Weak Short-Wave Trough To Cross Southern Arizona

Another cold morning here along the Rillito Wash with a low temperature of 28 F at the house.

Above is this morning's NAM analysis for 500 mb. The short wave and vorticity maximum west of Oregon and northern California is forecast by the model to dig southeastward and cross Arizona tomorrow and Saturday. The 500 mb forecast below is valid at 12 UTC on Saturday morning.

This system is moisture starved, but models forecast some light showers across parts of southern Arizona tomorrow into Saturday morning. Above is current NAM forecast of precipitation through noon on Saturday, while the early WRF-NAM from midnight forecasts some amounts of a quarter inch and bit more over in Cochise County. WRF forecast of rainfall below is also valid through noon on Saturday. 

Winter Arrives In Parts Of Country

Here in Tucson, something went awry in the airport temperature data - note that the 24-hour minimum temperature for the 18th is indicated above as 10 F - that would have been quite some record for here.

Far to the east, along Lake Erie, tremendously heavy, lake-effect snows buried parts of Buffalo and environs with as much as 6+ ft of snow. Photo during the event below (un-credited) conveys the unbelievable character of this event.

Frost In The Park

The low here at house this morning (Tuesday, November 18th) was again 25 F. There was a light frost on the grass area down at the Natural Resources Park, sparkling brightly as the sun rose.

The viewing was quite good early this morning, as there were only some patches of thin cirrus scooting by. I sat, bundled up, out in the courtyard for over half an hour, but the Leonids were a no show, while I was out watching. So it goes.

Freeze This Morning - Leonid Meteor Shower Tonight

It was very cold here in north Tucson along the Rillito Wash at sunrise this morning. Wore winter gear on my walk and was still a bit cold. Low temperature here at house fell to 25 F vs 36 or 37 F at the balmy NWS airport ASOS location. Other spots with readings in the 20s include the Sasabe RAWS site with a low of 25 F and the Empire RAWS site with a low of 24 F. In contrast, the low at the high-elevation, Mt. Hopkins RAWS station was only 38 F.

The Leonid meteor shower occurs tonight, with maximum activity expected after midnight. The early WRF-NAM forecast (above) of OLR indicates a forecast of mostly clear skies valid at 3 am MST. The WRF-GFS forecast is similar with only a bit of thicker cirrus out to the west. The outlook for the shower itself is that it will be a weak display this year.

Cold Front Now Across Most Of State

View of Catalinas (above) shows moderate Cu over the mountains at 7:30 am MST this morning (Sunday, November 16th). The MesoWest surface plot (below) is for 4:30 pm and indicates that the cold front has now crossed most of the state. The gusty winds (gusts are in red) were much as forecast by the WRF-NAM yesterday. Saw gusts over 40 mph earlier today at Yuma, and there were reports of blowing dust in Yuma and Phoenix areas.

Time-series of the surface observations for Luke AFB (above) and TUS (below) show that dewpoints have plummeted with the passage of the cold front. Luke Td is below 0 F and TUS has fallen into the teens. Places where the winds set will be very cold (for southern Arizona) the next two mornings. If the winds go calm here along the Rillito, I expect we'll see temperatures into the 20s. Low here at house ahead of the front this morning was a chilly 38 F.

Colder Air From The North Late Sunday

Chaotic skies this morning with a mixture of high clouds and low clouds hanging on the high mountain tops. Views here, from just after 8:00 am MST, are of Catalinas from campus (above) and from Kitt Peak looking south (below).

As a strong, short wave crosses northern Arizona on Sunday, cold air will invade the state from the Great Basin. The early WRF-NAM forecasts for 10-m winds on Sunday afternoon (November 16th) are shown above for 3:00 pm MST and below for 6:00 pm. The cold air brings quite strong winds out along the lower Colorado River area first on Sunday and then spreads across Maricopa County, with strong winds forecast for the Phoenix metro area by evening Sunday. It appears that freezing temperatures will return here along the Rillito in north Tucson early next week, bringing the second round of 32 F, or colder, morning lows for this Fall.

Bitter Cold Over Parts Of West

Very cold arctic air, particularly for early November, has plunged southward over the Plains, parts of the Northwest, and along the Front Range. View above (from Jack Hales weather site - link to right) shows the Tetons this morning. While surface plot below, centered on Denver, shows some of the cold temperatures this morning. Both graphics are from a bit before 8:30 am MST this morning, November 12th.

Cold Air Drainage Across The Tucson Area

First - a bit on recent minimum temperatures. Low here yesterday morning was 45 F and there was a light breeze blowing. But, down right along the Rillito only the tops of tallest trees were indicating wind, and it was calm and much cooler along the wash This morning it was calm both here at the house and down along the wash. Low at house was 37 F and it was markedly colder down along the wash - so much so that I wished I'd worn gloves.

Yesterday evening we drove out to Santa Rita Abbey (~ 5 miles north-northwest of the highways 82/83 intersection in Sonoita). I had noticed several years after the NWS started doing grid point, deterministic forecasts that the forecast low for the Abbey location was routinely too warm. Often times when we arrived out there just after dark, the temperature (from car's outside air thermometer reading) was often already colder than the forecast low for the next morning. So out of curiosity, re were the grid point forecasts improved now, I checked the forecast yesterday for the Abbey grid - it was for a low of 47 F. When we arrived up there (about 10 after 8:00 pm MST) the indicated outside air temperature was already down to 39 F - so things yesterday remained similar to a number of years ago. 

I also checked the grid point forecast for this morning's low at the Mt. Hopkins RAWS site: 46 F versus an observed low of 55 F. An east wind continued through the night there. It was calm at the Abbey, while a bit to the north-northeast at the Empire RAWS site the low was 38 F, with a light westerly drainage wind through the night. Point of all this is that the conditions related to observed low temperatures in this area are very much locally dependent upon wind, elevation and nearby land surface conditions (assuming clear skies, low humidity, and light winds at the larger scales).

All of this long-winded post was triggered by an NWS forecast discussion the other day that mentioned: THE WINDS MAY STAY UP IN SOME VALLEYS  OVERNIGHT...BUT AT THE VERY LEAST WINDS OVER THE HIGH TERRAIN WILL NOT ALLOW NORMAL DRAINAGE OF COOLER AIR INTO THE VALLEYS. This leads, of course, to the question: What is the "normal" drainage flow of cooler air into the valleys around Tucson? I feel that the answers to this question are not well documented or known. For example, the nighttime winds at the airport, under weak synoptic regimes, tend to be from the southeast. But what is not clear is at/from what elevations do these winds originate (for example, last night they certainly were not originating over the high terrain of the Santa Rita Mountains, where the potential temperature remained much warmer than those at the airport).

When I first moved here, I was puzzled by the obvious differences between drainage winds off the south faces of the Catalina Mountains versus those I had observed along the Colorado Front Range. Here at the house there seemed to be no drainage wind on nights with clear skies and light winds coming off the Catalinas; whereas, places I had experience with in Colorado had distinct drainage wind patterns from nearby high terrain into lower elevations. The foothills to the north of the Rillito wash are known locally to be a warm zone relative to lower elevations to the south. It was after a trip up to Summerhaven on Mt. Lemmon that the differences between here and mountains at higher latitudes began to sink in. I'll illustrate with some images snipped from Google.

The image above shows Pike's Peak and Colorado Springs, Colorado. The higher elevations west of Colorado Springs are dominated by rocks and barren terrain, above the tree line. The difference in elevation from the peak (14,114 ft MSL) to the airport (6,187 ft and ~19 miles from the peak at the southeast corner of image) is 7,927 ft.

In contrast, image below shows the Catalinas and Mt. Lemmon (9,159 ft MSL). Note that there is no high elevation tree line and that the Sky Island is covered by dense forest from lower elevations to the summits. The difference between the elevation of Mt. Lemmon and here at the house is 6,790 ft and the distance is about 14 miles. Thus, the elevation changes from the west side of Colorado Springs up to Pike's Peak and from here to Mt. Lemmon are fairly similar. 

The important difference is that the radiative surface characters are very much different (at least under clear, dry, quiescent conditions).
Cooler air that develops during the night at high elevations of the Catalinas can not drain very far down the slopes before compressional warming puts on the brakes. So, no routine cooling winds during the night/early morning here at house during the heat of May and June, at least not from the Catalinas to the north.

Above is an image showing the location of our house and nearby areas. There is some green apparent up in the foothills "banana belt". But a vast area to our east, along washes and toward Redington Pass, is very barren terrain that radiates very effectively. So my feeling is that the very cold temperatures observed along the Rillito develop in situ and over the barren terrain along the north and east slopes of the nearby washes. The image below emphasizes this by showing the area along and north of the Rillito wash within a few miles of the house. Probably the only way to study the local drainage regimes (say off the entire Catlinas Sky Island) would be to utilize a very-high resolution numerical model - one that could capture the terrain and surface character down at resolutions below 500 m.

Mt. Hopkins RAWS Records Gust To 88 MPH

Very nice lighting highlights just after sunrise this morning - above image from 7:00 am MST. As I walked I could see rotor clouds just downwind of Mt. Hopkins to the south. So I checked the RAWS site observations down there and found that the max gusts during last 24-hours was 88 mph, just a bit after midnight early this morning. This is the highest gust speed at Mt. Hopkins that I can recall. The exposed location of this RAWS station is downwind and just slightly south of Mt. Wrightson during easterly winds. Apparently a standing wave from Wrightson surfaces at Hopkins sometimes.

In the longer term, the ECMWF forecasts a very significant plunge of arctic air into much of the U.S. early next week. The surface forecast below is valid at 00 UTC on November 12th. Although the cold front has reached nearly to the Gulf of Mexico at this forecast time, the center of the high pressure is still up in Canada with SLPs of a bit over 1048 mb. I am sure that the media will be dusting off their "polar vortex" hype from last winter as this event unfolds.

Miscellany AM 5 November

Morning low temperature here at house this morning dropped to 41 F sometime before the winds kicked in, up 11 F from yesterday's low.

At longer ranges the ECMWF prevailed again, forecasting the current 500 mb closed low (analysis above for 12 UTC this morning) over Sonora much better than did the GFS. However this feature ended up a bit too far south to pull moisture from Vance back into southeast Arizona. Moisture from Vance, combined with slow-moving cold front in Texas, is bringing heavy rains to much of the state this morning, as per the surface plot below from 1345 UTC.

Finally, the early run of the WRF-NAM keeps fairly gusty east winds blowing across southeast Arizona through morning tomorrow. Bottom is 10-m wind forecast valid at 9 am MST tomorrow morning.

Gusty East Winds Bring Milder Morning Temperatures

I was busy with other things yesterday and did not pay enough attention to the weather, not even look at the WRF forecasts. So, a surprise this morning when I went out for the newspaper - gusty east winds, wind chimes making noise, and much milder temperatures. The above plot from MesoWest shows temperatures, winds, and gusts (red) at 5:45 am MST this morning. Quite a change from yesterday's near to below freezing temperatures. There are quite a few stations reporting gusts of 30 to 40 mph - Mt. Hopkins RAWS site with gusts to 47 mph, above, has hit 56 mph during past couple of hours. The 12 UTC WRF-NAM had forecast the winds fairly accurately - forecast below for 10-m winds at 8:00 am this morning. The model-based temperature forecasts had a low of only 60 F for TUS this morning - a bit overdone, but very good on the 24-hour trend at the airport. Can't take your eye off the ball in the weather world!

First Freezing Temperature Of Fall

Just a quick update - morning low temperatures did fall to freezing and below here in the north side of the City of Tucson, along the Rillito. The low at the house was 30 F, bringing out my winter hat for the morning walk. Yesterday the low ended up at 35 F, falling two degrees after I had read it prematurely. Quite a few lows in the 20s along the San Pedro and Empire RAWS station was at 29 F - coldest I noted was again at the Columbine RAWS on Mt. Graham with a cold 18 F.

Very Chilly Walk This Morning

Lowest temperatures here this morning for the Fall so far. Low at house was 37 F and I could see my breath while walking - first time for that in many months. I am having some challenges with my low temperature readings and I'll discuss that in a later post. Temperatures in the valleys fell to low 30s F this morning and I see that Empire RAWS site hit 32 F. Coldest lows were at the high elevations this morning with the RAWS sites at Rincon and Sollers (Mt. Lemmon) dropping to 29 and 30 F respectively. Coldest I saw was 22 F at the Columbine site on Mt. Graham. Still expect that temperatures will fall to freezing or below here in the north part of Tucson, along the Rillito tomorrow morning.

Yesterday the afternoon showers stayed mostly to the east of Tucson metro as forecast quite well by the early WRF-NAM. Rain fell at the highest elevations of the ALERT network and at the east edge, with 6 sites recording rainfall. The low elevation Tucson forecast zone remained essentially rain free yesterday. The plot of CG flashes above is for 24-hours ending at 11:00 pm MST last night. Some thunderstorm activity developed over the Rincons and Santa Ritas, but with most storm activity to the east in Cochise County.

Vance is now a Category 2 hurricane and is moving northward. The IR image below is from 1430 UTC this morning and indicates significant cold cloud areas north-to-northeast and southeast of the storm's center. The NHC forecast has changed little since yesterday, with moisture from Vance to have most impact over north-central Mexico and parts of Texas. The ECMWF has been bouncing all over the place and current versions continue to form a 500 mb closed low over Mexico, but that feature is now further south and east than in yesterday's forecast, and Arizona remains mostly under the influence of the ridging behind the short wave.

Pacific Front Moving Across Arizona Today

Heavy low clouds were hanging on the Catalinas at sunrise (above is view at 7:00 am MST) this morning as a cold front approaches from west-central Arizona. The early WRF-NAM continues to forecast most shower activity to remain east and north of the Tucson area (total precipitation forecast shown below is through midnight tonight (November 2nd). Morning sounding is moist below about 650 mb with only touch of possible CAPE. Front will be past Tucson area by mid-to-late afternoon, bringing in markedly cooler temperatures, especially in contrast to our warmth of last few days.

Winds have been gusty over south east Arizona past 24-hours with some sites hitting 35 to 45 mph. Art Douglas reports that a new weather station above him in the Huachucas has been gusting as high as 48 mph this morning, with 35 mph down at his location near the mouth of Ash canyon.

The longer term forecasts for mid-to-late week continue to markedly different in the GFS versus the ECMWF (operational members). The ECMWF forecasts a cutoff to form over northern Mexico and linger through the week, while the GFS forecasts a ridge to dominate at 500 mb. The ECMWF actually wraps moisture back into southeastern Arizona in a few days - so something to watch.

Far to the south Vance has been upgraded to a hurricane this morning and may continue intensifying during the next day or so. Visible satellite image above shows Vance at 7:30 am this morning. The morning forecast for Vance from NHC is shown below. The NHC anticipates Vance to eventually shear apart south of Baja as the storm tries to move north encountering the Southwest  trough or closed low in middle-levels. This lets a remnant low drift around at low-levels while middle-level circulation and moisture moves into north central Mexico and parts of Texas. I can not help wonder if Vance marks the end of the eastern Pacific season?

Backdoor Front Into Southeast Arizona This Morning

Chaotic skies this morning with a variety of cloud forms. Upper-level instability led to some buildups above 500 mb, and these were accompanied by virga and mammatus. View of Catalinas above is from campus at 7:00 am MST.

Strong cold front over the central U.S. this morning, with the far southwestern portion of high pressure pushing a backdoor front into southeast Arizona this morning. The main impact of this fast-hitting weather feature will be some gusty east to southeast winds later this morning. Then the trough moving in from the Pacific will dominate our weather for the next few days. The early WRF-NAM forecast from Atmo this morning, shown below, is for 10-m winds valid at 9:00 am this morning. The model forecasts showers and light precipitation with the cold front on Sunday, with rainfall amounts light except at highest elevations. The model forecasts some snow accumulations with the front on Mt. Graham and in the White Mountains. It appears that Monday night could be quite cold here in the north part of Tucson along the Rillito. If skies are clear with calm winds in this area, we will probably see our first morning with temperatures at or just below freezing - so quite a change on the way.

October 30th Quick Update

Features noted yesterday have moved eastward and northeastward during past 24-hours. The upper-level trough that will move across the Southwest is currently at about 30 N 125 W - above is 1100 UTC water vapor image this morning. This feature has mostly some high-level cloudiness with it.

Meanwhile, the NHC has begun forecasts for a new tropical depression (EP-21), that is currently south of southern Mexico.- IR image of 21-E below is from 1145 UTC this morning. The first NHC forecast for this system is shown at the bottom. The depression is forecast to strengthen eventually to hurricane strength as it heads north toward 20 N west of Cabo Corrientes. The global models indicate that the northward advance will be blocked early next week and moisture associated with this system may primarily affect north-central Mexico and perhaps the Southern Plains.

Edited to add: Forgot to mention that if this disturbance becomes a names TS/hurricane it will be "Vance".

A Mild October Coming To End

October has been very mild over much of the country this year. Here in the Southwest there has not been much to write about since the thunder, winds, and dust, with some light rain, back on October 19th. However, it appears that the first days of November will bring substantial changes to the western and eastern thirds of the country. A cold and intense coastal storm is being forecast by some of the models for the East and Northeast out around 96 to 120 hours.

Out here in the West a substantial 500 mb wave will dig southward and eastward from the Gulf of Alaska - centered this morning at about 50 N and 150 W in the above 14 UTC water vapor image. A weak upper-level short wave at about 25 N and 130 W has a substantial plume of upper-level moisture to its south and east that extends down to about 5 N. As this feature moves northeastward ahead of the major feature approaching the Northwest, it may bring some high cloudiness to southern Arizona to close out October. Temperatures over the weekend will be much cooler than the expected low 90s of the next couple of days. The northern part of the major wave will shear away to the northeast and leave behind a weak trough or closed low at 500 mb over northwestern Mexico. Both the closed low and a developing tropical disturbance, see this morning's NHC Outlook below, could come into play for parts of the Southwest and even Southern Plains early next week. The tropical system will be a wild card in the forecasts for next week, as currently model predictions of this feature are all over the place.

Isolated Showers Yesterday

There were isolated showers yesterday afternoon over higher elevations of eastern Pima County. The plot above shows CG lightning flashes through midnight last night - very large decrease in activity when compared to the same plot for Sunday in yesterday's post. Rainfall was measured at 9 ALERT stations (10% coverage) and these sites were in the Catalinas/Redington Pass area and also at the far south edges of network. The early WRF-NAM forecast yesterday was quite good again. Today's model run forecasts only light showers at the very highest elevations of eastern Pima County.

Sunday Was Fairly Active Thunderstorm Day For Much of Arizona

This morning was clear and crisp, as per view of Catalinas above, with temperatures this part of town in lower 50s around sunrise. Yesterday was an active thunderstorm day, especially for October, and the plot of CGs below is for 24-hours ending at 1 am MST this morning (Monday, October 20th). There were two severe storm reports locally - lines and poles down somewhere between Ft. Lowell and River and hail to size of golf balls near Tubac. I suspect that the lines were felled near here, because of power surges and blinking lights.

Across the ALERT network areal coverage of rainfall was more than 70% for 24-hours ending at 5 am MST this morning. There were 19 sites with more than 1/4 inch and 5 with more than 1/2 inch. Keystone Peak, off in southwest part of network had 0.83 inches. Largest amount I noted this morning was 1.14 inches, with thunder and gusts to 52 mph, at Safford. Here at house we had thunder, wind, dust, gusts of 40 to 50 mph, but just 0.14 inches of rain.

It was interesting that storms avoided the higher elevations of the nearby Sky Islands with little rainfall noted at spots like Mt. Lemmon, where POPs and amounts are usually much higher than at lower elevations. The early run of the WRF-NAM model was quite good for yesterday afternoon and it kept the rain at lower elevations here in the metro area. Its areal coverage also appeared fairly accurate.

This morning's sounding at TWC on campus (skew-T plot above) is much stabilized, compared to yesterday's. There is at most a sliver of afternoon CAPE at low elevations and the early WRF-NAM reflects this, with its forecast of only some very isolated showers on the mountains today.

Thunderstorms Across Metro Tucson

Showers and thunderstorm are moving across the Tucson area this afternoon and web cam view above shows high storm bases and light showers between the University and the western Catalinas. Here at the house we had strong outflows (estimated gusts 40 to 50 mph) about half an hour ago, with blowing dust and several power surges.

Composite radar image from NWS TUS below at 2:36 pm shows a number of strong storm cores over southeast Arizona with reflectivities of 50 to 60 dBZ.

Shown here are two products from the TUS radar that are available at the College of DuPage weather site. Thunderstorm tops (at 2:41 pm MST, above) are in the 26 to 32,000 ft msl range, with some higher tops indicated at long ranges (this may be a bit of an artifact due to beam width at those ranges). The 0.5 degree beam, dual-polarization hydrometeor type algorithm (below, also at 2:41 pm) indicates hail and graupel with the storms cores. Quite an interesting afternoon.

Possible Showers And Stubby Thunderstorms This Afternoon

Friday evening sunset - Katie took this photo with her phone as we were driving south on Highway 83. Ridge to west is the north end of the Santa Rita Mountains. Earlier in the afternoon showers over metro Tucson produced a couple of sprinkles and a Trace at the house.

Unusual and somewhat strange situation today. A weak short-wave and closed low is centered out near the north end of the GoC this morning in lower levels. The circulation has a pronounced tilt toward the northeast with height and very warm upper-tropospheric air is within the upper part of the circulation. This results in a very strong inversion just above 400 mb. The above skew-T plot is the early WRF-NAM forecast for Tucson valid at 2 pm MST this afternoon. The inversion is weaker in the model forecast, as would be expected, but still provides a formidable cap to convection rooted at lower levels. The forecast sounding exhibits CAPE from just above 700 mb up to just above 400 mb. Note that the lifted parcel temperature excesses are actually quite large around 500 mb, indicating substantial vertical accelerations in any convection that develops. Any thunderstorms that occur would be fairly short and stubby towers, with bases around 11,000 feet and tops around 26,000 ft MSL. However, the updrafts should be potent enough to produce some lightning and perhaps small hail or graupel. The graphic below shows the early WRF-NAM forecast of composite radar echoes valid at 3 pm MST this afternoon.