Quick-Hitting Event During Night

Last night's weather occurred very much as forecast by various models during past few days. This morning the cold front has moved off to the east, and there are only a few low clouds hanging over the Catalinas (as per Computer Science web cam view above).

Strong frontal passage occurred right around midnight, as per the T and Td time series (above) from Atmo. Dewpoint temperatures increased some an hour or so prior to the strong temperature fall, but rainfall occurred after the front passed. Winds on the roof of the Atmo building (below) gusted a bit over 60 mph when the front passed.

Precipitation amounts were very light behind the front - Atmo and TUS had 0.03", here we had 0.06", and DM reported 0.14. Winds during the night across southeast Arizona generally gusted to speeds of 30 to 50 mph. The RAWS station at Guthrie reported gusts to 63 mph. The WRF-GFS forecasts keep showers though the weekend confined to the northern half of Arizona. However, the model forecasts quite cold temperatures, especially tonight and Saturday night - tabular model forecast of highs and lows is shown below. Biggest question for here at house is whether or not low temperatures will drop into the teens.

Quick Look At Early WRF-GFS Forecasts

The next weather system is forecast by Atmo's high-resolution WRF-GFS model to move rapidly across Arizona late this afternoon and tonight. Warm temperatures and gusty south-southwesterly winds will precede a strong, Pacific cold front. The above forecast of composite radar images (from the midnight forecast run of the WRF-GFS) is valid at 1 am MST tonight and indicates showers across eastern Pima County.

The forecast surface plot (above) valid at midnight tonight indicates the cold front moving into the northwest portion of the metro area. The forecast for 1 am MST tomorrow (below) indicates the front has passed through the metro area, with light showers immediately behind the front.

Total precipitation through noon tomorrow (above) is forecast to be generally light with highest amounts over the mountains. The main impacts will be the winds and much colder temperatures. Thick middle and upper level cloudiness (see 8:30 am MST visible satellite image below) has already spread across southern Arizona and this will impact the degree of heating and boundary layer mixing that occurs today - it's currently a dreary, chilly morning here in north Tucson. Cold troughiness at 500 mb lingers over Arizona for at least the next 5 days, bringing what may be the coldest period of the winter.

Next Short Wave Off Northwest US

Above image is of water vapor in the upper troposphere at 14 UTC this morning (Wednesday, January 9th). The next short wave to affect the Southwest is off the the coast of the northwestern US and western Canada.     This feature is forecast by the models to dig into the Southwest and bring a strong cold front with it (front passes the Tucson area around midnight on Thursday night). The water vapor image seems to indicate that this system has a nice link with subtropical moisture from east of Hawaii. However, the 13 UTC eastern Pacific MIMIC-TPW product from CIMSS, at the University of Wisconsin, (below) indicates only a weak plume of higher PW currently with the system, indicating mostly middle-to-high level moisture and clouds. This system seems somewhat like the last two in that it has trouble bringing moisture (at least in the nodel forecasts) through the large, very dry area west of Baja.

This morning's forecast of Atmo's high-resolution, WRF-GFS model forecasts another light precipitation event - above is total precipitation for the period ending at 5 am MST Friday morning. The surface plot forecast from WRF-GFS valid at 11 pm tomorrow night (below) indicates the front, with light showers, approaching the Tucson metro area. The forecast sweeps the front across all of the metro area by midnight.

As substantial ridging occurs over the Gulf of Alaska from the 10th through the 14th, several  more short waves/vorticity maxima dig down the east side of the ridge into the Southwest. These all are very cold, with low thickness values, meaning that any showers they might generate would bring snow to low elevations in southeast Arizona - something to keep an eye on.

First Week Of New Year - Cold Mornings

A quick post regarding the cold start to the New Year. The lows here at the house (near the northern City Limits of Tucson) have been quite cold for the first nine days of 2013, as per:

(1) 24F (2) 26F (3) 25F (4) 35F (5) 25F (6) 29F (7) 26F (8) 27F (9) 28F

So, regardless of what happens later this week, it's been a frigid (for here) start of the year already.

Closed Low At 500 mb Continues To Head For Sonora

The 500 mb forecast from the ECMWF run at 00 UTC last evening is shown above - this panel is valid at 5 pm MST this afternoon. The models continue to forecast a glancing blow for Arizona, as the system digs into northwestern Sonora. The Atmo versions of the WRF model continue to keep precipitation mostly on the light side and south of the border. The forecast below shows total accumulated precipitation through 5 pm MST tomorrow (Tuesday, January 8th - forecast is on the larger domain 5.4 km grid).

On the 1.8 km, fine-mesh grid (above) the model forecasts very light precipitation amounts through 5 pm tomorrow, except for a few spots in Cochise County right along the border. Most activity forecast to be in northern Mexico, as with the last system - see photo in earlier post of snow in Mexico.

Next System To Clip Southeast Arizona

The weather system currently bringing some rains and mountain snow to southern California, is forecast to dig southeastward into northern Mexico. The models have consistently been forecasting this to be a fairly dry system once it moves into Mexico, until it progresses into Texas and picks up GoM moisture. The national meeting of the American Meteorological Society will be occurring in Austin and it looks to be a very soggy and perhaps stormy meeting.

The early run of Atmo's WRF-GFS is also similar with its forecast of very light precipitation accumulations over southeastern Arizona. Forecast above is for total precipitation through 6 am MST on Tuesday morning (January 8th). The new run of the NWS NAM forecasts slightly higher amounts for the southeast corner of the state - up to about a half an inch. The WRF-GFS starts the event out with strong and gusty east-southeast winds as pressures fall out to the west. Surface forecast below is valid at 10 am MST this morning.
The next system (around the weekend of the 12/13th) is forecast by the ECMWF to pick up considerably more Pacific moisture.

Saturday January 5th - Miscellany

Ron Holle reports that at his location in Oro Valley (elevation about 2700 ft MSL) he had only two morning lows of 32F or colder during 2012 - far different than here at the house. He's clearly in what is sometimes called the Catalina's banana belt. Any other reports from Tucson?

On the 31st of December we drove out to Santa Rita Abbey, northwest of Sonoita, about 3 pm. There were cold-air snow showers over most of the mountains. The heavy shower above is just off the southeast flanks of the Rincons. Photo below is of building showers over Mt. Fagan, to the west of Highway 83. Mt. Fagan is the northern-most peak of the Santa Rita Mountains (elevation 6189 ft MSL). The NWS Tucson radar was sited so that Mt. Fagan would "protect" Kitt Peak from its lowest elevation tilts. Unfortunately, this selection of the site to put Mt. Fagan between the radar and Kitt peak also resulted in a very large sector of significant blockage (by a feature of the Empire Mountains) to the southwest of the radar (which eliminates any low-level radar coverage of storms over Nogales and the International Border Crossing). It is fairly rare to see Mt. Fagan totally snow-covered.

Snow scene in northern Mexico posted by Paul Caiano to the MAPS list. He reports: "Southwestern storm making for a snowy scene in MEXICO! This is northern Chihuahua and a picture posted by webcamsdemexico. The train is the Chihuahua-Pacific railway (El Chepe) but no specific location was given. The peak elevation of that rail line is more than 8,000'."

 An earthquake of 7.5 magnitude occurred off the coast of extreme southeastern Alaska this morning.

The GFS and ECMWF are now fairly well in sync for the next short wave and closed low at 500 mb. The forecasts shown are from 00 UTC last evening and are valid at 5 pm tomorrow afternoon.

Comparison Of 96-hour Forecasts

There were some rain and snow showers along Arizona's southeast borderlands last evening and night, with heavier precipitation to the east across southern New Mexico. Web cam image above shows a view of Las Cruces, New Mexico, at about 8:30 am this morning.

Low temperature here this morning (Friday, January 4th) was 36F after three mornings in the middle 20s.

ECMWF and GFS 500 mb forecasts are shown (above and below) for 96-hours from 00 UTC last evening - the forecasts are valid at 5 pm MST next Monday afternoon. There is considerable difference between the models at this relatively short time range. The difference in forecast heights is on the order of 150 m over the northern GoC, where the ECMWF forecasts a fairly significant closed low. Will be interesting to watch how the forecast models converge with time. Edited to add: The 12 UTC forecast runs this morning show several of the GFS ensemble members trending toward the ECMWF, with fairly dry but deeper short-wave digging into the Southwest. Morning run of NAM is fastest, and most northerly of the models for this next short-wave.

Brief Summary Of 2012

I have just compiled a brief summary of 2012 weather here at the house. Photo above is of storms on the afternoon of July 24th - a situation that occurs far too frequently, with the house in the clear slot between storms.
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Precipitation

Total rainfall was 9.16 inches (about two inches below normal observed at the airport). By season: JFM = 1.03"; AMJ = 0.20"; JAS = 6.36"; OND = 1.57".

Number of days with measurable rain = 39; with 0.50" or more = 5; with 1.00" or more = 1 (1.57" on July 15th).

Number of days with thunderstorms = 32.

On two of these days the storms were marginally severe with wind gusts to about 60 mph.

I seem to recall that early one morning there was a brief period of very light snow - but I can't find it in my records.

Low temperatures

Days with a morning low of 32F or colder = 64 (freezing temperatures occur here along the Rillito much more frequently than they do at the airport!).

Days with a morning low of 25F or colder = 9 (coldest morning was 22F on March 3rd).
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This was a fairly typical year here in the far north side of the City of Tucson, with no extreme or significant severe weather events of any kind. During the summer there were thunderstorms here about every third day but but rainfall only on 25% of the days.

Art Douglas Storm Report

Art Douglas reports 5 to 6 inches of snow on the ground this morning at his place in Ash Canyon (south of Sierra Vista). Photos are of yard (above) and of his new house from a distance (below).

Quick Summary End Of Year Event

Showers began here at the house about 5:30 pm yesterday (Sunday, December 30th) and this morning 0.42" had fallen at house. Better than I had expected (web cam above shows what appears to be a fairly light snowfall on Mt. Lemmon), and light rain continues. It is hard to know quantitative details on precipitation across eastern Pima County, since many gauges experienced both rain and snow. Coverage was essentially 100%. Low here at house was 38F and rain was not close to mixing with snow. Morning sounding at NWS Tucson failed, so there is no hard information on local thickness. The NAM analyzes 534 dm but subjective analysis indicates that this may be too low (my estimate was 536 to 538 dm). Regardless the snow level did not come down as low as some model forecasts indicated. The NWS winter weather advisory (below) from yesterday was quite good, indicating snow level would stay at or above 3500 ft MSL. I also note that this advisory painted the exact area affected by the advisory, which is good. Happy New Year to all - more next year!

Cold And Damp Storm Approaching

This morning the 500 mb trough stretches from Montana deep into the subtropics west of Baja. The 12 UTC NAM analysis (above) indicates several vorticity maxima within the trough. The one over northern California will serve to broaden the base of the trough and slow it down just a bit. The NAM 24-hour forecast predicts the primary vorticity center to be located between Tucson and Yuma at 12 UTC tomorrow morning. The NCAR RAL 250 mb analysis for 12 UTC this morning is shown below. Upper-level winds appear to be strongest to our east and this would typically result in the trough moving quickly by. However, the jet west of the trough is centered somewhere just off the west coast, so it's character is not quite well-defined.

The Atmo WRF-GFS model apparently ran at midnight last night. However, I can only find the tabular forecast for southeast Arizona - above. Note that the WRF-GFS forecasts a fairly light precipitation event, with forecast to have the most precipitation at 0.47". It appears that this system is not pulling subtropical moisture into the Southwest, as the models had tried to do earlier in the week. The NAM model now forecasts thicknesses over or near Tucson tomorrow morning down to 534 dm - meaning the snow level will likely drop into at least parts of the metro area late tonight/early tomorrow. Assuming of course that the model has accurate forecast. Some roads and bridges in eastern Pima County could be slick at sunrise tomorrow. Caution advised.

Cold And Unsettled For End Of Year

It is a picture perfect afternoon today (Friday, December 28th) in Tucson with bright, clear skies, as the Computer Science web cam shows above. Temperatures are in the 50s, after morning lows below freezing (26F here at the house). We continue to be in a pattern that is causing considerable uncertainty in the computer model forecasts - both short and long-term. The next storm was forecast to be somewhat warmer and perhaps with a subtropical moisture connection. However, the models are now indicating another cold storm event, and one with uncertain moisture influx from the southeast to southwest.

The two ECMWF graphics above are valid at 72-hours (i.e., at 5 am MST Monday morning, the 31st) for 500 mb (top) and 850 mb (just above). The ECMWF forecasts a trough over the Southwest that has two embedded vorticity maxima at 500 mb - one over northern New Mexico and the other over southwestern Arizona. Moisture at 850 mb is fairly widespread in the ECMWF forecast and 1000-500 mb thickness values are below 540 dm, which means fairly low snow levels. Thickness values of 534 or less usually mean snowfall into the Tucson metro area.

Similar forecasts from the GFS are below, for comparison. The GFS forecast emphasizes the vorticity maximum over southwest Arizona. Both models forecast a trailing vorticity maxima that is advecting southeastward beneath the ridge over western Canada. This is a complex situation with subtle differences between the two models at 72-hours - one to monitor closely, given the model uncertainty. Note that GFS ensemble members forecast precipitation over southeast Arizona for the 12-hours ending Monday morning in 10 of 12 members - a fairly high probability for precipitation some time on the last day of the year.

Showers And Strong Winds This Afternoon

 
A band of convective showers (ahead of the short wave/vorticity maximum moving across southern Arizona) is moving across the Tucson metro area. Radar and view of Catalinas above are from around 4 pm MST. The showers produced a strong outflow and sharp temperature drop here at house - estimate brief gusts to 40 to 45 mph. The time series below (from Atmo) show the temperature drop and also indicate gusts to 70 mph on the roof of the Atmo building. Only a trace of rain here, but definitely a nasty end to the afternoon.

Subtropical Moisture Plume For New Years?

I've shown two panels from the 144-hour ECMWF operational model (valid time is 5 am MST on New Years Day). The model forecasts a 500 mb closed low (above) over northern Baja at this time. The 700 mb forecast valid the same time (below) indicates a broad plume of subtropical moisture from the south, as well as substantial moisture from the east, impacting Arizona. If accurate, this forecast indicates a wet start to the New Year for most of Arizona and New Mexico. There is considerable spread in the ensemble forecasts, however, a number of members are picking up a subtropical moisture plume. Will be interesting to watch how this situation evolves.

Summary Christmas Day

Christmas here in Tucson started out cool (low here at house 38F) and sunny. But by early afternoon the day had become very cloudy and dreary, as heavy, thick cirrus spread over southern Arizona from the Pacific - above image of campus and the Catalinas was at 2 pm.

The 500 mb pattern was much like the mid-range forecasts highlighted in earlier posts. Strong, negatively-tilted short-wave trough digging into north Texas (12 UTC analysis above) was main weather maker for the day. NWS watches and warnings for Christmas are shown below. Heavy snows from eastern Oklahoma into northern Arkansas occurred, with a severe thunderstorm outbreak over the lower Mississippi Valley (second graphic below).

This morning (Wednesday the 26th) the storm is impacting the Ohio Valley and heading for the east coast. The web cam image below shows early morning conditions in Indianapolis, Indiana, which was reporting heavy snow and 30 mph winds at the time of the image (about 7 am CST). Unsettled weather continues over the West and I'll take a look at the forecasts a bit later.