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FOUS30 KWBC 140127
QPFERD

Excessive Rainfall Discussion...Corrected Day 1.
NWS Weather Prediction Center College Park MD
825 PM EST Wed Feb 13 2019

Day 1
Valid 01Z Thu Feb 14 2019 - 12Z Thu Feb 14 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL CONTINUES OVERNIGHT ALONG
LARGE PORTIONS OF THE CALIFORNIA COAST AND IN THE UPSLOPE REGIONS
OF THE SIERRA...

The slight risk for excessive rainfall covers ongoing rainfall in northern-central CA and where rainfall develops overnight further
south in southern CA. The latest model consensus is for 1.5-3.0+
inch areal-average totals between 00-12Z Thursday across the
outlook regions.

There continues to be overall good agreement for widespread heavy
precipitation amounts in the ongoing atmospheric river event
affecting much of California this period. The axis of much above
average PW values of 2 to 3+ standard deviations above the mean
and 850 to 700 mb moisture flux anomalies of 3 to 5+ standard
deviations above the mean, which was focused initially into
central-northern CA early today will continue to push southward
and focus across southern CA tonight. This will support widespread
heavy rains in the upslope regions of the Sierra and into the
coast ranges of northern and central California and Transverse and
Peninsular Ranges of southern California.

Hourly rainfall rates of 0.50-0.75+ inch in clusters will
redevelop overnight across portions of the Slight Risk area --
especially after 06Z and north of the Bay area -- as depicted with
several of the latest HRRR runs. The increasing hourly rates and
areal coverage will continue into the day 2 time period after 1200
UTC Thursday. See the day 2 discussion below for additional
information.

Hurley


Day 2
Valid 12Z Thu Feb 14 2019 - 12Z Fri Feb 15 2019

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ORANGE...SAN BERNADINO AND RIVERSIDE COUNTIES IN SOUTHERN
CALIFORNIA...
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS COASTAL
SECTIONS OF SOUTHERN CALIFORNIA...

Update

Multiple models persist in depicting a strong signal of 3 to 5+
inches to fall over Southern California, particularly along and
south of the Transverse Range to Baja. Hi-Res guidance of 1-hourly
QPF suggests nearly 8 hours of rainfall rates of 0.75 - 1.5 +
inches over this region. Given scattered to moderate rain
currently falling, antecedent moisture will have lowered FFG over
southern California prior to the expected high rainfall rates as
the PW of the magnitude of 5 standard deviations advects in ahead
of and as the front pushes inland. This area will already have
elevated flooding risk prior to the Day 2 period and will be
highly sensitive to any additional rainfall. The southern bounds
of the Moderate Risk and High Risk was expanded southward to San
Diego and the surrounding area.


Previous Discussion

...Southern California...
Deep moisture and instability focused in an increasing upslope
flow ahead of a cold front is expected to feed low topped
convection that produces excessive rainfall during Day 2,
especially across the higher terrain and recent burn scars of
Southern CA. While there are some timing differences with the
front, there is good agreement with placement of the highest
rainfall. Based on this, the WPC QPF and Excessive Rainfall
Outlooks (EROs) were based on a blend of the 00z ECWMF/00z GFS/00Z
NAM. Some 00z WRF ARW and 00z NAM CONUS Nest were used to better
delineate the highest rainfall amounts in the higher terrain, as
well as determine hourly rainfall rates.

A closed mid level low off the central CA coast early on Day 2
evolves into a weakening long wave trough that crosses the CA
coast around 15/00z. Ahead of a cold front associated with the mid
level system, a 35/40 knot low level southwest flow focuses
1.00/1.25 inch precipitable water air (which is between three and
four standard deviations above the mean) on the Transverse and
Peninsular Mountains in Southern CA. Model soundings indicated
that 100/250
J/KG of MUCAPE will be present as front approaches, and the
instability could result in low topped convection coming across
from Santa Barbara county into San Diego county, especially
between 14/15z and 15/00z. There was a good model consensus for
2.00/4.00 inches of rainfall across the higher terrain in coastal
southern CA, with the bulk of the rainfall occurring in the
abovementioned time frame.

Just before the mid level trough crosses the CA coast, the 850-300
mb mean wind becomes aligned with the propagation vectors, which
would support training of any low topped convection that develops
ahead of the cold front. Training in the deep moisture plume could
easily support hourly rainfall rates of 0.50/1.00 inches (as seen
in the 00z WRF ARW and 00z NAM CONUS NEST output, with the
greatest rates between 14/15z and 14/21z) over the southwest
facing terrain. Training could also support local rainfall totals
over 5.00 inches (as indicated by the 00z NAM CONUS and 00z GFS),
especially over the favored upslope regions of the San Bernardino
Mountains. These rainfall amounts will pose a significant threat
of mudslides and debris flows here, and after collaborating with
WFO SGX, a High Risk was placed over the portions of eastern
Orange, far southwest San Bernadino and western Riverside
counties. The upslope flow and best moisture flux occur in this
area.

A Moderate Risk was placed over much of the remainder of the
coastal sections of Southern CA, with the highest rainfall amounts
over portions of Santa Barbara and Ventura counties. The greatest
threat for mudslides and debris flows here are over burn scar
areas, as well as southwest facing terrain of the San Gabriel
Mountains. Finally, a Slight Risk was placed over much of the
remainder of the Southern CA, as the two day rainfall amounts
could support mudslides and debris flows even outside of the burn
scars.

After the passage of the cold front 15/00z and 15/06z, the deep
moisture and instability should move east of the area, lowering
the flash flood threat later in the evening.

...Arizona...
Short wave energy crossing southern NV and northern AZ after
15/00z interacts with deep moisture to pose the threat for heavy
to excessive rainfall over portions of northwest into central AZ
during Day 2. There were some timing differences regarding the
timing and strength of the short wave as it crosses the area, but
there was enough agreement in the placement of the highest
rainfall amounts to use a blend of the 00z ECMWF/00z GFS as a
basis for the WPC QPF and Excessive Rainfall Outlook.

Short wave energy (the remnants of a long wave trough that came
ashore over CA earlier in the period) crosses southern NV and
northern AZ, with the best lift associated with it peaking around
15/06z. Ahead of the short wave, deep moisture associated with a
decaying atmospheric river (with precipitable water values between
1.00/1.25 inches, which is three to four standard deviations above
the mean) enters AZ before 15/00z. The combination of deep
moisture and synoptic scale lift is expected to result in a swath
of 1.00/1.75 inches of rainfall, with the highest amounts over the
Mogollon Rim.

There could be some residual instability across far northwest AZ
near 15/00z to result in local hourly rainfall near 0.50 inches
between 14/21z and 15/03z. Based on this, as well as collaboration
with WFO VEF, a Slight Risk area was placed over southern and
central Mohave county. Elsewhere, the deep moisture ushered into
central AZ on a 30/35 knot low level flow becomes entrained into
the synoptic scale ascent, forming bands of moderate to heavy rain
over the higher terrain. With no instability expected, a long
duration heavy rainfall is expected. Given some response in the
stream flow from the National Water Model, a Marginal Risk was
extended from northwest into central AZ.

Campbell/Hayes

Day 3
Valid 12Z Fri Feb 15 2019 - 12Z Sat Feb 16 2019

...The probability of rainfall exceeding flash flood guidance is
less than 5 percent...

...Tennessee Valley...

Locally heavy rainfall may develop over portions of the Tennessee
Valley and Southern Appalachians on Day 3 as Gulf moisture streams
along and over a frontal boundary extending across the southern TN
Valley. Precipitable water values nearing 1.25 inches will be
advecting through the Gulf states within a west/southwest flow of
25 to 35 kts. The lack of instability over this region may limit
organized convection.

Given the above, and a limited streamflow response in the National
Water Model, no excessive rainfall area was placed over TN/KY.
However, should there be a northward displacement in the heavy
rainfall swath, a Marginal Risk could be need here in later
forecasts.

Campbell


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt


$$




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