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AWUS01 KWNH 051729
FFGMPD
AZZ000-NVZ000-CAZ000-052327-
Mesoscale Precipitation Discussion 1049
NWS Weather Prediction Center College Park MD
128 PM EDT Tue Oct 05 2021
Areas affected...Far Eastern CA...Southern NV...Western and
Northern AZ
Concerning...Heavy rainfall...Flash flooding possible
Valid 051727Z - 052327Z
SUMMARY...Scattered areas of showers and thunderstorms are
expected to increase in coverage going through the late-morning
and early to mid-afternoon hours. The rainfall rates may be heavy
enough to result in some areas of flash flooding.
DISCUSSION...There is a well-defined upper-level low seen in
GOES-16 WV satellite imagery advancing east across far southern CA
and northern areas of Baja California. This is yielding rather
strong dynamic forcing downstream across areas of southern NV and
a large area of western and northern AZ where notably divergent
flow seen aloft is facilitating deeper layer ascent. In fact, the
latest Airmass RGB satellite imagery suggests a rather strong PV
anomaly with the core of the upper low, and the 12Z GFS does show
a 3 to 4 PVU (500/250 mb) magnitude tropopause fold attendant to
the upper low.
The leading edge of this PV anomaly is already favoring scattered
areas of showers and thunderstorms across areas of far eastern CA,
western AZ and southern NV where there is at least some very
modest instability in place (MLCAPE values of 500+ j/kg). Farther
east, there is also an area of showers impacting portions of the
Mogollon Rim and adjacent areas of northern AZ where a channel of
deeper layer and moist southerly flow is working in tandem with
orographic forcing and the divergent flow aloft for pockets of
locally heavier rain.
So far, the rates have been quite modest with the ongoing
activity, and generally under a 0.50" per hour. However, going
through the remainder of the late-morning time frame, and
especially by early to mid-afternoon, there should be a notable
expansion of shower and thunderstorm activity as the arrival of a
jet-streak rounding the base of the upper low yields robust
left-exit region jet forcing/dynamics for enhanced vertical
ascent. This will be coinciding with stronger diurnal heating of
the boundary layer with the expectation of seeing pockets of
MLCAPE exceeding 1500 j/kg materializing in the 18Z to 21Z time
frame. Some of this will also be related to the steepening
mid-level lapse rates associated with the upper low.
The downstream moisture environment will be rather favorable for
efficient convection capable of enhanced rainfall rates, and the
latest CIRA-LPW data shows notable subtropical/monsoonal fetches
of moisture advancing north across Baja California and adjacent
downstream areas of northwest Mexico in the 850/700 mb and 700/500
mb layers.
This coupled with the pooling of instability and stronger dynamic
forcing this afternoon should allow for convection that will be
capable of producing hourly amounts of as much 1 to 1.5 inches
with the stronger/deeper convective cores and much of this perhaps
falling in as little as 30 to 45 minutes. In fact, the latest 12Z
HREF guidance does show high probabilities (50% to 70%) of seeing
at least a 0.50" of rain in one hour more broadly across
west-central to northwest AZ in the 21Z to 00Z time frame as
convection reaches peak intensity/maturity.
Expect there to be concerns for some flash flooding as a result of
these heavier rainfall rates going through the afternoon hours.
There will as usual be concerns for any of the area burn scar
locations and dry washes which are highly susceptible to enhanced
runoff.
Orrison
...Please see
https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!rQG1bVxwcIMWZvlYPlyq1V-zfaHdrV8S6xzk-TIQyPUIMS4vQpB7hcu9fT190HCaFnLt= Fw7g$ for graphic product...
ATTN...WFO...FGZ...PSR...VEF...
ATTN...RFC...CBRFC...CNRFC...NWC...
LAT...LON 36971220 36311036 35390977 34291007 33101117=20
32781218 32671368 33081439 34361455 34781488=20
35411559 35951602 36611577 36951427=20
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