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AWUS01 KWNH 231932
FFGMPD
AZZ000-NVZ000-240125-

Mesoscale Precipitation Discussion 0667
NWS Weather Prediction Center College Park MD
329 PM EDT Tue Jul 23 2019

Areas affected...Central to Northwest AZ

Concerning...Heavy rainfall...Flash flooding possible

Valid 231925Z - 240125Z

SUMMARY...Scattered showers and thunderstorms developing near the
Mogollon Rim and adjacent areas of northwest AZ will be capable of
producing locally heavy rainfall rates and with some potential for
repeating cell activity. Some localized flash flooding will be
possible going through the afternoon hours.

DISCUSSION...Early morning cloudiness across much of Arizona has
given way to strong solar insolation and a destabilizing boundary
layer as a result. This is fostering MUCAPE values of as much as
500 to 1000 j/kg, with the greater instability pooled over
southern Arizona where surface dewpoints are higher.

However, the latest GOES-17 1-minute visible satellite imagery is
actually showing much of the CU development occurring much farther
north over the Mogollon Rim and adjacent areas of northwest
Arizona. This is all connected to better orographic ascent, and
terrain-enhanced circulations, but also proximity to a fairly
well-defined MCV that is advancing west across western Arizona.

This mid-level MCV is helping to foster somewhat stronger wind
fields around the northeast flank of its circulation, and thus is
enhancing the effective bulk shear values a bit with values of 20
to 30+ kts locally. Consequently, this added shear combined with
improving boundary layer instability and orographics is yielding
convective initiation and already there are scattered showers and
thunderstorms growing in coverage.

The latest HRRR guidance favors a bit more expansive convective
coverage going through the 21Z to 00Z time frame as peak heating
maximizes instability, and with the orientation of the convection
relative to the mean layer flow which is generally out of the
southeast around the deep layer ridge over the Four Corners
region, there will be an opportunity for the convection to locally
backbuild and repeat over the same area since the convection will
be tending to focus and locally anchor itself over the higher
terrain.

The low-levels of the column are rather dry as per 12Z RAOB
soundings, but the mid and upper levels are rather moist, and so
once the convection moistens up the lowest few thousand feet of
the column, the convection should become more efficient and
capable of producing locally enhanced rainfall rates. The added
shear factor as mentioned earlier will be a facilitator of
maintaining some organized updrafts that will be capable of
carrying out the process of column moistening and then greater
rainfall efficiency.

Rainfall rates may reach as high as 1.5 inches/hr within the
stronger and more sustainable cores, and some localized storm
totals may exceed 2 inches going through the afternoon hours. This
may result in some areas of flash flooding, and especially over
the more sensitive dry washes and any slot-canyons near and
downstream of the convection.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...FGZ...VEF...

ATTN...RFC...CBRFC...

LAT...LON   36991359 36711235 36111164 35061079 34481084
            34251149 34281203 34731323 35461416 36101439
            36721415


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