Quantitative Precipitation Forecast
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FXUS04 KWBC 190816

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Thu Apr 19 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 19/1200 UTC thru Apr 22/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1


A respectable amount of synoptic forcing will accompany a
shortwave trough moving into the mean larger scale troughing over
the Northeast this morning. Height falls, upglide on the 300 K
isentropic surface, and frontogenesis just north of the developing
coastal low will all support a broad area of rain and snow.
Moisture is, however, lacking in an overall sense, and the moist
plume will move quickly offshore after 12z. Cold air aloft and the
increasingly high April sun angle will support diurnal showers,
especially in the higher elevations of NY, PA, but with
precipitable water values falling to less than a half inch by
midday, this system will not produce a lot of liquid. The more
noteworthy impacts will be from accumulating snow in the early
morning over parts of NY, PA. See the WPC Heavy Snow Discussion,
QPFHSD, and local forecast products for details. WPC QPF was
derived from the GFS, UKMET, and SREF, and was supported by some
of our in-house ensemble tools. The 12z ECMWF was thought to have
been too suppressed southward with its QPF.

...Western U.S...

Despite a fairly classic appearance in loops of water vapor
imagery, the low entering the West Coast was not very anomalous at
only 1 standard deviation from climatology in the 500 mb height
field. Moisture is also scant, such that this system is more of a
wind producer and will enhance the wildfire potential in the
Southwest per Storm Prediction Center and local forecast office
products. There will, however, be some organized areas of light to
moderate rain and snow, especially in the northern portion of the
mid level circulation / the Sierras across the Great Basin, and
also where some southerly moisture return meets the difluent upper
flow pattern ahead of the trough / West Texas through New Mexico
into the western and central Colorado. Given well clustered model
solutions, a consensus approach should work well here. We leaned
on continuity and the non bias corrected version of our in-house
ensemble which is more generous with coverage of measurable
precipitation. We also leaned toward the WRF-ARW2 and GFS with
respect to convection and elevated convection expected in parts of
TX, NM this afternoon and evening. The CAPE axis sets up far to
the west, along I-25 per the NAM. Storms firing here may have a
difficult time making eastward progress into drier, more stable
air, but a few may tend to be more long lived near and south of
the Rio Grande within a broader field of moisture pooled along a
frontal zone.

Days 2/3

...Pacific Northwest/Northern Rockies...

A negatively tilted long wave trough crosses northern WA and
adjacent British Columbia during Day 2, reaching the Northern
Rockies later in the period. The next short wave in the flow
approaches the Pacific Northwest coast late on Day 3. There was
generally good model agreement with the evolution of the mid level
systems, so the WPC QPF was based on a blend of the 00z ECMWF/00z

Day 2...
A surface cold front associated with the negatively tilted long
wave trough crossing northern WA during Day 2 focuses 0.75 inch
precipitable water air in the 25 knot low level flow on the
Olympic Mountains and northern WA Cascades. While the best lift
and thrust of moisture ahead of the front are expected over
Vancouver Island and southwest British Columbia, local 0.50 inch
qpf amounts were placed over higher terrain of WA.

Day 3...
The short wave energy crosses the northern Rockies early on Day 3,
and the lift with the short wave taps some of the moisture that
survives from from the Pacific plume. The combination of moisture
and upslope flow is expected to produce local 0.25 inch qpf
amounts over the Rocky Mountain Front Range.

Further west, lift ahead of an approaching short wave should be
strong enough to focus the 0.50 inch precipitable water air on the
northern and central WA Cascades, mainly between 22/06z and
22/12z. Local 0.25 inch qpf amounts were placed over the northern
WA Cascades ahead of the strong short wave.

...Great Basin/Rockies/Central Pains...

A closed mid level low over UT/AZ early on Day 2 reaches eastern
CO/eastern NM by the end of the period. Lift with the closed mid
level low and upslope flow is expected to make the most of the
moisture in the column to produce locally heavy QPF in the higher
terrain, mainly during Day 2. The 00z ECMWF seems to be a tad on
slow side with the mid level system, so the WPC QPF was based more
closely on the faster 00z NAM/GFS.

Day 2...
As the closed mid level low trundles across the Four Corners area
during Day 2, a 25/35 knot low level south southeast flow
transports 0.75 inch precipitable water air into the western High
Plains from western NE into western KS and the Panhandle of OK,
mainly between 20/18z and 21/06z. Model soundings showed marginal
to moderate instability in the moisture plume extending from
western KS and eastern CO into western OK during this time. The
resulting convection is expected to form on the terrain and move
into the Plains after 21/00z. There was a good model signal for
0.50/1.00 inch qpf amounts extending across western NE into
western KS, and this formed the basis for the WPC QPF.

Further west across CO, the low level south southeast flow veers
as the mid level system approaches, funneling some of the moisture
from the Plains into the higher terrain. The upslope flow focuses
the moisture on the higher terrain, resulting in a qpf axis of
1.25 inches on the Front Range in CO, with lesser amounts (between
0.50/1.00 inches) over the CO Rockies into the San Juan Range in
southwest CO and nearby northern NM. Finally, moisture along the
Wasatch Mountains and Uintas in the upslope flow is expected to
produce local 0.50 inch qpf amounts.

Heavy snowfall is possible on Day 2 across the higher terrain of
the Rockies. Please refer to the most recent suite of WPC Winter
Weather products for more information.

Day 3...
Residual moisture in the upslope flow as the mid level closed low
passes over central and eastern CO during the first part of Day 3
should be sufficient to produce 0.10/0.25 inches of qpf over the
CO Rockies.

...Southern Plains into the Lower MS Valley...

A closed mid level low over eastern CO/eastern NM at the start of
Day 3 tracks toward OK/AR by the end of the period. An inverted
trough associated with the mid level system becomes the focus for
deepening moisture and instability, which feed convection that may
produce heavy to excessive rainfall, especially across the Lower
MS Valley. The 00z ECMWF continues that model`s slower motion with
the mid level system and its attendant surface trough, so the WPC
QPF was based more closely on the 00z GFS, with some 00z NAM
thrown in as well.

Day 2...
Moisture increases across OK and portions of central and western
OK during Day 2 ahead of a closed mid level low crossing the Four
Corners area. A 30/40 knot low level south southeast flow
transports 0.75/1.00 inch precipitable water over western and
central OK and the TX Panhandle. The developing convection may
become organized by lift associated with the closed mid level low,
and there should be enough mid level flow to keep the convection
moving east. There was a good signal from the 00z GFS/00z NAM for
an area of 0.25/0.50 inches of qpf over this area, with local 1.00
inch qpf amounts possible,

Day 3...
Surface low pressure that forms over west TX ahead of the closed
mid level system over the Rockies early on Day 3 weakens as it
tracks across east TX during the first half of the period. The
trough becomes the focus for increasing moisture, as a 30 knot low
level southerly flow transports 1.50 inch precipitable water air
into western and central LA/AR after 22/00z. Model soundings
showed generally marginal instability along and ahead of the
inverted trough (due in part to the time of day), but divergence
in the left front quadrant of a 110 knot jet streak crossing TX
into LA after 22/00z could allow the convection to become better
organized, partially compensating for the marginal instability.

As mentioned earlier, the 00z ECMWF appears to be too slow with
the surface and mid level systems, so the WPC QPF was based more
closely on the 00z GFS/00z NAM (though it is possible that the 00z
NAM may be a bit too deep, at least when compared to other 00z
guidance). As the surface and mid level system move eastward, a
large area of 1.00/1.50 inches of qpf was stretched from eastern
OK/northeast TX into southern AR/northern LA and far west central
MS. For the most part, three hour flash flood guidance are fairly
high (especially over
OK/northeast TX, where values are generally above 3.00 inches), so
a Marginal Risk was placed over this area to account for the

Given the 00z GFS placement of the highest qpf amounts over
northern LA, a Slight Risk wa placed here on Day 3. The National
Water Model shows the greatest stream flow increases here,
suggesting high flow before 22/12z. The Slight was placed over far
southern AR and far west central MS to account for 00z NAM
placement of the highest qpf amounts.

...New England...

Short wave energy on the back side of a closed mid level low
heading for the Canadian Maritimes during Day 2 will produce
upslope flow over northern New England. The upslope is expected to
produce qpf across the higher terrain. There was generally good
model agreement with the overall synoptic setup, so the WPC QPF
was based on a multi model blend, with some 00z NBM used to better
delineate higher terrain qpf maxima.

The boundary layer north northwest flow behind a closed mid level
low tracking toward the Canadian Maritimes during Day 2 lifts cold
and moist air up over the higher terrain extending from northeast
NY into western ME. Over the favored upslope areas, local 0.25
inch qpf amounts were placed, with the highest amounts over the
Green Mountains in VT and the northern White Mountains in NH.


Surface high pressure off the East Coast produces an easterly flow
that transports moisture and instability along a weakening
boundary during Days 2 and 3. Convection that develops is expected
to produce mainly light qpf amounts over land each day. Since
there was good model agreement with the overall synoptic setup,
the WPC QPF was based on a multi model blend.

Day 2...
A weakening boundary over central Fl during Day 2 focuses moisture
and instability in the easterly low level flow south of surface
high pressure over the East Coast. The low level flow transports
1.25 inch precipitable water air and marginal instability over
east central FL, where there was a good model signal for 0.25
inches of qpf. Locally higher amounts are possible along the coast
in the low level convergent flow.

Day 3...
The synoptic setup is essentially the same as Day 2, though there
could be more moisture in the column, as model soundings are
showing local 1.50 inch precipitable water air along the east
central FL coast on day 3. The mainly diurnally driven convection
could produce local 1.00 inch qpf amounts here, as the low level
easterly flow remains convergent.


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


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