Wednesday, March 04, 2015

500 mb Trough Moving Across Eastern Arizona This Morning

Significant cloudiness covers southeast Arizona this morning, as shown by 0715 am MST visible satellite image above. The 500 mb S/W is moving across eastern Arizona this morning (below is 12 UTC 500 mb analysis from NCAR), and there are light showers along and ahead of the trough.

The morning sounding from NWS TWC is plotted below (from SPC). Middle-level temperatures are colder than -20 C and moisture extends upward to above 500 mb. There is little resistance to vertical motion in the moist air and showers will continue until lower layers dry out during the afternoon. There may be a sliver of CAPE present in the sounding and surface heating, especially over the mountains, could lead to some thunderstorms during mid-day. After this system passes by the synoptic pattern over the Southwest becomes somewhat chopped up and ill-defined as we head into meteorological spring.

Tuesday, March 03, 2015

Brief Summary Of Yesterday's Event Over Eastern Pima County

View of Catalinas above is yesterday afternoon just before the Pacific cold front moved across metro area. Winds ahead of the front were very strong - estimated gusts of 40 to 50 mph here at the house. Rainfall was widespread with essentially 100% coverage over the ALERT network. There was one station see below that came in with 0.00 but that appeared to be perhaps a malfunction when compared to nearby gauges. The northern three sectors of the network are shown below - amounts were heaviest over the northern Catalinas and Redington Pass area with much lighter amounts across the City. We had 0.14" here at house.

Only thunderstorm activity in southern Arizona occurred far to the west (below shows detected CGs through 1 am MST today - March 3rd. As the showers passed through our area radar tops of the convection were in the range of only 14 to 20 thousand feet MSL.

The trailing S/W has now closed off to the southwest of San Diego, after kicking yesterday's closed low rapidly northeastward - water vapor image above is from 1330 UTC this morning. This feature will open up and cross eastern Arizona and northern Mexico during the next 36-hours - NAM 500 mb forecast below is valid at 5:00 am MST tomorrow morning. The WRF-NAM forecasts some showers along the Borderlands as this happens with some activity as far north as eastern Pima County.

Monday, March 02, 2015

Early March Weather Event Still Approaching

Water vapor image (above) from 12 UTC shows upper-level cyclone spinning west of San Diego this morning, as a trailing S/W digs southward toward it. The concurrent, blended PW analysis from CIRA at Colorado State (below) indicates that the AR that plowed across southern California and west to northern Arizona is now barely distinguishable, as a broad swath of tropical moisture is pulled northward - key question today is how much low-level moisture (mostly below 600 mb) will be pulled into southeastern Arizona before the event ends.

Radar echoes indicate most rainfall currently out along the Colorado River and extending into northern Arizona (above is Yuma composite radar display for 12 UTC this morning). Precipitation amounts exceed an inch at Kingman and the Grand Canyon (mostly snow so far), with Flagstaff reporting about 2 3/4" so far (mostly rain!). The 06Z WRF-NAM forecast from Atmo continues to weaken this event's impacts across southeastern Arizona - below is model's forecast of accumulated precipitation through the end of the main event (midnight tonight). However, yesterday's caution is still appropriate - model forecast soundings for metro Tucson during afternoon would support heavy, but not very deep, convective showers - so watch the radar a bit later today.

Sunday, March 01, 2015

Quick Look At Guidance Products/Forecasts

Only time for quick look at a couple of products re upcoming weather event. The graphic above is the QPF guidance from NWS WPC valid from 12 UTC this morning (March 1st) through 12 UTC Wednesday morning. The WPC's forecast amounts continue to decrease for southeastern Arizona as the event draws closer in time. The event, in their forecast, continues to be focused from San Diego across northern Arizona and into western Colorado. There is a secondary maxima forecast for northern Mexico.

For comparison I'll show the higher spatial resolution forecasts from the 00 UTC run of the WRF-NAM at Atmo. Above is the forecast on the 5.4 km grid through 12 UTC on March 4th and it is very similar to the NWS WPC guidance product above - almost looks as if the forecasters back there could have used this WRF forecast for their guidance.

Below is the same forecast but for the 1.8 km grid. Certainly the trend is not good for rain at lower elevations, with amounts over southeastern Arizona now focused on higher elevations in Pima County.

However, the event is still a day or so away (WRF forecasts main shower bands in Tucson area to occur tomorrow afternoon and evening). The soundings forecast for tomorrow afternoon for TWC are similar to other soundings this winter that produced nice amounts of rain - so we will need to keep an eye on tomorrow's evolution of the setting.

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.