Monday, February 20, 2017

More Light Showers/High Winds On Kitt Peak

Occasional light showers occurred over eastern Pima County Sunday (yesterday) and on into the night. Amounts from the northern 2/3 of the ALERT network are shown above for 24-hours ending at 06:00 am MST this Monday morning. Amounts were again generally around a tenth of an inch. Here at house we managed to get 0.19" as showers formed over the Tucson Mountains and moved our way a number of times (total here for the two days ended up at 0.24").

During coming week, short waves will tend to stay to our north and a brief warm up will be followed by cooler temps and some periods of west winds. Next possible precipitation at least a week away, according to GEFS forecasts.

I noticed that wind gusts were very strong out west at Kitt Peak on Saturday and Sunday, with 12-h maximum gusts ranging from 56 to 93 mph - as shown on the two time-series plots below. These are from the weather station on/at the Mayall 4-meter telescope, which is number 12 in photo above.

I don't know where this weather station is actually sited, which is of course information needed to assess exactly what these measurements might mean. I would guess that obstacle flow due to the telescope structure is amplifying the wind speeds. Any information from those in the know would be much appreciated.

Sunday, February 19, 2017

Some Web Cam Images

Here are some web cam images from this morning. Top two from local sources and bottom two from Jack Hales site.

Dreary morning here - above. Kitt Peak North view all iced up - below.

Lonely snowplow at work in Wyoming - above. Owens Valley, California, with the mountains covered in heavy snows - below.

Underwhelming "Event" For Southeast Arizona

The WRF forecasts were, unfortunately, right on in their forecasts of very light precipitation for lower elevations of southeast Arizona (see earlier posts). Yesterday morning's NWS forecast for QPF at the airport was 0.55" but observed was only 0.05". We received same amount here at house. My quick assessment remains that very strong, but downslope, winds were the main culprit. Radar loops yesterday seemed to indicate this also. There were undoubtedly other features at play also, but end result was not much.

There are light showers around this early am - above is composite radar at 06:37 am MST. Web cam views up in Summerhaven indicate several inches of snowfall there (below).

Shown here are ALERT amounts for 24-hours ending at 06:00 am - above is entire network, while below is zoomed on metro area. Except for the Sky Islands, amounts were generally less than a tenth of an inch. Western regions and Phoenix area did an order of magnitude better.

The morning sounding (below from SPC) indicates some moist instability below 600 mb, so showers continue possible for today - steering flow this part of town will be from the Tucson Mountains, which sometimes does pretty good here. 

Another "so it goes" event that was most exciting when the wind was gusting strongly here and there were some power surges.

Saturday, February 18, 2017

Status - AM Saturday February 18th

Saturday dawns mostly cloudy, but with a patch of blue beyond the Catalinas. Kitt Peak however is shrouded within dense cloudiness.

Current early morning forecast for the airport from NWS above. Winds are already gusty this morning. The 50% for overnight did not happen in eastern Pima County, but showers are nearby. This morning's 12 UTC 500 mb analysis below, from NCAR, shows the strong short-wave stretching from central California down all the way to west of the the end of Baja - unusually far south.

Radar above from NWS Tucson indicates light showers out to our southwest, but radar from Yuma below indicates widespread shower activity over northern GoC and lower Colorado River Basin (both radar depictions from just a bit after 07:00 am MST).

The midnight run of the WRF-GFS model at Atmo continues to forecast a large rainfall donut hole over lower elevations of all of southeast Arizona - forecast below is for total precipitation through 06:00 am on Monday morning. Light amounts likely being caused in model by strong, downsloping winds. But, I suspect that model may be a bit too dry - we'll wait and see.


Friday, February 17, 2017

Overview Of Developing Weather Situation

The global models have performed amazing well during past 4 or 5 days, and the strong trough they have been predicting is off California, digging toward northern Baja. Above is 00 UTC GFS forecast for 250 mb valid at 12 UTC this morning - note the intense jetstream on northwest side of the trough, which is causing the southeastward movement. The detailed, upper-level structure is complex, as indicated by 1330 UTC water vapor image just below. A surface plot from about 07:00 am MST (second below) shows moderate to heavy rains over much of California, with snowfall already into northwest Arizona.

The ensemble plumes shown here are for TUS QPF - above is from the 00 UTC GEFS, while below is from the SREF (available at SPC web page). Both sets of ensembles indicate 100 % POPs for tomorrow night, with some spill-over before and after. Amounts, however, are much more problematic. The GEFS has two clusterted sets of forecasts - one around 0.60", while the other is around 0.30" - this makes the average (black) somewhat of an artifact. The operational GFS is in the drier cluster.

The SREF models are mostly clustered around 0.25", but the 5 forecasts of more than 0.60" pull the mean toward higher amounts. The WRF variants from Atmo at 06 UTC have airport amounts close to the SREF average (black below).

The WRF continues to forecast very strong southerly winds this afternoon and tomorrow. The 06 UTC WRF-GFS forecasts here are of 10-m winds valid at 03:00 pm this afternoon (above) and at noon tomorrow (below). The forecast for tomorrow indicates very strong winds around the northern flanks of the Santa Ritas.

I also looked at the WRF-GFS forecast for total snowfall during the event (below through 11:00 am on Monday). While the amounts are likely overdone, the forecast indicates that snow will generally stay above the elevations of I-10. However, the high spots along Highway 83 from I-10 south to Sonoita may experience some snowfall. The model forecasts snow at Kitt Peak, although NWS grids only forecast rain there. So, many interesting details to watch this weekend.

Thursday, February 16, 2017

Weekend Event In WRF-GFS For Arizona And California

Models continue to be quite similar in their forecast of the evolution of the large-scale pattern through the weekend, so the smaller scale details will become important re what happens when and where. The forecast above is from the 06 UTC WRF-GFS forecast run at Atmo and is for total precipitation through 11:00 am MST on Sunday the 19th. The event is widespread for Arizona, but the model forecasts a considerable minimum over much of Pima County, with the Sky Islands of southeast Arizona capturing the heavier precipitation.

The 10-m wind forecast (below) indicates very strong, south-southeasterly winds at 02:00 pm on Saturday the 18th. These winds are downslope from the higher terrain along the Borderlands and appear to be the primary factor producing the donut hole in the precipitation pattern. The forecast sounding for Tucson (2nd below) shows a deep, dry boundary layer at the same time, with only a fairly shallow layer producing rainfall. The depth of vertical motion above the downslope winds will be very important as the event develops - deeper vertical motion below 500 mb favoring higher rainfall.

I also took a look at the total precipitation forecast from the 00 UTC WRF-GFS forecast, on the 5.4 km grid, and forecast above is of total event precipitation through noon on Monday, February 20th. Not much difference for Arizona in this forecast. The amounts of precipitation in California, however, are very significant. The structural failures at Oroville Dam have the state rushing to draw down the reservoir as quickly as possible, before the heavy precipitation begins with this event. The WRF-GFS forecasts extreme amounts above the dam (location shown below).

The model also forecasts very heavy amounts of rainfall in the northern portions of the LA Basin, across the Santa Barbara and Santa Maria areas and well north up the coast, reflecting the unusual influx of high moisture from the south-southeast. Much of southern California is under Flash Flood Watches as the event approaches.

Wednesday, February 15, 2017

WRF-GFS Forecasts For Weekend

The global forecast models now seem to be very much in agreement as to where the Pacific zonal flow will buckle, with the result being a strong closed low at 500 mb moving across northern Baja. There continues to be a large range in QPF forecasts for eastern Pima County, but that is to be expected. Forecasts shown in this post are from the 00 UTC WRF-GFS forecast run (on the 5.4 km grid) at Atmo last evening.

Winds this morning are gusting 30 to 35 mph from the east here along the Rillito. Up at the Mt. Hopkins RAWS site winds have gusted in the 40 to 60 mph range early this morning.

The forecast above for 500 mb is valid at 05:00 pm MST on 18 February (Saturday - four days out in time). The position of the low brings strongly difluent winds aloft over southeastern Arizona. Although PW at the same time (just below) is not extremely high, values do approach an inch over parts of southern Arizona. At the bottom is the forecast of total precipitation from the model through midnight next Monday night. The Sonoran Desert features a large donut hole in this forecast, while the Sky Islands of southeast Arizona do quite well - much to watch next few days.

Tuesday, February 14, 2017

East Winds Tomorrow/Another System On Weekend

First tomorrow's east winds - the 10 m wind forecast above is from the 06 UTC WRF-GFS run at Atmo last night and is valid at 10:00 am MST tomorrow morning. If it verifies, winds tomorrow would be gusty and over 30 mph across windy parts of metro tomorrow.

The forecast sounding for Sonoita (below) from the model run is valid at 05:00 am tomorrow morning. If this sounding forecast proves accurate, I'd expect winds after midnight gusting to 50 to 65+ mph at the Mt. Hopkins RAWS station.

Looking ahead to the weekend - above is the current am forecast for the TUS gridpoint, indicating 70% POPs for measurable rain next Saturday. This forecast event is produced by a negatively-tilted, short wave that is forecast by global models to dig into Arizona and northern Sonora.

The 06 UTC GEFS plumes (just below) for QPF at the airport on next Saturday are currently all over the place. The ensemble members forecast the 500 mb closed low to come ashore anywhere from northern to southern Baja, resulting in the large spreads in timing and QPF forecast.

The feature that would produce this event is currently embedded somewhere within a very strong, zonal jetstream far out in the central to west Pacific (around or west of the Dateline). The 00 UTC GFS analysis at 250 mb is shown at bottom. The fast and zonal flow makes predictability somewhat uncertain this far out in time. So another event in the models that we'll have to observe as it eventually tips its hand later this week.