Quantitative Precipitation Forecast
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FXUS04 KWBC 150815

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

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 15/1200 UTC thru Apr 18/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Southeast into the Mid Atlantic...

Convection is ongoing as of 06z across eastern MS into the FL
Panhandle. This convection should maintain as it moves across the
southeast today. As we head through the day instability should
increase out ahead of the line...with the trough to the west also
taking on more of a negative tilt with time. The combination of
this increasing instability and increasing upper level divergence
ahead of the trough...should allow for an intensification of the
line by mid to late morning into the evening hours. 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,
current convection is actually forming 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.

Would anticipate the line will remain most progressive over the
southeast...with the orientation of the line likely more favorable
for some training further north closer to the better mid/upper
level forcing. Thus kept our highest QPF amounts from the western
Carolinas northeast into portions of the Mid Atlantic. Strong
southerly flow ahead of the low should help enhance rainfall
totals some into the eastern slopes of the Blue Ridge and
Appalachians as well. Instability will be greater across
SC/NC...so certainly some chance the convective line really gets
organized there this afternoon...which could limit amounts a bit
further northwest over southwest VA. However, tough to get into
those details much ahead of time...but something to monitor. WPC
QPF leaned exclusively on the high res guidance for this warm
sector convection. Most of these solutions already seemed a bit
slow compared to the current radar...thus leaned more towards the
quicker high res solutions. Also incorporated some of the 0z HRRR
experimental, which was more robust with amounts over SC/NC than
the HREF component members were. As mentioned above, given the
better instability here, there is certainly a possibility that the
higher amounts do verify here.

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

A surface low will continue to slowly track east across the OH
Valley today. To the north and northwest of this low will see
continued wintry precipitation across MN/WI/MI. Did trend WPC QPF
a bit slower, with the closed low not as progressive as previously
forecast. This increases QPF amounts over portions of MN/WI. The
GFS may still be a tad quick with progressing precipitation
eastward, with the new 0z HREF closer in line with the 12z ECMWF
and 0z UKMET. WPC QPF is thus higher than the GFS solution, and
ends up pretty much in line with the HREF mean. Across northern NY
into northern New England saw better consistency with QPF amounts,
thus changes were generally minor here.

Days 2/3

...West Coast/Intermountain West...

A long wave trough crosses the West Coast during the first part of
Day 2, reaching the Rockies by the end of the period. The long
wave trough takes on a negative tilt as it heads for the Plains
during Day 3, as a short wave approaches the Pacific Coast late in
the period. For the most part, there was good model agreement with
the evolution of the mid level systems. so the WPC QPF was based
on a blend of the 00z ECMWF/GFS. Some 00z NBM was used to better
delineate higher terrain qpf amounts during Day 2.

Day 2...
As the long wave trough tracks from the West Coast to the Northern
and Central Rockies during Day 2, the dynamic lift associated with
the mid level feature, combined with upslope flow, is expected to
make the most of the moisture in the column. Closer to the coast
over WA/OR, the upslope flow is expected to focus the 0.50 inch
precipitable water air on the WA/OR Cascades, where there was a
multi model signal for 1.00/1.50 inches of qpf, with the highest
amounts over the northern WA Cascades.

Further south across CA and the Great Basin, the moisture is not
quite as robust (with precipitable water values generally between
0.25/0.50 inches). However the synoptic scale lift and upslope
flow over the Sierra Nevada range is expected to produce an axis
of 0.50/1.00 inches of qpf. The highest amounts are expected over
the central part of the range, where the low level flow is most
orthogonal to the terrain. Over east central NV and the Wasatch
Mountains in UT, local 0.25 inch qpf amounts are expected,
especially over the higher terrain.

Finally, some of the Pacific moisture survives to the Northern
Rockies during the first part of Day 2. Focused by an approaching
front, the upslope flow targets the higher terrain of the northern
Bitterroot Mountains in ID and the Rocky Mountain Front range in
MT. Local 0.75 inch qpf amounts were placed here, with lesser
amounts (generally else than 0.50 inches) over the Blue Mountains
in OR, the Sawtooth Mountains in ID, and the northern Absaroka
Mountains in MT.

Day 3...
The long wave trough moving from the Northern and Central Rockies
during the first part of Day 3 supplies sufficient synoptic scale
ascent for qpf across the region. Ahead of a frontal boundary
extending from western MT across WY into central and eastern CO,
the upslope flow makes the most of the dwindling moisture in the
column to produce local 0.75 inch qpf amounts across the northern
Bitterroot Mountains in ID and the Rocky Mountain Front range in
MT. Lesser amounts (between 0.25/0.50 inches of qpf) were placed
over the Grand Tetons and Wind River Mountains in WY, as well as
the Front Range in CO.

A short wave approaches the Pacific Northwest during the last part
of Day 3. Ahead of the short wave, a low level southwest flow
transports 0.50 inch precipitable water air to the WA/OR coast.
The upslope flow produces 0.25/0.50 inch qpf amounts across the WA

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

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

A negatively tilted long wave trough approaching from the Rockies
on Day 3 forms a closed mid level low over eastern
NE/western IA by the end of the period, as surface low pressure
tracks from the northern Rockies to the Central Plains. The 00z
GFS continues that models tendency to be too fast and too far
north with the mid level and surface systems. With that in mind,
the WPC QPF was based on a blend of the 00z ECMWF/NAM.

The southern end of a negatively tilted long wave trough tracking
from the Rockies to the Northern and Central Plains evolves into a
closed mid level low that reaches eastern NE/western IA by the end
of Day 3. Surface low pressure over WY early in the period becomes
better organized as the mid level system closes off, moving from
western KS into southwest IA between 18/00z to 18/12z.

Ahead of the surface and mid level systems, a 45 knot low level
south southeast flow transports 0.75 inch precipitable water air
north of a surface front over northeast NE into northwest IA.
Model soundings in the the increasing low level flow showed
500/1000 J/KG of MUCAPE in the same area, so elevated convection
is possible between 18/06z and 18/12z. A qpf bulls eye of around
an inch was placed over far northeast NE/far southeast
SD/northwest IA in associated with the increasing low level

At this time, it appears as though some of the qpf could fall in
the form of snow, which would affect the potential for local
runoff issues. For now, no excessive area was placed here due to
the precipitation type issues.

...Great Lakes/Northeast...

An elongated negatively tilted closed mid level tracks across the
Great Lakes and Northeast during Days 2 and 3, with surface lows
at each end of an occluded front that crosses the same area.
Strong inflow and deep moisture ix expected to produce heavy to
locally excessive rainfall across portions of southeast NY state
into New England during Day 2. Moisture wrapping around the
surface and mid level systems affects NY state and New England
during Day 3. The 00z GFS was again too fast and too far north
with its surface and mid level systems, so the WPC QPF was based
primarily on a blend of the 00z ECMWF/GFS. Some 00z NBM was used
to better delineate higher terrain qpf amounts during Day 2.

Day 2...
As the elongated closed low wobbles over northern NY state and the
Northeast on Day 2, a 65 knot low level southeast flow transports
1.00/1.25 inch precipitable water air (which is between two and
three standard deviations above the mean) from southeast NY state
across much of southern and eastern New England. The very strong
upslope flow focuses the moisture on the southern Green Mountains
in VT and the Berkshire Mountains between 17/12z and 17/18z,
producing an axis of 2.00 inches of qpf here. Some elevated
instability could increase rainfall rates briefly across eastern
NY state into southern and central New England, so a Marginal Risk
was placed over this area for Day 2.

As the occluded front associated with the closed mid level system
crosses eastern New England after 17/00z, the low level flow
strengthens to 80 knots, which produces a strong upslope flow
across the higher terrain of western ME, where there was a multi
model signal for 2.50 inches of qpf. Instability is nearly
non-existent as the low level flow reaches ME, so the heavy rain
is more likely going to result in a flood vs flash flood threat.
However, if rainfall rates increase enough, a low level flash
flood threat could develop in the presence of the strong upslope
flow. With this in min, the Marginal Risk for Day 2 was extended
across much of western and southern ME.

Day 3...
The elongated closed mid level low continues to wobble in place
over New England on Day 3. Moisture extends around the surface and
mid level systems across northern New England and northern NY
state. The highest amounts are expected across the Adirondacks in
NY state (with local 0.50 inch amounts). Elsewhere over northern
New England, qpf amounts are expected to remains between 0.10 to
0.25 inches. The exception could be over northern ME, where local
0.50 inch qpf amounts are possible over the rooftop of ME.


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


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