July 21st - Increased wind threat in Phoenix metro; heavy rain threat in south-central Pima county today

 Overview:

Not much has changed relative to the conditions from yesterday. Deep northerly flow is fairly well established across most of the state and we should once again see storms developing along the Mogollon Rim propagate southward into the population centers. Slightly better thermodynamic conditions in the Phoenix metro area this afternoon relative to Tucson should support a higher potential for wind gusts approaching severe limits, in particular across the northern and western sections of town. In Tucson, expect outflows from the storms over the White mountains to initially support storm development along the Rincon and Catalina mountain ranges. Interactions between mountain based convection and outflows from storms dropping southward from Pinal county will likely trigger new updrafts in and around the Tucson metro area much like yesterday.

On Monday, drier air is forecast to move in to the eastern half of the forecast area behind a shortwave dropping southward on the back side of the longwave trough situated over the Midwest/Great Plains. Along with the decrease in moisture, CAMs are indicating a decrease in storm coverage over the area.

Today:

Large-scale conditions:

Northerly flow continues to dominate the state between and upper-level high pressure system located near the AZ/NV border and the positively tilted long-wave trough situated over the central part of the country (red line). There is a general region of drier air associated with subsidence behind the trough apparent on the upper-level water vapor image stretching from Idaho into southwestern Colorado (brown line). A short wave embedded on the western side of this trough in Wyoming (black line) is forecast to propagate south-southeastward over the next 24 hrs which will cause some of this drier air to overspread extreme eastern AZ and western NM.


Upper-level Water Vapor and 500 hPa RAP analysis valid at 9 AM


Zooming In:

PWTR values this morning are on the high side of climatology with values of around 1.5" stretching near Tucson to 1.7" near Phoenix.

Precipitable Water Mesoanalysis Valid at 9 AM


Mid-level lapse rates are just below 7 C/km around Tucson, but increase to the NW to 7.5 C/km on the northwest side of the Phoenix metro.

Mid-level Lapse Rate Mesoanalysis Valid at 9 AM


The 12 Z soundings from Phoenix and Tucson highlight the greater potential for severe winds at Phoenix with generally steeper lapse rates, a deeper mixed layer, a higher PWTR value, and a larger DCAPE when compared to Tucson. 

12Z KTUS Sounding



12Z PHX Sounding


This Afternoon:
CAPE values are expected to peak in the metro areas this afternoon around 4 PM with values ranging from approximately 600 J/kg at Tucson to 800 J/kg at Phoenix.


UAWRF HRRR MUCAPE Valid at 4 PM


The forecast soundings from the UAWRF highlight the strong wind potential at Phoenix (right) with larger CAPE and DCAPE values as well as stronger mid-level flow.

UAWRF HRRR KTUS (left) and PHX (right) Soundings Valid at 4 PM




At this time, storms are forecast to be approaching the Phoenix metro area while outflows from storms over the White mountains should be just reaching the Rincons and Catalinas.

UAWRF HRRR Simulated Reflectivity Valid at 4 PM



As storms and outflows approach the international border, I expect rainfall rates to become more of an issue in an axis of higher CAPE in south-central Pima County.

UAWRF HRRR Simulated Reflectivity (left) and MUCAPE right Valid at 7:30 PM


While rainfall rates in the immediate Tucson metro areas may not be particularly intense given the lowish CAPE values, rainfall totals over the past couple of days may support some flooding concerns particularly in the extremee eastern portions of the city and national parks. As such, I should note that the WPC has included eastern AZ in a slight risk for excessive rainfall.....

WPC Day 1 Excessive Rainfall Outlook



While slightly further west, the SPC has indicated a marginal risk for severe winds


SPC Categorical  Day 1 Convective Outlook


My Take:

In the Tucson metro area, I expect a day similar to yesterday. Of course the exact evolution of the storms after development in the Rincons and Catalinas will depend on stochastic interactions and placements of outflows which will be impossible to predict. Areas near the mountains that have received significant rainfall over the past couple of days will have to be watched for flooding.

In the Phoenix metro area, I would be most concerned about the potential for strong to severe wind gusts. A deep mixed layer, steeper lapse rates, and large DCAPES all point to the potential for strong winds. The HREF indicates an 80% chance of winds exceeding 30 knots in storms and the potential for peak winds of 50-60 knots.

HREF 4 hr Max Wind and Prob > 30 knots Valid at 7 PM MST


Monday:

Drier air will push southward on the heels of the shortwave currently in Wyoming. Since this feature is difficult to locate at 500 hPa, I am illustrating this feature in the GFS forecast of the dynamic tropopause for tomorrow 

GFS Pressure and Winds on the Dynamic Tropopause Valid at 11 AM MST Monday


The corresponding decrease in PWTR is illustrated in a comparison of the UAWRF PWTR forecast for today (left) vs tomorrow (right).

UAWRF PWTR Forecast Valid at 4 PM Sunday (left) and 4 PM Monday (right)


The decrease in column moisture and surface dewpoints (not shown), result in a decrease in the intensity and coverage of convection around the area.

UAWRF Simulated Reflectivity Valid at 5 PM Sunday (left) and 5 PM Monday (right)






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