Quantitative Precipitation Forecast
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FXUS04 KWBC 160818

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Wed May 16 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 16/1200 UTC thru May 19/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Mid-Atlantic/Central and Southern Appalachians...

Southerly flow along the western periphery of the Bermuda high and
ahead of a weakening trough lifting out of the eastern Gulf of
Mexico will continue to channel deep moisture northward through
the region.  This moisture along with weak mid-level energy
lifting out ahead of the trough is expected to support some
orographically enhanced rainfall totals along the eastern slopes
of the southern to central Appalachians.  Guidance shows a fairly
good signal for south to north oriented training convection
producing some locally heavy amounts along the favored terrain
from far northeast Georgia and western North Carolina, to at least
as far north as southwest Virginia later today.  As energy
continues to lift north, weak surface to low level wave
development will enhance moisture convergence, which along with
some support from the right-entrance region of the upper jet, may
generate another round of heavy rains this afternoon and evening
from central and eastern Virginia to along a stationary boundary
draped across the northern Mid-Atlantic.


Onshore flow and low level convergence east of the surface wave
associated with the previously noted upper trough is expected to
support some moderate to heavy amounts across the eastern Florida
panhandle into southern Georgia today.  Heavy rainfall threat is
expected to wane by early Thu as the low weakens and lifts further

...Pacific Northwest/Northern California/Great Basin/Northern

A weak mid-upper level low is forecast to drift northeast from the
northern Great Basin into the northern Rockies this today.
Guidance shows PWS increasing to around an inch (over 2 standard
deviations above normal) near the center.  This moisture along
with daytime heating is expected to support scattered storms with
the potential for locally heavy amounts from central Oregon
northeastward into western Montana.  Meanwhile, an upstream low is
forecast to move into northern California, fostering additional
precipitation further south and west across northern California
into northern Nevada late Wed into early Thu.

Days 2/3...

...Pacific Northwest/Great Basin/Northern Rockies...

A long wave trough trundling across the Intermountain West during
Days 2 and 3 provides synoptic scale ascent for convection each
day. Deeper moisture and better instability over the northern
Rockies could result in heavy to excessive rainfall during Day 2,
while upslope flow south of a frontal boundary crossing the region
on Day 3 could focus convection for locally heavy rainfall. There
was generally good model agreement with the synoptic scale
features, so the WPC QPF was based on a blend of the 00z

Day 2...
As the long wave trough moves slowly eastward across the Great
Basin and Rockies during Day 2, and surface high pressure builds
south from western Canada, a low level southeast flow becomes
focused on the Northern Rockies. Model soundings over northern ID
and nearby western MT showed MUCAPE values in excess of 500 J/KG
in these areas between 17/18z and 18/03z. The flow associated with
the long wave trough keeps precipitable water values between
0.75/1.00 inches (which is about two standard deviations above the
mean), and the resulting convection is expected to produce an axis
of 0.75/1.25 inches of QPF over far northern ID and the Rocky
Mountain Front Range in MT. Three hour flash flood guidance values
are as low as an inch, and the National Weather Model showed high
stream flows, so a Marginal Risk was placed here for Day 2.

Day 3...
The long wave trough continues to move slowly across the Rockies
during Day 3, briefly spawning a closed mid level low over the
Northern Rockies. The synoptic scale ascent associated with the
mid level system , as well as marginal instability, is expected to
produce mainly diurnally driven convection across the higher
terrain of the Intermountain West. The best instability is
expected to stretch from southern ID into eastern WY, and local
0.50 inch QPF amounts are possible over the higher terrain.

...Central and Northern Plains...

Moisture and instability streaming north on an increasing low
level flow will become focused on a frontal boundary extending
from the CO plains into the Upper MS Valley during Day 2,
producing convection with heavy to locally excessive rainfall. As
the frontal boundary changes orientation, the focus for the
instability and moisture extends from the western High Plains into
the Upper MS Valley and far western Upper Great Lakes. While there
was generally good agreement with respect to the placement of
synoptic scale features, there are some differences in the
placement of the higher QPF amounts. With this in mind, the WPC
QPF was based on a blend of the 00z ECMWF/GFS/NAM.

Day 2...
A 30 to 40 knot low level southerly flow transports 1.25 inch
precipitable water air across the Central and Northern Plains,
ahead of a long wave trough moving slowly across the Rockies.
Model soundings showed a large reservoir of 1000+ J/KG of MUCAPE
available much of the afternoon/night. Convection is expected to
initiate in the axis of moderate to strong instability extending
across eastern CO/western and central NE, where local 1.50 inch
QPF amounts were placed. Both the 00z ECMWF/GFS showed local QPF
amounts in excess of 2.00 inches along this axis, but in different
locations. The storms are expected to be outflow dominated after
18/00z, but after that, local training could result in a flash
flood risk. Thus, a Marginal Risk was placed here to account for
the threat.

Further north across the Northern Plains, the moisture and
instability becomes focused on a frontal boundary extending from
central MT into ND and western MN after 18/00z. There is some
model signal for a cluster of storms or an MCS to form on the
front and track eastward. Depending on the placement of the front
(as there is still some spread concerning this), some of the
instability could be elevated. In any case, there is a multi model
signal for 1.50/2.50 inches of QPF over southern ND into northwest
MN. Three hour flash flood guidance values are as low as 1.00/1.50
inches along the ND/MN border, and the National Water Model showed
high stream flows before 18/12z. Because of this, a Slight Risk
was placed over much of southeast ND for Day 2.

Day 3...
As the long wave trough moves slowly eastward during Day 3, the
axis of deeper moisture and instability extends from western NE
across eastern SD into northwest MN, along and southeast of the
frontal boundary. Moderate to strong instability remains east of
the front, which should aid developing convection track northeast.
There was a strong model signal for an axis if 1.00/1.50 inches of
QPF over the aforementioned areas, and based on three hour flash
flood guidance values, a Marginal Risk was placed here on Day 3.

The exception is over far western NE/far southeast WY, where there
was a stronger model signal for 2.00/2.50 inches of QPF. Both the
00z NAM/GFS showed local 4.00+ inch QPF amounts here. Training of
cells would need to occur for these amounts to be realized, which
is possible near the front. Based on the above, a Slight Risk was
placed here for Day 3.

...TN/OH Valleys into the Mid Atlantic/Northeast...

Tropical moisture from the Gulf Coast states and the Southeast
becomes focused on a slow moving frontal boundary extending across
the TN/OH Valleys into Mid Atlantic during Day 3, moving slowly
into the Northeast states during Day 3. Deep moisture and
instability will feed convection that produces excessive rainfall
each day. There are still some differences in the placement of the
frontal boundary (and its associated instability), especially
during Day 2. The 00z EMCWF/UKMET seem to have the best handle on
the frontal position, keeping it further south than the 00z
NAM/GFS. Based on this, the WPC QPF was based on a multi model
blend, weighed more heavily toward the 00z ECMWF/UKMET.

Day 2...
Ongoing convection across the Mid Atlantic early on Day 2 should
weaken early as it exhausts any remaining instability. After that
time, a frontal boundary extending from the OH Valley across the
Mid Atlantic states will waver, as surface high pressure builds
across Quebec, and tropical moisture (with precipitable water
values near 1.75 inches, which is between two and three standard
deviations above the mean) continues streaming north in
association with a broad long wave trough extending from the OH
Valley to the Gulf Coast. A south southeast inflow increasing to
20 to 30 knots along and ahead of the surface front, which lifts
slowly north after 18/00z.

There was a fairly strong model signal for 2.00/3.00 inches of QPF
across eastern WV into northeast MD, though there are some
important differences in the model placement. As mentioned
earlier, a more southern solution is preferred with this front, as
instability will be the key to how far north the highest QPF can
get during the last part of Day 2. An axis of 2.00/2.50 inches was
stretched across north central VA into central DE, with another
axis of 1.50/2.00 inches extended down through the Southern
Appalachians into western NC.

Given the low flash flood guidance values over WV/nearby western
VA, a Moderate Risk was considered for this area for Day 2.
However, since there is still some important model spread on the
placement of instability and higher QPF, this was eschewed this
morning, in favor of a Slight Risk extending from west central NC
across eastern WV, much of northern VA/MD into central and
northern DE. A Marginal Risk was placed north of here to account
for model spread.

Day 3...
While there is better model agreement with the placement of the
front across the northern Mid
Atlantic/Northeast on Day 3, much of the 00z guidance may still be
too far north with the front. The surface high exits early in the
day, which could allow the front to move northward, but
instability could still be the key to where the best convection
occurs. Some of the convection is expected to be elevated,
especially across southeast PA across central NJ into the NYC
Metro area, but the depth of the tropical moisture could allow
elevated convection to produce heavy rainfall.

An axis of 1.75 to 2.25 inches of QPF was placed along this axis.
Three hour flash flood guidance has been lowering during the past
couple of day due to rounds of convection. The National Water
Model showed stream flows running high in these areas, so a Slight
Risk was placed in these areas for Day 3.


Deep tropical moisture associated with a mid level and surface
system tandem combines with in situ instability to produce
convection capable of heavy to locally excessive rainfall each
day, with the best chance of excessive rainfall over the east
coast of FL. There was generally good model agreement with the
placement of the synoptic scale features, as well as the higher
QPF amounts, so the WPC QPF was based on a multi model blend.

Day 2...
A broad long wave trough over the Gulf Coast and Southeast states
continues to focus tropical moisture and moderate instability
across the region during Day 2. Mid level forcing associated with
the long wave trough is expected across the Mid Atlantic states,
so much of the convection here is expected to be diurnally driven.
For the most part, QPF amounts are expected to remain below 1.00
inch, except where local training can occur with slow cell

The exception here may be the east coast of FL, where the mid
level flow parallels the coast. A 20 to 30 knot low level
southerly flow continues to supply 1.75+ inch precipitable water
to the coast, where model soundings showed moderate instability.
Much of the convection is expected to be diurnally driven, with an
area of 0.50/1.00 inches of QPF placed here. However, training
could produce local amounts over 3.00 inches over some of the
urban areas from Miami up through the Space Coast. Because of
this, a Marginal Risk was placed here for Day 2.

Day 3...
Similar conditions are expected during Day 3 across the area, with
the best low level flow expected once again over the east coast of
FL. Since training is possible over the urban areas of the FL east
coast, a Marginal Risk was placed here for Day 3.


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


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