Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
700 PM EDT Sun May 20 2018
Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 21/0000 UTC thru May 24/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr
...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.
In the 2030Z final issuance we moved things along toward the south
more quickly this evening based on trends, and using primarily the
mid afternoon HRRR runs. Confidence in the QPF details lowers
considerably after 06Z Mon as 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
...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. In the 2030Z final issuance we did make some stark changes
to the expected overnight rainfall, moving it westward in
accordance with the low level convergence signal in the RAP and
associated heavy rain signal in the HRRR. This makes some sense as
the global models predict a slight retrogression of the weakness
over the eastern Gulf tonight. This would place heaviest overnight
totals in the Everglades, parts of the Keys, and possibly up
toward Naples. At least this change represents a reduction of QPF
over southeast Florida where very heavy rain fell earlier today.
Still, we will maintain a Marginal Risk of excessive rainfall
stretched across all of south Florida overnight. 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.
...Southeast/eastern Gulf Coast/FL...
Deep moisture within confluent flow with corresponding lift
provided by the passage on an inverted 850 mb trough will support
bands of showers over the eastern Gulf coast....including the FL
panhandle inland to AL/GA on day 2 and 3. The NAM/UKMET/GFS
showed a closed low developing which led to strong low level wind
fields and resultant convergence/lift. The inflow off the gulf
advects the high moisture into the Florida panhandle across into
southern Alabama and Georgia, so combined with diurnal instability
and sea breeze initiated lift combined with convergence near the
low, the higher amounts are shown in these areas for Monday and
The models also depicted persistent, deep flow of moisture from
the Atlantic over portions of the eastern and central peninsula.
Onshore speeds are not terribly great, but the combination of PW
values approaching 2 inches and the shoreline convergence suggests
that some locally heavy rainfall amounts are possible from the
eastern FL peninsula up into coastal South Carolina.
Manual progs blended continuity with the GFS/ECMWF/UKMET. There
certainly model to model differences as well as run to run
differences within each model, so confidence in the details here
are below average.
Marginal risks for excessive rainfall were shown covering parts of
Alabama and Georgia in the proximity to the low-mid level
circulation. The portion of the marginal risk in the Florida
panhandle was expanded to include nearly all of the peninsula.
...Great Basin/Northern Rockies/Northern Plains...
A long wave trough will slowly move east out of OR and WA across
the northern Great Basin and the Northern Rockies by the end of
the forecast period.
The focus of rainfall develops with upper divergence maxima just
downstream from the trough/near the low level front in the ranges
of ID and western/central MT and northwest WY on Mon/Mon night.
Further south, showers continue in the proximity of the upper
trough and embedded closed low as it drifts out of CA towards
NV...with QPF maxima in the CA Sierra Nevada and favored terrain
of northern NV.
On Tue as the upper low ejects north across Utah to the ID border,
southerly deep layer flow advection moisture north across Wyoming
and the northern Plains. The warm/moisture advection impinges on
the ranges of northeast Wyoming, including the Bighorn Mountains.
Once again, the upper divergence maxima eject northeast across the
northern Plains Tue night, another round of showers/storms
develops. with the NAM/UKMET/Canadian showing higher amounts
over a larger area. T
The NAM intensifies the 700 mb low over North Dakota in response
to convective heating, usually a sign of convective/grid scale
feedback from convection, in this case in southeast ND. As a
result, the NAM solution was given least amount of consideration
once again in this forecast cycle. Otherwise, the manual QPF from
WPC was more in line with the latest GFS/ECMW/UKMET.
...West Texas/eastern NM...
East of the developing western U.S. trough, southeasterly
confluent low level flow over Eastern NM and across much of Texas
will advect moisture off the Gulf of Mexico inland. The airmass
will be characterized by a precipitable water value around
1.25-1.5 inches on Monday...which is 2 standard deviations above
climatology. This, combined with high CAPE and low shear, favors
slow moving storms. On day 2 with a couple of the models,
including the 00z GFS, show locally 2-3 inches of rain. The
excessive rainfall outlook was maintained at a slight risk
category on day 2, but not forecast on day 3 following the lower
amounts forecast by the models on Day 3/Tue.
WPC manual graphics tended to put more weight on the ARW core and
GFS for the QPF here early on day 2...with GFS/NAM getting the nod
once the CAM guidance ended.
...Great Lakes to the Ohio Valley/Central Appalachians/mid
A low-mid level wave over eastern Iowa on Sunday evening moves
towards the Great Lakes, with the deformation zone focus over
eastern Iowa/northern Illinois/northern Indiana.
The threat for showers in northern IL across southern Lake MI to
southwest lower MI will gradually shift towards the east during
the 2-day period, with slow moving cells possible near the
elongated cyclonic circulation.
More progressive showers/storms develop in an axis of the low
level jet with well defined 700 mb convergence near the jet maxima
crossing Ohio into PA Monday, supported by upper divergence
maxima. This continues downstream into NJ/southern NY in the zone
of warm/moist advection at 700 mb on Monday evening. Bands of
low level moisture convergence stream across much of Pennsylvania
and MD on Tuesday afternoon and evening, with diurnal instability
contributing to showers/storms.
Manual progs used a blend of the latest available ECMWF, the 12Z
NAM and GFS.
Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml