Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
541 AM EDT Sun Mar 18 2018


Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 18/1200 UTC thru Mar 21/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Central / Southern U.S...

Gulf moisture will continue to overspread the southern states,
butting up against a frontal zone deposited by the shortwave
exiting the east coast early Sunday. Moisture will wrap well north
of the surface front into the zone marked by a forming occlusion
and inverted trough structure over the central and southern
Plains. The environment is typical of spring, with steep lapse
rates supporting a convective mode to much of the precipitation
from Kansas southeastward, and even briefly convective or at least
with embedded thunder over eastern Colorado as cyclogenesis /
strong deep layer ascent occurs during the day and evening.
Nowhere is moisture transport especially focused, but there should
be several mesoscale pockets of heavier rainfall scattered along
the more general swath of widespread rain from Colorado to
Georgia. Upglide on the 300 K isentropic surface per 00z soundings
will support swaths of semi-organized heavier downpours anywhere
from southern Kansas to southern Alabama. This is especially true,
or will be enhanced because of the presence of a lead shortwave
preceding the main cyclogenesis, which was evident in satellite
imagery this morning over northern Mexico. This shortwave comes
into phase with the diurnal heating cycle by afternoon over the
southern portion of the Gulf Coast states, such that we may see a
surface-based MCS added onto the ongoing upglide-driven / elevated
convection. Expect also a surface-based event back in the deeper
mixing zone near the Red River Valley / Arklatex, near the surface
front. The 00z WRF-ARW depicted much of this thinking very well,
but was perhaps too aggressive over southern Louisiana where it
wanted to blow up an MCS early in the day. We used the 00z HREF
blended mean to temper the ARW, and also liked the broader detail
within the GFS solution. This approach kept a decent amount of
continuity, while also raising forecast precip amounts in several
pockets, including the strongly forced eastern Colorado /
northwest Kansas environment, and central/eastern Kansas in the
strongly difluent upper flow region that develops by nightfall.


...Western U.S. Including the Central Rockies up through the
Western Dakotas...

The 00z GFS was in line with the observational trends for
precipitation organized along a mid level frontogenesis zone in
eastern Utah and northeast Arizona, heading into western Colorado
early today. Much of the forcing then transfers out to the
developing cyclone over the Plains. Inverted troughing will extend
meaningful areal average QPF up into the western Dakotas with
time, and the cold upper level temperatures and cyclonic flow
continue to favor a showery regime over the northern Rockies and
Pacific Northwest (rain and snow). WPC favored the GFS, WRF-ARW,
and GEM Regional.



Days 2/3...


...California...

Ahead of a positively long wave trough off the West Coast on day
3, deep moisture associated with an atmospheric river crosses the
southwest CA coast late in the period, bringing with it the
potential for heavy to excessive rainfall. There is still some
model spread concerning the timing of the arrival of the deeper
moisture, with the 00z GFS seemingly a fast outlier, and the 00z
NAM possibly too far north with its depiction of the deeper
moisture and higher QPF. Since the 00z GFS/00z ECMWF were closer
with the placement of the higher QPF amounts, a blend of these
models comprised the basis for the WPC QPF.

A closed mid level low located near 33N 140W at the start of day 3
evolves into a positively tilted long wave trough as it moves
closer to the CA coast by the end of the period. Ahead of the long
wave trough, there is a multi model signal for deep moisture
associated with a well advertised atmospheric river (a feature
which the GFS has been showing for a few days now) to cross
southwest CA after 21/00z. Precipitable water values near 1.25
inches (which is between four and five standard deviations above
the mean) are ushered ashore by a 20 to 30 knot low level
southwest flow.

The flow become more orthogonal with time to the Transverse Ranges
in Southern CA, resulting in a better upslope component between
21/06z and 21/12z. Model soundings off the southwest CA coast
showed marginal instability in the moisture plume, with the best
synoptic scale lift expected late in the period. The ingredients
are apparently coming together for a significant rainfall event
for portions of Southern CA, which most 00z model solutions
showing some version of the above occurring, yielding between 1.00
and 1.50 inches of qpf along the southwest CA coast.

However, timing issues have introduced some uncertainty concerning
the arrival of the higher QPF amounts (with the 00z GFS the
fastest to the coast with the QPF, and the 00Z ECMWF slower).
Because of these timing differences, it is possible that the best
ingredients for a significant rainfall event occur after 21/12z,
or the end of day 3. After collaborating with WFO LOX concerning
the threat, a Slight Risk area was issued for much of the LOX CWA,
with the highest rainfall rates occurring between 21/06z and
21/12z. A Marginal Risk was extended up the coast over the Coastal
Range, to include the further north solution of the 00z NAM (which
was not favored at this time by WPC). Even with the timing
difference, it appears as though the heaviest rainfall amounts and
higher rainfall rates occur either late on day 3, or (more likely)
after that time.


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

A weakening short wave crossing the Northern and Central Plains
into the Upper MS Valley during days 2 and 3 will interact with
moisture for mainly light qpf amounts each day. The 00z ECWMF was
a bit slower than the 00z GFS with the mid level support, so a
blend of the two models was used for the basis for the WPC QPF.

Day 2...
As the weakening mid level system crosses northeast MT into ND/SD
during day 2, a low level east southeast flow introduces 0.50 inch
precipitable water air from NE into eastern NS/SD. While
instability is not present, there should be sufficient lift for an
area of 0.10 to 0.25 inches of qpf.

Day 3...
The weakening mid level system crosses northern IA into southern
MN during day 3. Moisture profiles are similar to those of day 2,
so an area of 0.10/0.25 inches of qpf was placed over these areas.


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

A closed mid level low over northeast OK early on day 2 opens up
into a weakening short wave that arrives along the Mid Atlantic
coast by the end of the period. Its attendant surface low tracks
along a frontal boundary extending across the Mid MS Valley across
the OH Valley to the Mid Atlantic coast becomes the focus for
moisture and elevated instability to produce locally heavy
rainfall. The short wave energy induces cyclogenesis late in the
period along the Mid Atlantic coast, resulting in additional heavy
QPF amounts. Additional short wave energy follows on day 3,
inducing surface low pressure development close to the Mid
Atlantic coast. There is still a fair amount of spread concerning
the placement of the highest QPF amounts along the front on day 2,
as well as the distribution of QPF with the surface systems along
the Mid Atlantic Coast. In an attempt to mitigate some of these
differences, the WPC QPF was based on a blend of the 00z ECMWF/GFS.

Day 2...
As the initial surface low tracks from northeast OK to the OH
valley during day 2, the frontal boundary associated with the low
becomes the focus for moisture. Precipitable water values near
1.25 inches are available along the front from east central MO
across southern IL into KY, where model soundings showed marginal
elevated instability. Resultant convection could produce an axis
of 0.75 to 1.25 inches of qpf along the aforementioned axis, with
local 1.50 inch amounts possible closer to the front and elevated
instability. The 00z GFS continues this axis into portions of
southeast VA, with amounts over 2.00 inches depicted.

While qpf amounts this high this far east may not materialize (as
the best instability is expected to remain south of that area),
the potential for higher QPF remains. Three hour flash flood
guidance values are as low as 1.00 to 1.50 inches over portions of
northern KY into much of WV, so a marginal risk was stretched from
east central MO across KY/southern WV into a portion of southern
and central VA (to account for the 00z GFS solution).

The short wave reaches the Mid Atlantic coast between 20/06z and
20/12z, and it spins up surface low pressure on a low level
baroclinic boundary extending along the NC coast. While there was
decent agreement with the placement of the surface low after
20/12z, there was still some spread concerning the northward of
the higher QPF amounts into MD/DE/PA and NJ. The 00z NAM appeared
to be too aggressive with its QPF placement, so the WPC QPF was
based more closely on a 00z ECMWF/GFS blend.

Day 3...
As the surface low tracks off the Mid Atlantic coast, it tosses
back moisture along the low level baroclinic zone into a portion
of the northern Mid Atlantic before exiting the coast. There is
some potential for banding with the surface and mid level systems,
but at this distance it is difficult to place where these may
occur. The 00z GFS appears to the outlier with lighter QPF amounts
across MD/DE/NJ during the first part of day 3, with the 00z EMCWF
further west with moisture along the low level baroclinic zone.
For now, the 0.50 inch QPF isohyet was placed along the southern
NJ and DE coasts, but changes concerning how far west this line
ends up could change with subsequent model runs.

It appears as though the second surface low will affect mainly the
southern portion of the Mid Atlantic coast during day 3. The 00z
NAM is a western outlier, and its solution would affect portions
of the Mid Atlantic into NJ with 0.50+ inch qpf amounts. Since its
mid level support is further west than the model consensus, its
qpf did not factor significantly into the WPC QPF.

Finally, moisture it funneled along an inverted surface trough
extending from SC across eastern TN into eastern KY. The moisture
interacts with lift ahead of an approaching long wave trough from
the OH/TN Valleys. There was good model agreement for local 1.00
inch qpf amounts over the higher terrain of eastern TN/western VA.

...Southeast...

Short wave energy tracking from the Great Basin to the lower MS
Valley during day 2 evolves into a long wave trough during day 3,
as a cold front extending from surface low pressure over the Mid
Atlantic states. Ahead of the front, moisture and instability will
feed convection that produces locally heavy rainfall each day.
There is still a fair amount of spread concerning the placement of
the maximum QPF, especially during day 2. In an attempt to
mitigate some of the differences, the WPC QPF was based on a multi
model blend.

Day 2...
An evolving long wave trough approaches the Southeast states on
day 2, and synoptic scale lift associated with the mid level
system is provided over a cold front extending from surface low
pressure over the Mid Atlantic coast into portions of
AL/GA/northern and western FL. Ahead of the front, model soundings
showed moderate instability and precipitable water values between
1.25/1.50 inches, and moisture and instability will feed organized
convection crossing the region.

Most areas are expected to receive between 0.50 and 0.75 inches of
qpf with the storms. with local 1.50 inch amounts possible. The
storms are expected to move with the strong mid level flow, and
the storm movement, combined with relatively high three hour flash
flood guidance values, should preclude an organized flash flood
threat on day 2.

Day 3...
Convection is expected to become better organized ahead of the
cold front across northern and central Fl during day 3. Ahead of
the cold front, precipitable water values near 1.50 inches and
moderate to strong instability is expected. The organized
convection is expected to produce an area of 0.75/1.25 inches of
qpf over portions of central Fl before the front exits the coast
between 21/06z and 21/12z. Again, the movement of the storms and
high flash flood guidance is expected to preclude an organized
flash flood threat on day 3.

Burke/Hayes

Graphics available at <a href="http://www.wpc.ncep.noaa.gov/qpf/qpf2.shtml"><u>www.wpc.ncep.noaa.gov/qpf/qpf2.shtml</u></a>