Thursday, December 05, 2013

Damp And Dreary Morning - 5 December 2013

NOTE - edited to add this morning's TUS upper-air sounding plot.

Light rains and cool conditions prevail across Tucson early this morning. The event early this morning has again exceeded expectations, especially mine based on yesterday morning's WRF forecast runs - I will come back to this later in this post. Below is Tucson NWS radar composite chart for 5:30 am this morning, indicating light showers continuing to the west and southwest of the metro area, as well as over town.

It is very hard to evaluate the ALERT data this morning. There are 10 sites that report no precipitation during the 24-hours ending at 5 am MST. However, the event has produced a mix of rain and snow and snow is not captured by the network. Two of these sites obviously received snow (Mt. Lemmon and Manning Camp) but the 8 others are at higher elevations in the east and southeast part of the network. If I had to guess, I'd say that precipitation coverage was essentially 100% during the early morning hours. For stations with rainfall, there were 40 sites with more than a quarter inch of rain and 9 with more than half an inch. The two sectors of the ALERT network that cover the metro area are shown above and below. Here at the house we had 0.13" through 5 am, while the airport had 0.34" and DM had 0.41". This was a more widespread event than expected, and one that was similar to several other light events this Fall.

If I recall correctly, the WRF under-forecast several of the light events this Fall because the PW available exceeded what was forecast. This may be the case again with this event. The time series of GPS PW above indicates that PW has increased over south-central Arizona to values around 3/4 of an inch. The 6 am WRF forecasts yesterday generally kept PW forecast values in the range of 0.5 to 0.6 inches. The Tucson skew-T forecast (from WRF-NAM) valid at 4 am MST this morning (immediately below) had 15 mm (0.59") of PW. The area forecast of PW (bottom) indicated considerable drying across almost all of Pima County by 5 am. I will try to take a look at the earlier events this fall to determine if my memory is correct.

Note - I added the morning sounding plot for TUS just below the forecast - observed temperatures are considerably colder than the forecast sounding and RH is high to saturated through a very deep layer.

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