Saturday, February 28, 2015

Various Aspects Of Impending Weather Event

There are many interesting aspects concerning the imminent weather event approaching southeastern Arizona as March begins. But first, the web cam view of the Catalinas at about 7:00 am MST this morning (Saturday, February 28th) shown above captures a heavy, cap-like cloud already sitting atop the mountains. Above that is a lenticular-like cloud with a nice train of K-H waves running down its top, nicely highlighted by the rising sun.

There continues to be considerable uncertainty regarding the timing and amounts of precipitation likely here in southeast Arizona (current grid forecast for the airport indicates 80% POPs for Monday and 70% for Monday night (but remember those POPs are for only .01" of rain or more). The 06 UTC run of the WRF-NAM at Atmo continues to indicate quite strong winds across portions of Arizona (including eastern Pima County) by mid-day today. However, the model forecast doesn't move much into metro Tucson wrt precipitation until mid-day next Monday. The graphic below show the model's forecast of composite radar echoes valid at noon on Monday the 2nd.

Graphic below shows latest QPF quidance forecast from NWS WPC that is valid for three days - from 12 UTC this morning through 12 UTC on Tuesday, March 3rd. This guidance continues to forecast maximum precipitation impacts from this event for Arizona to be focused in the Rim Country and northwest parts of the state.

The challenges of deterministic precipitation and QPF forecasts are well-illustrated by the concurrent probabilistic forecasts from WPC. Above graphic shows their probabilities for 1/2 inch or more precipitation from 12 UTC on the 2nd through 12 UTC on the 3rd of March - my eyeball estimate puts Tucson metro area in the 10 to 20% chance shading. Graphic below is for same period, but indicates WPC estimates of the probability of 1 inch of precipitation, or more, occurring. Appears that WPC may be ignoring the presence and scale of southeast Arizona's sky islands.

Finally, the really big challenge up in the northern half of state has to do with the snow level and how much of this event will be rain versus snow. David Blanchard posted (to the Albany MAP List) the interesting graphic above that was issued by the Flagstaff NWS Office last evening. The text discusses the two snow scenarios illustrated in an attempt to convey to the general public the real spread in what could actually unfold up there. I would guess that such frankness in their product (rather than just throwing out black and white numbers) is very useful to some users, while others may be puzzled or confused. But I think that it's encouraging to see see such scenarios being shown on Flagstaff's web site.
Contrast above, with current (9:30 am MST) high-res grid forecast for Los Alamos, New Mexico (below, from NWS Albuquerque).
Los Alamos is included within a Winter Storm Warning area for today. I consider this a highly inconsistent and actually quite bizarre forecast and have no idea how the general public could make any sense out of this gibberish.
  • TodaySnow likely before 11am, then rain and snow likely. Cloudy, with a high near 32. South wind around 15 mph. Chance of precipitation is 70%. Total daytime snow accumulation of 3 to 5 inches possible.

Friday, February 27, 2015

Weekend Will Begin With Increasing Winds

Winds were already kicking up yesterday afternoon here along the Rillito, and that will be the case again today. However, the WRF-NAM forecasts from 06 UTC run at Atmo indicate even stronger winds tomorrow and Sunday (March 1st). Shown here are the 10-m wind forecasts valid at noon on Saturday (above) and noon on Sunday (below). Strongest winds are forecast at higher elevations on both days.

The WRF model doesn't get much in the way of precipitation going here in eastern Pima County until Sunday night. But the way it is forecasting the situation to develop is quite interesting. These two graphics are also from the 06 UTC WRF-NAM, but are on the 5.4 km grid. The above is the forecast of PW valid at 5:00 pm MST on Sunday the 1st. There appears to be an atmospheric river (AR) along the surface cold frontal zone that reaches across Baja and into southwest Arizona. But of more interest for us is the broader plume of low-latitude moisture that is moving northward on both sides of Baja - this will serve to enhance the AR and provide a broad influx of moisture into southeastern Arizona.

The forecast of accumulated preciptation through 11 pm MST Sunday night indicates a substantial event has already occurred in the Rim Country and that things are just getting started in southeastern Arizona. So, March will certainly be off to a very interesting start, weatherwise, for us.

Thursday, February 26, 2015


Mike Leuthold has left a new comment on your post "March Will Start Off Stormy":

WRF-GFS goes crazy with it's QPF over the next week. As Pat Holbrook has observed in ID, usually model QPF is too high and comes into line once the event is a few days out. Still, 1/2 of 10 inches is still a lot.

The link above goes to the WRF-GFS QPF forecast from 00 UTC last evening (shown at top) and is valid for period ending at 5:00 pm MST on Monday afternoon, March 2nd.

March Will Start Off Stormy

A substantial trough will impact the Southwest beginning on Saturday and extending through the next five days or so - thus, an impending event that will be much different than the quick-hitting short wave of Tuesday the 24th. Above is the ECMWF four-panel forecast valid at 96-hours (5:00 pm MST on Sunday). Note that the global models are forecasting an influx of moisture from low-latitudes into this system. The GFS ensemble average and spaghetti charts for 500 mb valid at the same time are shown below.

The NWS Weather Prediction Center (WPC) makes national guidance forecasts of Quantitative Precipitation (QPF), and since this event is still quite a way in the future I'll show a couple WPC products. Above is their QPF forecast for days 4 and 5 (i.e., for the period 12 UTC Sunday through 12 UTC Tuesday). The WPC forecasts a very impressive event from San Diego, across Arizona, to the Colorado Rockies. Currently their forecasted maximum amount over Arizona is above 3 inches on the Rim near Payson.

The WPC also issues probabilistic forecasts for a wide range of precipitation amounts - these are called their PQPF forecasts. These forecasts are valid (understandably) for shorter, near-term periods. I've shown below their forecast of probabilities for at least 0.10" for the six-hour period ending at 5:00 am MST on Sunday morning - i.e., before the event really impacts most of southern Arizona. As the event approaches I'll show some more of these interesting products.

Wednesday, February 25, 2015

A Look At Dual-Polarization Data From Yesterday

I grabbed several graphics yesterday morning and never had a chance to discuss them - so here goes. Above is the abbreviated upper-air data plot for Tucson's morning 12 UTC sounding - note that there is low-level CAPE present that reaches up to a bit above 700 mb. Compare this observation to the forecast sounding (see post below that is titled Another Moisture-Starved...). The WRF-NAM forecast this structure quite well. The sounding indicates that the updrafts of showers would reach above 0 C but that showers building above the -10 C level would be unlikely (charge separation and electrification occurs efficiently within updrafts at temperatures of -10 to -20 C), unless the 700-500 mb layer cooled and moistened substantially (which it did - see 00 UTC sounding below). However, the lower layers dried significantly, so that there was only a sliver of CAPE locally during the mid-afternoon. So, thunderstorms stayed well off to the north and northeast yesterday - a couple of Cbs tried to develop over the Catalinas late, but surface heating was basically gone by then. 

The graphics above and below are from College of DuPage's radar page and are the Tucson NWS Doppler radar data from about 7:30 am MST. The reflectivity at 3.4 degree tilt is at the top, while the echo-tops product just above shows that the showers reached up to only a bit over 10,000 ft above the radar (well below 20,000 ft MSL). So the strong reflectivities (>40 dBZ) were likely due to graupel - which is what the co-incident hydrometeor product below indicated (hard to see but the GR color is there in echo cores). So it was an interesting situation yesterday, but one with the low-levels evolving out-of-sync with the middle-levels, and keeping thunderstorms well off to the north.

Finally, a brief summary of the event. At 6:00 pm MST about 80% of the ALERT stations had recorded 0.04" of precipitation or more and plot above is 12-hour precipitation from MesoWest at that time. So it was a fairly widespread, but light, rain event. The airport did quite well with 0.29" and Park Tank on Redington Pass came in with the highest amount I noticed at 0.39". Here at the house we had only 0.04" - which is now the total for February (we will have to hope that the next system manages to produce some rain here before the month ends). Coolest morning here of February also this morning with a low of 28 F.

Tuesday, February 24, 2015

Smell Of Moisture In Air This Morning

Amended to add: Showers here at house at 6:30 am - first rain in very long while.

Current TUS composite radar image (above, for a bit after 6 am MST) indicates light showers over the Rincons and a band of light showers approaching the Tucson metro area from the northwest.

While there has been little precipitation during the night, there is a nice hint of moisture in the air this morning. Only two ALERT sites up in the Catalinas had any measured rain during the night and that was only 0.04" at Oracle Ridge and Dan Saddle. The precipitation plot from MesoWest for past 12-hours (below) shows nothing in southeast Arizona, but light amounts of rain across Phoenix area.

The 06 UTC forecast from the WRF-NAM at Atmo is a bit more moist than past forecasts and indicates very light amounts for low-elevation portions of eastern Pima County through 5:00 pm this evening. Amounts forecast are somewhat higher for the surrounding mountains.

Monday, February 23, 2015

Re Gusty Winds Southeast Arizona This Morning

Art Douglas reported this morning:


Well the wind is blowing so hard I can not sleep.  Some wind reports below with the NWS criteria for a wind advisory. The station names in quotation marks are a joke in my opinion as they are not even close the area indicated:  names are all flat landers and the sites are on top of ridges!  Yes, I know the name is given to reflect the closest geographical settlement.........but still..........quoting weather for Miracle Valley that is on top of a high ridge is a joke.    Art

A Wind Advisory is issued when the following conditions are expected for 3 hours or longer.
1) sustained winds of 31 to 39 mph
2) wind gusts of 46 to 57 mph

"Miracle Valley"   47mph

Summerhaven  47mph

"Wilcox"           52mph
EW3391 Willcox, AZ (E3391)
Elev: 6739 ft; Latitude: 31.99917; Longitude: -109.33833
"Bowie"           49mph  

KG7KID-4 Bowie, AZ (AU921)
Elev: 5046 ft; Latitude: 32.14917; Longitude: -109.43867

Another Moisture-Starved Event For Southeast Arizona

Closed low at 500 mb this morning centered over Lake Tahoo (above is 06 UTC NAM 500 mb forecast valid at 12 UTC this morning - Monday, February 23rd). This feature will weaken and open toward the east as it moves across the Southwest during the next 48-hours.

Values of PW have increased to around 1/2 to 3/4 of an inch across lower elevations of southern Arizona, Analysis of total PW from CIRA (below) is valid at 12 UTC this morning and shows that values of an inch or more continue to lurk to our south over Sonora.

The WRF-NAM forecast run from 06 UTC this morning moves a band of light showers and sprinkles across eastern Pima County around mid-day tomorrow - forecast of composite radar echoes above is valid at 11 am MST on the 24th. The forecast sounding for TWC (below - valid at 10 am tomorrow) indicates very shallow low-level moisture, but does have a nice little area of CAPE below 700 mb. Showers in this layer would be very effective in getting a little rain to the ground because of the low cloud bases.

The WRF-NAM forecast for total accumulated precipitation through 5 pm MST tomorrow is shown above - model does not forecast much for southeastern Arizona. But through this winter its precipitation forecasts for southeast Arizona have been a bit low for the weaker systems.

Northern Arizona is having a wet storm with snow and some rain - Flagstaff has already measured 0.91" and Page has measured 0.69" of water equivalent, with mostly snow up there.

Meanwhile, off to the east, the DFW metroplex and north Texas have been hit by a significant sleet storm that is causing Monday morning chaos. Webcam shot below is of one of the expressways in Dallas. Over a thousand flights have been cancelled this  morning at DFW - glad I'm not flying today!

Sunday, February 22, 2015

Rain Before End Of Month?

Web cam shots at a bit after 7:00 am MST this morning show: above - some middle clouds and virga over the Catalinas and parts of Tucson; below - bright sunshine contrasts with shadows out at Kitt Peak.

As the west-east oriented short wave at 500 mb moves southward, there is some increased middle level moisture moving across southern Arizona this morning. However, as this feature moves slow across the Southwest values of PW will actually be decreasing. Primary impacts of this system, which has at best weak forcing for vertical motion, will be orographic precipitation at higher elevations from southern California eastward across Colorado. Graphic above is from 00 UTC WRF-NAM forecast on the 5.4 km grid and shows accumulated precipitation through 5:00 am on the 25th. Graphic below is from 06 UTC WRF-NAM on 1.8 km grid and shows forecast of PW valid at 11 pm MST on the 24th - DRY. Other WRF variants forecast similar outcomes, but some have very light precipitation falling at the highest elevations of southeastern Arizona. So, some measurable rain in the gauge here looks like a long-shot for coming few days - but models have been very unstable this year and I'll keep watching.

Friday, February 20, 2015

Cloudy Morning - February 20th

The day has dawned with fairly heavy middle clouds hanging over much of eastern Pima County (view of Catalinas above was from 8:14 am MST). The visible satellite image below (from 8:00 am) shows widespread cloudiness from Arizona and southern California southward into low latitudes, so limited sunshine at best today.

Water vapor satellite image from 6:00 am MST (above) indicates an upper-tropospheric cyclone at about 130W and 20N this morning. Widespread cloudiness has been associated with this feature for past several days and loops indicate that it is moving toward the northeast  currently. The CIRA blended total PW analysis (below) for the same time indicates values of PW of an inch or more creeping northward toward San Diego. It appears that the models have not captured the northward push of PW very well through the latest NAM forecasts. The forecast shown in bottom panel is from the 12 UTC NAM this morning and is for total precipitation through  5 pm MST next Monday, the 23rd. I suspect that the forecast underplays the potential for at least light showers across northern Sonora and parts of southern Arizona, especially higher elevations.

Thursday, February 19, 2015

Weather Miscellany February 19th

There are scattered cirrus over parts of southern Arizona this morning, and perhaps some middle clouds also (visible image above from 8:45 am MST). But, view south from Kitt Peak shows clear skies over Mexico (below from 9:03 am).

The 850 mb analysis above (from NCAR RAL) for 132 UTC this morning shows the very cold temperatures that cover eastern U.S. - Temperatures of 0C and below reach almost to Miami! Tucson has the warmest 850 mb temperature at 16C - making the gradient from southern Arizona to northern Michigan almost 50C - wow. Surface plot below (also from NCAR) shows some of the very cold temperatures at midmorning. Forecast high for Tucson area is 80+ and so it will be a nice day to be out and about down here in our corner of the Southwest.

Monday, February 16, 2015

Brief Look At Yesterday

There were quite a few thunderstorms around yesterday afternoon - above shows detected CG flashes through 7:00 pm MST. Across the ALERT network a bit more than 20% of the stations measured rainfall of 0.04 inches or more. Best amounts were off in the southwest sector of the network (below). Brawley Wash at Three Points had 0.43", the most that I found reported this morning. Both TUS and DM had light, but measurable rain. Douglas reported 0.36" and the RAWS station at Empire had 0.32". So, the event certainly did a bit better producing storms and rain than I had expected.

This morning there are some lingering clouds hanging on the mountains of eastern Pima County - image below is from Kitt Peak a bit before 8:00 am MST.. The 500 mb cyclone is now over Sonora, weakening and moving eastward - any final showers in Arizona will be off to the east, as this system bows out. 

A 500 mb trough is deepening over the central US and a trailing short wave is digging southeastward across Colorado and the Four Corners Region today. This feature will kick up westerly to northwesterly winds across southeastern Arizona this afternoon.

Sunday, February 15, 2015

Isolated Thunderstorms Around The Area

There have been some showers around metro Tucson today, with some spots receiving measurable rain. Just a Trace here at house. There was enough sunshine at mid-day to bubble up some thunderstorms, as shown by 2:45 pm coposite radar plot above from TUS and by detected CG flashes through 2:30 pm MST below.