Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sat May 12 2018


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


Day 1...

...Midwest to Mid Atlantic...

Rain and thunderstorms are expected to occur all along a
quasi-stationary frontal zone from Iowa to the New Jersey coast
and the DELMARVA this weekend. The difficulty is in deciphering
the mesoscale evolution of each precipitation area, as well as
getting the north to south placement and expanse correct. We used
observational trends and clues from the mesoscale guidance to
stitch together the 6-hour periods as best we could, and have
average or above average confidence in some of the details given
overall model agreement. The narrow corridor of usable instability
also helps matters. Most of the convection in this pattern has
been to the cool side of the surface boundary. Certainly some of
the storm this evening in the Mid-Atlantic states and perhaps the
Ohio Valley may sneak out into the warm sector, but expect further
upstream redevelopment to take place in the 850-700 mb frontal
zone this evening across Iowa and Illinois.

At 15Z we introduced a Slight Risk of excessive rainfall from
eastern Ohio through northern West Virginia into southern
Pennsylvania, valid through 12Z Sunday. Another Slight Risk area
remains valid back across eastern Iowa to western Ohio.

In the eastern Ohio Valley and Mid-Atlantic states deep layer
forcing will come into phase with the peak heating cycle, and a
round of strong to severe storms is expected this afternoon and
evening (see also Storm Prediction Center Day 1 Outlook). Although
the event will be overall characterized by progression, the early
development phase could easily see west to east training as deep
layer flow is nearly parallel to the stalled frontal zone in the
area. With 
moderate CAPE, greater than 1000 J/kg depicted in all the hi-res
models, and sufficient moisture around 1.00 to 1.25 inch
precipitable water, it would not take more than a couple of mature
thunderstorms rolling through any given basin to produce quick
1.50-inch or more rainfall. Flash Flood Guidance in portions of
this region is also fairly low, with only inch per hour or 1.50
inch per 3 hours needed to exceed. 
 
Elsewhere, Marginal Risk is maintained within the narrow frontal
zone extending through the rest of PA and southtward a bit into
MD/VA, although the storms should be more progressive, with
greater cold pool development by the time they head south.
Differential heating owing to low cloud cover near the coast and
extending into eastern PA may enhance the upglide contribution to
convective coverage later on, and the 12Z hi-res models are
favoring locally heavy rain over eastern PA into NJ.

WPC preferred many of the details seen in the 12Z WRF-ARW. This
includes the potential for an old MCV and associated forcing in
southern Ohio to spill over and spawn a few storms or a small
organized line of storms overnight in Virginia, separate from the
main event up across PA/MD/NJ. The WPC QPF through 00Z Monday was
based on a blend that included the WRF-ARW, HREF Mean, previous
WPC (continuity), and a small percentage 12z GFS.


...Florida...

From a QPF standpoint the Florida forecast is particularly
difficult because the QPF magnitudes vary so widely among the
models. It appears which produce heavier rainfall inland over the
southern half of the peninsula are relying on increased deep layer
ascent, especially mid and upper level, as a broad low continues
to take shape. The surface pattern and CAPE forecasts, however,
suggest troughing and rain-reinforced outflow will continue to hug
the coast of south and southeast Florida, with only gradual
advection of deeper low level moisture and instability inland. The
upper jet configuration should produce better lift just off the
east coast of the state, and also near the developing low center
near and north of Key West. In between, we tried to keep the
heaviest downpours / QPF near the surface trough and near the
coast, favoring solutions more like the GFS and the previous 00z
WRF-ARW2, along with a little bit of the 12z WRF-ARW and HREF
Means to capture the trend of increased QPF. So overall we played
it conservatively, but did raise amounts in the Keys and near
Miami.


...Western U.S...

The upper low over the Great Basin had produced heavy snow at high
elevations in the northern Rockies, and widespread coverage of
light to moderate rain at lower elevations. Some heavy snowfall is
still yet to come in the mountains in northern Wyoming and
southern Montana. The intensity should start to dwindle overnight
and Sunday as the system becomes increasingly stacked, with upper
jet speeds weakening during that time. Precipitation, nonetheless,
will continue, especially in the deeply moistened vertical profile
near the upper low center, from central/northern Nevada
northeastward, and especially tonight as some upper difluence
remains. Expect the precipitation to become more fractured on
Sunday except near the inner core of the upper low over Nevada
where some of the more consistent rains will occur. For QPF we
used our in-house ensemble minus the bias correction (which
preserves greater coverage of measurable precipitation) along with
the HREF Mean to incorporate higher resolution without the high
magnitude bias of the individual models. The resulting QPF was
somewhat heavier in Nevada, with more areal average half inch and
greater - and somewhat drier in Wyoming where the trend has been
to adjust precipitation westward with little to support areas that
try to spill out into the High Plains.

Burke