Quantitative Precipitation Forecast
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FXUS04 KWBC 182226

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
626 PM EDT Sun Mar 18 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 19/0000 UTC thru Mar 22/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Central / Southern U.S...

A rather active Sat evening through Sun from the Central Plains to
parts of the South/Southeast into northern FL. Multiple streaks of
heavier rainfall is anticipated over the next 24 hrs, as a lead
subtropical short wave streams through the southern tier of the
country, only to be followed up by a dynamic southern stream
trough. The combination of the two features will produce
widespread qpf from KS to FL/GA.

The subtropical short wave has already produced clusters of
showers and storms across LA into MS and southern AL today. Rounds
of convective clusters should persist, as the short wave arrives
and moisture pools near a surface stationary front Sat evening
into perhaps the overnight hrs. The latest global models and
hi-res suite indicate the prime spot being southern GA/FL
panhandle where increasing instability is likely and possible
areal avg 1-2 inch amounts with locally higher values.

Meanwhile, a vigorous short wave, moving through the Four Corners
region today, will eject overnight into the central/southern
Plains. The guidance indicates broad swrly upper difluent flow
ahead of the negative tilt short wave for synoptic scale heavy
rains across eastern/central KS. This area of enhanced vertical
lift invof of the 500mb vort will progress downstream through
central/southern MO into the TN valley before tomorrow aftn and
possible active convection near the wedging frontal zone from the
TN Valley into AL/western GA.

Overall the guidance was very similar with mass fields and WPC
felt comfortable using a blend of 60% hi-res guidance, 20% global
models (GFS/ECMWF) and 20% NBM (National Blend of Models)

...Western U.S. Including the Central Rockies up through the

Scattered areas of mainly light qpf is anticipated from the
Northwest through the northern Inter-Mountain west into the
northern Plains and Upper Midwest. Not much of an upper flow
regime with a closed low hovering just off the PacNW coast and
over the northern Plains, while an inverted trough extends from
the developing Plains cyclone into NE and the Dakotas. This is
where the most organized qpf is anticipated of up to a .50 inch,
some snow and rain mixed with snow. Overall WPC just followed a
blend of the latest model guidance.

Days 2/3...

...California into the Pacific Northwest..

A powerful upper low across the open waters of the eastern Pacific
will feature 500-mb height anomalies around 4 to 5 sigma below
climatology. Broad southwesterly flow should extend downstream
into the West Coast, while being accompanied by a well defined
subtropical moisture plume. This atmospheric river setup has the
potential to bring prolific rainfall amounts into Southern
California as well as over the Sierra Nevada ranges. Considering
the GFS modeled water vapor transport forecast, the IVT values
exceed 800 kg/m/s on Wednesday morning with the best moisture
fluxes taking aim at southern sections of California. Based on the
latest guidance, there is solid low-level support across the
region with a 35 to 40 knot 850-mb jet advecting 1.50 inch
precipitable water values toward the coastline. Additionally, such
support exists in the upper levels of the troposphere as the
southwestern U.S. will be on the right entrance region of a
powerful 250-mb jet streak. All and all, most parameters are in
place for a heavy rainfall producer on Day 3 onward. Multiple
inches of rain are likely across Southern California which would
easily be up to 25 percent of many locations annual rainfall
budget. This will sufficiently enhance the threat for flash
flooding and mudslides, particularly over the local burn scars. As
such, a slight risk area continues to be advertised on Day 3,
ending 21/1200Z across this region with a likely upgrade in future
days. The overall preference was primarily focused on impacts over
California although light precipitation does break over the
Pacific Northwest as well. The past several runs of the GFS appear
to be plagued by feedback issues, per the 2 to 2.50 inch amounts
in 6-hours which seems highly unreasonable. Generally took a
combination of the 00Z ECMWF/UKMET solutions which were slower and
a bit more conservative in comparison.

...Northern to Central Plains/Middle and Upper MS valley...

Multiple weak shortwaves are forecast to slip eastward across the
Dakotas into the Upper Midwest on Monday evening into Tuesday. A
broad area of light to locally moderate snowfall will likely break
out along a north/south oriented inverted trough extending into
the north-central U.S. While expected amounts are on the lighter
side of the equation relative to locations across west and east
coasts, respectively, multi-cycle trends show an uptick in the
overall intensity across the Upper Midwest into the Middle
Mississippi Valley. This allowed amounts to be bolstered a tad
relative to continuity. A blend of the 06Z GFS and 00Z ECMWF was
sufficient to handle such trends favoring a 24-hour QPF above 0.25
inches across southwestern Minnesota ending Tuesday evening.

...OH and TN Valleys/Mid Atlantic/Northeast...

A pair of systems will need to be monitored during the period with
the initial one being a flattening wave migrating from the Ohio
Valley into the mid-Atlantic on Tuesday morning. In spite of the
impulse being in the process of deamplification, a well defined
area of low pressure will sweep eastward along the Virginia/North
Carolina border with models showing better clustering than
previous days. Strong lift with the attendant shortwave will drive
a band of quick hitting precipitation through the mid-Atlantic on
Tuesday with some wintry p-types expected given appreciable
low-level cold air streaming northward from a dome of high
pressure to the north-northwest. The most concentrated activity of
heavy rainfall should be across southeastern Virginia, generally
closer to the parent surface low center. The approach here was to
follow a consensus of the 12Z GFS and 12Z ECMWF/UKMET with the
latter pair of solutions being a tad slower though.

On the heels of this system exiting the Delmarva Peninsula, the
more robust trough ejecting out of the southeastern U.S. has
proven to be a system to watch given large trends in recent model
cycles. Over the past day of runs, a vast majority of the
operational/ensemble guidance have made large west/north shifts in
their low track and corresponding QPF output. This particular
trend of course now brings measurable precipitation into the I-95
corridor across the northeastern U.S. Given plenty of cold air
already in place at the surface, wintry precipitation will be an
issue over this region. Please view the latest heavy snow/icing
discussion (QPFHSD) for more information on this threat. Decided
to make a northward adjustment in the updated QPF package which
nudged the activity northward as noted in the 12Z ECMWF/UKMET
solutions. A lot can change in the upcoming packages given how
quickly the models have navigated about this storm.

...Southeastern U.S...

Modest height falls sweeping through the Red River on Monday
evening will accelerate east-southeastward toward the Florida
panhandle during the following 24 hours. A marked increase in
amplitude of this shortwave trough is anticipated with seasonably
strong 500-mb winds in excess of 80 knots sweeping across the
Sunshine state. Strong dynamical lift with this system in
conjunction with ample buoyancy will support organized
thunderstorms pushing through the southeastern U.S. on Tuesday.
Based on the Day 3 outlook from the Storm Prediction Center, some
of these storms may be severe in nature with an enhanced risk
currently in place. While damaging winds should be a major issue
with cells that ignite, a heavy rainfall threat will also ensue
given the high precipitable water environment these storms will
have access to. Given the individual clusters should be booking, a
flash flood threat is not likely unless there are instances of
repeat convection. The manual QPF across the region was generally
driven by a split of the 06Z GFS/00Z ECMWF given agreed upon
timing and the preference to be on the wetter side of the guidance.


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


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