Saturday, April 19, 2014

Smell Of Moisture In The Air

There were quite a few showers, as well as some thunderstorms, over southeast Arizona yesterday afternoon, with some activity continuing through the night. Above is composite radar image from TUS a bit after 3 pm MST (Friday April 18th). Graphic below shows CG flashes detected from noon to 4 pm.

Rainfall was confined mostly to higher elevations, with many low elevation sites, including here at house, reporting Traces. Above 5,000 ft MSL almost all of the RAWS stations reported measurable rainfall yesterday but amounts were light. Only 3 RAWS stations had 2/10" or more, with Columbine (Mt. Graham) reporting the most at 0.28"

A time series of GPS PW for the last 14 days at Atmo (above) shows values values for past few hours that are nearly double the maximum value during the last 2 weeks. There was a distinct smell of rain in the air this morning, which was quite pleasant after days of very low RHs. The graphic below shows CG lightning flashes detected from midnight to 6 am - most thunderstorm activity at sunrise was in eastern Cochise county and southwest New Mexico. However, there were storms moving northward from Sonora into central Pima County.

The early run of the WRF-NAM at Atmo forecasts activity today to remain mostly to the south and east of the Tucson metro area; however, the early morning storms have developed further west than was forecast by the model. So, still some chance for storms across the metro area today. Graphic below is WRF-NAM forecast of rainfall through midnight tonight (Saturday, April 19th). The WRF-GFS forecasts appear considerably too dry for today, with hardly any activity over southeast Arizona.

Friday, April 18, 2014

Dreary Morning As Closed Low Approaches Northern Baja

The upper low west of northern Baja is quite distinct this morning, having developed much as the forecast models predicted. The 13 UTC water vapor image above shows the system and the large area of thick, high clouds that has overspread the Southwest. The 12 UTC 500 mb analysis (below from NCAR RAL)  indicates the 500 mb center west of San Diego, but apparently with two short-waves rotating around it. The NAM forecasts the vorticity maximum that is over northern Baja this morning to move rapidly northward across west-central Arizona today. The the main short wave moves across southern Arizona tomorrow and tomorrow night.

The models forecast precipitable water (PW) to increase substantially today and the morning sounding for Tucson indicates about 1/2 an inch of PW - highest value in quite a while. The MIMIC PW product for 12 UTC this morning (below - from CIMSS at Univ. of Wisconsin) indcates PW values of around an inch or a bit more over the northern GoC and off northern Baja. So this event appears not to be as moisture-starved as the last few have been.

The early WRF forecasts from Atmo are similar to yesterday; however, the GFS version is now considerably drier for southern Arizona than is the NAM version. Both versions forecast bands of high level showers/virga across Pima County today, with only scattered light showers reaching the surface. The graphic above shows the WRF-NAM forecast of accumulated precipitation through 11 pm MST tonight. Note the somewhat strange and distinct maximum over the Rincons.

Below, is the same forecast, but for the period ending at 11 pm tomorrow night (Saturday, April 19th). The model forecasts what would be a nice precipitation event for southeastern Arizona, given that it is late April. Current NWS forecasts are for 30% chance of rain tomorrow at TUS, but 60% chances for Mt. Lemmon. The WRF forecasts definitely keep the more significant rain amounts at higher elevations. So, I'll be waiting to see if the gauge here at house catches anything with this system (last rainfall here was on March 1st)..

Thursday, April 17, 2014

Sprinkles Or More Numerous Showers?

The early runs of the WRF model at Atmo last night have produced considerably different forecasts as another 500 mb short wave, in the southern branch of the flow, develops and moves across Arizona tomorrow and Saturday. Both the NAM and GFS bring a leading vorticity maxima north-northeastward across Arizona, followed by the main short wave on Saturday. Above graphic shows precipitation forecast by the WRF-NAM through 11 pm MST Friday night (18 April) and the same forecast from the WRF-GFS is shown below. The forecasts are quite different and leave the forecaster facing a dilemma - will the initial impact affect west-central Arizona or will it focus on southeast Arizona and the White Mountains? Note that the GFS version produces measurable rainfall over much of Pima County while the NAM version has forecast almost no rainfall for the county. Both versions forecast some additional, light showers on Saturday, mostly east of the Tucson metro area.

The difference results primarily from the initial influx of increased moisture associated with the leading portion of the short wave. The models forecast the most significant showers to be associated with the leading part of the short wave and the GFS version also forecasts development of higher values of CAPE across southeast Arizona. The situation appears to be one in which the different models have minor differences in the forecasts of development of the short wave in the southern stream of the flow and of the northward advection of moisture as this feature takes shape. The situation is further complicated by the current dryness in the southern stream. I see that the new NWS NAM forecast run this morning  shifts the main impact of this event to southeastern Arizona as the main part of the short wave passes. So each different model run seems to come up with slightly different forecasts. It appears to be a situation which will require careful monitoring of the PW tomorrow - so a tough forecast due to uncertainties that persist as the event draws near. The new WRF forecasts this morning should be interesting also.

Wednesday, April 16, 2014

Fire Weather Zones

With a wildfire burning in the Huachuca Mountains currently (the Brown Fire, shown in photo from yesterday, credit unknown), I thought that I would respond to a comment from early in April regarding the NWS Fire Weather Zones. The comment was:

FYI... WFO Tucson recently re-drew their Fire Weather Zones (effective 4/1), so the "regular" Watch/Warning/Advisory maps should look better with Red Flag Warnings now.

The new NWS Fire Weather Zones are shown below and Zone 152 is shaded orange-red to indicate a current Red Flag Warning. I assume that before the zones were re-defined a single zone covered all of southeastern Arizona, extending to the west of the Tucson Metro area. This led to some of the graphics on the NWS web page showing very large alerts for fire weather products, even if only a small portion of the area was impacted.

So, with the new zones, the home page of the NWS at Tucson indicates the above, i.e., a Red Flag Warning for Zone 152. A click on the "Detailed Hazards" tab is still needed to view the much smaller, actual area (below) that is under the Red Flag Warning. As the comment stated, the new zones do indeed reduce the extent of the warning on the NWS home page, but continue to indicate entire zones under the warning. It is not clear why the graphics used by NWS smear the details of some products on the main page that most users see first, and sometimes last. Note that this morning's NWS "Weather Story" does highlight the details of today's Red Flag Warning, and this feature catches the user's eye much better than does the detailed hazards tab.

Sunday, April 13, 2014

Fiery Sunset Over Rosemont Site - Friday Evening

Above is photo of sunset behind the northern Santa Rita Mountains on Friday, April 11th. Taken by Katie using her cell phone. View is from the Highway 83 Scenic Overlook, just east of the proposed Rosemont
mine site.

Saturday, April 12, 2014

Windy Afternoon

We were out northwest of Sonoita (almost directly downwind from Mt. Wrightson) this afternoon and it was very windy. While gusts around and over 30 mph were common today, I observed a few gusts early this afternoon that I estimate were in the 50 to 60 mph range. I see in the paper this morning (Sunday) that the flight performance of the USAF Thunderbirds was cancelled yesterday at DM due to the strong winds.
As for the extremely low dewpoints noted in an earlier post. Jack Diebolt sent the following:

Hi Bob!  I’ve been in KC for the past three years, but keep caught up on TUS weather via your blog.  I see you noticed the low TD/RH yesterday as well.  I placed a comment on NWS Tucson’s Web Page and got this response.. it also appears some others noticed as well!

Jack D

From TUS NWS (Facebook?) below. NWS seems to be saying the Vaisala RH sensor, in combo with ASOS software, can produce questionable results at low relative humidities.

Sorry for the small font - can't seem to get it larger.

Friday, April 11, 2014

Gila Monster Visit

A Gila Monster wandered through the back yard at midday today. Only the second one I've seen in the "wild."

Surface Time Series

Surface observation time series of T, Td, and RH from yesterday to go with previous post.

Friday April 11th - Couple Of Things

Short wave west of Baja (see MIMIC PW image for 12 UTC this morning - above) has a weak leading wave which will cross Arizona today. This feature has turned the Pacific moisture plume eastward across the Baja spur. So, this will be another moisture starved system, just as the models had predicted a couple of days ago. This morning's early WRF runs at Atmo are similar, with weak radar echoes forecast across eastern Pima County during middle of night - below is WRF-NAM valid at 11 pm MST tonight. Echoes remain aloft in the model forecasts. Gusty winds forecast for tomorrow afternoon.

Roger Edwards sent the above surface plot for around 3 pm yesterday afternoon, highlighting the extremely low dewpoint temperatures at Tucson and Douglas. A check of the surface observations shows that Tucson dropped as low as -39 F and Douglas -45 F. But, DM (only a couple of km from TUS) had a low dewpoint of only -9 F, while Mt. Hopkins hit -17 F and Rincon -14 F. So strange things going on at small scales. The TUS sounding below (released around 4 pm) does not show any air nearly so dry - except the layer right below 500 mb. So, if the dewpoint sensors at TUS and Douglas were accurate (? - I don't know how these sensors perform at ultra-low RH), then several pockets of strongly subsided air crossed portions of southeastern Arizona. Note that the dry adiabatic BL does extend upward to the dry layer aloft, so this scenario was possible.