Friday, April 17, 2015

Cold Morning In Spots

Another morning illustrating the difficulties in forecasting low temperatures across a region with complex terrain. Above plot is from MesoWest and shows some of the reported 24-h lows ending at 8:00 am MST this morning - the range in the lows on plot spans 27 F. Here at the house the low fell to 28 F (second coldest morning since back in January). At the observations page on the NWS site I found an additional 6 stations with lows of 32 F or colder - coldest at the RAWS site on Mt. Graham (Columbine) with 19 F. The NWS grid box forecast for where the house is was for 45 F - pretty good for spots a km or 2 north and south of here but way off for the Rillito Wash. I suspect that the algorithms used to adjust the model forecast temperatures for the grids are not very sophisticated - but it's hard to find any technical details.

Thursday, April 16, 2015


Very chilly here this morning - low was 36 F and northwest wind felt cold to me when I walked between 8:00 and 9:00 am. If winds are calm tonight along the Rillito, we'll have a late, hard freeze here in north part of the City tomorrow morning.

Meanwhile, things turning back to winter in Colorado today. Image above is up at Eisenhower Tunnel on I-70 west of Denver and view below is down at Boulder Creek near downtown.

Wednesday, April 15, 2015

Ugly Skies This Afternoon

Certainly nothing like Salt Lake City yesterday, but ugly, dusty skies prevail here this afternoon. Very appropriate for April 15th - Tax Day.

Rest Of Week Stormy Over Parts Of West

A cold, 500 mb low is centered just northeast of Salt Lake City this morning, with temperatures within the core of the system colder than -30 C. Although this system roared into the West yesterday, it is now forecast to take a somewhat long, strange trip before it exits onto the Plains. I show here a sequence of 500 mb forecasts from the 06 UTC run of the WRF-NAM at Atmo - times are in upper right corner and temperature color bar is at bottom. A trailing piece of the short wave digs south-southwestward, giving the system two circulation centers through much of the period covered here. By 0600 UTC on 18 April the closed low has only advanced to north-central New Mexico, with a strong vorticity maximum brushing through southeast Arizona late on the 17th.

Shown here are WRF-NAM precipitation forecasts for the period ending at 11:00 pm MST on the 17th. Precipitation produced by this system maximizes in Colorado, with the model forecasting a very significant event. Above is model's forecast of total precipitation, indicating the focus of the event over northeastern Colorado, north of the Palmer Divide. The model forecast of snowfall through the same period (below) indicates  maximum amounts of heavy snow remaining over the northern Foothills of the Rockies. Interestingly, the current morning forecasts from the Denver/Boulder NWS Office indicate a maximum precipitation probability of only 70% for both DIA airport and also west Ft. Collins on Thursday night - with snow accumulations of just 1/2 to 1 inch. Details of where the heavy, wet snow falls will be modulated by the surface to 500 mb temperature profiles - 500 mb temperatures of -30 C most always bring snow east of the Foothills, across the major megalopolis along the Front Range.

Thunderstorms In Eastern Pima County

Yesterday afternoon provided a reminder that summer thunderstorm season is slowly approaching, as there were buildups and some thunderstorms around late in the afternoon. I took these photos right around 5:30 pm MST - top is looking north toward the Catalinas and below is looking toward the southeast, where thunderstorms were developing and moving eastward.

Jack Diebolt sent the above radar image that was from a bit after 6:30 pm. Graphic below shows detected CG flashes during the evening through 11:00 pm MST (from Vaisala). The storm that moved eastward, south of I-10, was fairly strong during portions of its life. Pioneer Airfield (located southeast of the strongest cell) reported a NW wind gust of 54 mph at 8:03 pm. It is quite probable that the storm moving across the Cienega Grasslands north of Pioneer Field produced some wind gusts that reached severe levels (>56 mph). Two ALERT stations, impacted by the echoes shown above, recorded rainfall - Sahuarita had 0.35" and Davidson Canyon reported 0.24". So, an interesting afternoon.

Tuesday, April 14, 2015

Nasty Wind And Dust Storm In Utah Today

A very nasty day in northern Utah as cold front swept across with strong winds and blowing dust. State Capital building shown above. One hour of surface observations from SLC below - note the temperature drop of 22 F in 50 minutes. More details - see Jim Steenburgh's blog; link at right.

Thunderstorms To East Yesterday - Unsettled Rest Of Week

With colder mid-level air yesterday some thunderstorms developed over eastern Arizona during the afternoon. Visible satellite image above is for 6:30 pm yesterday and composite radar image below is for the same time. Graphic second below shows detected CG flashes through 6:00 pm - from Vaisala. Isolated buildups over higher elevations are again possible this afternoon.

A strong, 500 mb short wave now over the Northwest is forecast to move eastward and then dig almost due south, closing off near the Four Corners. Temperatures at 500 mb are very cold with this system - 500 mb forecast above is from 06Z WRF-NAM and is valid at 11:00 pm on the 16th. Currently precipitation is forecast to remain north with this system toward the Four Corners (below shows WRF-NAM forecast of precipitation on 5.4 km grid through 11:00 pm on the 16th). However, with such cold temperatures, some storms may be possible over higher elevations of southeast Arizona by the end of the week. April winds are a given with this system.

Monday, April 13, 2015

Quick Summary Of Yesterday's Near Miss

Yesterday's rain/no rain dividing line was very sharp across the ALERT network - as sjown by the plots of rainfall for 24-h ending at midnight last night - SW sector above and SE sector below. The rain and showers inched as far north as Sahuarita and the north end of the Santa Rita/Empire Mountains but basically missed the Tucson metro area. Not a drop here at the house, although there was a nice smell of moisture in the air several times. 

A broader view is provided by the MesoWest plot of 24-h rainfall through 6:00 am MST this morning (above). The model forecasts continued to waffle somewhat on the northern extent of the rain right through the 12 UTC forecasts yesterday morning. During the 24-hours leading up to the event Mike Leuthold's version of the WRF-NAM was the most accurate of the models with the northern extent of the rain forecast. The NWS GFS operational was the least accurate through the period keeping the rain along the border and into just the southeast corner of Arizona. The WFR-NAM forecast from Atmo at 12 UTC yesterday morning was quite accurate for the event's evolution - below is the model's forecast of rainfall through 5:00 am this morning.

Apparently convective cloud tops did not grow into cold enough temperatures aloft to produce lightning and thunderstorms. The CG flash plots show that closest thunderstorms were far south of the border in north-central Sonora. The WRF-NAM was also quite good wrt to thunderstorms.

Weather attention will now turn to an end of the week system that is forecast to dig into the Southwest - question will be whether it will come far enough south to bring us anything more than April winds.

Sunday, April 12, 2015

Showers This Morning Just South Of Metro Area

Heavy clouds this morning - vis satellite image above and radar composite below are both from about 7:15 am MST. Showers have gotten further north and west than any of the models were forecasting at the time of yesterday's post.

Rainfall has fallen across the southern portion of the ALERT network with 15 sites reporting rain - most of these are in the southwestern sector of the network (above at 7:15 am). Several sites around Green Valley have measured about a quarter of an inch so far.

The morning sounding from TWC (below, skewT plot for 12 UTC) is also better than the forecasts yesterday - with more precipitable water and a fair amount of CAPE. So, I will hope that the showers edge north this morning as the 500 mb low moves from north end of GoC eastward along the border.

Saturday, April 11, 2015

Another Moisture Starved System?

Nice view of Baboquivari Peak a bit after 7:00 am MST this morning, looking south from Kitt Peak.

The models are now very much in agreement this morning that tomorrow's weather event will just brush the southeast corner of Arizona and focus mostly in the Mexican mountains south of the border. Forecast of 500 mb winds and temperature (below from 06Z WRF-NAM forecast on 5.4 km grid) is valid at noon tomorrow (Sunday, April 12th) and shows the 500 mb cyclone centered over the border south of Tucson. Temperatures appear to be forecast in the -15 to -18 C range, with coldest 500 mb temperatures considerably to the south and east, over Mexico.

The WRF-NAM forecast of the TWC skewT valid at 6:00 pm tomorrow afternoon (above from 2.8 km grid) indicates cloud bases above 700 mb, a distressingly dry layer below that, and no CAPE. The forecast of PW valid at same time (below), shows the moisture-starved character of this event as currently forecast. Maximum PW over eastern Pima County struggles to reach 18 mm (7/10s of an inch). These forecasts from the WRF-NAM indicate an unsettled day over the greater Tucson area tomorrow, with: heavy cloud cover, mammatus, virga, sprinkles, and localized gusty winds. 

Finally, the WRF-NAM forecasts of total precipitation through midnight Sunday night are shown above for 5.4 km grid, and below for 2.8 km grid. The GFS version of the WRF is a bit drier, as is the latest NWS NAM forecast this morning. Regardless, best chance for light, but measurable, rainfall will be over the higher elevations of eastern Pima County.