Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Fri Apr 06 2018


Prelim Day 1 QPF Discussion
Valid Apr 07/0000 UTC thru Apr 08/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...West Coast/Intermountain West...

Ongoing atmospheric river event will continue to transport deep
Pacific moisture across portions of northern and central CA into
early Saturday morning. Low level flow focuses the deep moisture
on the higher terrain and the resulting heavy rainfall over the
higher terrain could produce flash flooding. WPC QPF was based
mostly on the 12Z HREF mean with adjustments per the GFS/ECMWF.
Trend in the 12Z hi-res guidance was just a bit farther south with
a heavy rain max just north of San Francisco Bay and pulled the
Moderate Risk area to the south as well. Larger Slight Risk area
encompasses much of northern California.

Morning sounding from OAK showed precipitable water value near
1.60" which was higher than any observed (1948-present) value in
the months of November-May. PW values may peak near 1.80" at the
start of the period tonight (00Z/07) which would be near the
highest value observed during any month of the year -- an
incredibly anomalous event. Warmth of the column is also of note
with freezing levels above 10,000-12,000ft. Recent RAP/HRRR
forecast soundings show an area of 100-400 J/kg of CAPE coming
ashore at the start of the period this evening which should add a
convective element to the precipitation character as both the
moisture and instability drop slowly south ahead of the surface
frontal boundary.

Combination of upslope flow and modest inflow along with plenty of
moisture supports several inches of rain for parts of the coastal
ranges and also the northern and central Sierra Nevada range with
lesser amounts (under 0.75") for the interior valley. This was
supported by the 12Z hi-res guidance and the 12Z HREF mean offered
a good starting point. After about 12Z Saturday, moisture axis
sinks southward and diminishes as the cold front pushes through
NorCal and then into the Great Basin later in the day. This will
split the precip areas up the coast into Oregon and also eastward
through Nevada into Utah.
Favored a modest compromise among the hi-res guidance with the GFS
as the ECMWF appeared too dry.


...Southern Plains across the Lower MS valley into the Mid
Atlantic...

Elevated convection north of a warm front over Arkansas into
Mississippi will push its outflow southward into warm/unstable air
surging northward out of the Gulf as evidenced by streamers into
southern Louisiana this afternoon. Convergence should help develop
additional convection south of the current area which is forecast
to sink southward as the surface cold front over Oklahoma and
north Texas dives southeastward. Models have had a good signal for
locally heavy totals in excess of several inches and continued the
small Moderate Risk area for northeastern Louisiana into west
central Mississippi for this evening into tomorrow morning.
Concern is for training ahead of the cold front with sufficient
moisture in place (PW values around 1.50" which is about +1.5
standard deviations) and CAPE near 1000 J/kg. Model spread
remained both N-S and E-W but the 12Z ARW and recent HRRR runs
captured the ongoing convective evolution the best. Front will
push diminishing activity eastward through the day on Saturday
with less amounts overall due to the quick progression. Cold air
to the northwest will help keep the system rather suppressed.


Fracasso