Quantitative Precipitation Forecast
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FXUS04 KWBC 190630

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
227 AM EDT Thu Apr 19 2018

Prelim Day 1 QPF Discussion
Valid Apr 19/1200 UTC thru Apr 20/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1


A respectable amount of synoptic forcing will accompany a
shortwave trough moving into the mean larger scale troughing over
the Northeast this morning. Height falls, upglide on the 300 K
isentropic surface, and frontogenesis just north of the developing
coastal low will all support a broad area of rain and snow.
Moisture is, however, lacking in an overall sense, and the moist
plume will move quickly offshore after 12z. Cold air aloft and the
increasingly high April sun angle will support diurnal showers,
especially in the higher elevations of NY, PA, but with
precipitable water values falling to less than a half inch by
midday, this system will not produce a lot of liquid. The more
noteworthy impacts will be from accumulating snow in the early
morning over parts of NY, PA. See the WPC Heavy Snow Discussion,
QPFHSD, and local forecast products for details. WPC QPF was
derived from the GFS, UKMET, and SREF, and was supported by some
of our in-house ensemble tools. The 12z ECMWF and still the 00z
ECMWF were thought to have been too suppressed southward with QPF.

...Western U.S...

Despite a fairly classic appearance in loops of water vapor
imagery, the low entering the West Coast was not very anomalous at
only 1 standard deviation from climatology in the 500 mb height
field. Moisture is also scant, such that on Day 1 this system is
more of a wind producer and will enhance the wildfire potential in
the Southwest per Storm Prediction Center and local forecast
office products. There will, however, be some organized areas of
light to moderate rain and snow, especially in the northern
portion of the mid level circulation / the Sierras across the
Great Basin, and also where some southerly moisture return meets
the difluent upper flow pattern ahead of the trough / West Texas
through New Mexico into the western and central Colorado. Given
well clustered model solutions, a consensus approach should work
well here. We leaned on continuity and the non bias corrected
version of our in-house ensemble which is more generous with
coverage of measurable precipitation. We also leaned toward the
WRF-ARW2 and GFS with respect to convection and elevated
convection expected in parts of TX, NM this afternoon and evening.
The CAPE axis sets up far to the west, along I-25 per the NAM.
Storms firing here may have a difficult time making eastward
progress into drier, more stable air, but a few may tend to be
more long lived near and south of the Rio Grande within a broader
field of moisture pooled along a frontal zone.



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