Mesoscale Precipitation Discussion 0565...Corrected
NWS Weather Prediction Center College Park MD
456 PM EDT Mon Jul 30 2018

Corrected for Areas affected

Areas affected...Southwest NM into southeast and central AZ

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

Valid 302045Z - 310215Z

Summary...Although today perhaps lacks synoptic or even mesoscale
organization, the combination of moderate instability and
unusually slow cell motions will fuel a risk of flash flooding.

Discussion...All indications from satellite and radar trends and
mesoscale models were that diurnal convection will again become
relatively widespread, forming first at the high terrain in AZ and
western NM. The region was marked by seasonably rich moisture, and
although PW values near 1.25 inches in the mountains (greater at
desert elevations) were not especially anomalous, there is some
concern over the degree of instability coupled with slow cell
motions.

At 2030Z the SPC mesoanalysis indicated a gradient of 500-1000
J/kg mixed-layer CAPE in the initiation areas, rapidly increasing
to over 2500 J/kg with southwestward extent into the population
centers (Phoenix, Tucson) and deserts. Moreover, the expected cell
motions, or mean 0-6km winds, were forecast by the RAP and other
models at near zero knots. Cell motions are often slow, but are
rarely near zero over a broad area such as is expected today. With
no real organizing influence, outflows will be required to
initiate new cells, but these outflows may be relatively
successful given the strong instability. Some enhanced 850-mb
westerly inflow is available as well, and 700-500-mb flow
increases in a narrow zone near and west of Tucson toward a
westward moving inverted trough feature.

The models generally agree to spotty heavy rainfall here,
including the University of AZ WRF. Rain rates should increase
over time as outflows become more numerous and perhaps yield some
cell collisions, although the flash flood threat will likely owe
to 1 or 2-hour rainfall exceeding an inch or two rather than
anything longer term.

Burke

ATTN...WFO...ABQ...EPZ...FGZ...PSR...TWC...

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

LAT...LON   35431230 35241134 34180998 34120898 34520765
            33680724 32750747 31680751 31070841 31191151
            32001148 33081191 34711271