Monday, August 31, 2009
Just a review of my rule-of-thumb regarding Tropical Storms and Gulf of California surges.
If a storm passes through the shaded box shown above moving with a westward component, then the winds and pressure rises east and southeast of the center trigger a surge. The surge usually occurs within about 24 hours after the storm moves north of 20N.
However, if a storm is moving to the north or north-northeast the situation becomes more complicated. In this case (or if the storm actually enters the GoC) the pressure gradient can point down the GoC and the only pressure rises are behind the storm. Forecasting the weather and storms ahead of the TS becomes complicated by many factors, including MCS developments as hot, unstable air flows into the circulation from mainland Mexico; the track and speed of the TS as it decays; the strength of easterly winds north of the system; etc.
It appears that Jimena is going to be one of these tough ones to outguess. Art Douglas notes that a large MCS may develop tonight along the first rainband over the southern GoC and act to push the subtropical air rapidly up the Gulf. So we'll have to wait and watch what this powerful Hurricane does.
Hurricane Jimena (IR image at 14Z above) will move north of 20N tonight and low-level moisture will push strongly into the lower Colorado River Basin. Note that Jimena has a very large cloud shield associated with it. Moisture continues a slow increase over southern AZ with quite a bit of cloud cover and a few showers over the southwest part of the state and drier conditions to the southeast. Impacts from Hurricanes moving along Jimena's predicted path usually occur sooner than expected here. Later, when the decaying storm moves north, the details are determined by the final path of the disturbance.
There was a slight uptick in storms yesterday - thunder and outflows around town with some rain on the Catalinas. Five of 93 ALERT stations had measurable rain but amounts were light. The really strong push of GoC moisture has yet to materialize, but the morning soundings at Tucson and Phoenix have very steep lapse rates and with the current amount of moisture present it appears that there will be CAPE in the lower elevations this afternoon. The NAM forecasts increasing easterly steering winds during the evening as the first disturbance over the GoC squeezes north - thus, there will be a good chance for storms with downbursts during the late afternoon and evening, and some of these may produce severe winds and dust.
Tomorrow, and beyond, depends upon a myriad of complex mesoscale details that will be determined by storms today, cloud cover, strength of easterly winds, exact track of Jimena, and etc. Hurricanes Nora in September 1997 and Javier in September 2004 provide nice examples of how the final impact of tropical storms can differ widely here in southeast Arizona.
Sunday, August 30, 2009
Photo of storm on north side of catalina Mountains on Sunday evening 30 August 2009. There have been rumbles of thunder, some outflows, and smell of rain this late afternoon and evening here at house. Observed CGs with this storm but it could not maintain itself across the Catalinas. A pleasant start to what will be very substantial changes next 48 hours. It appears this evening that a sustantial surge of low-level moisture is underway over the GoC and this fits with the WRF forecast discussion today - see
The probabilities of precipitation will increase markedly tomorrow and Tuesday as moisture increases and several short-waves approach from the south. An excellent shear profile for severe thunderstorms tomorrow if the CAPE increases.
Saturday, August 29, 2009
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
846 AM MST SAT AUG 29 2009
DISCUSSION...THE UNMODIFIED KTWC SOUNDING SHOWED A PWAT OF 1.03
INCHES...A LIFTED INDEX OF MINUS 6 AND A CAPE OF AROUND 1800 J/KG. ...
The morning Tucson sounding is shown above - it had about 0.4" of precipitable water but the GPS PW indicated that it actually had about 0.6". No lifted parcel in the sounding had the character described above. So, I assume that something went awry - perhaps an examination of a different sounding?
Chuck Doswell comments:
I think I'd argue that it's not YOU who is confused! You're being very generous, as usual. One wonders how this sort of erroneous diagnosis could have been done, of course. Unfortunately, the diverse tools (some of which are amazingly poor at what they purport to do) at a forecaster's disposal in today's offices makes it difficult to track down.
Friday, August 28, 2009
The 02Z IR image above shows that MCSs in Mexico yesterday evening developed far south of the border. There were some storms later in southwestern New Mexico, where there is a bit more precipitable water. Conditions were very suppressed in Arizona yesterday with record level heat.
This morning (28 August 2009) the 500 mb anticyclone is centered over Laughlin NV with very warm temperatures west and north of the center. Cooler, but very dry air continues advecting around the east and south quadrants of the high. It appears that this anticyclone is now acting as a lobe of the Pacific subtropical high, thus the very dry fetch. The high is forecast to sink a bit south and elongate west-to-east during the next 36 hours, keeping Arizona mostly under upper-level subsidence - more dry heat.
There is a distinct disturbance at the southern end of the Gulf of California, but in the short-term this feature moves off to the west and is replaced by what will perhaps be a stronger invert trough. There is also an upper-level trough moving southwestward across southern New Mexico and this feature will have little impact, except perhaps to enhance MCS development in Mexico tonight.
In the longer-term, the next couple of inverted troughs are forecast to move more to the north and west and, inconcert with troughing in the Pacific and much lower pressures in the Great Basin, will likely bring a surge of low-level GoC moisture back into Arizona by Sunday am. The deeper subtropical moisture may also shift north Sunday afternoon and Monday as the 500 mb anticyclone is pushed to the east. There may be a Tropical Storm thrown into the mix by Sunday. So, there's still hope that we will see some more storms as August closes out.
Thursday, August 27, 2009
Another monsoon day wasted in Phoenix. The 12Z/27 sounding [above] at Phoenix was about the ugliest thing I have seen and really says it all. Despite seeing some activity on the 21st, rainfall amounts were isolated and only a Trace was recorded at Sky Harbor. The weather pattern doesn't hold much hope for us folks up here through at least the weekend.
Another thing I wanted to mention was the incredible weather Phoenix saw at this time last year. One of the biggest thunderstorm outbreaks I can recall hammered the Phx metro on the night of the 28th. It was an interesting setup because not much activity was happening during peak hours, despite an atmosphere that was primed for severe weather. Things finally got going in a big way shortly after dark. The link below is from the Phoenix NWS and describes the event thoroughly.
I was in charge of the morning sounding from August 22, 2009. During the run the RRS antenna locked-up which happens a few times each year. In order to get the antenna to track again it needs to be reset. It is my understanding that during the reset process no data is collected. I know for a fact that data was missing between 665 mb and 615 mb in the raw RRS data. I don't know how this might impact the RAP plotting routine. On AWIPS the sounding plot looked much less "hygristorally challenged" than usual. Hope this helps.
Wednesday, August 26, 2009
Dewpoints continue on a downward trend, with 60s hanging on only in southwest Arizona at Yuma and Gila Bend. Precipitable water has fallen to values around three quarters to an inch. The 500 mb anticyclone has shifted south with its center over the central Gulf of California this morning. It will shift north over Southern California and then over southern Nevada during next 36 hours. The steering flow will become very favorable for moving storms toward the low deserts; however, the subtropical low-level air has been pushed down into Mexico again. So we'll be in a down mode with only some mountain storms to east and south until some feature acts on the GoC to push the moist air back north. Right now the only things in sight would be falling pressures over the lower Colorado Basin and MCSs moving out over GoC.
In the longer term the ECMWF keeps the subtropical air to the south through the weekend before bringing some subtropical disturbances north toward the Southwest. So, a not very exciting end to August as things appear now.
Tuesday, August 25, 2009
Looks like the RAP website data are slightly different from the SPC archive:
perhaps due to missing data? Even the Wyoming radiosonde data web site has data for that layer. What isnt clear is if there is some quality control occurring at the RAP website.
I should have mentioned the wind void on the RAP sounding plot - this gap was not a problem with the RRS data, as winds were observed through this layer.
There is some kind of software problem in the RAP plotting routine that sometimes causes winds not to be plotted through a layer - reason unknown, but I've seen many examples of this problem during last several years.
Good example of why, if something looks strange in a sounding, one should cross-check with the plots/data at other sounding access sites, e.g., SPC, Wyoming, FSL, etc.
Monday, August 24, 2009
Unfortunately, HMTs at some offices are in the practice of baselining from indoors...making it difficult to tell how the RH sensor is performing. Not being RRS-trained, it may not be possible to baseline outside. There's a GPS-repeater inside each office for baselining the GPS... I'm not 100% sure if the T/RH/Pressure are also baselined at that point.I'll look into this, unless someone with more knowledge can chime in.It would be awesome to send back hundreds upon hundreds of bad sondes to Lockheed if the sensors could be determined bad pre-release! :)
Yesterday was marked by storms that primarily stayed on or near the mountains. Only 39 of the 93 ALERT stations had measurable rainfall and these were the higher elevation stations. The mountain storms produced locally heavy rains - 11 ALERT stations had a half inch or more. Three stations in the Redington Pass area had more than an inch - that storm was visible from the house and produced a cool easterly outflow of about 30 mph here - otherwise a day with only anvils overhead. The Rucker RAWS station in the Chirachuas reported 1.98" for the highest amount I found this morning.
The piece of the 500 mb short-wave that came ashore last evening was stronger than expected and it is moving across Arizona today. Vertical wind profiles have become southwesterly and the dry air aloft is deepening. Precipitable water profiles continue a slow downward trend. The Tucson sounding this morning indicates a bit of CAPE for low elevations, but more drying or slight warming in middle levels would wipe that out.
The NAM has been too wet for this event, and I suspect that will again be the case today. Storms will again be mostly on mountains in eastern parts of state. Steering flow remains bad for low elevation regions - storms occur at the house under this kind of flow only if they develop nearly overhead.
The week begins with drying, and then there will be several features rotating around the 500 mb anticyclone as it shifts westward. Looks like the NAM predicted TS will track too far west to trigger a surge. It also appears that the S/W coming ashore today may try to track all the way around the anticyclone and come back our way from the east later in the week.
Sunday, August 23, 2009
Activity was considerably down yesterday, compared to Friday, with most storms occurring at higher elevations and on the flanks of the mountains. The sunshine we got wasn’t enough to heat up a well mixed boundary layer and it was quite a cool day. Additionally, the S/W that had passed by on Friday night was much stronger than the model runs suggested. Measurable rain fell at 44 of the 93 Pima County Alert Gauges, with quite a bit of that occurring before noon. Only 3 sites had half an inch or more and
This morning there is dry air aloft above 500 mb; the lower troposphere continues moist with moderate amounts of CAPE; the deep moist plume is completely to the east of
Saturday, August 22, 2009
Storms moved rapidly into the Tucson area yesterday afternoon between 2:30 and 3:00 pm. I shot the above photo at 2:45 pm looking toward the east. There were several reports of severe winds in south central Arizona and a majority of reporting stations had gusts of 40 to 50 mph. Rain coverage in the Tucso metro area past 24-hours was essentially a 100%. There were 8 stations that had 0.50" or more, and 1 station with more than an inch (DEQ at Ruthraff 1.14"). So, a nice storm event that produced winds of 40 to 50 mph and 0.39" here at house - my 50% chance yesterday was way too conservative.
The subtropical moisture/recycled moisture now covers most of Arizona with precipitable water amounts of around 1.6 to 2.0 inches indicated at lower elevation stations. It appears that the main vorticity lobe mentioned yesterday has moved past Tucson, so that storms today will be driven primarily by orography and local heating. The Tucson sounding has bad data in middle levels, but the data below 650 mb indicate that a little lift or heating will produce parcels with around 1000 m2/s2 of CAPE. Indeed, showers are continuing in many areas this morning.
Winds continue strong through most of troposphere, with some veering from the SSE to the SSW from lower to upper levels, so storms will be fast movers. Water vapor imagery indicates that drier air will be trying to spread northward at high levels but this should not impact the CAPE in the lower troposphere today. Stronger storms today carry a threat of very heavy rain rates and wet downbursts.
The NAM model keeps the moist plume over eastern Arizona through Monday, even though a large portion of the Pacific cutoff/trough comes ashore across the Great Basin. The NAM also indicates a Tropical Storm developing and moving to the southwest of the end of Baja, so that will be something else to watch.
Friday, August 21, 2009
Another day yesterday with no rain across the Pima County ALERT network. There were some brief, strong, high-based storms to the east of
The discussions of the last two days remain relevant. The wrinkles for today:
Dewpoints are up at some sites and down others, indicating the low-level moisture we have is sloshing around on local circulations.
Precipitable water values across southern
The Phoenix and Tucson soundings this morning show some CAPE but still indicate a layer of dry northerly flow around 600 to 800 mb. Cloud bases would be high and lots of kick needed to get things going at low elevations.
Heavy cloud covers much of southern
Regardless, it appears there will be adequate instability for mountain storms to organize and propagate into the deserts. There is a threat of severe downbursts and outflow winds today – especially if we can keep the sunshine into mid-afternoon. My forecast for the backyard – 50% chance of storms and at least some rain.
Tomorrow – things tomorrow will be heavily dependent upon how the situation evolves during the next 18 hours or so. The models do indicate the vertical wind profile becomes southerly and uni-directional, which is a bad profile for here since storms tend to anvil out overhead. Models predict greater chances for rain over the weekend, but we’ll have to wait and watch.
Thursday, August 20, 2009
Wednesday, August 19, 2009
Yesterday - several small, high-based Cbs developed southwest of Tucson late - probably in response to the very hot temperatures at surface and the realatively cool middle-level air. These were quite nice visually at sunset but I didn't get a photo. No stations in the Pima County ALERT network measured rainfall in past 24-hours.
Tuesday, August 18, 2009
1. 0.80 2009
2. 0.86 1900
3. 1.21 1960
4. 1.25 1960
5. 1.27 1995
6. 1.42 1940
7. 1.60 1996
8. 1.71 1939
9. 1.74 1978
10. 1.75 1991
Normal for this period is 3.88".
Monday, August 17, 2009
Sunday, August 16, 2009
Saturday, August 15, 2009
Friday, August 14, 2009
The subtropical low from
Northwesterly winds prevail this morning but the morning soundings at
Other miscellaneous – out in the Pacific, Guillermo has become a hurricane and is moving off toward the west. The
Thursday, August 13, 2009
Also to note, preliminary reports of a microburst occurred at Luke Air Base with wind gusts of 69 kts recorded at 22:22 to 22:26 in the Mesowest network.
Melissa Wagner, ASU
What a pleasant change – cool, light showers, and heavy clouds (see 7:20 am KEMX radar chart). The subtropical system from
Almost all observing stations in a large area of southeast and south-central
There is substantial
Wednesday, August 12, 2009
The cloud cover certainly won out yesterday. Slightly cooler temps, higher pressures and a retreat of the moisture back into
As for today – MCSs have formed and dissipated through several cycles in
The soundings this morning at
It appears that this transition has begun and that it will affect southern
Was also /very/ dry in
Tuesday, August 11, 2009
This figure for the last two weeks illustrates graphically why I have been referring to the "failed monsoon." This figure is from the Western Regional Climate Center (WRCC) at the University of Nevada. http://www.wrcc.dri.edu/anom/ari_anom.html
Yesterday storms occurred to south and southeast and also briefly on the Catalina and Rincon Mountains. A large MCS developed (see 03Z IR image above) with most of the active cells staying south along the borderlands (
The most interesting changes overnight occurred across south-central and southwestern Arizona where a surge of low-level moisture from the Gulf of California occurred, bringing some dramatic 24-hour increases at 6 am. As per:
Needles CA +33F;
So two issues for today will be: 1) how quickly will the cloud cover break up to allow heating? 2) will low-level moisture continue to slowly increase? Note that with precipitable water up to around 40mm in the PHX and TWC soundings we have had a nice increase, but the really moist subtropical air has not quite moved into
The western circulation center of the massive, mid-level anticyclone that covers the southern half of US and much of the subtropics has consolidated along the New Mexico/Arizona border. This provides a nice wind profile for moving mountain storms toward the deserts. The
Monday, August 10, 2009
Low-level moisture has slowly increased over southeast
Sunday, August 09, 2009
It was a crisp and very sparkling morning here at the house. I didn’t check the thermometer until 6:30 am and it was then at 64F – may have been lower. I see some stations away from the city were in the 50s. Unfortunately, we now have had 3 more consecutive days with no rainfall across the Pima County ALERT network.
Not much more to say about current conditions. Precipitable water amounts are less than half an inch across the state except for the far southeast where amounts are hanging in there at three quarters of an inch.
The morning upper-air charts show a large, inverted trough in the easterlies (present from 200 mb down to 700 mb that arcs from
So, there are some things that we should keep an eye on as the new week gets underway.
Friday, August 07, 2009
Thursday, August 06, 2009
The small S/W at 500 mb that was the focus of yesterday’s activity moved north further east than the NAM predicted and is over southwestern New Mexico this morning. The northward movement of this feature – combined with the onshore movement of the California cutoff – has realigned features at 500 mb. This morning there is one anticyclone center over the southern GoC and one over the eastern Colorado/New Mexico border. Arizona is now within the southwesterly flow regime around the cutoff low.
Dry low-level air is present over western Arizona; however clouds and light showers linger from Nogales into the Phoenix area. The upper-air charts and VAD winds indicate that drier air should spread eastward today and limit storm activity to the higher elevations of the state, basically east of Tucson to Flagstaff line. Both the Tucson and Phoenix soundings were modified by the cloud deck overhead. The Tucson sounding also had several superadiabatic layers that produced a false cooling in the layer from 520 mb to 450 mb.
The NAM forecast indicates that storm activity shuts down through the weekend, with anticyclone in southwest again pushed into northern Mexico, and the main lobe of 500 mb high pushed far to the east. A very large inverted trough (one of the few we’ve seen north of 20N this summer) is forecast to be moving westward across central Mexico over the weekend (see above 84-h NAM forecast), while the tail of the west coast trough is sheared off and left behind off the coast of southern California and northern Mexico. So, it appears that the larger-scale situation will again become very complicated as we start next week.
Finally, Tropical Storm Felicia was named by the NHC two days ago and was forecast to remain a TS – but this morning she is a Category 4 hurricane. Too far away to be of interest to us, but folks in Hawaii are probably keeping a close eye on her.
Wednesday, August 05, 2009
There apparently wasn’t enough rain from yesterday’s high-based storms to support much recycling, and the morning soundings remain quite dry below 600 mb. There will likely be
I note that the
Another dilemma this morning relates to the
My outlook for here at the house is that today is the best day since July 22nd and that measurable rain chances are about 50%.