Quantitative Precipitation Forecast
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FXUS04 KWBC 200610

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
209 AM EDT Fri Apr 20 2018

Prelim Day 1 QPF Discussion
Valid Apr 20/1200 UTC thru Apr 21/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Central U.S...

A closed low over the Southwest will progress slowly, in mid April
fashion, producing mountain snows and increasing coverage of
rainfall / some convective / on the high Plains and Plains.
Certainly the precipitation up through Thursday evening had been
sparse, with the system having no connection to sub-tropical
moisture. It will entrain Gulf moisture and should produce an
expanding coverage of rain and snow on Friday. Still, moisture
return is confined and hampered somewhat by extensive surface
ridging extending from the Great Lakes to the northern Gulf of
Mexico. The greatest intensity / longevity of precipitation should
occur along the inverted trough axis north of the low center
(generally northern Colorado and adjacent WY/NE), also in the zone
of max height falls downstream of the low center over western
Kansas, and finally in the marginally unstable environment
developing near / north of an advancing warm front into the Texas
panhandle, Texas South Plains, and western/southern Oklahoma.

Given this is a well defined, closed, medium wavelength system,
model differences are minimal, and a consensus approach to the QPF
should perform well. The inherited WPC forecast was already in
good shape, and we made minor adjustments, favoring the WRF-ARW2
and some of our in-house ensemble / consensus tools.

...Pacific Northwest...

A progressive, open trough will move onshore Friday evening and
overnight. Forcing will be strong, with 12-hour height falls of
150 meters, but also short-lived for this quick moving system.
Precipitable water values in the GFS just briefly spike above a
half inch. These factors will minimize areal average rainfall, but
given the time of year and intensity of the cold air aloft, one
could imagine some weak convective enhancement for a time this
evening over western Washington. Even with this occurring, showers
will be moving quickly, and any given location is unlikely to
exceed an inch of liquid for the event. WPC QPF is in line with
consensus, and changed very little since the previous model cycle.


Height rises in the Northeast should tend to suppress showery
activity relative to recent days, although there may be enough
residual low level moisture to shake out additional showers early
in the day over northern New England. Easterly onshore flow will
begin to promote similarly shallow rain showers along the east
coast of Florida, but based on the dry and stable look of the NAM
forecast soundings WPC took a conservative approach to coverage
and intensity.



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