Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Tue Mar 27 2018


Prelim Day 1 QPF Discussion
Valid Mar 27/1200 UTC thru Mar 28/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

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

A highly-meridional upper flow pattern is evident via the early
morning satellite WV/IR loops, with a closed mid-level circulation
over the Southwest-far nw Mexico, another closed upper low
northeast of Bermuda, with a broad ridge in between across the
central-eastern CONUS. 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 eastern
portions of the Southern Plains into the lower MS Valley.

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 closer to
the surface front from eastern OK into northern AR and southern
MO. WPC attempted to reflect this by incorporating some of the 00z
ECMWF, WRF-NMMB, 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 1.5-3.0 inch areal average amounts across
northeast TX-eastern OK into much of AR and southern MO. This idea
resulted in an increase in QPF farther north compared to
continuity (previous forecast), supported by the multi-model
consensus (including the WRF-ARW) of a bi-modal distribution of 2
max QPF axes (one n and another s).

Wherever small scale training occurs, there certainly may be some
5 inch totals. The nocturnal uptick in elevated low-level
southwesterly flow (while also aligning more parallel to the mean
850-300 mb flow) will maintain the highest risk of convective
back-building and cell training Tue morning across northeast TX
into eastern OK and northern AR, then again Tue night into Wed
morning a bit farther south to include portions of central TX into
the ARKLATEX, central-southern AR, and northwest LA. The day 1
excessive rainfall risk incorporated a MODERATE risk inherited
from yesterday's day 2 ERO, with the area expanded a bit to
encompass both rounds of more enhanced/organized and potentially
repetitive convection (and likely reduction in 1/3 hourly FFG
values with time), while maintaining the fairly broad SLIGHT and
MARGINAL risk areas where the short-term rainfall rates are not
expected to be as robust (FFGs a bit more challenging to
overcome), while also capturing the spread among the guidance
(especially CAMs).



...Pacific Northwest...

QPF trends from the previous forecast were minimal, however the
new day 1 totals remain 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.

Hurley/Burke