Quantitative Precipitation Forecast
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FXUS04 KWBC 142037

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
436 PM EDT Wed Mar 14 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 15/0000 UTC thru Mar 18/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1


The strong low currently over the Canadian Maritimes will be slow
moving for the upcoming day 1...lingering over New Brunswick- Gulf
of St Lawrence area.  Snowfall intensities in the comma
head/deformation precip band affecting portions of the northeast
are expected to decrease as per latest hi res simulated
radars---but with cyclonic low to mid level flow persisting
through day 1---snow showers will persist. Additional moderate to
heavy snowfall likely from northern to central NY to the east of
lake Ontario---east northeastward into the favored terrain of
northern New England.

Across the Great Lakes---a reinforcing shot of cold air expected
to sink southeastward across the lakes region tonight into
Thursday.  This should support increasing lake effect snow showers
developing from northwest to southeast downwind of all the lakes.
Model consensus is for light to moderate precip/snowfall totals.

Rockies---Great Basin---central to southern California

The elongated full latitude mid to upper level trof will be
pushing inland from California---into the Great Basin---Southwest
and through the Rockies day 1.  Above average pw values ahead of
these height falls and the associated surface frontal boundary
will support widespread moderate to locally heavy precip totals
through the central to northern Rockies into eastern portions of
the Great Basin and across central to southern California.

Northern California into the coastal Pacific Northwest

Stronger height falls associated with a closed low off the B.C.
coast will be dropping southeastward along the Pacific Northwest
coast during Thursday.  This will help strengthen the low level
southerly to south southwesterly flow from northwest California
into the coastal Pacific Northwest.  This will support increased
shower activity Thursday across these areas with the greatest
precip potential from the northern Sierra into the northwest
California coast range and the far southwest Oregon coast range
where the upslope component to the strengthening low level flow
will be the greatest.  Northward into the coastal Pacific
Northwest the low level southerly flow will have a primarily
terrain parallel trajectory--with less precip amounts expected.

Elsewhere---light precip amounts possible over south TX as the low
level southerly flow strengthens off the western Gulf of Mexico.
Scattered showers also possible across southwest TX ahead of
shortwave energy moving east northeastward across northern Mexico
into southwest TX.

Days 2/3...

WPC QPF was based on a blend of the ECMWF/UKMET.

...West Coast to the Rockies...

A deep upper-level low near the Pacific Northwest Coast will
slowly weaken as some short wave energy ejects eastward from
Thursday evening to Saturday evening.  Circulation around the
upper-level low will stream moisture into Central California with
precipitable water values of 0.50 inches with the low-level flow
of 35 to 40 knots on Thursday evening that will slowly move
southward to Southern California by Friday afternoon with the flow
slowing to 15 to 20 knots.  The moisture plume will continue to
move southward into Northwest Mexico by Saturday evening.  The
system will produce maximum qpf amounts ranging between 2.00 to
2.30 inches over the Sierras on Thursday evening into Friday
evening.  As the moisture plume moves south, the qpf over the
Sierras will decrease with maximum qpf amounts ranging from 0.50
to 0.90 inches on Friday evening into Saturday evening with a
secondary maximum over the Wasatch mountains ranging between 0.25
and 0.55 inches.

...Central/Northern High Plains to the Ohio/Tennessee Valleys and
Central Appalachians to the Southeast...

Short wave energy ejected from West Coast upper-level low will
coalesce over the Central Plains by Friday morning moving to the
Ohio Valley/Mid-Atlantic by Saturday evening.  The associated
baroclinic zone over the Southern High Plains will, likewise, move
eastward to the Southern Mid-Atlantic by Saturday evening.  The
storm will draw moisture from the Western Gulf of Mexico into the
Central Plains with precipitable water values as high as 0.75
inches on a low-level flow of 45 to 50 knots on Thursday evening.
The precipitable water values will weaken to mostly 0.50 inches
with the low-level flow of 30 to 20 knots by Friday evening.  The
maximum qpf amounts will range from 1.25 to 1.50 inches over the
Central Plains from Thursday evening to Friday evening.  A
secondary maximum will develop over parts of the Lower Mississippi
Valley with maximum qpf amounts ranging from 0.40 to 0.50 inches.
As the system moves eastward, moisture will move into the
Ohio/Tennessee Valleys with precipitable water values of 0.75
inches.  The system will produce maximum qpf amounts ranging from
0.35 to 0.50 inches on Friday evening into Saturday evening over
parts of the Ohio/Tennessee Valleys.  WPC used a blend of the
ECMWF/UKMET as a starting point for the manual guidance with the
GFS being faster with the system and the ECMWF/UKMET being more
consistent with the qpf placement.


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


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