Wednesday, February 08, 2017

Warm Followed By Strong Front And Possible Showers

Interesting situation for southeast Arizona as 500 mb short-wave digs in from the Pacific over the weekend. Still a few days away but I took a look at model forecasts from both 00 and 06 UTC last evening/night. The 06 UTC GEFS plumes for T at the airport shown above (current am forecast for TUS is 86 F for Thursday and 88 F for Friday - both of which would be records - see NWS Weather Story at their home page).

The QPF plumes for airport are shown below - all but one member forecasts at least 0.01" with the spread reaching from 0.00 to 0.70" - ensembles forecast the event during Saturday night into Sunday morning. Second graphic below is from 00 UTC WRF-GFS and shows that model's forecast of precipitation through noon on Sunday.

The 500 mb evolution is shown here (using the 5.4 km grid forecasts from Atmo's 00Z WRF-GFS run). The forecast above is valid at 07:00 pm MST on Friday the 11th, with the short wave just approaching the lower Colorado River Basin. By 05:00 am on Monday the 13th the trailing portion of wave has closed off over Arizona and New Mexico, with center of circulation forecast to be just southeast of Tucson. The ECMWF operational model is much more progressive with this feature and moves it quickly east of Arizona - so we'll see what actually evolves.

The WRF-GFS forecasts a pronounced, low-latitude atmospheric river (AR) moving southeastward into Arizona from the Pacific at 07:00 am on Saturday. This moisture intrusion is forecast to be weakening rapidly during the day and forecast below shows it just reaches into southern Arizona at 07:00 pm. This feature may well prove to be key as to how the event actually develops - will it weaken rapidly or hold together a bit longer? 

At the bottom is the model's forecast of the sounding at TWC valid also at 07:00 pm. This sounding has CAPE of almost 300 J/kg and PW of 21 mm. The convection with this sounding would just reach into the -10 to -20 C thermal layer, so it may prove too warm for thunderstorms, although this remains a long way out in time.

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