July 28th - Inverted trough increases potential for significant Monsoon activity from Saturday through Monday

 Overview:

A potentially busy forecast period ahead, particularly for the Saturday through Monday timeframe after a relatively quiet Friday. Today is likely to be similar to yesterday, with any storms that develop in the higher terrain having some difficulty making into the valley. Chances for activity in the immediate Tucson area should increase tomorrow with the potential for earlier initiation as heights in the upper levels start to fall as the trough approaches. On Sunday, the trough should be situated just southeast of the Tucson area, and the focus for heavy rainfall will likely shift to just west of the immediate Tucson area. By Monday, this feature is forecast to turn northward as the upper-level high pressure system that has been plaguing the area shifts eastward, and the steering flow becomes southerly. This should set up one last round of more widespread activity. I should say that the details of the convection forecast will be heavily influenced by the previous day's convection. Consequently, I will be somewhat broad in my discussion for the weekend, and I will update the forecast daily as conditions evolve.

Current Conditions:

                  16Z water vapor image (weather.cod.edu)

I have highlighted some of the major features and streamlines on the water vapor imagery from this morning. Of immediate concern is the upper-level high pressure system centered along the Arizona/New Mexico border this morning. The proximity of the high-pressure system will likely keep things on the hot/quiet side today. Meanwhile, the upper-level low/inverted trough can be seen in the Gulf of Mexico, near the international border of Texas and Mexico. This is the feature expected to move eastward and then turn northward over the next several days. 

          12Z GFS PWTR anomaly 00h Forecast (tropicaltidbits)

The precipitable water anomaly is shown here to illustrate a couple of points. The first is that a psuedo dry line continues to be situated in extreme southeastern Arizona. This boundary has served as a demarcation line for significant convective activity for the past few days, The second point is that precipitable water values in the vicinity of the inverted trough are still below climatological values. We usually associate these inverted troughs or psuedo "easterly waves" with tropical levels of moisture. However, in this case, persistent subsidence associated with anticyclonic vorticity advection on the east side of the upper-level high, has left the area anomalously dry. While the water vapor imagery above shows the area moistening, the potential for wide-spread excessive rainfall is not as high as we would normally expect given the pressure field.

                   16Z TUS sounding (spc.noaa.gov) 

This morning's sounding indicates less impressive thermodynamics than the past couple of days. The impacts of last night's convection have left the upper levels of the atmosphere with a moist adiabatic lapse rate and somewhat saturated. This is highlight by the cirrus cloud deck present over southeastern Arizona as can be seen here:

    16:40 Z cirrus near-infrared satellite (weather.cod.edu)

While precipitable water values in the sounding are near climatological values at 1.26", the forecast CAPE and DCAPE are about half of the values from yesterday at 401 J/kg and 944 J/kg respectively. While the SPC mesoanalyses, continue to underrepresent precipitable water values in the region by .1-.2", both the 12Z WRF and HRRR initializations of the UAWRF are within 0.04" of the observed value. 

Forecast for today:

The combination of the cirrus cloud deck and an unfavorable high location

           12Z UA_WRFHRRR 500 hPa wind/temp valid at 21Z 

should delay any convective initiation outside of Rim locations until later in the day.

           12Z UA_WRFHRRR sounding valid at 23Z 

Instability is maximized at around 4 PM, although mixed layer and most unstable CAPE values are under a 1000 J/kg. This is about the time that models initiate convection in the higher terrain of the Catalinas and Rincons north and east of Tucson.

         12Z UA_WRFHRRR sim reflectivity valid at 00Z/29th

Zooming in, we can see the development over the Catalinas, and some of the outflow winds working into the valley, but the reflectivity cores quickly dissipate in a manner similar to yesterday.

12Z UA_WRFHRRR sim reflectivity and station plots 00Z/29th

12Z UA_WRFHRRR sim reflectivity and station plots 02Z/29th

In my opinion, the HREF from SPC does a good job of outlining the preferred areas today.

                  12Z HREF 4 hr max wind valid at 02Z/29

While storms and strong winds moving into the valley cannot be definitively ruled out, higher chances will remain confined to the higher terrain north and south of Tucson in Pinal and Santa Cruz Counties.

Saturday:

Things get a little more interesting on Saturday as the trough and ridge axes move west throughout the day. The ridge axis (indicated by the jagged line will move west of the Tuscon area by 00Z on the 30th. This will place Tucson in increasing cyclonic flow aloft in a quadrant of the wave conducive for ascent on a larger scale.

            12Z GFS 500 hPa heights/vort valid at 00Z/30th

Additionally, steering currents aloft will be from the northeast favoring initiation along Mogollon Rim, as well as the White and Catalina ranges. Storms and outflows should propagate to the southwest. While instability may not be as large as I would like,

        12Z UA_WRFHRRR sounding valid at 00Z/30th 

with values less than 1000 J/kg, increasing mid-level flow and synoptic-scale conditions favorable for ascent will hopefully lead to storms propagating into the valley.

12Z UA_WRFHRRR sim reflectivity and station plots 00Z/30th

12Z UA_WRFHRRR sim reflectivity and station plots 01Z/30th

The potential risk area for heavier precipitation is again highlighted by the HREF

             12Z HREF 3 hr max precip valid at 03Z/30th

which highlights the potential for 1" accumulations running from the Tucson metro area southwestward.

Sunday:

My concern for Sunday is that the area of favorable conditions will be moving into a region of potentially very high precipitable water values associated with surges from the Gulf of California.

          12Z GFS 500 hPa heights/vort valid at 00Z/31th

The wave pattern should become slightly more compact, increasing the forcing for rising motion. Meanwhile, precipitable water values will be approaching 1.75" in western Pima and Yuma Counties.

                 12Z UA_WRFHRRR PWTR Valid 00Z/31th

 12Z UA_WRFHRRR Total Precip Valid 12Z/31th

The green area highlights the region of precipitation on Saturday while the red area highlights precipitation on Sunday. If there is a region where I think amounts could be underdone, it is the red highlighted region based on the above. Of course, all of this will depend on the specifics of the previous day's convection and cloud field, but in my mind represents the largest potential risk.