Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
249 PM EDT Sat Apr 07 2018


Prelim Day 1 QPF Discussion
Valid Apr 08/0000 UTC thru Apr 09/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Pacific Northwest/Northwest CA...

A strong closed mid level low off the Pacific Northwest coast
moves onshore and weakens as it moves onshore and inland near the
WA/BC border this evening. Low level moisture fluxes on the nose
of an 850 mb jet forecast to be 50-60 kt by the 12z gfs and upper
level divergence combine with upslope flow to produce heavy to
locally excessive rainfall over windward terrain tonight. There
was good model agreement with the evolution of the mid level
system, so the WPC QPF was based primarily on a multi-model and
ensemble blend, including the 12z HREF mean.

There was a multi model signal for an axis of 2.00 inches of qpf
in the OR Coastal range into the Olympics in WA, and also in the
southern WA and northern OR Cascades.  This much rain could result
in local runoff issues, especially across the coastal ranges late
in the period. The strength of the low level jet and resultant low
level convergence drops as you head south across northern CA, so
rainfall amounts subside in concert.

...Rockies/Great Basin/Northern Plains...

Short wave energy in a fast mid level flow provides synoptic scale
ascent over a frontal boundary extending from western MT to WY,
and surface low pressure develops on the front over WY. The
combination of moisture from a Pacific plume, upslope flow and
synoptic scale lift is expected to produce locally heavy qpf
amounts over the CO Rockies. There was good model agreement with
the overall synoptic setup, so the WPC QPF was based a multi-model
and high res ensemble (HREF) mean over the terrain. The initial
activity has developed over UT, which persists into the evening
until it moves east overnight in tandem with the upper jet
existing UT and crossing CO.  Precipitable water values over the
the CO Rockies rise to between 0.50-0.75 inches, which is 3-3.5
standard deviations above the mean. 

A sfc low pressure that develops over WY is forecast to move east
southeast to near the SD/NE border Sunday.  Precip develops in the
band of low level frontogenesis north of the low track....with the
models indicating a quarter to half inch liquid equivalent across
northeast MT to the SD/ND border and then southwest MN Sun.  See
the latest winter weather graphics and discussion for potential
snow amounts.

...Southern Mid Atlantic/Southeast coast/Florida...

Short wave energy spins up low pressure on a frontal boundary over
SC.  Ahead of the front and surface low, moisture and instability
will feed convection that produces locally heavy rainfall on the
southeast coast.  A trailing cold front moves east across GA and
into the coastal waters tonight and south across the Fl peninsula
tonight and Sunday.  Ahead of the front, a 40 knot low level
southwest flow focuses 1.50 inch precipitable water over FL/GA
into eastern SC and NC.  Divergence in the right rear quadrant of
a 150 knot jet streak will help organize the convection, which
moves off the coast this evening.

The front drifts south across central Florida Sunday, with showers
and storms developing, supported by low level moisture convergence
along and south of the front, aided by sea and lake breeze
boundary convergence and deep moisture with forecast soundings
indicating layer sfc-700 mb relative humidity of 70-80 percent. 
With deep westerly flow, afternoon convection moves east towards
the east coast.  Slightly more weighting was given to the 12z NAM
maxima along the southeast coast.

Petersen