Saturday, April 25, 2015

Tough Forecast For System This Weekend

First, much of yesterday was quite cloudy and some light showers developed mid-morning and moved across parts of the Catalinas. The northern sector of the ALERT network recorded light rainfall from these showers (above, 24-hours through 6:00 am MST this morning). Not even a sprinkle here at house.

As a strong, 500 mb short wave digs southward toward northern Arizona, forecasting exactly what might happen across lower elevations of Pima County has been quite a challenge. The fairly crude resolution of the NWS NAM and GFS models result in a smearing of forecast precipitation southward toward the border. This is due to the coarse resolution of the terrain in those models.

The high-resolution WRF model variants run at Atmo, forecast the precipitation event with this system to remain mostly in the mountains and higher elevations to our north. Here are two forecasts from that model for comparison. The WRF-NAM forecast above is from the 06 UTC run and shows the forecast of accumulated precipitation through 11:00 pm tomorrow night (Sunday, April 26th). Over eastern Pima County the focus is over the Catalinas and Rincons, with little forecast for low elevations of the metro area. The similar WRF-GFS forecast below is from the 00 UTC run and is even more restrictive with it's forecast, indicating just a touch of precipitation in the nearby mountains.

However, the WRF-NAM forecasts of the evolution of the vertical profiles over the campus, sounding-release site (TWC) indicate that conditions briefly become fairly favorable for convective showers, as the strongest portion of the Pacific cold front comes by. The forecast sounding below, valid at 1:00 am MST tonight, has almost an inch of precipitable water and CAPE of over 500 J/Kg.This is the most favorable sounding I've seen in the model forecasts for quite some time.

The sounding indicates to me that there is a better chance for showers in the metro area than the WRF models are indicating above. However, the most favorable conditions last only for a brief period - so we'll have to hope for the best as this event unfolds. Note that current NWS forecasts of POPs for precipitation are: 40% tonight and 30% tomorrow for the airport grid box; but only 60% tonight and 50% tomorrow for the grid box that Mt. Lemmon is within.

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