Quantitative Precipitation Forecast
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FXUS04 KWBC 171029

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
627 AM EDT Thu May 17 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 17/1200 UTC thru May 20/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...

...Mid Atlantic, Southeast and FL...

Widespread showers and thunderstorms will again be common today
across much of the Southeast into the Mid Atlantic. Broad
troughing over the area with embedded shortwaves and broad upper
divergence will provide the synoptic support for convection. PWATs
will remain well above normal, with much of the area above the
climatological 90th percentile, and some near record values for
this time of year. The broad synoptic support, well above normal
moisture, and ample instability...will all set the stage for
another day of locally heavy rains.

The most pronounced wave is across south GA as of 06z...and will
slowly track north northeast into the Mid Atlantic. In the lower
levels a west to east boundary is forecast to drift slightly
southward and set up across central VA into the Delmarva.
Anticipate the heaviest rainfall totals during the day 1 period
will end up focused near this boundary. As the aforementioned wave
approaches from the south, should see gradually increasing 850 mb
moisture transport into the front. The greatest instability should
also focus along/south of this front. Thus the expectation is that
the highest rates will generally be along/south of the front...and
the front should act as a focus mechanism for some backbuilding
and west to east training. Overall, the high res guidance was in
good agreement on this evolution...although some latitudinal
differences with the front, and thus max QPF axis, were noted. WPC
generally went for a middle ground solution, accounting for all
the 0z HREF members and the 0z Experimental HRRR. Ended up with a
swath of 3"+ near the front across central VA into the Delmarva.
The 0z HREF depicts moderate to high probabilities of 5"+
amounts...suggestive that localized amounts of this magnitude are
certainly a possibility. Given the stationary front, PWATS, and
forecast instability...localized amounts of this magnitude do seem

...Lower/Mid MS Valley, OH/TN Valley into the Central Gulf coastal

Main forcing here will be the presence of a mid/upper level shear
axis and embedded shortwaves. The presence of these features,
combined with moderate instability, should result in scattered to
widespread convective development. Weak shear will limit the
organization of storms, and most storms should thus be short
lived. However, will likely see some cell mergers given the
expected widespread nature of activity, along with some localized
organization along outflows. Also, weak mean storm layer winds
along the shear axis will also support slow moving cells. PWATs,
while not as anomalously high as further east, will remain above
average. All of this suggests, that while an organized widespread
flash flood threat is unlikely, isolated flash flooding will again
be probable today across this region. WPC did increase our QPF
amounts quite a bit from our previous forecast over this
area...and is generally in line with the 0z HREF guidance.

...Central and Northern Plains...

Moderate to high instability will develop along/ahead of a
boundary over the western Plains today. The approach of a
mid/upper level trough from the west will result in increased
upper level divergence over this boundary, and increasing 850 mb
moisture transport. These factors will combine to produce a
favorable environment for convective development by this
afternoon. The increased southerly flow will also result in
increasing PWATS, with values into the well above normal range by
thus evening. The combination of high cape/moisture suggest the
likelihood of intense storms capable of producing heavy rainfall
rates. In general think storms will be moving quick enough off to
the east to prevent any extreme rainfall totals. However does
appear like there may be two areas to watch for the potential for
locally heavier amounts. One across ND, where a stationary west to
east boundary could act as a focus mechanism for training for a
period this afternoon/evening. The other is across NE, closer to
the core of 850 mb moisture transport. The increasing low level
jet and moisture transport corridor this evening may allow for
some backbuilding of convection for a period over NE. Although the
corridor of moisture transport appears to progress enough to the
east with time to suggest that long duration training/backbuilding
is less likely.

WPC QPF across this region relied heavily on a blend of the 0z
HREF members, with a bit more weight given to the ARW members.

...Great Basin, Northern Rockies, MT...

Troughing over the area will continue to support widespread shower
activity into Thursday. Multiple shortwaves embedded within the
trough will help focus convective activity. PWAT values near these
shortwaves will be in the well above normal range. Thus locally
heavy rains will be possible with each wave, with at least weak
instability forecast as well. Thus anticipate locally heavy totals
across northern NV into southern ID, eastern WA into northern ID,
and much of MT. Anticipate mainly pulse convection given low
shear...although slow storm motions and some localized
organization along outflows will still allow for pockets of heavy
rains and some flash flood threat.

...Southern Plains...

Isolated to scattered convection is likely today ahead of the dry
line over TX, possibly aided some by the approach of the
subtropical upper jet. PWAT values above normal will support some
locally heavy rains. Not thinking we will see much in the way of
organization, but as usual, anticipate some brief repeat cell
activity will be possible resulting in some localized heavier

Days 2/3...

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

A long wave trough extending from the Northern Rockies to the
Southwest states at the beginning of Day 2 weakens and takes on a
negative tilt across the Pacific Northwest and Northern Rockies
during Day 3. Moisture and instability associated with the mid
level system feed convection that could produce heavy to locally
excessive rainfall, especially during Day 2. There was generally
good model agreement with the synoptic scale systems, so the WPC
QPF was based on a blend of the 00z ECMWF/GFS.

Day 2...
As the long wave trough over the Rockies weakens early on Day 2,
it begins to take on a negative tilts as it reorients itself over
the Pacific Northwest by the end of the period. High pressure
building south from west central Canada will focus the low level
flow on the Northern Rockies, where precipitable water values of
0.75/1.00 inches (which is about two standard deviations above the
mean) will feed convection developing in the marginal to locally
moderate instability. The convection is expected to produce areas
of 0.50 inches of QPF across the Blue Mountains in northeast OR
and nearby central and northern ID, mainly between 18/18z and

Three hour flash flood guidance values here are generally above
these values, but short term training could produce local amounts
over 1.00 inches (which is depicted by the 00z GFS) could result
in runoff issues. Additionally, the National Water Model is
showing high stream flows here, so a Marginal Risk was placed here
for Day 2.

Day 3...
As the mid level system weakens over the Pacific Northwest, any
synoptic scale forcing is expected to diminish. However,
instability remains in place, though the moisture profiles are not
expected to be as robust as Day 2. Convection developing over the
terrain could produce local 0.50 inch QPF amounts over places like
the Grand Tetons in WY, and the Uinta Mountains in UT, but
elsewhere lesser amounts are expected.

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

Short wave energy ejected from a weakening long wave trough over
the Rockies during Day 2 crosses a frontal boundary extending from
eastern CO into the Upper MS valley. Moisture and instability
along the front feed convection that produces heavy to excessive
rainfall. The short wave energy then crosses the Central Plains
into the Mid and Upper MS Valley during Day 3. There is generally
good model agreement during Day 2, but there is growing spread in
just how quickly the short wave energy crosses the region during
Day 3. The 00z ECMWF/UKMET are slower than the 00z NCEP guidance,
and based on trends, the WPC QPF was based more closely on the
non-NCEP guidance during Day 3.

Day 2...
Short wave energy exiting ND early in the period interacts with
moisture and instability along a frontal boundary extending from
the Upper MS Valley into the Central Plains to produce an area of
1.00/1.50 inches of QPF over much of eastern ND. While the
activity will be exiting, there could still be enough QPF for a
Marginal Risk early in the period, as three hour flash guidance
values are as low as 1.50 inches.

The potential for flash flooding is expected to shift southwest
along the front into central Plains and eastern plains of WY and
CO. Model soundings showed a plume of instability extending from
the southern High Plains into western NE/eastern CO and eastern
WY, peaking between 19/00z and 19/06z. The strong convection may
be outflow dominated at first, but as the column moistens (as a
low level southeast flow transports 0.75/1.00 inch precipitable
water air (which is about two standard deviations) into the

There was a multi model signal for 2.50 inches of QPF over
southeast WY and nearby western NE, with lesser amounts extending
into northern NE and far south central SD. Locally higher amounts
are possible where short term training exists. Three hour flash
flood guidance values 1.00/1.50 inches over far southeast WY, so a
Slight Risk was placed here for Day 2. A Marginal Risk was
extended over portions of the Central Plains to account for
activity along the front.

Day 3...
Short wave energy ejected from a weakening long wave trough over
the Rockies during Day 2 crosses the Central Plains and Northern
Plains, well as the Upper MS Valley, during Day 3. Lift with the
energy spins up surface low pressure over southeast CO, which then
tracks into KS and NE. There is increasing spread on the placement
of the surface low and front on which it forms. The 00z
ECMWF/UKMET indicated a slower solution, and may be onto a trend.
A slower solution would allow deeper moisture and instability for
form on the southeast side of the front extending from the surface
low into northeast MN and western MI.

The non-NCEP guidance placed a 2.00/2.25 inch QPF maximum over
northeast NE/southeast SD and far northwest IA in the deepening
moisture and instability, with local amounts over 4.00 inches. By
contrast, the NCEP guidance was much more scattered with its QPF
field, suggesting that the moisture and instability becomes
wrapped around the mid level system. For now, the non-NCEP
solutions were favored, and a Slight Risk area was placed in the
aforementioned area. However, given the spread in the guidance,
changes are possible (with the placement of the highest QPF, as
well as the Slight Risk area) with subsequent forecasts.

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

Deep moisture streaming northward from the Southeast states and FL
rides up and over a frontal boundary extending from the OH Valley
across the northern Mid Atlantic states during Day 2. There is
still some spread concerning just how far north the front gets
during Day 2, and instability parameters suggest that a more
southerly solution is better than a more northerly solution. With
this in mind, the Day 2 WPC QPF was based on the more southerly
00z GFS/ECMWF guidance. On Day 3, the front moves north across New
England as short wave energy drives a cold front across NY states
into the eastern OH Valley. There was better model agreement on
Day 3, so the WPC QPF was based on a multi model blend.

Day 2...
Ongoing convection ahead of a short wave or MCV is expected to
exit the northern Mid Atlantic coast early during Day 2. After
that time, short wave energy in the deep layered south southwest
flow provides synoptic scale lift over a slow moving frontal
boundary extending from the OH Valley across central and northern
VA. The best instability remains here for the first part of the
day, as strong surface high pressure builds across Quebec and
northern New England. After 19/00z, the front edges north as the
surface high crosses the northern New England coast.

A 25 to 30 knot low level south southeast flow transports 1.75
inch precipitable water air and marginal instability to the front,
which becomes elevated over the front into PA and NJ. Because
instability appears as though it will be the limiting factor, the
highest WPC QPF was extended northern VA/northern MD/DE and far
southern NJ, with amounts near 2.00 inches. The 00z GEFS has a few
3.00 inch amounts over central and northern VA, so a Slight Risk
area was placed here. A Marginal Risk was extended from central NJ
across eastern WV into western VA/western NC and eastern TN, where
slow moving cells in the deep moisture could cause isolated flash
flooding issues.

Day 3...
As the surface high move out over the Atlantic, and mid level
ridging builds along the Mid Atlantic coast, the surface front is
allowed to move into New England. While a 25/35 know low level
south southwest flow transports 1.50/1.75 inch precipitable water
into this area (which is almost three standard deviations above
the mean), instability likely ends up being the limiting factor
for a more widespread heavy rainfall/flash flood threat. Following
a general model consensus, an axis of 1.00/1.50 inches of QPF was
placed over the NYC Metro area and southern New England. Three
hour flash flood guidance values where are high, but given the
deep moisture in place, and a frontal boundary approaching NY
state and northern New England, a Marginal Risk was placed here
for Day 3.

Finally, moisture streaming northward ahead of the Mid Atlantic
mid level ridge will combine with instability to produce lines of
northward moving of convection that impact eastern SC and portions
of eastern NC. Three hour flash flood guidance values are fairly
high here, but given the potential for training in the deep
moisture channel, a Marginal Risk was placed here for Day 3.


Deep moisture ahead of a weakening long wave trough over the OH
Valley and Southeast states and mid level ridging off the Mid
Atlantic coast is expected to feed convection riding north across
the east coast of FL into the Southeast during Day 2/3. The
combination of moisture and instability could result in heavy to
locally excessive rainfall, especially across the east coast of
FL. There was generally good model agreement with the overall
synoptic setup, so the WPC QPF was based on a multi model blend.

Day 2...
The weakening long wave trough over the Southeast states and mid
level ridging off the Mid Atlantic coast will funnel 1.75/2.00
inch precipitable water air along the east coast of FL, as well as
across GA into SC. Spokes of lift riding around the trough could
produce local support for clusters of thunderstorms in the
moderate instability, and lines of storms are expected to cross
the east coast of FL into coastal SC. Training along the northward
moving lines could produce local 3.00+ rainfall amounts in these
places. While three hour flash flood guidance values are generally
above these values. short term training in the deep moisture could
result in isolated flash flooding issues, so a Marginal Risk was
placed over the east coast of FL for Day 2.

Day 3...
Similar conditions are expected on Day 3. The long wave trough
weakens and becomes absorbed in the westerly flow over the
northern Mid Atlantic states, but the mid level ridge along the
Mid Atlantic coast moves further west. The flow ahead of the ridge
will continue to supply deep moisture to the east coast of FL, as
well as the SC coast. Like Day 2, training along the northward
moving lines could produce local 3.00+ rainfall amounts in these
places during Day 3. A Marginal Risk was placed over the east
coast of FL to account for the isolated flash flood threat.


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


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