Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Mar 26 2018


Prelim Day 1 QPF Discussion
Valid Mar 27/0000 UTC thru Mar 28/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Central and Eastern U.S...

A multi-day heavy rain event will initiate this evening in the
southern Plains and lower Missouri Valley. A low was closing off
in the lower portion of a deep, large scale trough. The pattern
will remain only slowly progressive, with periods of repetitive /
training convective systems anticipated from OK/TX eastward and
northeastward. The conveyor belt for this weather system, marked
by PW anomalies of 2 to 3 standard deviations above climatology,
will extend up into the Ohio Valley, resulting in a very lengthy
and broad swath of heavy rain, but with the most intense storm
totals and hourly rates occurring upstream in the more unstable
airmass over the Plains and lower MO/MS valleys.

For the Day 1 period, the WRF ARW continues to emphasize warm
sector convection to such a degree that it greatly limits the
expanse of heavier rain northward. There is, meanwhile, plenty of
evidence from a multi-model approach, to expect that synoptic
forcing will yield a secondary swath of higher amounts overnight
from central and northeast Oklahoma into southern Missouri. WPC
attempted to reflect this by incorporating some of the 12z GFS,
WRF-NMM, and HREF Mean into the forecast. The result ends up being
a bit washed out in the 24-hour totals, but has the effect of
broadening the 2-4 inch areal average amounts while eliminating
some higher amounts (around 5 inches in the previous forecast).
The convection will likely try to fight through the diurnal
minimum in the morning over OK/AR, and then come back with
enhanced intensity in the afternoon. Wherever small scale training
occurs, there certainly may be some 5 inch totals. Excessive
rainfall risk is maintained in the Slight Category as this is the
initial onset of heavier rain, and there is some uncertainty as to
exact placement, but the Slight category will need to be expanded
northward based on these changes to QPF.


...Pacific Northwest...

QPF trended downward somewhat, but remains heavy in the favored
mountain terrain, with strong onshore flow on the southern
periphery of an incoming 125 knot jet streak. A rather impressive
short wave trough moving rapidly across the nern Pacific towards
British Columbia will yield zonal moist onshore flow into
Washington and extending south into Oregon. This upper disturbance
and lower heights will not dig much at all, due to the mammoth
upper ridge over the eastern Pacific and extending into the
northwest quad of the country. However, an anomalous long fetch of
Pacific moisture flux over-top the upper ridge will persist
through Monday night across the Pacific Northwest. Overall WPC
followed a blend of the GFS/ECMWF along with the 00z high-res
package, which yielded areal avg totals of 1 to 2+ inches with
locally higher values in the coastal ranges, Olympics and
Cascades. QPF was based on a 50/30/20 blend of the 12z GFS /
WRF-NMM / WPC Previous.

Burke/Hurley