Saturday, September 02, 2017

Significant Low-Level Moisture Increases Overnight

Nice sunrise this morning, as middle and high clouds northeast of a decaying TS Lidia moved over southern Arizona (IR image below is from 6:00 am MST).

As Lidia moved northward the area of higher moisture accompanying the storm moved into all of southern Arizona, as winds at low-levels shifted from easterly to southerly directions. Here in Tucson this happened around 10:30 pm - as per T and Td times series from Atmo (above).

The morning sounding plot for TWC (from SPC below) shows the low-level moisture reaching just above 850 mb at sounding time. This sounding has some CAPE if mixed to 700 mb. The two keys for this afternoon and evening will be how much low-level moisture increases during the day, and how much heating there will be. Tucson sounding has 1.17" of PW verses the very high value of 2.70" for the morning Empalme sounding. The vertical wind profile has  excellent steering flow, as well as favorable upper winds for organized and gusty storms - if they develop.

The morning 500 mb analysis (above from SPC) the anticyclone centered far to the northwest centered near Nevada's Black Rock Desert. We are definitely in Lidia's circulation here in southern Arizona.

The 06 UTC WRF-GFS forecasts some thunderstorms in eastern Pima County this afternoon and it's forecast of PW at 5:00 pm this afternoon indicates that the dry easterlies stay well to our north and east. The model forecast also increases PW to about 1.50 by afternoon. All of this indicates a better chance for afternoon showers than the models were indicating yesterday, a good trend for us in the new forecasts.

1 comment:

  1. Anonymous9:25 AM

    Heating seems to be an issue as mid and high clouds will be around all day...of more interest is the contrast between the ECMWF and GFS with Irma. If it hits the U.S it could be as destructive as Harvey.