Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
304 AM EDT Mon Apr 16 2018


Prelim Day 1 QPF Discussion
Valid Apr 16/1200 UTC thru Apr 17/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Mid Atlantic into the Northeast and Great Lakes...

As of 06z a line of heavy showers and embedded thunderstorms
stretches from southern PA into eastern NC...with this line
continuing to progress off to the northeast through the day
Monday. This remains a very dynamic system with the negatively
tilted mid/upper level trough supporting strong upper level
divergence. Instability is marginal, but anticipate approximately
100-500 J/KG of MUCAPE to be present from the Mid Atlantic into
southern and eastern New England. This should be enough to
continue supporting embedded heavier convective cores within the
broader rain shield. While the line will remain progressive off to
the northeast limiting any extreme rainfall amounts, will continue
to see some short lived south to north training of individual
cells embedded within the line. PWATs and moisture transport both
remain anomalously strong...and this short term heavy rainfall
rates remain likely.

Model agreement was very good with the evolution of this more
convective precipitation as it moves northeast. Thus WPC mainly
just blended the 0z HREF members along with the HRRR to derive QPF
totals through day 1. The mid/upper forcing becomes a bit less
pronounced by later this morning into this afternoon as the mid
level trough begins to lose some of its negative tilt and the
upper jet weakens. Thus would anticipate we will see the overall
intensity and width of the convective axis weaken some as it
progresses Northeast through the day. Still looking at a
widespread 1-2" of rain from the eastern Mid Atlantic into
southern and central New England...with locally heavier totals
into the 2-3" range. Best chance of these localized higher totals
appear to be from portions of northern NJ into southeast NY into
southern and central New England, where we are seeing some WAA
showers develop out ahead of the main convective axis...and also
where orographic impacts are maximized in the strong southerly
flow. Rainfall rates should locally peak around 0.5"-0.75" in an
hour...with up to around 1.5" in any three hour period.

Further northwest under the closed mid level low will continue to
see precipitation into the Great Lakes region. Did increase
amounts some here over our previous forecast, with the closed low
continuing to be slower to exit. Again followed close to a blend
of the 0z HREF members here.


...West...

A trough will continue moving east into the Great Basin by Monday
night. This will continue to support precipitation over much of
the west, with as much as 1-3" over western WA, around 1" in the
northern Rockies and up to 1" in the central Sierras.

Chenard