Thursday, September 20, 2018

Significant Rains Yesterday

Yesterday's widespread heavy rains left behind very moist air that led to fog and stratus fractus this morning. Above photo from about 6:30 am MST along Rillito - Finger Rock through the fog. Below is view from campus at 7:15 am.

The event yesterday (somewhat similar to the June event) resulted from a tropical moisture intrusion interacting with an advancing trough in the westerlies. Also playing a role was TD-19 (as per NHC); however, Art Douglas sent along observations from Mexico that indicate the system was likely a tropical storm in the lower third of GoC. 

Plot of detected CGs above (from Atmo and Vaisala 24-hours thru 10:00 pm last night) shows that thunderstorm activity with the event was quite limited and mostly occurred before noon yesterday morning.

Significant amounts of rain fell across the entire ALERT network - plots ending at 5:00 am for north portion above and south portion below. There were 45 sites that recorded over an inch of rain and 16 had over two inches. The two reports of more than three inches came from Samaniego Peak in the Catalinas and from Altar Wash at Highway 286. Here at house we had 0.65", while airport had 0.76", as did Atmo (DM ASOS has been out of service a couple of days). The Rillito was flowing this morning and apparently had reached bank-to-bank levels at a couple of spots between USGS gauge at Dodge and the Campbell Ave. bridge. Plot second below shows the flow peaked at about 2700 cfs around midnight.

The WRF forecasts indicate a quiet day today, with perhaps some storms over mountains off to east. While low-level moisture persists, dry air is intruding across Arizona at middle and high levels and this will cap deep convection. The MIMIC plot below is from 6:00 am and is the analysis of PW in the layer 700 to 500 mb - very dry air west of the 500 mb trough.

The longer-term global forecasts indicate some hope for another intrusion of tropical moisture before the month closes out.

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