Friday, February 15, 2013

Dreary Or Sunny?

Edited to add: Now that the sun's up, the webcam view of Catalinas above shows that it's definitely a dreary start to the day. The north edge of the cirrus is visible at left side of the view.

This morning (15 February) a large area of high cloudiness from west of Baja is intruding into southern Arizona (above is water vapor image from 5:30 am MST). Thus, one forecast issue is how cloudy it will be and when during the day. The "when" question is probably easiest, since thin cirrus is already overhead. The how cloudy (i.e., how dreary) issue is more difficult.

I have examined the early WRF-GFS forecast from Atmo and show above total outgoing long-wave radiation and below total incoming solar radiation (both in watts per meter squared). Both forecasts are valid at 10 am MST this morning. the northern edge of the clouds is fairly sharp both in the forecast and in current satellite imagery. So, the model forecast indicates dreary along the Borderlands this morning but mostly sunny up in Phoenix. However, by 2 pm the model forecasts sunny skies across all of southern Arizona except the far southeast corner, near Douglas.

The forecast is tough re the timing, since flow in the upper-troposphere is quite strong. The timing is quite important for a number of practical reasons. This time of year solar in put is highest from around 10 am to 2 to 3 pm. If thick cirrus is overhead during this time of day, then the output from solar panels will be way down. The standard NWS ASOS observations won't shed any "light" on this issue, since clouds above 12,000 ft agl are not detected and automated surface observations will report "clear" even when it's quite overcast.

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