Normal Sunday, Manic Monday?

Overview:

A diffuse center of high pressure at 500 hPa stretches from southern Colorado into southeastern Arizona. Further to the east, a 500 hPa low-pressure system is dropping southwestward in the northerly flow on the east side of the upper-level high pressure system. For today, the proximity of the high pressure system should mean another fairly hot day with temperatures in the Tucson and Phoenix metros just south and north of 110 F respectively. A subsidence inversion associated with the high pressure should along with a moisture profile close to the climatological norm, should mean that convection today will largely be focused on the Sky Islands around Tucson. Weak mid-level flow and marginally negative synoptics will mean that propagation into the valley will be problematic with the one caveat centering around DCAPES sufficient for significant outflow boundaries. The collision of outflow boundaries in the Valley could result in storm formation although this is a relatively low probability (20-30%) event. Meanwhile over in the Gulf of Mexico, Tropical Storm Debbie looks to be rapidly organizing in a favorable environment just west of the Florida coastline. Expect Debbie to pose a significant problem for the panhandle of Florida as well as the Southeast US over the next several days.

For tomorrow, the upper-level currently over Texas is forecast to propagate southwestward into New Mexico. Upper-level heights will fall slightly in response, and the 500 hPa flow is forecast to strengthen and shift to northeasterly. By mid-afternoon southeastern Arizona should be experiencing weak synoptic-scale forcing for ascent, which combined with the northeasterly flow will likely result in convection forming over the Mogollon Rim and propogating into the greater Tucson area by early evening. Increasing shear may allow some of the convection to organize and there will be an increased threat for strong winds moving through the area, especially after 5 PM.

Sunday:

Large Scale Overview:

The upper-level water vapor and overlaid 500 hPa mesoanalysis does a good job of summarizing the synoptic-scale conditions. There is an upper-level high dominating the Southwestern US with weak flow over southeastern Arizona in response to our proximity to the ridge axis. A shortwave upper-level low is seen centered over north-central Texas with relatively sharp cyclonic curvature in the wind field. Out east, Tropical Storm Debbie is readily apparent in the comma-shaped plume of deep water vapor just off of the west coast of Florida. 

Upper-level water vapor and 500 hPa mesoanalysis valid at 10 AM MST

Tropical Storm (soon to be hurricane) Debbie update:

As of 11 AM MST, the NHC reported Debbie had 65 mph sustained winds and was just below hurricane strength. Satellite pictures indicate that the storm is organizing with convection now wrapping around the center of circulation.

Visible Satellite image valid at 10 AM MST

In addition, Debbie is moving through a region of favorable shear and very warm sea surface temperatures in excess of 30C (86F), leading to the expectation that Debbie will continue to rapidly strengthen ahead of making landfall in the panhandle of Florida.

SSTs; 850-200 hPa wind shear; and forecast track of Debbie valid at 10 AM MST

Once inland, steering flow is expected to collapse leading to the circulation meandering around the Southeast with the potential for rainfall amounts in excess of 20".

WPC 5 day rainfall forecast

Closer to home:

Dynamics:

By this afternoon, expect the ridge axis at 500 hPa ridge axis (the locus of maximum anticyclonic curvature as defined by the wind field and identified by the jagged line) to be situated over or just east of eastern Pima county, with the 500 hPa trough axis (the locus of maximum cyclonic curvature as defined by the wind field and identified by the smooth line) located over western Texas. The proximity of the ridge axis to and just east of Tucson should provide weekly hostile synoptic-scale conditions for convection and is part of the reason that prospects for valley convection should remain on the low side of climatology today.

UAWRF-HRRR 500 hPa wind valid at 4 PM MST

Thermodynamics:

This morning's sounding from Tucson indicates a Precipital Water value of 1.3", which is close to the climatological average. Boundary layer moisture is shallow with dewpoint temperatures just above the surface at 50 F and below. Mid-level flow is weak, with wind speeds in the 700-400 hPa layer at 10 knots or less. Further aloft, somewhat stronger flow from the southeast may cause anvil shading from storms developing to our southeast. While lapse rates are very steep, limited mixed layer moisture should result in somewhat limited instability....

5 AM MST KTUS observed sounding

as indicated by the UAWRF forecast sounding for Tucson valid at 4 PM with CAPE values of less than 500 J/kg. The mixed layer is extremely deep with cloud bases expected to be around 600 hPa. With such a deep and dry mixed layer, DCAPES of around 1400 J/kg should be sufficient for significant outflows development from convection over the higher terrain. Intersecting outflows in the value could help overcome poor dynamics as well as improving surface moisture, but that is stocahstic in nature and a low-probability occurrence.

UAWRF-HRRR KTUS forecast sounding valid at 4 PM MST

Precipitation Forecast:

The 12Z UAWRF indicates storms developing over the Rincon and Quinlan mountain ranges by 4 PM. 

UAWRF-HRRR forecast refectivity valid at 4 PM MST

Updrafts are expected to weaken as the move into the valley by around 6 PM

UAWRF-HRRR forecast refectivity valid at 6 PM MST

HREF ensembles also support the idea of scattered convection across the Sky Islands, as a composite of greater than 40 dBZ reflectivities from all ensemble members indicates scattered convection around Tucson, with more focused and widespread activity near the Quinlan mountains. 

HREF ensemble members with reflectivities > 40 dBZ valid at 2 PM MST

Given the weak wind flow and slow propagation, heavier rainfall totals may be in problem in in higher terrain although ensemble probabilities of greater than 1" are generally less than 30%

HREF probability of 3 hr QPF > 1 " valid at 5 PM MST

Monday:

Dynamics:

I expect improving synoptic-scale conditions tomorrow as the ridge and trough axes move eastward, leaving southeastern Arizona in a region of mid and upper-level cyclonic vorticity advection. Also, expect upper-level heights to fall and the flow to strengthen from the northeast as the upper-level low moves into Chihauhau, Mexico  by 4 PM tomorrow.

UAWRF-HRRR 500 hPa wind valid at 4 PM MST

Thermodynamics:

While boundary layer moisture will still somewhat of an issue tomorrow, slightly better surface dewpoints should lead to CAPE values of 500-1000 J/kg. Additionally, shear profiles and DCAPE values in excess of 1800 J/kg support some convective organization and the possibility of intense outflow winds.

UAWRF-HRRR KTUS forecast sounding valid at 5 PM MST

Precipitation Forecast:

Expect convection to be ongoing by 4 PM over the Santa Rita mountains although the southwestward propagation will mean this will not impact the Tucson metro area. More to the point is the cells that are developing across eastern sections of the Rim.

UAWRF-HRRR forecast refectivity valid at 4 PM MST

By 7 PM, outflows should be approaching the eastern slopes of the Catlinas where new and potentially intense updrafts should be initiated. The terrain will undoubtedly distrupt the propagation of these cells southwestward, but the enhanced shear and organization may allow storm propagation through portions of the valley.

UAWRF-HRRR forecast refectivity valid at 7:30 PM MST

HREF ensembles also support the idea of a more coherent line of convection developing along the Rim with activity approaching Tucson around 6 PM

HREF ensemble members with reflectivities > 40 dBZ valid at 2 PM MST

HREF ensemble members with reflectivities > 40 dBZ valid at 6 PM MST