Friday, October 31, 2014

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.

Thursday, October 30, 2014

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".

Wednesday, October 29, 2014

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.

Tuesday, October 21, 2014

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.

Monday, October 20, 2014

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.

Sunday, October 19, 2014

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.

Friday, October 17, 2014

Bands Of Very Light Showers Over Arizona

A weak, middle and upper-level short wave is moving into Arizona today from the Pacific. There are several bands of very light showers ahead of this feature. The graphic above is TUS NWS radar composite chart for 8:22 am MST this morning - showers are moving generally northward while the bands are inching eastward. Visible satellite image from 8 am below shows that most of Arizona has heavy cloud cover this morning.

Perspective from the water vapor image, at same time above, indicates the upper-tropospheric vorticity center is probably near Yuma this morning. The 12 UTC TUS sounding plot below indicates that cloud bases are very high - near 500 mb (i.e., around 16,000 ft msl) with temperatures at cloud base being around -6 to -10 C. Today's short wave will be followed by another on Sunday and yet another Wednesday - so some cloudy periods and perhaps more light showers off and on for next five days or so.