Thursday, December 18, 2014

More Light Showers Moving In

More showers approaching Tucson early this morning - December 18th. Composite radar chart above is from a bit before 6:00 am MST. These showers are quite light compared to some of the moderate to heavy showers that passed through the area yesterday afternoon. There were also a few CG flashes in eastern Pima County, so there were isolated thunderstorms mixed in with the showers. Here at the house we ended up with 0.51", which was a fairly typical amount at the lower elevation ALERT stations. Heavy precipitation on mountains with snow occurring at highest elevations. Web cam from Summerhaven below is from early afternoon yesterday. Couple of stations on Redington Pass had over an inch of rain. So quite a nice event.

Wednesday, December 17, 2014

Light Rain Across Tucson Area Before Sunrise

EDITED AT 07:30 MST TO ADD - Just checked the rain gauge and total here so far is 0.12". At 06:00 am MST ~90% of the 92 stations in the ALERT network had measured rainfall, with amounts generally less than 2/10ths of an inch. 

Woke up to rainfall here at 5 am MST (above composite radar plot from NWS TUS radar is for 6 am). The showers and rain developed with the first Pacific, middle and upper-level, short-wave that had been forecast by various models to cross southern Arizona during the past night. Models varied in forecasts of precipitation with this first wave. The water vapor satellite image below is also for 6 am. It shows that the short wave, in the upper half of troposphere, is already along the Arizona/New Mexico border. But there is a smaller-scale feature passing over eastern Pima County at the time of the image, and the showers seem to have developed with this feature. The next Pacific short wave is just off the coast of southern California.

A plot of detected CG lightning flashes through 5 am MST is shown above and indicates considerable thunderstorm activity over northern Sonora. There had been a couple of flashes just on this side of the border at the time of the plot. 

Plot of the morning sounding from NWS Tucson is shown below from the Storm Prediction Center in Norman, Oklahoma. The tropopasue is nearly saturated through its depth (red line shows measured temperature along the track of the balloon and green line shows dewpoint temperature). The temperature profile indicates the top of the troposphere (topopause) is at about 350 mb (~ 8 km above the ground level at the NWS office). Winds observed along the path of the balloon are plotted vs height at the right, and are in knots, Precipitable water was calculated to be a bit more than 2/3rds of an inch (precipitable water is the total depth of water vapor measured through the atmosphere by the sonde instrument). It is the amount of PW and the profile of RH that help to determine the likelihood of clouds and precipitation. This is a general overview of what this graphic shows, in response to a question a few posts ago. So, the showers are not surprising given the very moist character of the atmopshere this early morning.

The SPC provides a nice interactive page to answer questions about what's plotted on the diagram as well descriptions of the parameters they calculate from the data. This can be found at:

Tuesday, December 16, 2014

Very-Long Range Model Forecasts For Christmas

The short-term forecasts have considerable fluidity as the model forecasts are a bit volatile. So, I thought I'd look all the way out to Christmas Day, just out of curiosity. By 240 hours out, the GFS ensemble spaghetti plots do indeed look like a bowl of the same.

So here goes. Above are the ECMWF 500 mb and surface forecasts valid at 5 pm MST on Christmas Day. Below are the same two forecasts from the GFS. Both are from the operational versions of the models. The ECMWF is shown "on the top" for obvious reasons.

The ECMWF forecasts the most significant weather action to be over the West with a strong cold front crossing the Southwest, a strong low pressure system over Colorado, and nasty upslope snow from eastern Colorado north to Montana.

The GFS patterns are generally similar but considerably damped. The model forecasts an east-west cold front across the U.S., no significant low pressure systems in the interior, and scattered areas of light snow across the northern half of country. The small area of heavier snow forecast southwest of Chicago seems to be right where the ECMWF forecasts the greatest 500 mb height rises that would likely be accompanied by considerable downward motion. 

In eight to ten days, or so, we'll know how these forecasts actually played out. Cheers.

Monday, December 15, 2014

Unsettled Weather To Continue This Week

The weather here in the Tucson area and southeast Arizona will continue unsettled as we head toward Christmas. This is well indicated by the current NWS forecast probabilities of precipitation (POPs) at the airport - Wed 50% Wed Night 70% Thur 70% and Sun 30%.

However, the global forecast models (GFS vs ECMWF) have considerably different details within their respective forecasts. Shown above are the 500 mb forecasts valid at 5 pm MST on Wednesday (GFS top and ECMWF bottom). The short wave and vorticity maximum forecast by the ECMWF to be over the Texas Panhandle is much weaker and faster in the GFS forecast. However, the short wave over the Southwest is essentially similar in both forecasts, although the GFS has considerably more small-scale structure and a more complex vorticity field. So, the exact timing and details of what will happen here is not well-defin, even for the short term.

The forecasts below are longer-term and are valid at 5 pm MST next Sunday, December 21st. The closed low forecast to be over the Southwest by then is slower and considerably stronger within the ECMWF forecast. The ECMWF forecast, were it to verify, would lead to a stormy, wet and cold day for us next Sunday. The ECMWF also forecasts a significant coastal storm to impact the middle Atlantic coast next Sunday; whereas, the GFS forecasts much higher heights and zonal flow over that region. So, considerable uncertainty about the details of the weather for much of country persists through the week - keep a close eye on things if you plan to be travelling next weekend.

Sunday, December 14, 2014

Brief Overview - Sunday Morning

When it rains in desert people sometimes lose their grip on reality (photo by Mike Christy, Arizona Daily Star).

Graphics here show 24-hour precipitation amounts across the ALERT network (for sectors covering the Catalinas, Rincons, and metro Tucson) at 7 am MST this morning. Coverage over eastern Pima County was 100%, but with a large range in amounts, as the figures illustrate. If there was snow, it was at the highest elevations of the Catalinas and Rincons. The highest rainfall measurement was just over two inches at the Mt. Lemmon ALERT station. One site southeast of here indicates only 0.04". If that's a good measurement, then gradients were over 2 inches across a fairly short distance. Here at the house we got 0.41", thanks to several fast moving, moderate showers that seemed to develop just upstream on the Tucson Mountains and then come right across our location. Art Rangno out in Catalina reports about 2/3rds of an inch. This has been yet another event where the skill and accuracy of the model predictions was very impressive, out to and just beyond 7 days!

Also a damp and frosty morning here at house with the temperature a couple of degrees below freezing - too early though to know exactly the low will come in at.

The details of the global models (at least ECMWF and GFS) are considerably different in our area, but both models forecast another event in the Wednesday/Thursday time frame and then another around next weekend. I note that the GFS ensemble members all seem to forecast precipitation over southeast Arizona for Wed/Thur, and the current NWS for the airport reflects this with 50% POPs Wednesday and 60% Wednesday night.

Saturday, December 13, 2014

Showers Out To The West Currently

At a bit past 6 am MST this morning there were light to moderate showers (composite radar from TUS above) out to the west and northwest of Tucson - from Sells to Marana and on north into the Phoenix area, where coverage was more substantial. The TUS morning sounding (below) has moistened considerably below 700 mb and deeper moisture will move into eastern Pima County as the front approaches this morning.

Friday, December 12, 2014

Quick Look At Tomorrow's Weather Event

As I had mentioned a number of days ago, the Winterhaven Festival of Lights seems to draw in several winter storms during its two week run. This year things are right on track, as the Festival kicks off tomorrow - Saturday, December 13th. This will be a quick-hitting frontal event, as the forecast models have been predicting for more than a week now.

There has been a pronounced atmospheric river (AR - aka the Pineapple Express) ahead of and along the Pacific front. This has brought a fairly warm and very wet storm to California, with snow levels at fairly high elevations. The above graphic (CSU CIRA blended PW analysis for 12 UTC this morning) shows that the AR may be losing some of its organization. But, PW values of an inch or so have nosed up the GoC and are a bit higher along west coast of Baja. So, the system will pull some decent PW northward at lower levels to feed showers and moderate convection along the front as it crosses southeastern Arizona tomorrow.

The forecast sounding for TUS (below, valid 11 am MST tomorrow) is from the WRF-NAM early run (both versions are fairly similar this morning) and indicates a bit less than 1 inch of PW that is mostly below 600 mb (there is also a bit of CAPE forecast, again below 600 mb).

The WRF runs forecast the front to cross eastern Pima County during the late morning/early afternoon. Graphic above is of WRF-GFS forecast composite radar echoes valid at 10 am Saturday morning. The WRF-GFS forecast below shows the models forecast of accumulated precipitation through 5 pm MST tomorrow afternoon. The model forecasts the event to just brush southern Arizona with light rainfall, keeping the main impacts to our north in eastern Maricopa County and the Rim Country, although the high elevations near Tucson pick up heavier amounts than the low elevations.

The WRF model has, for reasons that are not obvious, underplayed the forecasts for the last several precipitation events, and this will likely be the case again with tomorrow's event. I expect that the lower elevations of eastern Pima County will pick up some higher rainfall amounts than the model is currently forecasting.

Finally, to put this weather event into a regional perspective, the graphic at the bottom is the WRF-NAM forecast for accumulated precipitation through 5 pm tomorrow, on the 5.4 km grid. California continues to catch the brunt of this storm, with very heavy rains forced by both the southern coastal ranges and the Sierra Nevadas.