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.

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