Slight Risk of a High End Event Today
Overview:
Complicated forecast today due to a series of small-scale vortices forecast to propagate west-northwestward between an upper-level low situated over the Gulf of California and a broad high pressure system centered in northeastern Texas. The feature most pertinent to today's forecast is a mesoscale vortex (MCV) located near the border of Chihauhau and Sonora Mexico with the northern extent reaching into extreme southwestern New Mexico. This feature is forecast to approach the greater Tucson area between 3 and 6 PM. Convection is already developing ahead of this feature more aggressively than forecast over Cochise and Santa Cruz counties. Cirrus blowoff is spreading rapidly NW and may act to minimize destabilization over the Tucson metro. Anvil shading not withstanding, the MCV could provide enough ascent and shear in given sufficient destabization for convection to organize in the as we approach late afternoon. Given the potential for organization, the Storms Prediction Center (SPC) has issued a marginal risk for severe winds today in extreme southern Arizona from Tucson westward while the Weather Prediction Center (WPC) has issued a slight risk for excessive rainfall for the same region. Clouds and the potential for showers will continue overnight as a series of disturbances continue to propagate through the area. Details for the forecast tomorrow will heavily depend on the evolution of today's event, but I would expect a downturn in activity in the wake of the numerous disturbances progged to move through the area today and tonight.
Today:
Large Scale:
Broadly, southeastern Arizona is situated in a region of moist southeasterly flow between the 500 hPa cyclone located over the Baja Peninsula and high pressure over the Southern Plains. A series of four disticnt disturbances/convective clusters can be seen oriented from north to south stretching from just south of the New Mexico border into the Tropical Eastern Pacific. The southernmost disurbance is likely to become a tropical system in the next 48 h per guidance from the National Hurricane Center.
Zooming In:
A closer look shows a mesoscale inverted ridge (black line) situated between two MCVs or inverted troughs (redlines) centered in Yuma county and near the Sonora-Chinhauhau border in Mexico.
The mesoscale ridging has allowed skies to clear this morning west of Cochise county although some cumulus buildups are already present as of 10 AM over the Santa Rita and Quinlan mountain ranges.
Mid-level lapse rates increase from west to east across Arizona with values from Tucson westward supporting marginal to moderate instability.
A closer look at the 12Z Tucson sounding shows PWTR values near the climatological average with a dewpoint profile supporting a mixed boundary layer dewpoint value in the low-mid 50's in the absence of advection throughout the day. A deep dry layer above 600 hPa also supports DCAPE values in excess of 1500 J/kg although the mid-levels are likely to moisten ahead of the approaching MCV. Lapse Rates of 6.5-7 C/km support a forecast CAPE of 1700 J/kg, although boundary layer mixing should keeps us from fully realizing those values.
This Afternoon:
The UAWRF and other CAMs are consistent in indicating convective initiation in the higher terrain around 1-2 PM ahead of the MCV forecast to be near the AZ/NM border by 12 noon.
The UAWRF and most CAMs indicated more widespread convection by 3 PM from the Santa Rita mountains westward with this morning's MCV in Cochise county at 2 PM. Modification of the 500 hPa by the convection is Santa Cruz and is already underway...
and a new MCV starts to develop in central Pima county even as the original MCV approaches the area by 6 PM. The UAWRF shows convection firing ahead of the vortex in Tuscon.....
The nature of the convection over Tucson ahead of the MCV will heavily depend on how and if convection earlier in the day modifies the environment. Ideally, moist outflows will bump surface dewpoints and CIN development from anvil coverage/surface cooling will be minimal. Provided that convection can and does develop over the Tucson metro in the 4-7 PM time frame, enhanced flow from the MCV should provide enough shear from some organization of convection as forecast by both the HRRR....
and the UAWRF, although the UAWRF is more aggressive in creating convective inhibition from outflow cooling
Overnight/Tomorrow Morning:
Disturbances are forecast to continue to propagate through the area overnight into tomorrow morning continuing the chance of clouds and showers overnight. The UAWRF is the most aggressive of the CAMs showing a coherent MCV over Santa Cruz tomorrow morning....
with widespread showers/storms continuing south and west of the Tucson metro area.
My Take:
This is a complicated forecast. The longer that Tucson can remain sunny and on the hot side, the better the odds for strong to severe convection to develop with the arrival of the MVC. Cirrus blowoff from convection that is already developing to our southeast does concern me, but the forcing from the MCV and the enhanced shear leaves us with at least the potential for heavy rains.
DCAPES should decrease as the MCV approaches but convection off of the higher terrain to our west ahead of the MCV will still have a significant evaporative potential and carry the highest probability of severe winds.
The disturbances that are progged to come through the area overnight either have yet to develop or are not in range of US CAMs or mesoanalyses and as such confidence in timing/intensity is low, but there is enough of a signal to support chance probabilities of precipitation overnight.
Tomorrow:
If we ignore the potential complicating factors from today, tomorrow should be a fairly climatological type of day with southeasterly flow of 20-25 knots....
and CAPE values of 750-1000 J/kg.
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