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AWUS01 KWNH 252255
FFGMPD
AZZ000-UTZ000-NVZ000-CAZ000-260400-
Mesoscale Precipitation Discussion 0675
NWS Weather Prediction Center College Park MD
654 PM EDT Thu Jul 25 2019
Areas affected...Portions of eastern California and the Great
Basin of Nevada
Concerning...Heavy rainfall...Flash flooding possible
Valid 252300Z - 260400Z
Summary...Slowly waning coverage of showers and thunderstorms will
persist heavy rain rates near 2"/hr through the evening. Slow
northward motion of these storms may produce localized amounts of
3" of rainfall. This rain on top of areas with very low FFG may
create flash flooding.
Discussion...Regional radar mosaic this afternoon shows expansive
coverage of showers and thunderstorms from far western Kansas,
southward into northern Mexico, through California, and across
most of the Four Corners region. The broad anticyclonic
circulation of these cells clearly indicates the strong ridge
aloft, which is pumping rich theta-e air into the region and
driving PWATs as high as 2 inches near the CA/AZ border, with
1-1.5 inches common throughout the discussion area. These PWATs
are as much as 3 standard deviations above the climatological
mean, indicative of the tremendous monsoon moisture available for
heavy rainfall.
In addition to the anomalous moisture, instability has climbed due
to heating this afternoon, with recent RAP analyzed SBCape as high
as 1500 J/kg in southern NV, with 1000 J/kg well north of there.
This combination of PWAT and SBCape is creating a thermodynamic
environment ripe for convection, which is being tapped by modest
diffluence within the RRQ of an upper level jet streak, as well as
low-level convergence due to outflow interactions and subtle MCV
circulations evident on GOES-16 WV imagery. Additionally,
low-level convergence along an elongated trough analyzed across
California will focus moisture to drive renewed and persistent
thunderstorms. These features together will persist convection at
least through peak heating, with slow decrease thereafter due to
loss of instability, although the presence of the large and
meso-scale ascent may allow thunderstorms to persist well after
nightfall. The thunderstorms that do develop and persist will
likely move extremely slowly, as the weak forcing in the vicinity
of the ridge aloft will keep storm motions less than 10 kts to the
N/NE.
Recent runs of the HRRR highlight well the continuation of
convection before waning begins during the evening. This is echoed
as well by subtle reduction in moisture flux on the RAP, and by
ensemble QPF probabilities on the latest WoFS runs. There will
remain potential for rain rates of 2"/hr through the evening, but
weakening instability combined with reduced PWAT as the upper
ridge shifts slowly westward should bring an end to the flash
flood risk tonight. Although flash flooding is likely ongoing
presently, the slow forecast wane in intensity and coverage
prompted the use of the flash flooding possible tag with this MPD
update.
Weiss
...Please see www.wpc.ncep.noaa.gov for graphic product...
ATTN...WFO...FGZ...HNX...LKN...REV...SLC...VEF...
ATTN...RFC...CBRFC...CNRFC...
LAT...LON 39041551 38991475 38831437 38801424 38411383
37701312 37431277 37181239 37001194 36691140
36421109 36141104 35741107 35561126 35421177
35361233 35381299 35551363 35881417 36181477
36241534 36181603 36011671 35791794 35941849
36291884 36831910 37351924 37911863 38331805
38621748 38831673 38961609
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