Quantitative Precipitation Forecast
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FXUS04 KWBC 201858

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Sun May 20 2018

Prelim Day 1 QPF Discussion
Valid May 21/0000 UTC thru May 22/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Central-Northern Intermountain West...

The highly-amplified, initially negatively tilted upper trough
along the west coast early will evolve toward a deepening/closed
upper low over southern CA-NV on Monday as an upper level jet
streak moves through the trough base. During this time, shortwave
ridging builds across southwest Canada, thus putting the northern
Great Basin within a `col` region both aloft and at lower levels.
Given the broadly difluent upper flow, along with the departing
area of confluent upper flow (toward south-central Canada), the
models continue to depict focused areas of upper level divergence
during the day 1 period from the Sierra Nevada northeast to the
northern Rockies to include the intermountain region. Modest
deep-layer instability (MUCAPES 1000-1500 j/kg) along with the
moist environment (PW anomalies ~2 standard deviations above
normal) will generate widespread additional areal-average totals
between 0.25-0.50" this afternoon through Monday, with locally
higher totals between 1-1.5" per the high res CAMs.


The models, including the high-res CAMs, continue to play catch-up
with the overnight MCS activity and airmass modification, with
renewed convection forming across central and south-central TX
along outflow boundaries (south of the main front). Quite a bit of
adjustments were made to the QPF this afternoon through evening --
mainly to account for the convective trends -- thus early on the
forecast was initialized with the more recent HRRR/experimental
HRRR guidance and NBM. Confidence in the QPF details lowers
considerably after 06Z Mon, however departing MCV energy and
ensuing shortwave ridging would favor less convective coverage on
Monday (mainly confined along the Gulf coast where the axis of
higher PWs will linger).



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