Friday, November 25, 2016

Hopkins Wind; Grid Forecasts; Sunday Rain

The winds at Hopkins yesterday were just about as anticipated (see previous post) with 6 hours having RAWS observations with gusts over 30 mph - max was 36 mph at 9:19 pm MST. The WRF sounding forecasts were again quite good, indicating a marginal wind situation because of shallow depth of low-level easterlies.

There was considerable reference to the high resolution, deterministic, grid point forecasts produced by the NWS in the previous posts discussing restructuring. These are available for every hour out to seven days. This morning provides an example of far awry even the first hour can be under a mundane synoptic setting. Above graphic shows the start of hourly forecasts for TUS beginning at 6:00 am this morning. The observed temperature (red) at that time was 57 (11 F warmer than the grid forecast); Td (green) was 12 F (8 F lower than the forecast). Winds (purple) were from the correct direction but a bit stronger than forecast (observed at 6:00 am was ESE at 14 with gust to 21 mph at 25 minutes after the hour). This is a huge and resource intensive forecast exercise, but not one that would inspire much confidence in users.

Looking ahead to the potential rain event - the TUS grid forecast for 3:00 am early Monday morning is: T = 42 F, Td = 37 F, POPs for that hour 57 %. That forecast may prove better than the hour one forecast.

The 06 UTC GEFS plumes for QPF at TUS are shown above. The plumes indicate some chance for three different precipitation events through December 3rd, but with main event occurring Sunday night. Amounts forecast by the ensemble members range from 0.0" to 0.4" and indicate about 80 to 90% chance of measurable rain at the airport (i.e., 0.01" or more - current NWS chances are at 60%, similar to grid point forecast).

The 06 UTC WRF-GFS forecast of precipitation through 11:00 am Monday morning is shown below. This version of the WRF forecasts a donut hole over much of Tucson area and focuses the precipitation over higher elevations of the Sky Islands. The NAM version is wetter and forecasts 0.3" at airport. The moisture for this event comes in from the northwest with the 500 mb short wave and thus is not great and also is quick htting.

Will be interesting to observe how this event actually transpires, since it is almost three days away.

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