Quantitative Precipitation Forecast
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FXUS04 KWBC 112222

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
621 PM EDT Sun Mar 11 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 12/0000 UTC thru Mar 15/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Lower Ohio Valley - Tennessee Valley - Southern-Central
Appalachians - Mid Atlantic - Southeast...

The well-defined shortwave over the mid MS Valley will pivot
across the TN Valley and eventually off the southeast coast.
Meanwhile, the northern stream wave - dropping south of Hudson/St.
James Bay early this afternoon - will drop across the Great Lakes
and Ohio Valley toward the end of the period (00Z Tue). These
features will become more in phase by Monday, and in doing so,
lend support to a southern "Miller-B" cyclone evolution as the
surface low across the TN Valley weakens with time upon crossing
the mountains and gives way to cyclogenesis off the mid Atlantic

The latest WPC reflected noteworthy changes with the 12Z guidance
from the previous model cycle(s) - particularly a boost in amounts
across central SC-NC where the model consensus depicts a widening
area of coupled southern stream/northern stream dynamics. At the
same time, we have curtailed amounts a little north of the VA/NC
border during Day 1 in light of the upper shortwave ridge/lower
level anticyclonic flow lingering a while longer. Did not go as
dry as the ECMWF (see PMDHMD).

...Georgia / Florida...

As of early evening / 22z, a linear MCS associated with the
leading edge of large scale height falls was marching steadily
across southern Georgia and northern Florida. Steering flow will
generally keep the line moving eastward. Intensity may fall off
over southeast GA and northeast FL where the MCS will outrun the
supply of better instability, but it is then likely to flare up
again offshore to the east when the outflow finds the warmer
near-surface layer over the ocean. Some activity may survive on
the tail end over the Gulf as well, but with time our attention
will focus primarily on south Florida, where advection of moisture
/ axis of greater PW located south of the Keys / and some warm
advection should lead to scattered showers - some of which were
already ongoing at 22z. As the vigorous upper shortwave over the
southeast U.S. digs in this direction, lower to middle level flow
will become gradually more and more convergent, and coverage of
rain/thunder should increase toward morning, particularly from
south of Tampa Bay southward, with the potential over central
Florida being a bit more limited by the presence of broad low
level troughing stretched across the peninsula which will deflect
the Keys PW axis eastward without as much northward progress. On
the large scale the models form a decent consensus aside from the
very dry GFS solution. There is a lot of variance in the smaller
details, but when using the large scale forcing mechanisms as a
guide, WPC QPF was close to resembling the WRF-ARW2 and NAM.

Days 2/3...

...The West...

Multiple shortwave spokes are expected to wrap around a large
closed low which will edge slowly closer to the West Coast Monday
night through Wednesday. While there is large scale agreement on
the progression and depth of the large scale pattern, smaller
scale details, such as those individual spokes, have yet to be
ironed out. A blend of the 12Z NAM, 12Z GFS and 00/12Z ECMWF was
used for QPF across the West with adjustments made based on hi-res
data where available.

One of the shortwaves is expected to near the northern
California/southern Oregon coast beyond 00Z/13 with decent model
agreement outside of the farther west 12Z UKMET. A surface low is
forecast to deepen at the head of an occluded front and track
north near latitude 130 W, while the front bows out to the east
making landfall along the northern California/southern Oregon
coast Monday night. Ahead of the front, roughly 50 to 65 kt
850-700 mb flow is forecast to be present (according to the 12Z
GFS), parallel to the coast up north and a little more
perpendicular to the coast farther south toward San Francisco bay.
Precipitable water values are expected to peak in the 1.0 to 1.1
inch range along the coast but wind and PWAT values decrease as
the progressive front continues down the California coast during
the day on Tuesday. 12Z NAM_nest...which may be a worst case
scenario....supports hourly rates peaking in the 0.25 to 0.50+
in/hr range Monday night into Tuesday, but the progressive nature
of the anomalous moisture flux should limit any widespread
flooding concerns with no risk on the excessive rainfall outlook
at this time. For what it`s worth, the 12Z NAM_nest 24 hour QPF
ending 00Z/14 shows localized 3-5 inch totals from Coastal Ranges
north of North Bay into the Santa Cruz and Santa Lucia ranges, but
it tends to have a high bias in the terrain and limited to zero
instability should preclude these higher amounts given the
progressive nature of the boundary. WPC expects areal average
values for these ranges to be in the 1 to 2 inch range, locally
higher. Two-day areal average liquid equivalents for the Sierra
Nevada ranges are expected to peak in the 3-6 inch range
supporting a combination of lower elevation heavy rain and
mountain snow.

A second notable shortwave spoke is forecast by the 12Z NAM/GFS to
near the northern California coast on Wednesday along with a burst
of heavier precipitation, but given the ECMWF/UKMET/CMC show less
support for this feature a blended approach was taken at WPC.

Weakly anomalous, +1 to +2 standard deviations above the mean, are
expected to be transported inland beyond the Sierra Nevada into
the Great Basin with strongly diffluent upper level flow and left
exit region divergence setting up atop the surface cold front.
This general idea continues downstream, but moisture availability
and dynamics weaken into the downstream ranges of the Rockies and
Wasatch, still resulting in moderate to locally heavy rain/snow
for a few inland locations of the Intermountain West.

...Carolinas to New England...

Two main upper waves are expected to impact the Northeast U.S.
early this week with a smaller scale shortwave swinging out across
the Mid-Atlantic coast while acquiring a negative tilt Monday
night, while a larger scale mid-level trough reaches down to its
northwest over the northern Mid-Atlantic states. Models continue
to struggle with the details of a strong coastal low, which is
expected to continue strengthening northward through the western
Atlantic Monday night, with the latest consensus taking the
surface low just southeast of the 40N/70W benchmark Tuesday
morning. The 12Z ECMWF/UKMET adjusted back to a low a bit farther
offshore compared to the 12Z NAM/GFS which are closer to the
coast. WPC leaned in the middle of these, but with more weight
toward the 12Z NAM/GFS solutions given better ensemble support for
this track. The 12Z CMC also agrees more with the 12Z NAM/GFS
versus the 12Z ECMWF/UKMET. Future wobbles in the low track are
expected which will have major impacts on where the heaviest
liquid equivalents fall, with the max of 2-3 inches currently
expected over southeastern Massachusetts. The 12Z NAM appeared too
heavy here, given the expected steady movement to the surface low
and duration/degree of moisture wrapping back around to the west.

As the surface low nears offshore New England, there is a clear
model signal for a deep layered frontogenesis arcing from
south-southwest to north-northeast over southeastern New England
between roughly 06Z/13 and 00Z/14. Strong banding with potential
convective elements appear likely, but the exact location of this
heavy precipitation axis is still uncertain. A strengthening
deformation axis should impact eastern and northern New England
while light to moderate snow falls farther west into the interior
Northeast. 850-700 mb winds of 30-40 kt inland and peaking in the
60-70 kt range along the coast will support orographic influences
as winds back from southeast to east and eventually northeast.

The larger scale upper low referenced in the first paragraph of
this discussion will ensure plenty of cold air to support lake
effect/enhanced snow showers downwind of the Great Lakes with a
focus southeast of Lakes Erie and Ontario and wrap around moisture
into northwestern New England. Further info on snowfall can be
found in the QPFHSD discussion.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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