Quantitative Precipitation Forecast
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FXUS04 KWBC 260853

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
453 AM EDT Thu Apr 26 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 26/1200 UTC thru Apr 29/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Tennessee Valley into the Southeast and Mid-Atlantic...

Another compact mid-upper low will pivot across the lower MS and
TN Valleys today, then northeast into the mid Atlantic region
tonight. This feature, along with a more open shortwave in its
wake approaching the ARKLATEX Thu night, will reinforce the
longwave trough/height falls across the Gulf coast region Thu
night into Fri. Despite the largely curved upper flow and thus
favorable Qs forcing ahead of the initial deep-layer circulation
(upper difluence, dpva), the dynamical support will remain
transient given the continued steady progression of the shortwave.
This along with the modestly anomalous moisture parameters
(including PW and 850-700 mb) and minimal deep-layer instability
will restrict the QPF potential to some degree.

The models, in particular the high-res CAMs, denote two discrete
areas of heavier rainfall. The first is over southern TN into
northern MS and central-northern AL-GA during the day, aided by
the focused low-level moisture flux convergence along and north of
the quasi-stationary w-e surface frontal boundary, along with the
diurnal heating and resultant uptick in surface-based instability
(albeit weak as GFS and ECMWF forecast CAPES top out below 1000
j/kg). Using a multi-model consensus for the QPF, highest
areal-average totals over this region ranged from 0.50-1.00", with
little variance in terms of the upper-bound amounts per the
individual high-res CAMs (1.5-2.0"). Short-term rainfall rates in
this region are therefore expected to remain below FFG, as
supported by the latest HREF exceedance probabilities.

The other area of concern is across eastern portions of the
Piedmont into central NC-VA. 850 mb moist southerly inflow
increases to 30-35 kts Thu night/early Fri morning as the compact
circulation lifts across the area. The uptick in the low level
inflow, along with aligning parallel to the mean 850-700 mb flow,
will allow for an enhanced potential for repetitive linear
convective segments, as depicted per the high-res simulated
reflectivity. This scenario is also manifested in the multi-CAM
consensus in terms of the deterministic QPF -- with an axis of
1.5-2.0" noted over a 3-6 hour period. Instability is rather
meager (500 j/kg tops), however given the antecedent moist
soils/current FFGs, could be enough to allow for isolated areas of
excessive short-term runoff. As a result, a MARGINAL risk has been
noted over this area in the day 1 Excessive Rainfall Outlook
(ERO). The MARGINAL risk area also encompasses the highest (50+
percent) probabilities of 1 hour QPF exceeding 1 inch per the
latest (00Z) HREF.

...Northern New England...

Another compact, deep-layer circulation currently traversing the
lower Great Lakes early this morning will swing across northern
New England today, becoming more negatively tilted as a separate
(also compact) shortwave lifts nne along the northeast coast.
Favorable upper level forcing, deep-layer moist isentropic ascent,
and anomalous PW/low-mid layer moisture flux anomalies within a
robust warm conveyor belt will lift north of New England late
today. Additional QPF after 12Z Thu will average between
0.25-0.50+ inches across northern New England per a multi-model

Days 2/3...

...Mid-Atlantic to the Northeast and Great Lakes...

As a weakening shortwave trough lifts through the region late Fri
into early Sat ahead an amplifying trough over the Great Lakes,
precipitation will continue to move north from the northern
Mid-Atlantic region into southern New York and New England   Over
the past 24-hrs, models have moved into better agreement with this
system and therefore the WPC QPF largely reflects a blend of the
GFS, ECMWF and NAM CONUS nest into early Sat.

Further west, a well-defined shortwave diving into the base of the
previously noted trough is expected to produce a stripe of light,
to perhaps moderate, amounts from the Arrowhead Region of
Minnesota early Thu into the southern Great Lakes region by Fri
morning.  Showers, producing mainly light amounts, are expected to
push across the central Appalachians, northern Mid-Atlantic region
and Northeast Saturday to early Sun as a deep upper low closes off
over the Great Lakes and then rotates east -- sending a surge of
colder air through the region.  Here also, WPC QPF largely
reflects a blend of the GFS, NAM and ECMWF.

...Pacific Northwest to the Northern Rockies...

A strong upper ridge is forecast to slowly give way to an upper
low moving into the region, supporting shower and thunderstorm
development across western Washington, Oregon and northwest
California on Fri, with precipitation beginning to develop further
east into the northern intermountain region by early Sat along a
low-mid level front as it lifts north.

Precipitation will continue to expand further east into the
northern Rockies by early Sun as the low continues to rotate
further inland.  A negatively-tilted shortwave lifting out ahead
of the upper center, along with low-mid level frontogenesis, may
support some locally heavy amounts across portions of central to
northern Idaho Sat night into Sun morning.

With the guidance in generally good agreement with the
larger-scale features, WPC QPF gave more weight to the NAM and GFS
through the period, discounting the EMCWF, which was a relatively
dry outlier.

...Southern Rockies...

Mean trough axis over the Midwest on Thu is expected to shift east
into the eastern U.S. by Sat.  Increasing moisture, afforded by
return flow is expected to support shower and thunderstorm
development across central Colorado to New Mexico as it interacts
with weak shortwave energy embedded within difluent flow out ahead
of the western U.S. low.  Here also, the EMCWF may be showing its
dry bias and therefore, while not going quite as heavy,
significant weight was given to the more robust amounts of NAM and
GFS in spots.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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