Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Mon May 28 2018


Prelim Day 1 QPF Discussion
Valid May 29/0000 UTC thru May 30/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Southeast/Southern Appalachians/southern mid Atlantic...

There is expected heavy rainfall of 4-8 inches along and to the
right of the track of Alberto, with the main core of heavy rain
expected in the Florida panhandle, Alabama, and western Georgia. 
A secondary maxima develops in the upslope regions of the Blue
Ridge / southern Appalachians over the western Carolinas and
northeast Georgia. Models indicate some build of up instability
just upwind, and low level flow steadily increases into the
mountains. The flash flood threat will likely increase in this
area tonight into Tuesday. Farther east along the coast, broad
upper difluence and warm advection will continue tonight in the
eastern Carolinas with lift aided by an inverted trough.
Showers/storms persist into the evening near the front in VA, and
should redevelop in the mountains of western VA/WV Tue.

The NHC Alberto forecast track is closer to the NAM, with the GFS
and Hurricane WRF tracking the system west of the NHC track,
leading to their QPF axes further west.  Consequently, manual QPF
gave the last amounts of weighting to the GFS and Hurricane WRF,
and more to the HMON hurricane model and the 12z NAM/High Res
Ensemble mean QPF.

...Rockies / central to northern Plains/upper MS Valley...
The long-lived upper low over the Great Basin will eject toward
the Plains during this period. Instability and upslope flow will
continue to favor eastern Wyoming and southeast Montana for
showers and thunderstorms tonight, with locally heavy rain
possible. An inverted low level trough persists in this area this
afternoon/evening, and convergence along the trough combines with
upslope flow to produce rainfall.

A shortwave rotating around the base of the upper trough emerges
over the high Plains of CO. A strong upper divergence max will
generate shower/storms and a potential MCS, feeding off deep
southerly flow that advects GFS forecast 1.5 inch PW values
combined with strong low level convergence this evening over
western KS.
 
The GFS has backed off some of the prolific 10-11 inch bulls eyes
of prior runs but continues showing heavy rain potential in
western KS.  The GFS axis/orientation is similar to the 12z high
res windows and 12z NAM/UKMET, so the GFS QPF can now be included
as part of the overall blend. A very warm boundary layer will
promote cold outflows, but the enhanced inflow above the surface
could support a slow moving system producing very heavy rain. Much
of the hi-res guidance does paint heavy rain over western Kansas
and Nebraska.  Expect another active convective cycle from
northwest Kansas to southeast Wyoming and western South Dakota, as
the airmass has already moistened and the ground has as well from
prior rainfall.
WPC QPF blended the High Res Ensemble Mean with the NAM/UKMET/GFS
for the details, with the primary adjustment higher amounts
tomorrow afternoon in central KS to eastern NE as the next diurnal
destabilization occurs in the moist airmass in an area of low-mid
level moisture convergence.

Return warm/moisture advection develops with bands of 850 mb
convergence extending northeast out of the central Plains to and
across the upper MS Valley overnight and Tue. During Tue lapse
rates are favorable in the 7-8 degrees c/km range in NE to MN
along with deep southerly flow, so diurnal destabilization leads
to increasing shower/storm development.  Manual QPF amounts were
increased from continuity to match the high res ensemble and
global model mean QPF.

...New England...
The models show showers occurring ahead of a cold front this
evening/overnight. Sufficient low/mid level moisture and cold air
aloft is forecast to support a few hundred J/Kg of CAPE over
northern Maine and northern New York / New England. Showers are
also supported by frontal circulation-driven ascent. With the
frontal passage, cooling and drying occurs Tue with the loss of
ascent, so no additional rain is expected during the day in
northern New England. Modest, barely measurable amounts are
possible in southern New England.

Petersen/Burke