Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
546 AM EDT Thu Mar 22 2018
Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 22/1200 UTC thru Mar 25/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr
On Day 1, the models target California for the heaviest
precipitation with several inches of liquid equivalent
precipitation in the coastal ranges of southern CA and along the
spine of the CA Sierra Nevada range. One distinct area is within
the plume of deepest moisture streaming onshore into southern CA
into the southern CA Sierra Nevada ranges. The integrated water
vapor imagery is well above normal and the slow movement of the
max precipitable water values in the streaming streaming onshore
in the counties north of Los Angeles combines with 850 mb
convergence and 300 mb divergence to produce areas of heavy rain,
with ascent aided by windward terrain. The continuation of the
high moisture and water vapor transport into interior southern Ca
leads to maxima along the foothills and mountains of interior CA
with heavy Sierra Nevada snows, with several feet likely over the
next day and a half. For further details regarding the excessive
rainfall potential, please refer to the latest QPFERD.
A second area covers northern CA to the OR border as the northeast
Pacific upper low and trough drift towards the coast... with a jet
max streaming onshore and inland into the ranges of ID and
adjacent western MT. The upper jet moving into NV Thu afternoon
supports upper divergence maxima streaming northeast from the CA
Sierra across NV into UT with a maxima near the northern
Wasatch/Uintas of UT, driving precipitation in windward terrain.
QPF was derived from a combination of the latest high resolution
guidance -- particularly the HREF means and the 04Z NBM. See the
winter weather forecast amounts and snow probabilities for heavy
snow potential in the mountains of CA.
...Pacific Northwest/Northern Rockies...
As a strong closed mid level low weakens and comes ashore over the
Pacific Northwest during Days 2 and 3, short wave energy ahead of
the closed low taps Pacific moisture to produce locally heavy qpf
amounts across portions of the Pacific Northwest into CA. There
was generally good model agreement with the overall synoptic
setup, so the WPC QPF was based mainly on a blend of the 00z
A strong closed mid level low off the Vancouver Island at the
start of Day 2 drops south, reaching a position off the OR coast
by the end of the period. Short wave energy ejected from the
closed low early crosses the Northern Rockies. There is some
Pacific moisture in the flow as it reaches the Northern Rockies,
and upslope flow is expected to focus the moisture on the higher
terrain. Areas of 0.25/0.50 inches of qpf were placed over the
Blue Mountains in OR, as well as the northern Bitterroot Mountains
A strong short wave ejected from the closed mid level low comes
ashore over southwest OR during the second half of Day 2. Ahead of
the short wave (and its attendant surface low), a low level
southwest flow focuses 0.50/0.75 inch precipitable water air over
the Klamath Mountains of southwest OR and the Shasta and Siskiyou
ranges in northern CA. There was a multi model signal for 1.25 to
2.50 inches of qpf over this area, with the highest amounts over
the northern Klamath Mountains. Snow levels drop sufficiently with
the short wave passage as to preclude a flash flood threat here on
The closed mid level low off the OR coast at the start of Day 3
weakens into a short wave as it crosses northern CA and northern
NV before reaching the Northern Rockies by the end of the period.
The low level southwest flow becomes focused on the northern
portion of the CA Coastal Range, as well as the Sierra Nevada
range in northern CA. There should be enough moisture in the low
level flow to produce axes of 1.00 inches of qpf across the
Shasta/Siskiyou Ranges. There was a multi model signal for an axis
of 1.50 inches of qpf across the favored upslope areas of the
Sierra Nevada Range. Snow levels should be low enough in all areas
to prelude the threat for flash flooding on Day 3.
Snow is expected over the higher terrain of the West Coast and the
intermountain West on Days 2 and 3. Please refer to the most
recent suite of WPC Winter Weather products for more information.
...Central and Southern Rockies/Great Basin/Southwest...
Short wave energy tracking from Southern CA crosses the Central
and Southern Rockies during Day 2, bringing with it some of the
Pacific moisture for locally heavy QPF amounts. For the most part,
there was good model agreement with the the track and timing of
the short wave energy. With this in mind, the WPC QPF was based on
a blend of the 00z ECMWF/GFS.
As short wave energy tracks from Southern CA across the Central
and Southern Rockies during Day 2, it brings with it some of the
deep moisture from the Pacific plume. Precipitable water values
ahead of the short wave briefly peak at 0.50 inches across
portions of the Four Corners area, and model soundings showed
marginal instability (due mainly to steepening lapse rates). Low
level flow on the back side of surface low pressure forming over
eastern CO focuses the moisture ion the CO Rockies, where there
was a multi model signal for 0.25/0.50 inches of qpf. Further west
across UT and AZ, local upslope flow is expected to produce
generally 0.10 to 0.25 inches of qpf over the Wasatch Mountains in
UT, as well as the Mogollon Rim in AZ.
...Northern Plains/Upper MS Valley/OH Valley/Mid Atlantic...
Short wave energy tracking from central CA reaches the Northern
and Central Plains during Day 2. Lift with the short wave spins up
surface low pressure on a frontal boundary extending the Central
Plains into the Mid MS Valley. The low tracks across the lower OH
Valley during Day 3, before weakening. There was good model
agreement with the synoptic scale features, so the WPC QPF was
based on a blend of the 00z ECMWF/GFS.
Ahead of short wave energy tracking from central CA to the
Northern Plains, a 35 to 45 knot low level southeast flow focuses
Pacific based moisture on the 295 K isentropic surface across the
Northern Plains into the Mid and Upper MS Valley. The best
isentropic lift occurs between 23/18z and 24/00z (west) and
between 24/00z and 24/06z (east), and model soundings showed some
elevated instability across portions of southern IA into northeast
MO and west central IL. The combination of moisture and lift is
expected to produce an elongated axis of 0.75/1.25 inches of
stretching from western ND across eastern SD/southwest MN over
much of IA into northernmost MO and much of central and southern
IL. The 00z GFS showed maximum amounts closer to 2.00 inches
across southwest IA, but this could be overdone given the limited
amount of instability available in the moisture plume.
The short wave energy tracks from the Upper MS Valley across the
lower OH valley into the Mid Atlantic states during Day 3. Its
weakening surface low tracks along a similar path, and the track
of the surface low is expected to keep instability south of the
track across portions of KY into eastern TN. Model soundings over
this area showed mainly elevated instability (with MUCAPE values
below 250 J/KG), with little of this instability surviving the
trip over the frontal boundary. South of the surface low and its
associated frontal boundary, a 40 to 45 knot low level southwest
flow transports 1.00 inch precipitable water air into the lower OH
Valley, mainly after 25/00z. There was a multi model signal for an
axis of 0.75 to 1.50 inches of qpf extending from central IL into
southeast VA and northeast NC, with the highest amounts over
northern and eastern KY (where the best instability is expected).
At this point, with little in the way of instability, no excessive
area was issued for northern and eastern KY.
Snow and ice are expected along the main qpf axis extending from
the Northern Plains into the Mid Atlantic states during Days 2 and
3. Please refer to the most recent suite of WPC Winter Weather
products for more information.
Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml