Tuesday, July 28, 2015

Ill-Defined pattern Continues For Southeast Arizona

There were several isolated thunderstorms in southeastern Arizona yesterday afternoon as per detected CG flashes above through 1:00 am MST this morning (from Vaisala and Atmo). Thunderstorm activity increased more markedly across northern Mexico - thus, a bit of an uptick from preceding day. There was one report of 0.04" of rain across all of the Pima County ALERT network and basically the break in summer storm activity this side of border continued for yet another day.

The larger-scale pattern this morning continues to be ill-defined and has some pro indications for increased storm activity, but also carries considerable cons with it. The water vapor image below is from 6:30 am this morning. The most distinct circulation feature in the upper-troposphere is a cyclonic circulation centered just west of Baja at about 25 N. There is a skinny snake of dry air air along the east side of this circulation. The circulation appears to be moving westward, or even a bit west-southwestward this morning. Over Texas a broad, diffuse, inverted trough continues in the upper-levels. The most distinct circulation center embedded in the broad trough appears (via loops) to be centered south of the Big Bend. The latest NAM forecasts weaken this feature and move it west-southwestward.

The skewT plot above is from SPC for the 12 UTC data from TWC. Pros - PW is up and over 1.60 inches; CAPE is moderate and considerably up from recent days. Cons - Indistinct steering flow this morning due to L/V winds from 500 to 300 mb; upper-level flow is from south, which is quite negative since anvils spread ahead ahead of storm cores and stabilize the low-levels.

SkewT below is the same plot but for PHX upper-air data. Conditions at Phoenix are much different than those here at Tucson. Pros - none that I see (note that PW is considerably lower due to the dry air just above the surface). Cons - No CAPE, plus all those listed for Tucson.

Last evening there were only forecast runs for the WRF-NAM at Atmo, with the GFS versions missing. The 06z forecasts from WRF are forecasting a big storm day (at least in comparison to past past week or so) for eastern Pima County. The forecast sounding above is for TWC at 2:00 pm MST - the CAPE is indicated to be over 1600 J/Kg (appears to be considerably too high for the mixed layer conditions), even though PW is down some (~1.50 inches) due to the northwest winds in the BL. The model forecasts a much more organized southerly flow to be in place above the BL this afternoon - but will this actually happen? The new NAM forecast seems to indicate continued L/V middle-level winds this afternoon. The model forecasts storms to move fairly quickly toward the north; however, if the steering flow remains weak, storms will hug the higher elevations and only come off if there are organized outflows. There should definitely be an increase in thunderstorm activity today, but many locales will probably be "anviled out" due to the southerly winds at upper-levels.

The two forecasts below are from the 06Z WRF-NAM. The first is of composite radar echoes valid at 4:00 pm this afternoon and the bottom is for accumulated rainfall though midnight tonight. The 00 UTC version of the WRF-NAM forecasts kept most thunderstorms south of the Tucson metro area from southeast to southwest, and with much less rainfall. All-in-all, a very difficult day for precise forecasting.

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