Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
444 PM EDT Fri Sep 18 2020
Valid 16Z Fri Sep 18 2020 - 12Z Sat Sep 19 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF WESTERN OREGON AND SOUTHWEST WASHINGTON...
...Western Oregon and Southwest Washington from the Klamath
Mountains into the Eastern Willamette Valley, Cascades, and
Western Columbia River Gorge...
Deep low pressure currently along the southern OR coast will shift
northeast up the OR Coastal Ranges, then linger over northern OR
tonight as the attendant trough swings into the northern Great
Basin Friday. Difluent flow east of this system, and left exit
region jet forcing will maintain a favorable dynamical profile
from the Willamette Valley through the Cascades through this
afternoon, before the upper trough axis and surface front pushes
east of the Cascades. Within the compact Warm Conveyor Belt (WCB)
will be sufficient instability (MUCAPE around 1000 J/kg)
persisting over the outlook area through this afternoon. The
instability along with PW values around 1.0-1.2" (around 2
standard deviations above normal) will make for a favorable
thermodynamical setup prior to the frontal passage -- supportive
of isolated hourly rainfall rates of 0.5 to 1.0 inch within the
strongest cells through the afternoon per the latest high-res CAM
guidance. 12Z HREF probabilities of hourly rainfall rates
exceeding 1" are between 30 and 40% within the Marginal outlook
area, again mainly through this afternoon/00Z Sat. These short
term rainfall rates would likely lead to localized flash flooding
and debris flows in the vicinity of recently burned areas around
the Archie Creek, Holiday Farm, Beachie Creek, Lionshead, and
Riverside fires in OR, as well as the Big Hollow fire in WA.
Valid 12Z Sat Sep 19 2020 - 12Z Sun Sep 20 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN ROCKIES...AS WELL AS PORTIONS OF THE EAST COAST OF
...2130 UTC Excessive Rainfall Discussion...
How the models handled the large scale synoptic pattern changed
little in the 18/12Z cycle...both in terms of mass fields and
precipitation. As a result, no changes were needed to the
previously-issued Marginal Risk area.
...East Coast of Florida...
A few high-resolution guidance members, led by the NAM NEST, show
a blossoming of 1 to 3 inch isohyets clustered along the east
coast of the Florida peninsula early in the period. Still think 2
inch per hour rainfall rate are attainable and that isolated
rainfall amounts of 2 to 4 inches are possible where cells train
or if there are repeated rounds of convection impacting the same
location. Only change was to expand the boundary of the Marginal
Risk area a bit.
...0830 UTC Excessive Rainfall Outlook Discussion...
Deep moisture from the eastern Pacific and marginal instability in
place will fuel low topped convection capable of producing heavy
to locally excessive rainfall across portions of the Northern
Rockies during Day 2. Ahead of a negatively tilted tracking from
the Pacific Northwest into the Northern Rockies, the mid level
flow transports 0.75/1.00 inch precipitable water air (which is
between two and three standard deviations above the mean) across
ID/MT/WY ahead of a frontal boundary snaking along the terrain.
Model soundings showed 500/1000 J/KG of MUCAPE in place, and the
combination of moisture and instability should support at least
scattered low topped storms between 19/20z and 21/03z from west to
Difluence ahead of the mid level trough could allow storms to
become organized for a time from eastern ID across southwest MT
into northern WY, though it is unclear just how widespread the
convection becomes. Deep moisture in the column could certainly
support storms with hourly rainfall rates of 0.50 inches (which is
shown in most of the 00z high resolution guidance) in these
locations during the convective window mentioned above. As the mid
level trough approaches, storm motions could drop below 10 knots,
especially over MT/WY before 20/00z. Any short term training or
cell mergers could allow for local rainfall amounts approaching
Three hour flash flood guidance values are generally between
1.00/1.50 inches, and given the moisture in the column, these
amounts are attainable. Based on that, a Marginal Risk was placed
over a larger portion of the Northern Rockies for Day 2.
...East coast of Florida...
Deep moisture and instability along and behind a front crossing
the FL Peninsula could support convection in the lower level
northeast flow that produces heavy to locally excessive rainfall
across portions of coastal northeast and central FL during Day 2.
Strong surface high pressure over the Northeast will push a
surface front across the northern and central portion of the
peninsula. Along and just behind the front, a low level east to
northeast flow funnels 2.00/2.25 inch precipitable water air
across the region, with the low level convergent flow providing a
focus for development of convection in the 1000/2000 J/KG of
SBCAPE in the column (per model soundings).
Generally between 19/15z and 20/03z, waves of convection in the
low level flow are expected to come ashore across northeast and
central FL. Given the depth of the moisture in the column, hourly
rainfall rates could exceed 2.00 inches, and much of the high
resolution guidance (led by the 00z NAM CONUS Nest and the 00z WRF
ARW), shows this potential in the abovementioned timeframe. Local
3.00/4.00 inch rainfall amounts are possible where short term
training occurs. Since this is a flow regime that can support
heavy rainfall along the FL coast, a Marginal Risk was placed here
for Day 2 (which was collaborated with WFOs JAX/MLB)
The Day 3 outlook will be updated by 2030Z.
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt