Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EDT Mon May 07 2018


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


Day 1

...Northern to Central Plains...

A 700 mb wave and potential embedded closed circulation cross the
northern Plains tonight into Tuesday. Return warm/moisture
advection develops across the northern Plains.  As well defined
upper difluence moves across the northern Plains, convection
should increase through the region with lift in the positive
850-700 mb theta-e advection zone and pre-frontal convergence. 
Increasing pw values--up to 1.25-1.5 inches Tuesday afternoon...2
to 2.5 standard deviations above the mean....supports increasing
amounts of rainfall with showers/storms Tuesday.  Multiple models
indicate potential for a strip of 1 inch rainfall in portions of
the eastern Dakotas and MN. Difference persist because there are
multiple spokes of embedded cyclonic shear axes within the 700 mb
circulation, so rainfall maxima are forecast with each cyclonic
vorticity maxima rotating around the common circulation.  Manual
progs placed the rainfall in the middle of the cluster of
solutions, with less weighting on the maxims Tue in eastern ND and
northern MN owing to typical moisture intercept from convection
further south.  consequently, more weighting was given to the 12z
NAM/UKMET and less on the GEM regional and GFS.

...Central to Southern Appalachians/coastal NC to southeast VA ...

The mean mid to upper level trough drifts southeast across the
central to southern Appalachians later today into this evening and
then east of the mountains Tuesday. Diurnal instability underneath
this trof---mu cape values 500+ j/kg-- will support scattered
diurnal showers through the central to southern Appalachians this
afternoon and evening.

Light to moderate precip totals in the .10-.25" range depicted
from western NC/northeast TN---Southwest VA---central to eastern
WV into far western MD and far southwest PA.  High resolution
model forecasts indicate locally heavier totals are possible,
which could produce some runoff issues where ffg values are low. 
Neighborhood probabilities from the hi res guidance...ARW...NMMB
and NAM Conest all show a chance of exceeding 1 and 3 hour ffg
across these areas. 
A marginal risk area continues on the day 1 excessive rainfall
potential outlook across much of WV into Southwest Virginia for
this afternoon into early evening.

On Tuesday the upper trough drifting east with the upper low
drifting across central VA and NC takes the peak upper divergence
maxima out of the mountains in tandem with the movement of the
upper low.  The models forecast less rain on Tuesday as a result
in the mountains.  With a separate upper difluence/embedded upper
divergence maxima/700 mb convergence maxima along the coast of NC
to adjacent VA, the models indicate showers in this area as well. 

...Pacific Northwest/Northwest California...

The models indicate a deep layer circulation over the northeast
Pacific that heads in the direction of Vancouver Island late
Tuesday-Tuesday night.  A trailing frontal boundary move towards
the coastal of the Pacific northwest and northwest CA late
Tuesday.  Pre-frontal convergence an and enhanced band of moisture
with 700 mb relative humidity increasing to 90 percent leads to
showers across these areas.  Amounts are light with good agreement
among the model solutions, so a consensus approach was taken for
QPF.

Petersen