Cold Mornings And Life Under The Ridge

The past week we have mostly been under the 500 mb ridge, or under the influence of very week disturbances passing by mostly to the north and northeast. With mostly clear skies, the last 9 mornings have been quite cold here along the Rillito. The morning lows have ranged from 21 F to 30 F each morning for these 9 mornings - with 21 F on the 6th to 30 F on the 8th - this morning (Monday, January 13th) coming in with frost and 26 F here at the house.

I took a look at the WRF-NAM early run to see whether the model forecasts strengthen the easterly winds, as high pressure dominates over the southern Plains and the Southwest. The early WRF-NAM does forecast easterly winds increasing to 15 to 20 mph over parts of the metro area tomorrow night (Tuesday, January 14). because of this the model forecasts (see graphic below) the low temperatures to be bouncing around again. For tonight the model forecasts low temperatures west of the Catalinas to be around 10 F warmer than down at the airport. However, the forecast lows for Wednesday morning are generally 5 F or so warmer, but with the airport warming by 11 F. The string of consecutive days below freezing here at the house may continue for a few more days (it tends to go calm and stable here many nights when the winds blow at TUS and Davis-Monthan).

The GFS-ensemble forecasts for 500 mb are shown here with averages on left and spaghetti plots on right - above is at 84-hours (valid 12 UTC on Thursday) and below is at 168-hours (valid 00 UTC next Monday). We basically remain under the ridge through the next week. By the end of the week the GFS spread is greatest again over the Southwest, as a number of weak 500 mb short waves are forecast to break through into the southern part of the ridge. Little weather to talk about - cold mornings some spots, mild afternoons, and continuing drought.