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


Prelim Day 1 QPF Discussion
Valid Mar 21/0000 UTC thru Mar 22/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

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


Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 20/1200 UTC thru Mar 23/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...eastern Ohio Valley/Central Appalachians/Mid
Atlantic/Northeast...

All the ingredients were coming into place for a heavy snowfall
from Washington, DC to Philadelphia and New York City.  Refer to
the WPC Heavy Snowfall graphics and discussion for winter weather
amounts and probabilities. The storm will also produce heavy
rainfall over the coastal mid Atlantic, with rain changing to snow
in coastal areas of the northern mid Atlantic.

The models have come in better agreement with cyclogenesis in the
southern mid Atlantic with the low intensifying over the coastal
waters early Wed. Snow redevelops along the existing low level
frontal zone in southern PA and expands across NJ into the metro
New York City area.  The strong lift, followed by enhanced
moisture fluxes on easterly low level flow off the ocean, leads to
1.5 to 2 inches liquid equivalent precipitation in NJ, with over
an inch to Long Island and into southeast PA and northern MD and
DE.

The WPC QPF followed the model consensus, and blended the high
resolutions windows with the 12z NAM Conus Nest, and 12z Canadian
regional model.  The 12z ECMWF trended south with its QPF axis in
the mid Atlantic in response to its axis of low pressure shifting
south, so no the northern edge amounts remain uncertain in
northern PA/southwest NY/New England.

The high res windows and ensembles have concurred with the 12z nam
to produce one secondary maxima in the Ohio Valley centered on IN
and western Ohio as 850-700 mb convergence maxima develop west of
the circulation in the Oh Valley.  There is potential for banded
snow in this area.  Another area of heavier precip is where the
circulation produces 700 mb convergence in southern WV to adjacent
VA with lift aided by upslope flow. Given good multi-model
agreement, a blend of the models/ensembles was used.


...California...

The models continue to advertise widespread rain in California and
 heavy rain in portions of southern CA, particularly the coastal
ranges.  Secondary maxima develop in the interior mountains.
Please see the risks of excessive rainfall /
flash flooding graphics and discussion. The models show an
atmospheric river with moisture transport tonight increasing into
California, with the 12z nam showing precipitable water values
increasing to 1.25-1.5 inches in portions of coastal southern CA. 
A well defined 870-700 mb convergence maxima moving onshore
between Santa Maria and Los Angeles,
This results in heavy rain developing Wed near Santa Barbara and
nearby ranges.
As the upper jet across north into northern CA, widespread rain
develops in northern CA. Lift from well defined upper divergence
maxima in central CA continue overnight and drift north into
northern CA in tandem with the upper jet maxima on Wed.

Due to good large scale agreement, a consensus approach to the QPF
was used, including the high resolution ensemble forecast mean for
the day 1 forecast. This approach usually does well along the West
Coast.

Rainfall rates tonight gradually ramp up to half to three fourths
of an inch in six hours, increasing to 1-1.25 inches in 6 hours
during the day on Wed.  

Petersen