Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Mon Jun 04 2018
Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Jun 05/0000 UTC thru Jun 08/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr
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
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.
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).
Day 2 and 3...
...Northern Rockies to the Northern and Central Plains...
In general, the Day 2-3 period in the Central CONUS will be marked
by a broad mid-upper level ridge, with the ridge axis extending up
the Plains roughly along 100W longitude. Much of the rainfall in
this period (from Tuesday Night through Thursday) should be
positioned near or north of the I-80 corridor, around the northern
periphery of the elevated mixed layer (generally outlined by the
+12C isotherm at 700mb). Some models wanted to extend some light
QPF further down the Plains, even into West Texas, likely the
result of some isolated convection. While this can't be ruled out,
the environment should generally be hostile and there was not
sufficient confidence in the precision of any localized QPF maxes
to include rainfall further south than central Kansas.
Some of the more focused heavy rainfall will likely be from North
Dakota into Minnesota from Tuesday Night into early Wednesday, as
a stronger cold front pushes into a highly unstable air mass. This
appears to be a favorable setup for MCS development, and it should
drop increasingly to the southeast and south around the periphery
of the ridge with time into Wednesday Night and Thursday -- either
as a continuous cluster of thunderstorms, or with new convection
regenerating on outflow. This will be a favored area for heavy
rain given the likelihood of organized convective clusters (strong
instability and wind shear), but convection should also tend to be
forward propagating. Therefore, a Marginal Risk was maintained in
this area for Day 2, and introduced into southern Minnesota and
Iowa on Day 3. The timing of the QPF maximum was a blend of the
timing of the GFS and ECMWF. However, the QPF was not necessarily
directly blended given the differences in timing. Went close to
the ECMWF and internal bias-corrected pseudo-ensemble for the QPF
magnitude, but shifted it slightly faster.
Elsewhere in the region, QPF generally followed a broader model
blend, with a slightly greater weight placed on the 12Z ECMWF.
A broad, digging trough should push into the Gulf Coast region
during the Day 2-3 period. And there is likely to be a
(relatively, for this region in the warm season) focused ribbon of
rainfall ahead of that trough in the plume of higher PWATs. Model
QPFs showed greater disagreement on the 00Z cycle, but have
converged more favorably on the 12Z cycle. Therefore, a blend of
the GFS and ECMWF was generally used, with a hi-res model mean
also incorporated early on the Day 2 period. The resulting QPF was
adjusted slightly to focus rainfall more offshore during the
overnight periods, and over land during the daytime periods, as is
typical with the diurnal cycle around the Gulf in the warm season.
A Marginal Risk was introduced for Louisiana and adjacent far
southern Mississippi and Alabama on Day 2. Hi-res models showed
potential for 1-2 in/hr rain rates with the 12Z HREF showing small
areas of around 10% probability of FFG exceedance.
Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml