July 1st-Thinning AM cloud deck clearing the way for high values of instability today

Overview:

The synoptic-scale pattern remains largely unchanged from yesterday, The trough axis over California has shifted into the intermountain region to our north, but the flow in our area is still being dominated by the Southern Plains high-pressure system. Deep southerly flow extending well south into Mexico has help advect moisture from an overnight convective complex northward into southern Arizona resulting in an increase in the precipitable water as compared to yesterday. On the mesoscale level, a remnant cyclonic circulation in the 700-500 hPa layer is apparent over extreme southwestern Arizona. This feature is expected to weaken while crawling eastward today and may play a role in initiating convection over eastern Pima county into this evening. For tomorrow, Arizona will be situated in a weak region of cyclonic shear in-between the 500 hPa Southern Plains high and a developing high pressure system off of the West Coast. The exact details of the flow will depend on convective activity over night given the relatively weak gradient in the region. A bit higher aloft, southwesterly winds will be strengthening at 250 hPa which may supply a bit of dynamic help with divergence in the equatorward jet entrance region. With sufficient moisture still in place, scattered convection will still be possible.

This Morning:

The Big Picture:

Our weather is still being dominated by the Southern Plains high pressure system and the southern extent of the trough that has pushed into Nevada as of this morning. The visible satellite image overlayed with the 500 hPa RAP Mesoanalysis provides a good summary of synoptic-scale conditions.

Visible Satellite Imagery and 500 hPa Mesoanalysis valid at 8 AM MST

(weather.cod.edu)

The cloud bands in the west are situated ahead of the trough axis in the region of differential cyclonic vorticity advection. Further to the south, we can see a region of clouds along the West Coast of Mexico which are the remnants of a large convective complex that formed overnight. Moisture from this feature has been advected northward around the western periphery of the Southern Plains high into southern Arizona as can be seen in the PWTR mesoanalysis below.

PWTR and 850 hPa Moisture Transport Mesoanalysis valid at 8 AM MST

(spc.noaa.gov)

Zooming in: 

We can see the remnant area of cyclonic circulation situated just northeast of the Gulf of California.

500 hPa Mesoanalysis valid at 8 AM MST

(spc.noaa.gov)

While this feature may play a role in today's weather by influencing cloud cover and convective initiation, it should be noted that this already weak feature is expected to become more diffuse throughout the day.

Mid-level clouds from this morning have mostly dissipated, although some high clouds are still prevalent in parts of central and eastern Pima county.

Visible Satellite Loop valid at 9 AM MST

(weather.cod.edu)

This morning's sounding indicates deep moisture with lapse rates conducive to the development of CAPE.

12Z KTUS sounding

(spc.noaa.gov)

Precipitable water values are close to 1.8" which continues our trend of significantly higher than normal moisture values. CAPE values as of 5 AM were already at 1000 J/kg, thanks in part to an increase in the surface dewpoints temperature overnight. While downdraft CAPES this morning were less than a 1000 J/kg, mid-level drying in combination with surface warming should see those values increase considerably throughout the day. The forecast surface based CAPE is abouth 1500 J/kg, easily besting yesterday's value. The wind profile indicates decent propagation of storms to the north/northeast. Exceptionally light winds above 400 hPa should mean that anvil shading shouldn't be too problematic today.

CAM forecasts:

CAM models are generally showing scattered convection from central Pima eastward into Cochise County by this afternoon as suggested by the HRRR ensemble reflectivity map.

HRRR ensemble 'paintball' showing member reflectivities greater than 40 dBZ valid at 

4 PM MST

(spc.noaa.gov)

The general lack of clustering indicates that there is not much of an organizing principle or feature to focus the convection. However, the HRRR ensemble mean MUCAPE forecast suggests an axis running from Cochise and Santa Cruz counties northwestward into the greater Tucson area.

HRRR ensemble MUCAPE valid at 4 PM MST

(spc.noaa.gov)

And is roughly aligned with region of > 30% probability of greater than 1" accumulations.

HRRR ensemble max QPF valid at 5 PM MST

(spc.noaa.gov)

UAWRF:

The 12Z run of the UAWRF focuses most of the activity today into tonight just northeast of 1-10 which includes a significant portion of the Tucson metro area

UAWRF 12Z HRRR total precip valid at 10 PM MST

(atmo.arizona.edu)

while the 15Z intitalization is less aggressive overall

UAWRF 15Z HRRR total precip valid at 10 PM MST

(atmo.arizona.edu)

The reasons for the discrepancy are hard to discern as forecast values of CAPE, cloud cover, as well as the handling of the cyclonic feature near the Gulf of California this morning seem quite similar between the 2 runs. 

My take:

The lack of cloud cover should result in decent instability today. Mixed layer CAPES are currently around 500 J/kg and should reach or exceed 1000 J/kg by this afternoon. Convective inhibition should be overcome in by about noon resulting in convective buildups starting in the higher terrain of the Quinlan and Santa Rita mountains. Without significant forcing or convective organization, coverage will be scattered with about a 30-40% chance of convection in the greater Tucson area. Given the forecast sounding from this afternoon, any convection that does develop will be capable of strong winds and heavy downpours,

UAWRF 12Z HRRR forecast sounding for KTUS at 4 PM MST

(weather.arizona.edu)

with a greater than 70% chance of winds in storms that do develop exceeding 30 knots.

HRRR ensemble probability of exceeding 30 knots winds valid at 5 PM MST

(spc.noaa.gov)

Tomorrow:

The synoptic pattern will remain roughly the same for tomorrow

12Z GFS 500 hPa heights/vorticity valid at 11 AM MST

(tropicaltidbits.com)

with moisture still in place

12Z GFS PWTR and MSLP valid at 11 AM MST

(tropicaltidbits.com)

although PWTR values may be a bit less than today pendng any unexpected contributions from overnight convection in Mexico.

The one difference may be increased flow from the southwest at jet level, which could pose a problem for anvil shading from storms that develop to our southwest.

12Z GFS 250 hPa flow valid at 5 PM MST

(tropicaltidbits.com)

but expect roughly the same over pattern of scattered convection 

UAWRF 12Z HRRR reflectivity valid at 4:30 PM MST

(atmo.arizona.edu)