Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Tue Mar 20 2018


Prelim Day 1 QPF Discussion
Valid Mar 20/1200 UTC thru Mar 21/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...East Coast...

All the ingredients were coming into place for the latest in a
series of Nor'easters to produce a substantial swath of snowfall -
enough to disrupt life/commerce in the big cities. Refer to the
WPC Heavy Snowfall Discussion for winter weather details. The
storm will also produce heavy rainfall over the coastal mid
Atlantic. The event will be stretched over two days, through
Wednesday. The latest WPC Day 1 QPF trended significantly upwards,
now a full half inch heavier liquid equivalent heavier over the
areas where MD meets PA, and extending to central New Jersey. More
generally we are now a quarter inch heavier over the majority of
the Mid-Atlantic states down through southeast Virginia, and
nearing Long Island for QPF valid through 12z Wednesday.

The 00z upper air analysis depicted a very energetic and fairly
uncommon pattern over the southern states, with very short wave
spacing between a lead wave which drove a severe weather outbreak
over the South on Monday and an upstream wave that will amplify
quickly over the same region today. Rather than move out and
reinforce the offshore thermal gradients, the models now agree
that the movement of the lead short will be dictated by the
amplification of the trailing wave such that they are more in
phase with one another and become part of a relatively
consolidated upper air amplification over the Appalachians.  This
allows for cyclogenesis very near to the Mid-Atlantic coast,
peaking Tuesday night and Wednesday. Additionally, with the lead
wave now forecast to lift through the eastern Ohio Valley, it not
only induces a broad scale warm-advection / frontogenetically
forced precipitation event on Tuesday, but also acts to solidify
an elevated baroclinic zone over northern MD / southern PA and
across New Jersey. Strong lift during the initial setup phase,
followed by continual easterly low level flow off the ocean, into
the terrain, and then finally enhanced broad scale by the
developing cyclone and broad deep layer height falls going into
Wednesday, makes for a likely swath of hefty liquid equivalent
precipitation in this zone.

Other heavy amounts, although mainly in the form of rain, are
expected over the Virginia tidewater region and DELMARVA
peninsula, especially early Tuesday in association with the lead
wave. The system makes an attempt at forming a TROWAL back through
the Ohio Valley, and with nearly saturated air circulating around
the gradually deepening upper low, expect at least quarter to half
inch liquid amounts to occur back west.

The NAM sometimes does a good job on Day 1, especially along the
East Coast. Its QPF, at least through 12z Wed, was consistent with
global model mass fields / synoptic thinking. Given a decent model
consensus as well, we were able to use our in-house ensemble
products which had trended upwards with amounts. Generally the GFS
appears to have been too conservative leading into the event, and
the ECMWF has a known dry bias even in strongly forced events, but
its QPF did trend wetter. WPC used a 40/30/30 blend of our
ensemble / 00z NAM / 00z HREF blended mean.


...California...

A long advertised trough promises to bring widespread rain and
some heavy rain into California. It still appears as though the
bulk of the intense rain rates will occur on Day 2, beginning
Wednesday morning - please see the Day 2/3 portion of the
discussion, including an escalating risk of excessive rainfall /
flash flooding. The ECMWF evolution of the system had been slower,
but the 00z run started to catch up to the global consensus. With
good large scale agreement, a consensus approach to the QPF,
including bias-corrected tools, usually does well along the West
Coast. For simplicity, we used the same QPF blend here as we
employed elsewhere on Day 1, a 40/30/30 percent blend of our
ensemble / 00z NAM / 00z HREF blended mean. This may have produced
a result just slightly too heavy, given the WRF-ARW influence on
the HREF mean. The WRF-ARW is doing something we keep seeing east
of the Rockies, with a tendency to produce very heavy swaths of
convective precipitation along the southern edge of synoptic
forcing. This intensity, which would affect southern California on
Day 1 if the WRF-ARW were correct, seems unlikely given the 850 mb
flow really does not increase until Day 2. There is also a
pronounced lead shortwave passing by Southern California Tuesday
evening, with a brief period of subsidence in its wake. Therefore,
we will maintain only Slight Risk probabilities for excessive
rainfall on Day 1, leading into a more dangerous heavy rain event
on Day 2.

Burke