Mesoscale Precipitation Discussion 0609
NWS Weather Prediction Center College Park MD
829 PM EDT Tue Aug 07 2018

Areas affected...Southeast Arizona

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

Valid 080028Z - 080628Z

Summary....Flash flooding is possible over southeast Arizona over
the next several hours, with some indications of an increasing
threat.

Discussion...Convection continues to expand late this afternoon
across southeast Arizona. Mean winds are weak, favoring slow storm
motions and a localized flash flood risk. The last several runs of
the HRRR depict a scenario that may result in an increasing flash
flood threat into the evening hours.

The general idea is that the convection forming across the higher
terrain will generate a strong outflow boundary over the next few
hours. This outflow would then push south southwest into the
remainder of southeast Arizona. Meanwhile there is a consistent
signal for a broader increase in 850mb southerly flow moving north
out of Mexico and into Arizona over the next several hours. This
flow should advect moisture northward and sustain instability over
the area. As any southward dropping outflow interacts with this
increased southerly flow...the potential is there for increasing
convective development. Meanwhile, winds at 700mb are forecast to
gradually increase out of the north...setting up a decent lower
level wind shear profile...which may also help sustain convective
activity. 

Thus the combination of expanding slow moving cells as of 0z,
expected increasing 850 mb southerly flow and moisture transport,
and the interaction of this increased flow with southward dropping
outflow boundaries...supports the idea that the flash flood threat
may increase over the next several hours. Confidence in this
evolution is not high...but the parameters in place do warrant a
close monitoring of the situation as we head through the evening
hours.

Chenard

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

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

LAT...LON   33761043 33510974 33370912 33260882 33000878
            31670919 31250967 31101035 31181097 31241140
            31491204 31841195 32381157 32871128 33361094
            33611079