Quantitative Precipitation Forecast
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FXUS04 KWBC 192050

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
449 PM EDT Mon Mar 19 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 20/0000 UTC thru Mar 23/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern U.S...

A very dynamic southern stream shortwave will move through the
Tennessee Valley today, inducing a widespread precipitation event
from the Plains to the east coast...and some concentrated severe
weather per the Storm Prediction Center Day 1 Convective Outlook -
associated with a well defined frontal wave and the extensive
back-bent occluded front. Areal average precipitation amounts will
be greater north of the warm front and along the occlusion, where
low level WAA/moisture convergence is maximized. Down along the
front and into the warm sector storms will be moving quickly, with
70-80 knot westerly 500 mb flow...thus QPF will generally be less
in the warm sector. Overall this still looks like more of a severe
than flash flood event...with storms expected to be progressive in
nature. However, still appears like isolated 2" amounts in an hour
or two remain possible...which may approach FFG over the risk
area. Could see some 1-1.5" totals further east into portions of
NC and southern VA with embedded heavier convective
cores...although instability here is weaker and elevated in
nature. Thus not currently thinking rates will be high enough to
pose a flash flood risk here. Further north across the Mid
Atlantic and OH Valley precipitation will be stratiform in nature.

Convection has expanded in coverage over the last few hours over
the FL Panhandle...as suggested by earlier HRRR runs and the 12z
runs of the ARW and ARW2. This activity is forming ahead of a weak
mid level wave and on the nose of an enhanced axis of 850 mb
moisture transport. Storms will remain quick moving, however a
broad area of convective development will allow for some repeat
convection. Some areas of the FL panhandle saw several inches of
rain overnight, and with the potential for localized 1-4"
additional rain today...some flash flood issues could arise in
more susceptible locations.

For QPF WPC utilized a multi model blend, incorporating both the
latest global models and high res CAMs. The one consistent trend
seen amongst all guidance was to push the northern extent of the
precipitation shield over IN/OH further north. This makes sense
given the track of the well defined shortwave currently over MO.
QPF was also shifted a bit north across the northern Mid Atlantic
given the consistent trend seen. From the southern Mid Atlantic
into the southeast changes from continuity were more minor...with
the general idea the same from our previous forecast...just some
of the details changed based on the latest consensus. Over FL,
preferred sticking closer to the ARW/ARW2...which both appeared to
have a better handle of the ongoing convection over the Panhandle.


Rain will begin to move into central and southern CA by the end of
the day 1 period...although the heavier rain should hold off until
Tuesday night and last through mid week. Some model differences
were already apparent by 0z Wednesday...with the GFS and high res
CAMs quicker to bring in locally heavy rain onto the coast.
Meanwhile the 0z ECMWF and 12z GEM regional were similar in
depicting a slower and further north axis through day 1. For now
opted to split the difference and go with a consensus in between
the two solutions.

Days 2/3...

...West Coast/Rockies/Great Basin...

Ahead of a strong mid level system approaching the West Coast
during Days 2 and 3, deep moisture associated with an atmospheric
river will affect much of CA, especially from late day 2 and much
of day 3. The deep moisture and strengthening upslope flow over
portions of central and Southern CA could result in significant
rainfall and the potential for flash flooding, as well as
mudslides and debris flows in burn scar areas.

While there is fairly good agreement among models with respect to
the overall synoptic setup, there is still some spread concerning
where the moisture becomes focused. In an attempt to mitigate some
of the differences (mainly between the latest runs of the
ECMWF/GFS), the WPC QPF was based on a blend of these models and
some of the ensemble means, weighted more heavily toward the 12z
ECMWF and NAM.  Was able to incorporate some of the 12Z UKMET in
the 00Z HPC QPF package after it shifted towards the EC/NAM/GFS
consensus position.

A closed mid level low located near 35N 135W at the start of Day 2
evolves into a positively tilted long wave trough as it moves
closer to the CA coast by the end of the period. Ahead of the long
wave trough, there is a multi model signal for deep moisture
associated with a well advertised atmospheric river (a feature
which the GFS has been showing for a few days now) to cross
southwest CA.  Precipitable water values near 1.25 inches (which
is between four and five standard deviations above the mean) are
ushered ashore by a 20 to 30 knot low level southwest flow.

The flow become more orthogonal with time to the Transverse Ranges
in Southern CA, resulting in a better upslope component by 21/12Z.
Model soundings off the southwest CA coast showed marginal
instability in the moisture plume, with the best synoptic scale
lift expected late in the period. The ingredients are apparently
coming together for a significant rainfall event for portions of
Southern CA, which most 00z model solutions
showing some version of the above occurring, yielding between 1.00
and 1.50 inches of qpf along the southwest CA coast.

The strong positively tilted long wave trough off the West Coast
approaches the Pacific Northwest during the first half of day 3.
A strong mid level closed low forms off the WA coast, and the low
level southwest flow ahead of the mid level and surface low
transports 0.75 inch precipitable water air over the western
WA/western OR. The best upslope flow occurs over the OR Cascades,
where local 0.75 inch qpf amounts are expected. Some of the
moisture survives the trip over the terrain into the Northern
Rockies, where upslope flow produces local 0.25/0.50 inch qpf
amounts over the Blue Mountains in OR, the Sawtooth Range in ID,
and the Grand Tetons of WY.

Further south, the low level southwest flow becomes focused on
central and Southern CA ahead of a cold front associated with the
low off the Pacific Northwest coast. The best lift associated with
pieces of short wave energy in the mid level flow occur over the
central CA coastal range into the Transverse Ranges in Southern CA
after 22/00Z. The low level southwest flow increases to 30 to 40
knots, transporting 1.25/1.50 inch precipitable water air (which
is approaching five standard deviations above the mean) from the
central CA coast to the mountains north of LA.  The low level flow
becomes orthogonal to the terrain, resulting in strong upslope
component, putting the ingredients in place for excessive rainfall
over a large area.

The model QPF and the signal for excessive rainfall remained solid
so no changes were made either to the QPF or the previously-issued
excessive rainfall outlook.

...OH Valley/Mid Atlantic/Northeast...

Short wave energy tracking out of the Northern Plains and short
wave energy in the southern stream crossing the Lower MS Valley
aid in carving out a long wave trough extending from the OH Valley
into the Mid Atlantic states during Day 2, then takes on a
negative tilt over the Northeast by early day 3.

The mid level lift supports the development of a pair of surface
lows impacting the northern Mid Atlantic and Northeast. There is
still a fair amount of model spread concerning the evolution of
the mid level systems, but one consistent signal in the 19/12Z
guidance was a northward/westward shift in the low track and
associated QPF.  Overall, the models have been struggling with the
evolution of the system and it can not be ruled out that there
will be another shift the other way.  But nudging the WPC QPF that
way should put the forecast in a position where subsequent changes
should be fairly minor.

Day 2...
The first short wave in the northern stream tracks toward the Mid
Atlantic coast by the start of the day 2 forecast.  Banding of
precipitation associated with the surface low looks as though it
will remain along the DE/southern NJ coast, where an area of 0.50
to 1.25 inches of qpf was placed. Further west, a weakening
surface low over OH/WV and its frontal boundary serves as a focus
for moisture ahead of the short wave crossing the region, and a
large area of 0.25 to 0.50 inches of qpf was placed over
OH/KY/eastern TN across much of VA/MD.

As the southern stream short wave reaches the Mid Atlantic states
on day 2, it helps develop a negatively tilted long wave trough
extending from the OH Valley to the Mid Atlantic coast at the
start of day 3. A second surface low forms off the NC coast early
in the period, then tracks northeast with a closing mid level
system as to moves near the New England coast.

The WPC QPF used the 12Z GFS/12Z NAM/00Z and 12Z ECMWF along with
the ECMWF/GFS ensemble QPF.  This resulted in a stripe of heaviest
precipitation from parts of eastern PA northeastward across
southern New England.  Changes in the western edge of the more
significant QPF could occur with subsequent model runs.


Deep moisture and strong instability streaming ahead of a cold
front crossing FL early on day 2 that will feed late-day
convection along and ahead of the front.  Given that the models
were in generally good agreement in terms of the timing of the
front and its departure from the FL peninsula, a general model
compromise was used.  Because of the speed of the convection,
flash flooding is not expected with the storms.


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


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