AWUS01 KWNH 071441
FFGMPD
IDZ000-ORZ000-WAZ000-072040-

Mesoscale Precipitation Discussion 0880
NWS Weather Prediction Center College Park MD
1040 AM EDT Mon Aug 07 2023

Areas affected...Portions of the Pacific Northwest

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

Valid 071440Z - 072040Z

SUMMARY...Scattered slow-moving clusters of heavy showers and
thunderstorms may produce instances of flash flooding going
through the early afternoon hours.

DISCUSSION...The latest GOES-W IR satellite imagery shows a mid to
upper-level low gradually digging across the Pacific Northwest
which will be interacting with an anomalously moist and
increasingly unstable airmass going through the day. The latest
satellite trends are showing a gradual expansion of slow-moving
shower and thunderstorm activity with locally cooling convective
tops across areas of northeast OR through central to northeast WA.

Over the next several hours heading into the afternoon, a
combination of relatively steep mid-level lapse rates and diurnal
heating should lead to a more pronounced destabilization of the
column and should support an increase in the coverage of shower
and thunderstorm activity. Increasingly divergent flow aloft ahead
of the height falls will further drive ascent that will be
conducive for seeing at least scattered clusters of slow-moving
convection given the rather weak steering flow.

Some of the greater concentration of convection heading into the
afternoon hours may tend to be over southeast WA and into
northeast OR near the ID border where eventually stronger
instability and relatively focused low to mid-level convergence is
expected to set up. The 06Z HREF guidance supports locally heavy
rainfall totals over these areas and some elevated probabilities
of seeing the 100-year ARI exceeded for a 6-hour period.

PWs across the region are rather moist and running as high as 2 to
3 standard deviations above normal, and this coupled with the
increasing instability profiles with time should favor convective
cells with increasingly heavy rainfall rates. The 06Z HREF
guidance shows rather high probabilities (>50%) of seeing 1+
inch/hour rainfall rates with the evolving convective cells going
into the afternoon hours, and some of the strongest cells may
produce 1.5 to 2 inches in an hour given the anomalous moisture
pooled over the region.

Some storm total amounts may locally reach 2 to 4 inches through
early afternoon, and this may produce instances of flash flooding.
The more sensitive burn scar areas, and urbanized locations, will
be most susceptible today to runoff problems and concerns for
flash flooding. Will continue to closely monitor.

Orrison

ATTN...WFO...BOI...MSO...OTX...PDT...

ATTN...RFC...NWRFC...NWC...

LAT...LON 49051912 49011742 47841607 46101521 44821603
44521754 44721883 45401968 46572010 47312046
48052050 48682002

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AWUS01 KWNH 072030
FFGMPD
MTZ000-IDZ000-ORZ000-WAZ000-080200-

Mesoscale Precipitation Discussion 0883
NWS Weather Prediction Center College Park MD
429 PM EDT Mon Aug 07 2023

Areas affected...Southeast WA...Northeast OR...Central ID...West
to Southwest MT...

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

Valid 072030Z - 080200Z

SUMMARY...Slow Moving/Merging cells along deformation zone pose
localized 1-2" totals through evening. Widely Scattered to
Scattered instances of flash flooding are possible, particularly
over steeper terrain.

DISCUSSION...GOES-W WV suite depicts a drying arc along the upwind
edge of the mid-level trof axis with nose of the 70-80kt 300mb jet
streak having recently rounded the base into SE OR and the Western
ID Snake River Plain. Older shortwave center over central WA is
transferring energy to the shortwave at the cyclonic rotor of the aforementioned jet. This is providing very strong left exit and
anticyclonic diffluence aloft across SE WA into central ID to
provide solid broad scale ascent.

Full insolation across much of the area has allowed for solid
instability to build across this area, though dense cirrus within
the jet has enhanced a differential heating boundary bisecting OR
from W to East and across the Snake River Plain of W ID, the
deeper return moisture through the Columbian Plateau could not
filter eastward to enhance deeper layer moisture though 700mb RH
remains above 70% and CIRA LPW shows older lingering moisture axsi
across the region with .25-.4 through much of the area in both the
850-700 and 700-500 layer (only exception was a bit of low level
drying in the SE Sawtooth and SW MT ranges, but with surface Tds
of upper 40s to lower 50s, there should be ample moisture for
above average rainfall rates. The key will be moisture advection,
and as the older 700mb low in WA weakens, and transfers southeast
across NE OR into ID, flow should accelerate toward 15-20kts at
cloud base increasing convergence expanding convective coverage
across the shear axis between the circulations and across the
older weak 700mb low in west-central MT. This should delay cell
motions with potential for outflows to collide after first/second
up/downdraft cycles. As such, rain rates up to 1"/hr are possible
and with mergers/collisions some spots of 1-2" totals are possible
across the area of concern, particularly where divergence/outflow
aloft can maintain updraft a bit longer than typical pulse
convection.

Combined with recent spotty heavy rainfall over the past few days,
similar widely scattered to scattered instances of flash flooding
are considered possible through late evening with timing from west
to east timed with best DPVA/left exit to nose of 3H jet streak.

Gallina

ATTN...WFO...BOI...MSO...OTX...PDT...PIH...TFX...

ATTN...RFC...MBRFC...NWRFC...NWC...

LAT...LON 48671667 48421608 48021560 47251420 46841277
46591150 45961118 44821124 44041179 43661416
43461592 43461774 43851908 44721888 45441812
45731824 46681848 47181919 47751932 48401835
48661758

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