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


Prelim Day 1 QPF Discussion
Valid Apr 06/1200 UTC thru Apr 07/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1


...West Coast/Intermountain West...

An atmospheric river transports deep Pacific moisture across
portions of northern and central CA during Day 1. The increasing
low level flow focuses the deep moisture on the higher terrain,
and the resulting heavy rainfall over the higher terrain could
produce flash flooding. For the most part, the WPC QPF was based
on a blend of the 00z GFS/12z ECWMF, augmented by the 00z WRF ARW
and the 00z HREF mean.

Ahead of a short wave located near 38N 145W early on Day 1, an
atmospheric river transports deep Pacific moisture toward the
northern CA coast (where the most recent blended TPW product
showed 1.75 inch precipitable water air sitting just off the
northern CA coast). The low level flow ahead of the short wave
(and associated surface front) increases to 25 to 35 knot from the
west southwest between 06/18z and 07/00z, as the low level flow
becomes orthogonal to the terrain over the northern CA coastal
range, as well as the northern and central Sierra Nevada Range.

The low level flow transports 1.50/1.75 inch precipitable water
air (which is about five standard deviations above the mean) north
of SFO in northern CA, with lower values the Sierra Nevada Range.
During this time, the 00z HREF mean showed an area of 100/250 J/KG
of MUCAPE coming ashore with the deep moisture plume in the 07/00z
to 07/06z time frame. This is expected to add a convective element
to the precipitation character during this time, as both the
moisture and instability drop slowly south ahead of the surface
frontal boundary.

There was a multi model signal for 4.00 to 5.00 inches of qpf
across the Coastal Range in Northern CA, as well as across the
favored upslope flow regions of the northern and central Sierra
Nevada range. Given the deep moisture and instability in the
upslope flow, these amounts seem plausible. It should be noted
that the 00z HREF mean and the 00z WRF ARW showed locations with
6+ inch rainfall amounts, but since the inflow is not as high as
50 knots for a longer period of time, these amounts may end up
being maximum amounts in the higher terrain.

The short wave forms a closed mid level low just off the Pacific
Northwest coast by 07/12z, and the moisture plume drops south
across central CA toward the end of the period. The highest
rainfall amounts toward the end of the Day 1 are expected to drop
south into the central Sierra Nevada range.

Further east, some of the Pacific moisture survives the trip into
the Great Basin and the Central and Southern Rockies. Upslope
ahead of an arctic front stretched from the Northern Rockies into
CO will be augmented by low level convergence, which is expected
to produce areas of 0.75/1.25 inches of qpf over the CO Rockies
during Day 1.


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

Moisture and instability focused on a frontal boundary extending
from the Southern Plains across the Lower MS Valley into the Mid
Atlantic states will feed convection that produces heavy to
excessive rainfall over portions of the Lower MS Valley during Day
1. The WPC QPF was based primarily on a blend of the 00z GFS/12z
ECWMF (as well as the 00z WRF ARW and 00z HREF mean), as these
models showed a qpf maximum further south, closer to the best
instability.

Moisture and elevated instability along a low level boundary
extending from far northeast TX into southern AR will feed
convection extending from eastern OK into central and northern AR
and the western TN Valley during the first part of Day 1. There
should be sufficient convective coverage to produce an axis of
1.50 to 2.50 inches of qpf along the aforementioned areas through
06/21z or so. As the convection drops south on an outflow boundary
toward a developing cold front, a 35 knot low level southwest flow
transports 1.25/1.50 inch precipitable water along the front from
the Southern Plains into the Lower MS and western TN Valley.

There is still some model spread concerning the placement of the
qpf maximum ahead of the front, with the 00z NAM apparently too
far north (and furthest away from the best instability). The 00z
GFS/12z ECWMF were closer with the placement of the best
instability, keeping it south of the AR/LA boundary after 07/00z,
and the 00z HREF mean also keeps the best instability south of
this line. Model soundings showed moderate instability near the
front, and given the position of the best surface based
instability, an axis of 3.00 to 4.00 inches of qpf was extended
from northern LA across west central MS.

Three hour flash flood guidance values here are generally between
2.00/3.00 inches, and the focuses instability along and south of
the front could result in some training in the 07/00z to 07/06z
time frame, before surface low pressure forms on the front over
MS/AL, and drags the front south. Deep moisture and marginal
instability could result in an axis of 1.50 inches of qpf into
portions of eastern TN, but drier air to the north (supporting a
growing model consensus) suggests that the northern extent of the
qpf could be shunted south into central and southern VA by the end
of day 1.


...Great Lakes/Northeast....

A short wave tracking out of the Upper Great Lakes early on Day
crosses northern New England by the end of the period. Moisture
focused on a surface low associated with the mid level system will
produce a swath of qpf across the region. The WPC QPF was based
mainly on a blend of the 00z GFS/12z ECWMF.

Regional radars showed an increasing area of precipitation ahead
of a short wave over WI early this morning (based on the GOES-16
mid level WV loop), and the mainly warm air advection induced
precipitation shield is expected to expand across the Great Lakes
into the eastern OH Valley during the first part of Day 1.
Moisture ahead of the short wave (and its attendant surface low,
which tracks across Quebec and northern New England into New
Brunswick) is expected to extend into the Northeast after 06/18z,
with the best mid level lift between 06/18z and 07/00z.

For the most part, qpf amounts are expected to remain below 0.25
inches, with some local 0.35 inch amounts possible in the higher
terrain of eastern NY state, central and southern VT into NH, as
the broad upslope flow could enhance precipitation rates.

Hayes