Friday, December 12, 2014

Quick Look At Tomorrow's Weather Event

As I had mentioned a number of days ago, the Winterhaven Festival of Lights seems to draw in several winter storms during its two week run. This year things are right on track, as the Festival kicks off tomorrow - Saturday, December 13th. This will be a quick-hitting frontal event, as the forecast models have been predicting for more than a week now.


There has been a pronounced atmospheric river (AR - aka the Pineapple Express) ahead of and along the Pacific front. This has brought a fairly warm and very wet storm to California, with snow levels at fairly high elevations. The above graphic (CSU CIRA blended PW analysis for 12 UTC this morning) shows that the AR may be losing some of its organization. But, PW values of an inch or so have nosed up the GoC and are a bit higher along west coast of Baja. So, the system will pull some decent PW northward at lower levels to feed showers and moderate convection along the front as it crosses southeastern Arizona tomorrow.

The forecast sounding for TUS (below, valid 11 am MST tomorrow) is from the WRF-NAM early run (both versions are fairly similar this morning) and indicates a bit less than 1 inch of PW that is mostly below 600 mb (there is also a bit of CAPE forecast, again below 600 mb).



The WRF runs forecast the front to cross eastern Pima County during the late morning/early afternoon. Graphic above is of WRF-GFS forecast composite radar echoes valid at 10 am Saturday morning. The WRF-GFS forecast below shows the models forecast of accumulated precipitation through 5 pm MST tomorrow afternoon. The model forecasts the event to just brush southern Arizona with light rainfall, keeping the main impacts to our north in eastern Maricopa County and the Rim Country, although the high elevations near Tucson pick up heavier amounts than the low elevations.

The WRF model has, for reasons that are not obvious, underplayed the forecasts for the last several precipitation events, and this will likely be the case again with tomorrow's event. I expect that the lower elevations of eastern Pima County will pick up some higher rainfall amounts than the model is currently forecasting.

Finally, to put this weather event into a regional perspective, the graphic at the bottom is the WRF-NAM forecast for accumulated precipitation through 5 pm tomorrow, on the 5.4 km grid. California continues to catch the brunt of this storm, with very heavy rains forced by both the southern coastal ranges and the Sierra Nevadas. 



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