Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sun Apr 15 2018


Prelim Day 1 QPF Discussion
Valid Apr 16/0000 UTC thru Apr 17/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Southeast into the Mid Atlantic...

A very dynamic, highly amplified spring system continues to
migrate eastward, with a well-defined, elongated QLCS progressing
through the southeastern U.S. this afternoon. Deep-layer
instability, largely surface-based, continues to be on the uptick
ahead of this line over the southeast and lower mid Atlantic
region, while the increasing upper level divergence ahead of the
trough (especially as the trough becomes more negatively tilted)
should maintain the widespread coverage of moderate-heavy rainfall
through the new day 1 period. In general the convective line will
remain progressive in nature, likely limiting the potential for
any extreme rainfall amounts. However, deep layer mean winds
parallel to the line supports at least some south to north
training of cells embedded within the line. Also, current
convection is actually forming ahead of the true cold front, thus
the potential exists for additional storms to fire on the true
cold front as well, which would result in multiple rounds of
locally heavy rainfall in any one location. Overall, this looks
like a widespread 1-3" rainfall from the Southeast into the Mid
Atlantic region...with the aforementioned training and repeat cell
potential allowing for localized 3-5" amounts. Impressive 850 mb
moisture transport and PWAT vales exceeding the climatological
90th percentile certainly support these totals and the potential
for high rates. Thus at least some flash flood risk exists where
any cell training or repeat convection is able to occur.

The orientation of the line will likely be more favorable for
short term training and repeat cells from northwest NC into
portions of WV/VA/MD closer to the negatively tilting trough and
better mid/upper level forcing. Strong southerly flow ahead of the
low should help enhance rainfall totals some into the southern and
eastern slopes of the Blue Ridge and Appalachians as well. Thus
will carry carry a Slight risk from northwest NC into eastern WV,
western and northern VA into MD. It is across these areas where
the best chance for localized 3-5" totals exists to go along with
a bit lower FFG. For this issuance opted to include the
DC/Baltimore urban corridor as well. Dry conditions of late should
help, but this corridor is still more susceptible to flash
flooding, and note several HREF members and the 12z extended
experimental HRRR showing localized 3-5" totals in the
vicinity...and most of this would fall over a short period. Thus
while not a sure bet, think the conditional threat for flash
flooding is high enough to warrant the slight risk inclusion at
this time.

Instability will be greater across SC/NC...so certainly some
chance the convective line really gets organized there this
afternoon...which could block moisture transport and limit amounts
a bit further northwest over southwest VA and far northwest NC.
Tough to nail down these details though with much lead time, so
will need to monitor trends. Given this potential, also opted to
include the Charlotte metro in the slight risk, given their
susceptibility, and to account for the possibility of a further
south focus.

WPC QPF leaned exclusively on the high res guidance for this warm
sector convection. Most of these solutions continue to seem a bit
slow compared to the current radar, thus leaned more towards the
quicker high res solutions. Also incorporated some of the 0z HRRR
experimental, which was more robust with amounts over SC/NC than
the HREF component members were. As mentioned above, given the
better instability here, there is certainly a possibility that the
higher amounts do verify here.


...Upper MS Valley---Great Lakes into northern NY and northern New
England...

A surface low will continue to slowly track east across the OH
Valley today. To the north and northwest of this low will see
continued wintry precipitation across MN/WI/MI. Continued to lean
toward the slower model solutions in terms of lifting the TROWAL
east of the area -- which is close to a blend of the ECMWF and
UKMET along with the WPC in-house bias corrected ensemble QPF.
Significant, widespread winter weather hazards continue with this
dynamic spring system -- please refer to the latest QPFHSD for
further details.


...West...

A closed mid level low will move into the West today...with a
focused plume of moisture out ahead of it producing some 1-2" QPF
amounts from far northwest CA into southwest OR, the OR Cascades,
and the CA Sierras. WPC QPF ends up close to the HREF mean.

Hurley/Chenard