Mesoscale Precipitation Discussion 1020
NWS Weather Prediction Center College Park MD
535 PM EDT Mon Sep 27 2021

Areas affected...Northern and Central Arizona, far Western New
Mexico

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

Valid 272134Z - 280300Z

Summary...Slow moving thunderstorms rotating around an upper low
will increase in coverage through the evening. Rainfall rates may
at times reach 1"/hr, producing pockets of 1-2" of rainfall. This
could produce isolated flash flooding.

Discussion...The GOES-E visible satellite imagery this afternoon
clearly indicates a broad cyclonic circulation across New Mexico
and Arizona around an upper low drifting slowly to the northeast.
Spokes of vorticity will continue to rotate around this feature as
well, providing locally enhanced forcing into an environment
favorable for heavy rainfall. PWs across the region range from
0.75 to 1.25 inches according to recent GPS observations, near and
above the 90th percentile for the date. Instability measured by
SBCape has climbed to 1000-15000 J/kg despite abundant cloud
cover, likely in response to breaks leading to heating, and steep
mid-level lapse rates beneath the upper low. Within this
environment, showers and thunderstorms were developing, with rain
rates as high as 0.5"/hr in stronger convection.

As the afternoon progresses, the upper low should drift only
little to the northeast, maintaining an environment supporting an
expansion of convection, aided additionally by increasing upper
diffluence to the northeast. Cyclonic flow around the upper low
will remain weak, especially near the center, with 850-300mb winds
progged to be just 5-10 kts, with similar propagation vectors.
This suggests the potential for slow moving and training
convection, which may feature rain rates climbing above 1"/hr as
depicted by recent HREF probabilities. At the same time, effective
bulk shear of 20-25 kts supports at least modest organization
potential, and several of the CAMs including the UA WRF RR suggest
a cluster of storms dropping southward towards the Mogollon Rim
into the evening. While most of the models depict scattered QPF of
around 1", there are several which suggest local amounts above 2"
are possible, especially if this cluster organizes as forecast.

Isolated flash flooding will be possible anywhere a slow moving
thunderstorm can produce 1"/hr rates. However, the greatest threat
appears to be along the Mogollon Rim and across parts of Yavapai,
Gila, Coconino, and Navajo counties where MRMS 48-hr rainfall has
been 1-2+", and 40cm soil moisture is above the 98th percentile
according to NASA SPoRT. Much of this activity should wane
nocturnally with loss of heating, reducing the flash flood threat
shortly after nightfall.

Weiss

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

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

LAT...LON   36941182 36921088 36541001 36070915 35720854
            35200824 34850827 34270853 33800879 33530914
            33500994 33691102 34031197 34291277 34601310
            35271321 35851326 36411306 36831238