Quantitative Precipitation Forecast
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FXUS04 KWBC 201917

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
317 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).

...Lower-Mid Mississippi Valley into the Tennessee and Ohio
Valleys and Mid Atlantic...

Another region where the model guidance continues to lag the
observational/mesoanalysis trends. Leaned more toward the
experimental HRRR early in the period (through the overnight) with
the QPF ahead of the mesoscale-convective aided vort centers
across western NE and eastern OK-KS early this afternoon. WPC
focused the highest areal-average totals where the airmass was
able to destabilize Sun afternoon (following the
thinning/dissipation of nocturnal debris from last night`s
convection)  Moderate to high instability will develop, and will
generally be uncapped given the broad troughing
remaining over the area. PWATs, while not extreme, will continue
to run above normal. Multi-cellular convection will be most
organized downstream of the aforementioned shortwaves, with the
modest deep-layer instability in place. The activity farther north
will likely become better organized given the 30-40 kt 0-6km bulk
shear; however, slower cell motions (Corfidi vectors 5kts or less)
would support locally heavy rainfall into central and southern
MO-IL where bulk shear values are 20-25 kts or less.


Will be another wet period across Florida, with moisture remaining
high. As usual, low confidence with the details today. The
potential will certainly be there for heavy rates and some
flooding concerns, just a question as to if/where we see enough
instability and localized convergence to generate more intense
convection. Some signs another weak mid level shortwave wave may
be approaching the state again on Monday, as the southeasterly
850mb flow will remains strong enough to support the persistence
of organized convection shall it develop.



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