will build a new boundary layer up to about 750 - 800 mb but this will have some CIN above that would need outflow help at low elevations; upper-level wind profile is quite negative and would again stream anvil clouds from south over the metro area (indeed, several of the WRF forecasts for the afternoon show this happening); and some of the WRF forecasts begin drying out our low-levels from the northwest this afternoon.
I have looked at the WRF runs from both 00 and 06 UTC last evening - I can not really find a variant that does not seem to have some problems. Some appear too dry, some too cloudy, some too stable, and etc., so I'll wait for Mike's discussion of the new 12 UTC forecast runs. However, none of the four versions forecast thunderstorms at low elevations of eastern Pima County. The 00 UTC WRF-NAM forecast of composite radar echoes for 4:00 pm this afternoon (below) is as active as it gets in that forecast. Other versions have more storm activity on the mountains right around Tucson. I tend to feel that these early forecasts from WRF are a bit conservative, and that we'll see more thunderstorm activity nearby this afternoon - which is probably a reasonable guess, given yesterday's total lack of nearby thunderstorms.
Posted by Bob Maddox at 8:02 AM
Thursday, July 23, 2015
The morning showers obscure the Catalinas at 6:36 am MST (above). Appears to have been a bit more measurable rain here at house, but airport has reported moderate rain, 76 F, and south-southeast winds at 15 mph. The showers do not reach much above 25,000 ft MSL and there have been no lightning flashes detected. The visible satellite image below (from 1340 UTC) shows a thick cloud mass covering much of northern Mexico and eastern Arizona, which will cause difficulties this afternoon, as temperatures will likely not make much of a rebound.
The TWC morning sounding plot (below from SPC) is very moist, and there will be substantial CAPE present later today at spots where there is some sunshine, especially at higher elevations. The issue, of course, will be whether morning outflows completely dominate and shut down afternoon activity. Winds remain quite light, except in upper-half of troposphere - another day with marginal steering winds but potential for heavy local rains and perhaps wet downbursts.
The WRF-NAM forecast from 00 UTC last evening appears to have forecast the early morning showers most accurately of the various models. It forecasts limited thunderstorm activity during the afternoon, as opposed to the 06 UTC runs which forecast a very active afternoon across eastern Pima County. Setting is one in which we'll just have to watch the situation evolve and also take a look at the new 12 UTC WRF forecasts from Atmo.
Posted by Bob Maddox at 7:15 AM
A line of light showers is moving slowly northeastward across the Tucson metro area this early morning. The radar composite chart above is from 5:35 am MST and view of Catalinas at 5:49 am (below) shows the leading edge of clouds with the shower band.
The PW has continued to climb through the night and values are now about the highest they've been in past five days - time series of GPS-estimated PW on campus above indicates values now just over 1.80 inches. There were light showers over the metro area yesterday afternoon but the thunderstorms stayed locked onto higher terrain where they formed. Here at house we had a brief shower, no lightning or thunder, and I found 0.02" in the gauge. This brings the July total here to only 0.22" - one of the driest Julys to date, since I set up the gauge here. Across the ALERT network only 8 sites had rainfall yesterday, also slight amounts except at Florida Canyon where a thunderstorm dumped a quick 0.83".
The skewT plot (from Univ. of Wyoming) for the TWC sounding at 00 UTC yesterday afternoon is shown above. There was only a sliver of CAPE in mid-levels to support the showers. Winds below 400 mb remained light and variable, helping to account for the nearly stationary storms. The plot of detected CG flashes for yesterday through midnight (below from Vaisala/Atmo) shows small circular clusters of lightning with the storms in eastern Pima County.
Posted by Bob Maddox at 6:07 AM