Mesoscale Precipitation Discussion 0557
NWS Weather Prediction Center College Park MD
520 PM EDT Sun Jul 29 2018

Areas affected...Far southwest NM across portions of central and
northern AZ

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

Valid 292120Z - 300320Z

SUMMARY...Monsoon convection will expand in coverage this
afternoon. Slow cell motions in a high moisture environment could
result in a flash flood threat, especially over the higher
terrain, in areas that received heavy rainfall Saturday/Saturday
night.

DISCUSSION...The GOES-16 clean IR loop and regional radars across
AZ and western NM showed convection increasing in coverage over
the higher terrain, mainly from southwest NM across the Mogollon
Rim. The CIN in place is starting to erode, allowing the
convection access to 500-1000 J/KG of MLCAPE over this axis. As
the remainder of the CIN is exhausted, initiation will continue
across the higher terrain, as the cells begin to fill in along the
Rim.

The convection is feeding on a 1.00/1.25 inch precipitable water
gradient stretched across the terrain, and the KFSX radar is
already showing hourly rainfall rates near 1.25 inches over far
southern Navajo and northeast Gila counties. With the Southwest
states under mid level ridging, synoptic scale forcing is expected
to remain largely absent.

The mid level ridging in place also ensures slow cell motions,
especially when the convection is over the higher terrain. The
combination of high moisture content air and very slow cell
motions (under 10 knots in most places) could allow some of the
cells to produce hourly rainfall rates near 1.50 inches. It should
be noted that some of these areas (especially over the Mogollon
Rim) received between 1.00 and 3.00 inches of rainfall in the last
24 hours, and the heavy rainfall over wet soils could enhance the
flash flood threat.

The convective extent is expected to peak between 29/22 and
30/01z, before the cells starts moving slowly south southwest with
the mid level flow. It should be noted that much of the available
high resolution guidance is not bullish with the convection but
local 2.00/3.00 inch rainfall amounts (which occurred last night),
are not out of the question. Because of the uncertainty in overall
coverage, flash flooding is considered possible.

Hayes

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

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

LAT...LON   36681323 36571196 35061037 34250984 34310887
            34330823 34300799 34210773 34060744 33790724
            33440711 33080705 32650710 32270734 32090776
            32270885 32831034 33241128 33941230 35321364