Tuesday, January 17, 2017

500 mb - Cold, Colder, And Then Much Colder


The global models continue to be very consistent, forecasting a series of Pacific short waves and cold fronts to cross the Southwest during the coming week. I've taken a quick look at 00 UTC WRF-GFS forecasts out through a week, even though that's a bit dangerous. The GEFS plumes for QPF at TUS (above from 06 UTC last night) forecast two 100 % events at the airport - one on Thursday night, rapidly followed by a more significant event Friday night into Saturday, and then there's a more scattered event at the start of next week.Several of the GEFS members are forecasting some snowfall at the airport - this was the case with an earlier event this winter that melted away as it got closer in time.



The forecasts of the temperatures at 500 mb associated with these short waves is very impressive and very cold over Arizona. The forecast at top of these three panels (all on the 5.4 km grid) is valid at 03:00 am MST on 20 January; the one directly above is for 10:00 am on the 21st; and the forecast below is valid at 10:00 pm on the 23rd (next Monday). Temperatures with these short waves range from -25 C to colder than -35 C. I don't remember when 500 mb temperatures colder than -30 C last occurred at TWC. 



Just for fun, here are the model's forecasts of precipitation (from 00 UTC on the 1.8 km grid) out through the next week (i.e., 05:00 pm on Tuesday the 24th. Total precipitation shown above.. while forecast below is for snowfall during that period. The model forecasts some snow over the grasslands east of the Santa Ritas and also along I-10 at several places east of Tucson. Will watch how all this unfolds in the real atmosphere during what should be an interesting week.


Monday, January 16, 2017

Event Summary And Week Ahead


The weekend event is mostly over, although there could still be some sprinkles today. Rainfall (and snow) was most significant over the northeast half of the ALERT network, with amounts near and over an inch common. The plot above covers the entire event through 06:00 am MST this morning. The plot below (from weather.graphics and Vaisala) shows CG flash density for the past 48-hours - quite a bit of thunderstorm activity over parts of Arizona and much more so to our east. There was thunder here at the house Saturday afternoon but most of the rain here (0.90" - this was the most rainfall here for an event since that of August 9th, so it was very welcome) fell Saturday night and during the day yesterday. Airport reported 0.57" for this event.



The large-scale pattern remains very unsettled with several fast-moving short waves forecast to impact the Southwest from mid-week through the coming weekend. The forecast above (from the 00 UTC WRF-GFS on 5.4 km grid) is for 500 mb and is valid at 05:00 am on Saturday the 21st. Note the very cold temperatures over Arizona. Below is the forecast from the same run for total precipitation (on the 1.8 km grid) through midnight next Saturday night. The very long-range global forecasts zeroed in on this big change beginning a couple of weeks out - quite amazing to this old dinosaur forecaster.


Sunday, January 15, 2017

Sunday Morning Update



Two webcam views from campus yesterday afternoon about 15 minutes apart. We had thunder and light showers at house as this was happening.

Current rainfall (at 08:30 am MST) for past day across parts of ALERT network, showing strong southwest to northeast gradient in the amounts. Here at house there was 0.62" in the gauge at 07:00 am. Some snow up in Summerhave shown in bottom view.



Friday, January 13, 2017

Wide Variety Of Weather This Holiday Weekend


The GEFS plumes for QPF at TUS (above, from 06 UTC last night) have become very clustered in time with precipitation from tomorrow afternoon (Saturday) into Sunday, with bulk of rainfall on Saturday night. The amounts however, continue to have a large spread ranging from around a third of an inch to almost two inches - so the details continue to be very important in determining what actually occurs.

The 06 UTC WRF-GFS forecast from Atmo forecasts a period of strong southeasterly winds tomorrow afternoon - forecast below is valid Saturday at noon. So it appears there'll be wind, rain, a slight possibility of thunder, and winter conditions in the Sky Islands - quite a mix. 



The model's forecasts of upper-air soundings, shown on these skewT plots, illustrate the variety of conditions for the next 60-hours or so. The sounding above is for the airport and is valid at 01:00 pm tomorrow afternoon. It could support light showers, perhaps a rumble of thunder, and strong, gusty winds.

The sounding below is for Sonoita and is valid at 02:00 am on the 15th. This sounding could bring snow showers, along with gusty winds to that region or nearby places that are just a bit higher in elevation.

At the bottom is the forecast sounding for Mt. Graham valid tomorrow at 04:00 pm. This sounding could support a brief period of thundersnow, as well as very strong winds of 40 to 60 mph. The forecast soundings at Mt. Graham indicate two distinct periods of snow as this event unfolds. The WRF-GFS forecasts Sky Island snow amounts of 10 to 12 inches and more, and even brings 3 to 5 inches of snow to Kitt Peak. 



Thursday, January 12, 2017

Models Continue To Forecast Wet Weekend


Forecast models continue to indicate a wet weekend for southeast Arizona, as closed 500 mb low digs south along the California and Baja coasts and then turns northeastward across our part of the state. The TUS plumes for QPF from the 06 UTC GEFS (above) have become quite consistent with 100 % POPs for Saturday afternoon and night, although amounts in the forecasts still have large spread. The operational GFS (blue) is now quite close to the ensemble average and indicates nearly half an inch at airport.

The 06 UTC WRF-GFS forecast for composite radar echoes at 5:00 pm MST on Saturday (just below) is quite impressive for eastern Pima County, and the model's forecast for total precipitation through 11:00 am on the 15th (Sunday - second below) indicates a decent, widespread event with maximum amounts up in the White Mountains.



The details will depend upon the actual track that the 500 mb closed low takes (it is currently along the northern California coast). The 500 mb forecast from the WRF-GFS valid at 5:00 pm on Saturday (below) places the low to our southwest, just south of the border. This would be a good track for precipitation here. However, a similar situation, and forecasts, earlier this winter went astray as the low dug further south before turning northeastward - so we'll have to keep close watch as the current low heads south.


Tuesday, January 10, 2017

Damp Weekend On Tap?


It was a mostly cloudy day yesterday - above view from campus was at 5:00 pm MST. Unfortunately, the automated observing system used by NWS indicates that yesterday was perfectly clear (see ASOS observations below). I think that surface observations were actually more accurate, and useful to aviation and other users, back in the 1960s, when I was learning to take observations at the St. Louis Weather Bureau. Higher technology can sometimes make things easier by taking humans out of the loop, but result in degraded quality. Note - the ASOS system does not detect clouds above 12,000 AGL, thus it was "clear" yesterday.



The current NWS forecast for the airport (above) indicates chances for showers and measurable rain at the airport from Thursday night through Sunday, with highest chances of 40 % for 0.01" on Saturday.


The ECMWF and GFS global models are in agreement, forecasting a closed, 500 mb low into the Southwest - the two forecasts here are valid at 5:00 pm Saturday afternoon. The ECMWF (above) has the low further north than does the GFS (below), which has the circulation center over southern Sonora. The ECMWF forecast would bring better chances of rain to our part of the state - but the weekend is still several days off.

The GEFS plumes for QPF at the airport (below from 06 UTC forecasts last night) forecast 100 % chance of measurable rain at the airport but there is considerable spread in the timing of the event (as reflected in NWS forecast above), as well in amounts of rainfall possible. The operational GFS (blue) is very dry, possibly because of its southerly track forecast for the low. Another let's wait and watch situation.



Monday, January 09, 2017

Heavy Clouds Replace Yesterday's Sun And Warmth


Heavy weather, snow and rain, continues over much of the West as we start a new week. The IR satellite image above is from a bit after 7:00 am MST this morning. Heavy clouds have extended eastward over most of Arizona, and rains have slipped south to southern California, as an atmospheric river (AR) from low-latitudes makes landfall there (MIMIC PW from 12 UTC this morning below).



The dreary views from campus and campus and Kitt here - above and below - about 8:30 am this morning. Yesterday, with abundant sunshine, the high temperature at the airport reached 81 F.



The global models are forecasting a 500 mb low into the Southwest by the end of the week, with perhaps a chance of some precipitation in our part of the state. The ECMWF 500 mb forecast above is valid at 5:00 pm on the afternoon of Friday the 13th. Below are the 06 UTC QPF plumes for the airport - these indicate a mixed bag for the end of week and weekend, as many of the members forecast at least a chance of rainfall at the airport.