Friday, April 18, 2014

Dreary Morning As Closed Low Approaches Northern Baja

The upper low west of northern Baja is quite distinct this morning, having developed much as the forecast models predicted. The 13 UTC water vapor image above shows the system and the large area of thick, high clouds that has overspread the Southwest. The 12 UTC 500 mb analysis (below from NCAR RAL)  indicates the 500 mb center west of San Diego, but apparently with two short-waves rotating around it. The NAM forecasts the vorticity maximum that is over northern Baja this morning to move rapidly northward across west-central Arizona today. The the main short wave moves across southern Arizona tomorrow and tomorrow night.

The models forecast precipitable water (PW) to increase substantially today and the morning sounding for Tucson indicates about 1/2 an inch of PW - highest value in quite a while. The MIMIC PW product for 12 UTC this morning (below - from CIMSS at Univ. of Wisconsin) indcates PW values of around an inch or a bit more over the northern GoC and off northern Baja. So this event appears not to be as moisture-starved as the last few have been.

The early WRF forecasts from Atmo are similar to yesterday; however, the GFS version is now considerably drier for southern Arizona than is the NAM version. Both versions forecast bands of high level showers/virga across Pima County today, with only scattered light showers reaching the surface. The graphic above shows the WRF-NAM forecast of accumulated precipitation through 11 pm MST tonight. Note the somewhat strange and distinct maximum over the Rincons.

Below, is the same forecast, but for the period ending at 11 pm tomorrow night (Saturday, April 19th). The model forecasts what would be a nice precipitation event for southeastern Arizona, given that it is late April. Current NWS forecasts are for 30% chance of rain tomorrow at TUS, but 60% chances for Mt. Lemmon. The WRF forecasts definitely keep the more significant rain amounts at higher elevations. So, I'll be waiting to see if the gauge here at house catches anything with this system (last rainfall here was on March 1st)..

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