Mesoscale Precipitation Discussion 0995
NWS Weather Prediction Center College Park MD
601 AM EDT Tue Oct 23 2018

Areas affected...central/southeastern AZ into southwestern NM

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

Valid 231000Z - 231600Z

Summary...An increase in convection starting around 12Z across
central and southeastern AZ into southwestern NM is expected to
pose a localized flash flood threat this morning. Maximum rainfall
totals of 1-2 inches will be possible through 16Z.

Discussion...GOES 16 water vapor imagery at 0930Z showed a well
defined shortwave west of the northern Baja peninsula of Mexico,
moving toward the east. A jet streak was inferred to the south of
the shortwave with GOES 16 Derived Motion Winds (DMV) showing a
90-110 kt core over the peninsula which was slightly stronger (10
kt) than RAP analyses and the DMV vectors suggested the jet streak
was a bit more compact than the RAP depiction. MUCAPE was
500-1000+ J/kg over southeastern AZ into southwestern NM as of 09Z
via the SPC mesoanalysis page with recent GPS precipitable water
values ranging between 0.8 and 1.1 inches across the region.

As depicted in short term forecasts from the RAP, as the shortwave
continues to approach the Southwest, increased cyclonic vorticity
advection and upper level diffluence will overspread central AZ to
the AZ/NM border, along with upper divergence associated with the
left exit region of the aforementioned 90-110 kt upper level jet
streak. The increased forcing for ascent and pool of instability
(which should only maintain or slowly increase in magnitude over
the next several hours) are expected to support an increase in
convection starting somewhere around 12Z over eastern AZ via
recent runs of the experimental HRRR and Arizona Regional WRF
model forecasts.

Layered PW imagery from CIRA has shown an increase in moisture in
the 700-500 mb layer over the AZ/NM border, emanating northward
from Hurricane Willa located west of central Mexico. This combined
with mid-upper level moisture advecting downstream of the Baja
peninsula shortwave trough is expected to have eroded mid-level
dry air noted in the 00Z EPZ and TUS soundings. Increased deep
layer relative humidity, sufficient CAPE above RAP forecast LFCs
near 700 mb and near unidirectional flow above 700 mb should help
to support periods of training rain. Rainfall rates of 0.5 to 1.0
in/hr are expected with localized 1-2 inch totals through 16Z.
Given areas from southern AZ into southwestern NM have seen
200-400 percent of normal rainfall over the past week, flash flood
susceptibility may be higher than average.

Otto

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

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

LAT...LON   35311118 35221019 34380927 33940886 33250809
            32060792 30930828 30831126 32601177 33721219
            34621240 35051205