Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
203 PM EST Tue Feb 27 2018


Prelim Day 1 QPF Discussion
Valid Feb 28/0000 UTC thru Mar 01/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Southern / Eastern U.S...

We are setting up for another heavy rain event going into Tue
night and Wed. This is initially driven by warm advection and a
pocket of rich moisture that had pooled along the tail end of the
frontal boundary in the western Gulf. Stronger deep layer forcing
will not arrive until Wednesday, but the region from the TX Gulf
Coast up though Illinois was already upstream of shortwave
ridging, and experiencing gradual height falls along with
strengthening warm advection. Upglide on the 295-300 K surface,
along with relatively steep mid level lapse rates owing to a
strong westerly component to the flow and an unseasonably mild
column overall, will yield swaths of convectively enhanced
rainfall. Widespread convection was initiating as of 18z in the
central U.S., and will spread eastward. There is fairly strong
model agreement as to an axis of heaviest amounts, areal average 1
to 3 inches. from the ARKLATEX through central Tennessee through
00z Thursday, and a secondary axis farther up the sloped frontal
surface extending from eastern Kansas to southern Illinois. With
convection being elevated, and the pattern suggestive of
progressive warm advection that quickly washes through a given
area, it may be difficult for much training to set up anywhere
until later on Wednesday when frontogenetical forcing and upper
jet dynamics will become more persistent ahead of the arrival of
the longer wavelength trough out of the Southwest states. Through
this evening, however, persistent / training convection may set up
along the edge of deeper moisture moving into northeast Texas and
northern Louisiana. WPC favored, in particular, the WRF-ARW2 and
NSSL WRF solutions. Both models depict hourly accumulation rates
that could briefly exceed 0.75 inches. This presents the
possibility of renewed flash flooding, given recently saturated
ground.


...Great Lakes and Northeast...

While the primary jet axis is farther south, there is a distinct
northern corridor of enhanced gradients / stronger flow in up
through the mid levels. As moisture returns northward along the
295 K isentropic surface, it should produce measurable
precipitation over much of the lower Great Lakes tonight, and then
again in a heavier fashion beginning Wednesday night / Day 2 / as
troughing phases and forcing strengthens. In the Day 1 period the
ECMWF ensemble probabilities and SREF were in better agreement
with the overall consensus, allowing us to introduce a stripe of
0.10" and greater QPF in multiple swaths over WI/MI and northern
IL/IN.


...Western U.S...

A well defined closed low over southern California will open up
and push steadily eastward. With no real connection to
sub-tropical moisture, its effects will be limited, but a showery
regime will persist under the upper low through evening, and
rain/snow is expected out ahead of the trough through the
4-corners area, although accumulations will be somewhat minimal as
the system weakens / shears out on its way downstream.

Farther north a stronger trough will dig along the Pacific
Northwest coast. Still there is no real connection to sub-tropical
moisture, but PW values will gradually increase, especially by the
end of the period. This is a large scale feature well handled by
the models, so we favored a consensus QPF approach, although some
of the detail from the NAM and the HREF was especially useful.
Areal average amounts may push above one inch through 00z Thursday
in the coastal ranges of western WA/OR.

Burke