Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun Mar 18 2018


Prelim Day 1 QPF Discussion
Valid Mar 18/1200 UTC thru Mar 19/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Central / Southern U.S...

Gulf moisture will continue to overspread the southern states,
butting up against a frontal zone deposited by the shortwave
exiting the east coast early Sunday. Moisture will wrap well north
of the surface front into the zone marked by a forming occlusion
and inverted trough structure over the central and southern
Plains. The environment is typical of spring, with steep lapse
rates supporting a convective mode to much of the precipitation
from Kansas southeastward, and even briefly convective or at least
with embedded thunder over eastern Colorado as cyclogenesis /
strong deep layer ascent occurs during the day and evening.
Nowhere is moisture transport especially focused, but there should
be several mesoscale pockets of heavier rainfall scattered along
the more general swath of widespread rain from Colorado to
Georgia. Upglide on the 300 K isentropic surface per 00z soundings
will support swaths of semi-organized heavier downpours anywhere
from southern Kansas to southern Alabama. This is especially true,
or will be enhanced because of the presence of a lead shortwave
preceding the main cyclogenesis, which was evident in satellite
imagery this morning over northern Mexico. This shortwave comes
into phase with the diurnal heating cycle by afternoon over the
southern portion of the Gulf Coast states, such that we may see a
surface-based MCS added onto the ongoing upglide-driven / elevated
convection. Expect also a surface-based event back in the deeper
mixing zone near the Red River Valley / Arklatex, near the surface
front. The 00z WRF-ARW depicted much of this thinking very well,
but was perhaps too aggressive over southern Louisiana where it
wanted to blow up an MCS early in the day. We used the 00z HREF
blended mean to temper the ARW, and also liked the broader detail
within the GFS solution. This approach kept a decent amount of
continuity, while also raising forecast precip amounts in several
pockets, including the strongly forced eastern Colorado /
northwest Kansas environment, and central/eastern Kansas in the
strongly difluent upper flow region that develops by nightfall.


...Western U.S. Including the Central Rockies up through the
Western Dakotas...

The 00z GFS was in line with the observational trends for
precipitation organized along a mid level frontogenesis zone in
eastern Utah and northeast Arizona, heading into western Colorado
early today. Much of the forcing then transfers out to the
developing cyclone over the Plains. Inverted troughing will extend
meaningful areal average QPF up into the western Dakotas with
time, and the cold upper level temperatures and cyclonic flow
continue to favor a showery regime over the northern Rockies and
Pacific Northwest (rain and snow). WPC favored the GFS, WRF-ARW,
and GEM Regional.

Burke