Sunday, September 20, 2015

How Wet Will It Be?

First a brief summary of yesterday. The WRF-NAM forecast that I posted on Friday (valid for yesterday) was indeed very optimistic - but each run that followed forecast less and less chances of storms at low elevations. There were storms around yesterday - above image shows heavy rain shaft south-southeast of Kitt Peak and 24-hour flash density chart below (from Vaisala and Weather,Graphics for period ending at 6:00 am MST this morning) shows that storm activity shifted westward again on Saturday.

Across the ALERT network 10 sites measured rainfall - these were at high elevations and around the southern edges of the network - Keystone Peak (west of Green Valley) had 0.83" and Mt. Lemmon came in at 0.35". The NWS POPs on Friday were actually pretty good for yesterday - looks like the low-elevation zone had zero coverage (although there were strong outflow winds and a bit of dust) while coverage for the high-elevation zone was probably about 50 percent.

For today the WRF forecasts little storm activity this side of the border. But then the situation turns wet as the subtropical disturbance approaches for the south-southwest.  The forecast above from NWS is for the airport grid and has very high POPs for Monday through Tuesday. Below is a weather story graphic for this morning's forecast - note the confidence statements. I think this is a very gutsy forecast, since it is essentially based on the model forecasts for a feature that is heading our way out of an observational data void. Those who have been around the Southwest very long know how the small-scale details can mess up even a short-term forecast.

Will end up for today by showing various model and guidance products for the approaching event. The above forecast is from the 06 UTC NWS NAM for total rainfall though 11:00 am on Wednesday the 23rd. Below is same forecast from the 06 UTC WRF-NAM at Atmo - showing much more, terrain-influenced detail and a very large range in rainfall amounts forecast.

Above is same forecast from the 06 UTC WRF-GFS, with an even larger range of amounts in its forecast. Below is the current QPF guidance for Day 1-3 from the WPC national center.

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