Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
226 AM EDT Tue Apr 10 2018


Prelim Day 1 QPF Discussion
Valid Apr 10/1200 UTC thru Apr 11/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr




...West Coast/Rockies...

A short wave just off the Pacific Northwest coast early on Day 1
crosses Vancouver Island, as a weaker short wave tracks from
northern CA to the Northern Rockies. Each short wave taps Pacific
moisture for locally heavy qpf amounts. Another short wave
approaches the Pacific Northwest late in the period. For the most
part, there was good model agreement with the timing of the short
waves, so the WPC QPF was based on a blend of the 00z GFS/12z
ECMWF. Some 00z HREF mean was utilized to better delineate the
higher terrain qpf maxima.

Ahead of the first short wave (at the base of a long wave trough
off the Pacific Northwest coast) that tracks from off the OR coast
to Vancouver Island during the first part of Day 1, a 30 to 50
knot low level southwest flow transports 0.75/1.00 inch
precipitable water air over northern CA into western WA. The
upslope flow, combined with the lift associated with the short
wave, is expected to produce a stripe of 0.75 to 1.25 inches of
qpf along the higher terrain of the OR/WA Cascades, mainly before
11/00z. Locally higher qpf amounts (nearing 2.00 inches over the
Olympic Range in WA) are possible, as the upslope flow is most
parallel to the terrain in western WA.

A piece of short wave energy streaks from northern CA across the
Northern Rockies between 11/00z and 11/12z. Some of the Pacific
moisture plume survives into the Northern Rockies, with
precipitable water values between 0.50/0.75 inches (which
approaches two standard deviations above the mean). Model
soundings showed marginal instability (mainly in the form of
MUCAPE values between 100/250 J/KG) between 10/18z and 11/04z,
which could add a convective flavor to the precipitation. The
combination of moisture, instability and upslope flow is expected
to produce local 0.50/0.75 inch qpf amounts over the central
Bitterroot Range in ID and the Grand Tetons and Wind River Ranges
in WY. Lowering snow levels should mitigate the threat for flash
flooding. though low level heavy rainfall and snowmelt could
result in some runoff issues.

Another short wave in the flow approaches the Pacific Northwest
late in the period, with most of the qpf with this piece energy
occurring near or after 11/12z.


...Great Lakes/Northeast...

Short wave energy tracks from the Upper Great Lakes across
northern New England during Day 1, and the short wave pushes along
a weak surface reflection. Lift associated with the short wave is
expected to produce mainly light qpf, as the airmass in place is
relatively dry. For the most part, there was good model agreement
with the timing of the short wave, though there were some
differences in the qpf coverage across New England. In an attempt
to mitigate some of the differences, the WPC QPF was based mainly
on a blend of the 00z NAM/00z GFS/12z ECMWF.

The short wave tracking from the Upper Great Lakes reaches
northern New England during Day 1. The airmass in place is very
dry (with 00z soundings showing precipitable water values under
0.15 inches), so qpf amounts associated with this feature are
expected to remain below 0.15 inches across MI/northern WI into
the UP of MI. Further east, moisture from an offshore low could
become funneled into northern New England ahead of the short wave,
and the moisture in an upslope flow over VT/NH/western ME could
produce local 0.15 inch qpf amounts here as well. 


...Southeast/FL...

Deep moisture and instability along and south of a quasi
stationary boundary extending across north central FL will feed
convection that produces heavy to locally excessive rainfall
during Day 1. While there is some model consistency on the
placement of the front (at least through 11/00z), there are still
some significant differences in the placement of the maximum qpf,
due mainly to the presence of mesoscale boundary into central FL.
In an attempt to mitigate some of the qpf placement differences,
the WPC QPF was based on a blend of the 00z WRF ARW and the 00z
HREF mean, then adjusted for radar trends in the near term portion
of the forecast.

Surface based convection continues to fire along the west central
coast of FL early this morning, ahead of a wavy front extending
from northwest to southeast across central FL. East of the front,
the low levels have cooled and stabilized, as cold pool influences
have pushed the front south of KMLB. In the near term, the surface
based convection is expected to drop slowly south (through at
least 10/15z) as cooler/more stable air to the north pushes south.
This should result in a west to east line of at least broken
convection from north of the KTPA area to south of KDAB, and
mirrors the overall presentation during this period of the 00z
HREF.

After what looks like a break in the convection through the mid
morning hours (based mainly on radar and satellite trends), the
forecast problem becomes whether there will be sufficient
instability left in place for allow surface based convection to
fire once again. This would occur after 10/15z or so, and there is
quite a bit of model spread as to where (and even whether) this
occurs.

For now, a blend of the more reliable high resolutions models
(including the 00z HREF mean and the 00z WRF ARW) suggest that
there should be enough instability to refire convection over
central FL. However, whether the convection will produce locally
excessive rainfall remains problematic. The WPC QPF followed the
idea of convection dropping south with time, but slowly as the
front through 11/00z remains quasi stationary.

North of the front this morning, elevated convection remains
possible as MUCAPE values hover between 500-1000 J/KG through
10/18z. Rainfall amounts here should be lighter, as the isentropic
lift north of the front is not that strong.

After 11/00z, as a short wave approaches from AL, surface low
pressure is expected to organize off the Mid Atlantic coast.
During this time, divergence in the right rear quadrant of a 130
knot jet streak stretching from the Mid Atlantic into southern New
England is expected to help the convection become more organized.
The threat for flash flooding south wane during this period, and
become negligible as the short wave pushes the surface front off
the southeast FL coast by 11/12z.


Hayes