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


Prelim Day 1 QPF Discussion
Valid Apr 26/1200 UTC thru Apr 27/1200 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
consensus.

Hurley