Quantitative Precipitation Forecast
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FXUS04 KWBC 152258

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
657 PM EDT Sun Apr 15 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 16/0000 UTC thru Apr 19/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Southeast into the Mid Atlantic...

A very dynamic, highly amplified spring system continues to
migrate eastward, with a well-defined, elongated QLCS progressing
through the eastern U.S. this afternoon. Deep-layer instability,
largely surface-based, continues to be on the uptick ahead of this
line over the southeast and lower mid Atlantic region, while the
increasing upper level divergence ahead of the trough (especially
as the trough becomes more negatively tilted) should maintain the
widespread coverage of moderate-heavy rainfall through the new day
1 period. In general the convective line will remain progressive
in nature, likely limiting the potential for any extreme rainfall
amounts. However, deep layer mean winds parallel to the line
supports at least some south to north training of cells embedded
within the line. Also, convection has shown a tendency to form
ahead of the true cold front, thus the potential exists for
additional storms to fire on the true cold front as well, which
would result in multiple rounds of locally heavy rainfall in any
one location. Overall, this looks like a widespread 1-3" rainfall
from the Southeast into the Mid Atlantic region...with the
aforementioned training and repeat cell potential allowing for
localized 3-5" amounts. Impressive 850 mb moisture transport and
PWAT vales exceeding the climatological 90th percentile certainly
support these totals and the potential for high rates. Thus at
least some flash flood risk exists where any cell training or
repeat convection is able to occur.

The orientation of the line will likely be more favorable for
short term training and repeat cells from northwest NC into
portions of WV/VA/MD closer to the negatively tilting trough and
better mid/upper level forcing. Strong southerly flow ahead of the
low should help enhance rainfall totals some into the southern and
eastern slopes of the Blue Ridge and Appalachians as well. Thus we
maintain a Slight Risk of excessive rainfall from northwest NC
into eastern WV, western and northern VA into MD. It is across
these areas where the best chance for localized 3-5" totals exists
to go along with a bit lower FFG. We opted to include the
DC/Baltimore urban corridor as well. Dry conditions of late should
help, but this corridor is still more susceptible to flash
flooding, and we note several HREF members and the 12z extended
experimental HRRR showing localized 3-5" totals in the
vicinity...and most of this would fall over a short period. Thus
while not a sure bet, we think the conditional threat for flash
flooding is high enough to warrant the Slight Risk.

...Upper MS Valley---Great Lakes into northern NY and northern New

A surface low will continue to slowly track east across the OH
Valley. Areas to the north and northwest of this low will see
continued wintry precipitation across MN/WI/MI. WPC continued to
lean toward the slower model solutions in terms of lifting the
TROWAL east of the area -- which is close to a blend of the ECMWF
and UKMET along with the WPC in-house bias corrected ensemble QPF.
Significant, widespread winter weather hazards continue with this
dynamic spring system -- please refer to the latest QPFHSD for
further details.


A closed mid level low will move into the West today...with a
focused plume of moisture out ahead of it producing some 1-2" QPF
amounts from far northwest CA into southwest OR, the OR Cascades,
and the CA Sierras. WPC QPF ends up close to the HREF mean.

Days 2/3

...West Coast/Intermountain West...

At the start of the Day 2 period (0-6z Tues), the long wave trof
has come ashore generally neutrally tilted centered along 120W.
The northern portion of the wave/vorticity has progressed into
southern Canadian Rockies with a trailing trof to the base of the
large scale trof...crossing the Absaroka Range to just west of the
Tetons.  The remaining limited mid-level moisture will slowly
progress eastward though orientation to the terrain supports
decent orographic enhancement particularly through the
Wasatch/Unita/Wind River and Big Horns where broad .25 to .5" are
expected over higher terrain with a few spots in the .6" range in
most ideally orientated positions (such as S Big Horn Range and
southern Wasatch).  Mainly focused on a GFS/UKMET/ECMWF blend
enhanced by the 12z NMMB mainly across UT to WY for 00-12z on
Tues...though eventually shifting a bit toward slower solutions of
the ECMWF/UKMET toward Wed as the base of the wave exits into the

Strong onshore flow in the wake of the northern shortwave will
support westerly flow of 30kts to 45ks at 85-7H at or near
orthogonal to the Cascade range and though moisture is fairly
limited WPC forecast QPF are still in the range of 1.0" through
00z Wed in Washington and .5" in Oregon.

Heavy snowfall is possible across the higher terrain of the West
Coast and Intermountain West during Days 2 and 3. Please refer to
the latest suite of WPC Winter Weather products for more

End of Day 3...
A QPF respite is expected across much of the Northern
Intermountain West as the base of the large scale trof ejects into
the central Plains late Tuesday into early Wed.  However, another
compact wave is already approaching the West Coast aiming toward
the OR/CA coast with broad troughing extending north into Western
Washington toward midday Wednesday.  Light orographic ascent will
affect the Olympic range with up to .10" by the end of the
forecast period; however the focus will be with southwesterly
flow/warm conveyor ahead of the wave by 18z Wed leading to
increased mid-level shield precipitation across SW Oregon before
expanding north and eastward. The main forcing/QPF is expected to
reach NW California toward 00z Thurs.  The models have come into
better agreement over the last 3-4 cycles though a recent slight
southward/strong shift was noted in the 12z GFS.  This reduced
some of the higher confidence in a higher confidence non-UKMET
blend, but still the agreement is tight enough to continue to
support this blend.

...Northern and Central Plains/Upper MS Valley...

The base of the long wave trof progressing through the West Monday
into Tuesday, emerges early Wednesday into the Central Great
Plains with a negative tilt orientation.  This spurs fairly rapid
surface cyclogenesis and rapid precipitation development across
Nebraska to N Iowa midday Wed, eventually occluding late in the
day and past 00z Thursday.  The 12z GFS did trend slower but run
to run spaghetti plots show it remains the fastest solution and
was discounted from preference.  The 12z NAM, like its
predecessors was also quite cold in the lower thermal profiles and
supports the fastest deepening, typical of a known negative bias
at Day 3.  The 12z ECWMF shifted a bit faster and matches the
UKMET within the slow to center of guidance packing, but also
trended a bit deeper/stronger.  Strong MUCAPE to 1000 J/KG will
support some elevated convective nature to TROWAL of the system
enhancing QPF from 09-18z before reducing in intensity with the
occlusion of the low by 00z, leading to an area of 1.0 to 1.5" QPF
from NE Nebraska to NE Iowa.   At this time, it appears a portion
of the QPF will be wintry in nature (snow/ice/mix) and likely
reduce any flooding threat so no excessive rainfall area is

...Great Lakes/Northeast...

The large closed low will continue to progress through the eastern
Great Lakes/northern Mid-Atlantic with the surface wave/triple
point refocusing in a tricordal wobble in the NH/ME border
vicinity.  This allows for a slight slowing of eastward
progression of the deepest southerly moisture flux into the higher
terrain of western ME, where multiple model signal remains for QPF
of 2.5".  While much of the instability will be exhausted (less
than 50 J/KG), the IVT values remain will above 2-3 StDevs of
normal and has the potential for inundation flooding (vs. flash
flooding)...still the magnitude is high enough for the area to
remain within the Day 2 (12z to 12z Mon-Tues) Marginal Excessive
Rainfall Outlook.  WPC forecast was mainly driven by a blend of
the GFS/ECWMF/UKMET and some Hi-Res CAM guidance such as the

The southerly warm conveyor belt will shift out of the CONUS late
Tuesday though the vast nature of the upper low will support
internal vorticity centers returning across the Great Lakes into
northern NY/New England Tues into Wed increasing forcing/cooling
aloft to activate some NW to SE oriented Lake Effect Belts as well
as some weak comma head return moisture in N NH/ME by later in the
Day Wed.  Still this QPF is not substantial but please refer to
WPC Winter Weather products for more information.


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


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