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Quantitative Precipitation Forecast (QPF) Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1846Z Jun 04, 2018)
 
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Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
246 PM EDT Mon Jun 04 2018


Prelim Day 1 QPF Discussion
Valid Jun 05/0000 UTC thru Jun 06/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


...Southern U.S...
From this afternoon through Tuesday the predicted rainfall over
the CONUS is low on coverage and not especially heavy on average.
One of the heavier stripes of expected rainfall extends from near
the Red River down to New Orleans / Biloxi, straddling a
stationary frontal zone. Low level moisture will pool and
gradually deepen near the front, and a mid level trough slicing
through the mean larger scale ridge position should support better
organized clusters of thunderstorms by late tonight into Tuesday.
In the early hours of the forecast, however, we were somewhat
pessimistic as to how much convective coverage would be realized.
The MCS that rolled out of the Texas Hill Country this morning had
dissipated, and morning RAOBS in the southern Plains and lower
Mississippi Valley revealed a lot of very warm and dry mid level
air with weak to mid range lapse rates, as well as shallow low
level moisture.

WPC sided with the HRRR and WRF-NMMB which suggest if any
organized afternoon convection is to form it is more likely to
occur with eastward extent, ahead of the remnant MCV and toward
the cooler - roughly 8 C at 700 mb - mid level temperatures in far
east Texas and Louisiana. Much of the remainder of central Texas
up through Oklahoma may remain capped, although with strong
heating and holding onto some continuity, we do indicate some
widely scattered activity.

Expect coverage to increase overnight, but it may wait until later
in the night over OK/TX, as the low level jet veers to induce
stronger warm advection / more steeply crossing the mid level
isotherms. Even still, the core of the low level jet will be
located farther west such that flow speeds in the area of deep
forcing near the mid level trough will likely remain light, less
than 25 knots at 850 mb. Wherever activity does develop, however,
there will be potential for very slow movement initially, and some
locally heavy rainfall amounts. Given the limiting thermodynamic
factors listed above, WPC leaned as far north and east as possible
from late tonight into Tuesday, using a majority WRF-NMMB and NAM
CONUS Nest - which line up well with the expected north side of
the synoptic front per the NAM. This approach resulted in lesser
QPF over Texas and greater QPF downstream into Louisiana and the
central Gulf Coast.

...Northern Plains...
A sharp, but low amplitude shortwave trough will eject through the
northern Rockies this evening and tonight. Primary height falls
will occur up in Canada, but some deep layer forcing and a
generally difluent jet structure will coincide with an instability
axis over parts of central and eastern Montana, leading to at
least scattered thunderstorms. Given the flow strength, cells will
be moving quickly, so there is relatively little risk of excessive
rainfall in this region, but most-unstable CAPE values above 1000
J/kg and PW values just shy of an inch open up the possibility of
a quick accumulation of an inch or more in some isolated
locations, owing to brief training or cell mergers. Guidance is
all pretty agreeable as to the areal average rainfall. Downstream
the event will increasingly become dependent on warm advection
overnight in North Dakota, and as the more surface-based activity
diminishes out of Montana, there may be a local minimum of
coverage / intensity in western ND, picking up farther east with
stronger warm advection. This regime eventually leads to likely
MCS development the following night, Tuesday night, downstream
into eastern ND per the Day 2 forecast.

...Great Lakes / Northeast / Mid Atlantic...
The main warm conveyor and deep moisture will remain offshore
ahead of east coast troughing. Continued height falls, however,
working with sufficient residual moisture, will lead to a rainy
period for northern New England, and a sharp shortwave trough
diving through the lower Lakes into the Ohio Valley will bring
some showery / thundery weather as well. The more stratiform area
over New England should be well handled by a consensus QPF
approach. From the Lakes into the Ohio Valley the synoptic forcing
per the global models supported leaning on QPF from the ECMWF and
the WRF-NMMB (which was preferred in other regions as well).

Burke