Wednesday, February 20, 2019

20190220 Utility/Pima County

Remarkably, the forecast track of the storm and overall precipitation amounts have not changed all that much since the last discussion on Monday.  However, it is uncertain how much snow may fall in Tucson. This forecast is going out to both the utilities and Pima County as there is something for everyone.  I'll start with the precipitation as once again, some truly remarkable amounts fall over the mountains due to a combination of strong upper low plus lifting by air forced up the sides of mountains. Many mid and high terrain locations will see at least 1.5 inches with isolated areas above 3 inches.  Mt Lemmon and Mt Graham may see as much as 4-6 inches of liquid water. However, it's not going to fall as rain like last time and I'll get to that later.   Even the lower elevations do well with widespread 1/2 to over on- inch amounts. The below plot is an ensemble average/most likely of the last 6 or so Arizona Regional WRF runs with amounts in solid colors.  The contours are maximum amounts/worst case amounts.

Temperatures are much colder with this storm and the snow level will be as low as 2500 ft.  At high and mid elevations, snow amounts will be extreme especially because the snow to water ratio will be high, 10:1, at least.  Amounts for Mt Graham and Mt Lemmon go off the top of the scale of the plot, above 50 inches. Of note to APS, a moderate amount of snow accompanied by cold temperatures will impact the Cholla power plant.

Precipitation starts over the state during the morning hours tomorrow and continues throughout the day.   There is just enough instability for a few weak thunderstorms to form over much of southern Arizona which will result in some isolated heavy rain, occasional lightning, and small hail.

The big question is regarding snow in Tucson.  Model runs say it's about a 50-50 chance. What is most likely is a cold rain with snow mixed in and then changing over to all snow for a few hours on Friday morning when the cold front moves through with accumulations of an inch or two of wet snow.  I now think it's unlikely for any significant accumulation above a few inches.


The forecast remains on track for a major winter precipitation event over Arizona starting tomorrow morning.  The accuracy of the model runs from two days ago is remarkable as the most recent forecast has the deep and cold trough moving into far southern Californa during the day on Thursday much like was predicted 2 days ago.

Moisture is limited especially compared to the last storm as PW is around 12-18mm.  However, the Herman rule is in effect as it’s well below the -21C threshold at San Diego.  -21C or colder means that the low-level moist marine air is able to mix deeply and above coastal mountains of SoCal and northern Baja and allow it to move into Arizona.

With the increasing cold air aloft and an increase in moisture, a little CAPE is present over southern Arizona during the day Thursday which may result in a few weak storms accompanied by small hail and occasional lightning.

Precipitation continues into the night over much of the southern ½ of the state along with a few weak storms over the far south.

The big question in Tucson is if it will snow?  My guess is most precipitation will be rain with maybe some snow mixed in at times and then changing over to all snow sometime during the early morning hours when the front moves through.  Maybe an inch or two?

Accumulated precipitation is expected to be significant for the lower elevations for the Tucson and Phoenix areas with over an inch in some areas.  High elevations of SE Arizona are again forecast to receive extreme amounts up to 6” liquid equivelent.

Some plume diagrams from the time-lagged ensemble system are below.  There is quite a spread in the Phoenix area.

Extreme amounts of snow are predicted above 7-8k feet with more than 50 inches (white).  The scale only goes up to 50” but with a 10:1 snow to water ratio, amounts could be as high as 60”.

Monday, February 18, 2019


Another highly unusual weather event is expected to impact Arizona and eastern Pima county starting early Friday morning.  Another strong and cold short wave drops down into the western CONUS longwave trough. Normally, I wouldn’t forecast this far in advance but the global models have been very consistent the past few days bringing this trough far to the south. Below is the EPS from the 0Z run which shows remarkable consistency among the ensemble members.
The 0Z GEFS location is nearly identical but not quite as deep.  

The operational ECMWF moves the trough far to the south along the AZ/Sonora border during the day Friday which is an ideal location for a big snow event for SE Arizona.  

This system is much drier compared with last week’s storm but with such cold air and strong dynamics, precipitation amounts are again expected to be quite high with widespread mountain precipitation over 2 inches and lower elevations around .5 to one inch.  Note that the below plot also includes amounts from the current storm so subtract off about ½ inch over the Rim and White Mountains.

The big story with this storm is the low snow levels, the significant accumulation of snow at lower elevations of SE Arizona, and the big accumulations at high elevations.   

Most of the WRFGFS runs over the past few days forecast significant accumulations in Tucson with some areas possibly receiving 4-8 inches of snow.  Timing is good for a snow event as a strong cold front moves through just before dawn changing the rain over to snow. Note that the station plot does not account for snow.  It’s also possible that all the precipitation falls as rain but I’m leaning snow as the ECMWF is a little deeper and colder than the GFS. The event will be in the range of the WRFNAM soon and it will be interesting to see what those runs have to say.  

20190218 TEP

Another highly unusual weather event is expected to impact SE Arizona and eastern Pima county starting early Friday morning.  A very cold low-pressure system moves slowly through the state bringing widespread precipitation along with a strong cold front.  This system is much drier and colder than the previous system but has much stronger atmospheric forcing thus lower elevations will see as much or more precipitation.  The really important part of this storm is the very low snow levels and right now, it's likely that the lower elevations of eastern Pima County will see significant snow accumulations with 2-4 inches with more at the higher elevations, near the mountains, maybe in the 4-8 inch range.  Mt Lemmon snowfall is expected to be extreme with 2-4 feet possible.  I've included TEP in on this email as there may be enough heavy wet snow to bring tree limbs down on power lines in parts of Tucson.

Precipitation amounts are generally .5" to over 1 inch in the lower elevations with 2-4 inches in the mountains which will all be snow.  It will be cold in the mountains so the snow to liquid ratio will be above 10:1. 

I do not normally send out a forecast this far in advance but confidence is fairly high in the model forecast as all the runs have been quite consistent recently plus this is such an unusual and high impact event, I thought it important to alert Pima County and TEP well in advance.   The set up for snow in Tucson is excellent as the forecast has the cold front coming through around 4-6am, right at the coolest part of the day.  Rain should rapidly change over to snow and continues through the morning. Note that the station model on the below plot shows rain as I never programmed in snow.

Tomorrow, the event will be within the range of the ensemble prediction system thus I'll have a better idea of what may actually happen.