Monday, June 08, 2015

Complicated Forecast As TS Blanca Moves North Over Baja

Heavy middle and upper-level cloudiness from Blanca has spread over southeastern Arizona during the night. Above is view south from Kitt Peak at 6:10 am MST and below is visible satellite image from 6:00 am. Won't be much sunshine today, which will affect possible development of thunderstorms.

Blanca made landfall on the Baja peninsula before sunrise - the storm's track was apparently a bit too far east to trigger a classical Gulf Surge and the moisture fields are moving more slowly northward with the decaying storm. The blended PW analysis for 4:00 am (above, from CIRA at Colorado state University) shows values of an inch and more just intruding into southern Arizona.

However, the 12 UTC sounding from TWC this morning (below) indicates that the boundary layer (i.e., below 600 mb) remains dry with light and variable winds. The forecast is complicated by several things:

The models forecast varying degrees of easterly winds below 500 mb today and tomorrow, which means downslope flow fighting the low-level moisture and keeping highest values out to the west.

Warm middle-level temperatures plus limited sunshine combine to limit CAPE - in fact the WRF-NAM forecasts develop almost no CAPE during much of this coming event.

The NWS NAM and GFS model forecasts for precipitation remain very different, even at the short time frames we're now dealing with. Graphics below illustrate the differences in the models' precipitation forecasts.

These two forecasts (all shown here are from 00 UTC runs last evening) show total precipitation forecast by the NWS NAM (above) and from the WRF-NAM run at Atmo for the period ending at 5:00 pm MST on Wednesday. Similar but with more detail, as expected, in the high resolution forecast. Neither forecast does much over the metro Tucson area.

These are the same forecasts but from the NWS GFS (above) and from the WRF-GFS below. The NWS model forecasts are hugely different for Arizona at just 72-hours, which I find rather discouraging. The WRF-GFS forecasts a very significant rainfall event for the metro area, which would be amazing for so early in June.

But, with such model variance, we'll just have to rely upon metwatch and see what actually happens. Old tropical storms often prove extremely fickle for our area - remember Odile last September?

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