Quantitative Precipitation Forecast
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FXUS04 KWBC 172144

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
544 PM EDT Sat Mar 17 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 18/0000 UTC thru Mar 21/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

Southern Appalachians into the southern Mid-Atlantic

The relatively small closed low initially over the upper MS valley
region expected to continue to weaken day 1 as it presses east
southeastward from the upper OH valley into the central
Appalachians and southern Mid Atlantic tonight into early Sunday.
The model consensus is for precipitation ahead of this closed low
to refocus across the southern Appalachians this evening and move
fairly quickly east southeast across the southern Mid Atlantic
tonight.  The quick movement likely to be a detriment to any
widespread heavy totals with model consensus for areal average
moderate to possibly isolated heavy amounts across these areas.

Southern Plains---lower MS Valley---central Gulf coast

Height falls in the southern stream flow expected to push east
northeastward out of northern Mexico into the southern Plains this
evening---eastward toward the lower MS valley by early Sunday.
There is a fairly consistent signal in the hi res guidance for
convection to develop late this afternoon over portions of north
central TX and move fairly quickly downstream.  Despite the
expected fast movement---intense rainfall rates over a short
period will support locally heavy to isolated excessive rainfall
amounts---especially in the first 6 to 9 hours of the upcoming day
1 time period---with rainfall intensities and amounts expected to
decrease after this as the initial activity moves toward the lower
MS valley/central Gulf coast region.  The marginal risk area on
the latest excessive rainfall potential outlook over portions of
northeast TX was not changed with runoff issues possible in the
most intense heavy rain cores---especially if they occur over any
urban areas in the vicinity of Dallas to Waco.

Late Sunday morning into Sunday afternoon---another round of
precip expected to break out over portions of the Southern Plains
in an increasingly diffluent pattern ahead of additional southern
stream height falls moving northeastward from northern Mexico and
stronger upstream height pushing east from the southwest into the
southern high plains.  This next area of precip looks like it will
form farther to the north than the lead area---from north TX into
eastern OK Sunday afternoon---pushing into the lower AR river
valley toward the end of the day 1 period---with model consensus
for .25-50" amounts.

Western U.S.

The broad mean upper trof initially over the large portions of the
western U.S. will become increasingly separated into distinct
circulations day 1.  The northern portion of this trof will be
comprised of three fairly slowly moving circulations.  One moving
very slowly southward off the Pacific Northwest coast---while one
over the northern Rockies and another over southern
Alberta/southern Sask combine near the MT/Alberta/Sask border
region. Meanwhile---stronger and faster moving height falls will
eject out of the base of the trof tonight---pushing from southern
California---through the Southwest and emerging into the southern
high plains Sunday afternoon.  Overall---there is good model
agreement on what will be a large region of precip associated with
each circulation center.  Model consensus is for areal average
moderate totals---with locally heavy amounts possible in the
favored terrain regions from the Mogollon Rim of AZ---the central
Rockies of CO---through the Wasatch of UT and into the northern
Rockies of north central ID into western MT.

Days 2/3...

...Northern/Central Rockies into the Northern/Central Great

An upper low sweeping out of the central Rockies will remain a
focus for active precipitation over the center of the country
extending up into the northern Plains. The primary area of
interest will likely be northeastern Colorado given a favorable
low-level moist, upslope flow on Sunday evening into the overnight
hours. Eventually by Monday morning the low-level flow will turn
more parallel to the Rocky Mountain chain which should
considerably decrease the coverage of precipitation over the
region. During the 19/0000Z to 19/0600Z window, some solutions
show anywhere from 0.50 to 0.75 inches of liquid which should
mostly be in the form of snow. Additionally, a north/south
convergence zone extending across the Northern Plains will also be
a focus for active precipitation. This area may persist slightly
longer given weak upstream height falls aiding in further lift on
Monday. Farther to the south, a quick-moving, locally intense band
of convection may spawn in close proximity to the deep surface low
pushing along the Kansas/Oklahoma border late Sunday into Monday.
The progressive nature of this wave should limit a flash flood
threat but cannot rule out brief local issues given the extent of
deep convection near the cyclone. As a whole, favored a
combination of the 12Z NAM/GFS with the 00Z ECMWF back toward the
terrain while utilizing a blend of the 12Z/06Z GFS with the 00Z
ECMWF for the warm sector activity.

...Lower/Middle MS Valleys and TN/OH Valleys into southern states
and much of the Eastern Seaboard...

As the mentioned upper low slides eastward toward the Ozarks, the
system should evolve into more an open wave and accelerate toward
the lower Ohio Valley on Monday evening. Models vary with the
intensity of this feature with the 12Z NAM being more robust which
supported a heavier swath of precipitation streaking through the
mid-Atlantic on Tuesday. Ensemble low plots support a wave
emerging off the southeastern Virginia coast on Tuesday morning
with quick progression toward the northeast thereafter. While
progressive in nature, solutions favor rather potent omega fields
supporting a quick burst of precipitation tracking from the lower
Ohio/upper Tennessee valleys into the central Appalachians and
mid-Atlantic region. Please check out the heavy snow discussion
(QPFHSD) for more information on the wintry aspects of this
system. At this point, felt the 12Z NAM was displaced too far to
the north while favoring a consensus formed by the 12Z/06Z GFS and
00Z ECMWF given building model agreement. However, confidence
remains only average given how much the model runs have fluctuated
in recent cycles.

Within the warm sector, active convection should continue from the
Ozarks eastward into the mid-South and southeastern states. Much
of the activity will focus along the advancing cold front with
additional thunderstorms likely within the downstream warm sector.
Further, an amplifying trough sweeping through in the wake should
provide a further impetus for convection over the region. While
the guidance is scattered a bit with QPF solutions, some of the
high-resolution guidance shows heavier swaths, particularly the
12Z HRW-ARW with higher amounts from central Mississippi eastward
into southern Georgia. As such, felt confident in keeping the
inherited slight risk on the excessive rainfall outlook. The
manual QPF produced followed a similar approach to the
mid-Atlantic region by favoring a combination of the previous two
runs of the GFS (12Z/06Z), with the 00Z ECMWF.

...West Coast...

A powerful mid/upper-level low out of the central Pacific will be
quite anomalous in nature with 500-mb height departures on the
order of 3 to 4.5 sigma below climatology. Steady mid-level
southwesterly flow will push through the state of California
accompanied by an impressive subtropical moisture connection. An
atmospheric river should unfold with 1.50 inch and above
precipitable water values streaming toward the southern/central
California coast on Tuesday evening. While the best vertical
motions lurk just offshore by 21/0000Z, some quicker solutions
would bring this heavier slug of precipitation inland late on Day
3. Although the 12Z NAM solution was not recommended, it did show
1 to 1.50 inch 24-hour amounts reaching the San Francisco Bay Area
by Tuesday afternoon. However, per the Model Diagnostics
discussion, stuck closer to a non-12Z NAM consensus while leaning
more closely on the 12Z GFS/00Z ECMWF output. Based on the QPF
from the medium range, some prolific amounts appear primed to
reach much of California by mid-week.


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


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