Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
1252 PM EDT Mon Mar 12 2018


Prelim Day 1 QPF Discussion
Valid Mar 13/0000 UTC through Mar 14/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1...

...California...
Precipitation moves into northern CA late tonight as a cold front
with good low- to mid-level frontogenesis moves ashore and moves
steadily into central CA with time. Given the well defined nature
of the front and good mid/upper level forcing forecast...would
expect to see a broad area of steady precipitation.  The
combination of precipitable water values of just over 1" and MU
CAPE values of 200-300 J/kg should yield hourly precipitation
totals up to 0.6" across the favored terrain of northern CA into
the Sierras late tonight into Tuesday morning.  The 12z HREF
probabilities of 0.5" an hour agree with the rule of thumb, with a
spike above 50% in the northwest CA coastal ranges 07-08z and
across the Siskiyou and Sierra Nevada intermittently from 06z-18z.
 Model agreement was good, allowing for a multi model blend for
QPF (used a 12z GFS/00z and 12z UKMET/12z ARW/00z ECMWF
compromise).  This keeps reasonably good continuity from WPC's
previous forecast.  See our winter weather suite of products for
more concerning this system.

...Mid Atlantic into the Northeast...
A developing coastal low intensifies as it moves offshore the Mid
Atlantic coast.  Frontogenesis in the 650-850 hPa layer,
eventually within the system's comma head, is expected to be the
main player for heavy precipitation with this system.  The
combination of 100-200 J/kg of MU CAPE and 0.5-0.7" precipitable
water values late tonight and early Tuesday morning should be able
to yield hourly precipitable totals up to 0.3-0.4".  The 12z HREF
probabilities of 0.25" agree, showing 50-90%+ chances of such from
06z-20z.  With the best frontogenesis stuck at any one spot of
southeast MA for 9-12 hours, this should be able to achieve total
liquid equivalent above 2" with local amounts of 3-4" possible. 
The QPF was based on a blend of the latest GFS/UKMET/ARW/ECMWF
which allowed for some sense of continuity across southeast New
England while scaling back amounts across Maine.  See our winter
weather suite of products for more concerning this storm.

Roth