Quantitative Precipitation Forecast
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000
FXUS04 KWBC 162059
QPFPFD

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


Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 17/0000 UTC thru May 20/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...


---Eastern U.S.----

There is not expected to be any significant changes to the very
wet pattern affecting large portions of the eastern U.S.  A very
large area of much above average pw values---1.5 to 2.5+ standard
deviations above the mean will persist along and to the east of
the elongated mean trof stretching from the OH Valley---into the
TN Valley and central to eastern Gulf Coast region.  Widespread
showery conditions will persist from Florida--northward through
the Southeast---Southern to Central Appalachians---Mid-Atlantic
into southern NY state and southern New England.  The href mean
and in house hi-res mean qpf was used primarily for qpf details
day 1 to mitigate the typical run to run and model to model qpf
detail differences.  The combination of the much above average pw
values and broadly difluent upper flow to the east of this mean
trof will support widespread moderate to heavy totals across these
areas.  Focus areas for the heaviest precip likely through the
Southern to Central Appalachians from northeast GA---the Upstate
of SC---western NC---western VA into eastern WV where uvvs will be
enhanced by topography.  The persistent west to east oriented
stationary front between the OH and TN valley---into the
Mid-Atlantic will also help focus a potential heavy precip axis
through the Mid-Atlantic into far southern NY state and coastal
southern New England.  Excessive rainfall wise---a broad marginal
risk area is maintained across these regions---with slight risk
areas along and north of the stationary front from the Central
Appalachians into the Mid-Atlantic and over the Southern
Appalachians in the vicinity of the above mentioned precipitation
maxima.

...Lower MS Valley into the Central Gulf coastal region...

Height fall expected to drop southeastward on the back side of the
mean trof position stretching from the OH-TN Valleys into the
Central Gulf coastal region.  PW values are not as anomalous in
this area as points to the east of the above mentioned mean trof
position---generally around 1 standard deviation above the mean.
Convection enhancing over the Southern Plains this afternoon ahead
of these height falls expected to weaken by the beginning of the
upcoming day 1 period as they push toward the Lower MS Valley.
This activity will likely re-ignite later day 1--especially
Thursday afternoon over the Lower MS Valley-Central Gulf coast
region.  There is a lot of model qpf spread here---leading to low
confidence.  Moderate areal average amounts depicted with the
likelihood of locally heavier totals where convection does become
most organized.

...Northern California...Great Basin...

Models are similar day 1 with the eastward push of the north
central California closed low moving into the Great Basin.
Showers will remain active to the east and northeast of this
closed low from northern California---eastern Oregon---northwest
NV.  This will support moderate to isolated heavy precip totals
across these areas where pw values are expected to remain much
above average---2 to 2.5 standard deviations above the mean.  No
changes planned to the marginal risk area depicted across these
areas to the north and northeast of this closed low.

...Northern Rockies...

A farther northeast closed low will be moving slowly northward
from the Northern Rockies into the northern High Plains this
period.  Shower activity likely to continue to the north and
northwest of this system---enhanced by developing north
northeasterly upslope flow into the northern Rockies.  Moderate to
heavy totals likely from northern Idaho into northwest MT.

...Northern Plains into the Upper MS Valley...

Shortwave energy pushing to the northeast of the Northern Rockies
closed low will enhance uvvs along and north of the west to east
oriented front forecast to lie across the Northern Plains into the
Upper MS Valley.  The best chance for shower development along and
north of this boundary will be Thursday afternoon ---with this
activity likely to enhance further after the day 1 time period
into the beginning of the day 2 time period.  For the day 1
period---the timing of the convective development will likely keep
the areal extent of potentially heavy rain fairly small---with an
expanding area likely day 2 heading into convective max time early
Friday morning.



Days 2/3...


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

A long wave trough making its way 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 eastward into the Northern Plains 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 12Z ECMWF/GFS/NAM.

As the long wave trough moves 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 over western
Montana and parts of northern Wyoming.  Increasing low level
moisture will help boost the static stability over the region,
with MUCAPE values in excess of 500 J/KG in these areas by early
in the Day 2 period.  The flow associated with the long wave
trough keeps precipitable water values between 0.75/1.00 inches
which is expected to produce an axis of 0.75/1.25 inches of
precipitation from far northern ID eastward across the Rocky
Mountain Front Range in MT.

Three hour flash flood guidance values remained as low as an inch,
and the National Weather Model showed high stream flows, so a
Marginal Risk was placed here for Day 2.

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 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.  There was a convergence
among the NCEP global models and hi resolution models in the
placement of the axis of heaviest rainfall.  As a result, the WPC
QPF was based on a blend of the 12Z ECMWF/GFS/ARW.

Day 2...
A 30 to 40 knot low level southerly flow will transport an airmass
with precipitable water values around 1.25 inch 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 12Z
GFS/NAM/ARW showed local QPF amounts in excess of 2.00 inches
along this axis,  with some convergence on the placement of the
heaviest rainfall being along a northeast to southwest oriented
axis over parts of North Dakota extending back into western South
Dakota.

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. The signal for a
cluster of storms/possible MCS were still pretty strong in the 12Z
NCEP global models and hi resolution ARW core with spot maximum
amounts now running 2 to 4 inches.

Since three hour flash flood guidance values remain as low as
1.00/1.50 inches along the ND/MN border, and the National Water
Model showed high stream flows before 19/00Z, some minor
adjustments were made to the slight risk area and the Marginal
Risk area that surrounded it...but the changes did not reflect a
fundamental change in the previous forecast reasoning.

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 with the
NCEP runs still being insistent on amounts locally exceeding 4
inches.  Was very suspicious of their solutions in this part of
the country when 6 hourly amounts were 3+ inches during the
overnight hours considering how unusual that would be at that time
of day.  Certainly, the upslope flow will continue to feed
moisture from the plains into the upslope region, and there could
well be some elevated convection, so certainly continued to
forecast rainfall in the area. but we cut back the QPF there for
those reasons.  There was no reason to make many modifications to
the slight risk, other than expanding the slight risk into the
upslope region of the Cheyenne Ride.

...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 12Z EMCWF/GFS seemed to have the best handle on
the frontal position.  Thus the WPC QPF was based on a multi model
blend based on discussions with the WPC model diagnostician.

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.


...Southeast/FL...

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.

Oravec/Bann

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

$$





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