Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
208 PM EST Thu Mar 01 2018


Prelim Day 1 QPF Discussion
Valid Mar 02/0000 UTC thru Mar 03/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Eastern U.S...

Northern and southern streams were in the process of becoming
phased Thursday afternoon. Ascent was shifting toward the
developing cyclone in the Ohio Valley / lower Lakes, which will
hand off to vigorous coastal cyclone development tucked into the
climatologically favored area off Long Island from tonight through
early Saturday. As such, the connection to the CAPE axis /
convective weather over the southern states will dissipate, with
the exception of some smaller scale convection and a few showers
in the surging frontal zone. The main precipitation story will
shift up north. Already moderate rainfall was developing over the
OH Valley and Great Lakes. Warm advection will contribute to the
initial expansion of rainfall into the Mid-Atlantic and southern
New England states overnight. This event will become more
intensely focused as the coastal low develops, leading to deep
frontogenesis and anomalous easterly onshore flow from the
Atlantic. All these ideas fit well with the previously issued WPC
forecast, which we retained in part, and blended in the 12z GFS
and 12z WRF-ARW. The GEFS re-forecast data, based on analog
events, suggests a swath of 3 to 4 inch areal average QPF through
Friday from eastern northern New Jersey to Long Island to eastern
Massachusetts. This gave us one more reason to push toward the
WRF-ARW magnitudes and raise QPF somewhat. The predicted rates of
perhaps 1.5 inches in 3 hours are of increasing concern,
especially given expected river levels and onshore flow backing up
drainages. Also concerning to see these kinds of rates in the
models a good 24 hours in advance of the event. We therefore are
considering an upgrade to Moderate Risk of excessive rainfall for
a sliver of coast from Long Island to eastern Mass, focusing on
the early Friday period.

QPF across the eastern states was based on roughly a 40/30/30/
blend of the previous WPC / 12z GFS / 12z WRF-ARW.


...Western U.S...

Another phased cyclone will move south along the west coast of
North America through the period. Heavy rain focuses along the
California coastal mountain ranges early in the period before 850
mb onshore flow relaxes...and a frontal zone pushes farther
inland. Watch for mesoscale discussions detailing an flash flood
risk.

There is a good QPF consensus among the models out west. WPC QPF
was based on a blend of the previous WPC forecast / 12z NAM / our
in-house ensemble, resulting in fairly little change. This blend
matched a little better to NDFD in some areas. Heavy snows
expected in the Sierras especially - see QPFHSD, heavy snow
discussion.

Burke/Bann