Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Apr 05 2020

Valid 12Z Sun Apr 05 2020 - 12Z Tue Apr 07 2020

...California braces for significant mountain snows, heavy rain, potential
flash flooding, and gusty winds as a strong Pacific storm system slams
into the state today and Monday...

...Warming trend to engulf much of the eastern two thirds of the U.S. the
first half of the week...

...Showers and thunderstorms forecast to flare up in the Mississippi and
Ohio Valleys late Monday and into Tuesday...

California is feeling the initial impacts of an intensifying storm system
off the West Coast this morning with heavy showers tracking through
portions of central California. This is only the start of what will be a
busy Sunday as copious amounts of Pacific moisture stream out ahead of the
storm this morning. In the Sierras, snow will accumulate in a hurry as
snow falls fast and furious up and down the mountain range today and into
Monday. Snow will be measured in feet while the combination of heavy snow
and gusty winds lead to treacherous travel and whiteout conditions.
Meanwhile, rain will all heavily at times, especially for those along the
coastal range and the Southern California mountain ranges. Rainfall totals
are forecast to exceed 3 inches in the higher elevations with locally
higher amounts possible. Even lower elevation areas will see plenty of
rain with urbanized flooding possible. Moderate and Slight risks for
excessive rainfall are in place for portions of Southern California today
and a Slight risk remains in place for far Southern California on Monday.
Look for conditions to improve state-wide by Tuesday morning, although
abnormally cool temperatures will remain in the forecast on Tuesday.

While temperatures stay cool along the West Coast, folks from the front
range of the Rockies to the East Coast will warm up through the first half
of the week. Temperature anomalies in the Midwest will jump to 10 to 15
degrees above normal by Tuesday with daytime highs soaring into the 80s.
To the south, morning temperatures ranging between the upper 60s and low
70s in the Deep South could tie or break daily record warm low
temperatures Tuesday morning and into the latter half of the week. The
East Coast will also enjoy more spring-like temperatures with the lone
exception being New England as a cold front drops their temperatures
seasonally cooler levels by mid-week. Spring lovers in the central and
eastern regions will want to enjoy this stretch of warmer weather while
they can as cooler temperatures are forecast to return the second half of
the week.

The return of warmer temperatures in the Midwest and across the Gulf Coast
will also mean a chance for spotty showers and thunderstorms late in the
day on Monday. As a warm front in the the Mississippi Valley lifts east
towards the Ohio Valley and Mid-Atlantic on Tuesday, areas of showers and
thunderstorms will follow suit leading to wetter conditions from the
Mid-Atlantic on south into the Southeast. The Ohio Valley in particular
could be contend with severe thunderstorms and the potential for localized
flash flooding on Tuesday.


Mullinax


Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
135 AM EDT Sun Apr 05 2020

Valid 12Z Wed Apr 08 2020 - 12Z Sun Apr 12 2020

...Significant precipitation potential over central and southern
California through Wed...

...Overview and Guidance/Predictability Assessment...

A broad upper-level ridge across the northeast Pacific Ocean is
forecast to expand during the extended forecast period, promoting
a broad region of downstream cyclonic flow across much of the
CONUS. An upper-level low initially off the coast of southern
California on Wed (day 3) is forecast to slowly move east across
the Southwest through late in the week, eventually getting
absorbed into a broad northern stream trough forecast to develop
across the central/eastern U.S. The speed with which the upper low
moves east and interacts with the northern stream remains the
point of greatest contention among the guidance. Models have
generally followed into the same camps as recent days, with the
GFS consistently quite a bit faster than other guidance, and the
UKMET on the slow side (the CMC has waffled between the two
extremes, but has spent more time on the slow side of the spread).
Trends among ensemble members suggest a slow convergence toward
the center, which has been consistently occupied by the ECMWF.

In the northern stream, model consensus remains relatively good on
a shortwave expected to dig across the Upper Midwest/Great Lakes
and Northeast Wed-Thu night, with a deepening low pressure system
expected to cross eastern Canada, and a surface low expected to
develop off the coast of Maine Thu night. Guidance spread is
higher with the next northern stream shortwave, forecast to dive
southeast across western Canada Fri/Fri night, and into the U.S.
northern tier on Sat. There is a quite a bit of spread on this
feature will amplify and dig a bit more southwesterly into the
eastern side of the Pacific ridge (GFS) or will be less amplified
and more quickly move east into the north central U.S. (ECMWF).
The ECENS and GEFS ensemble means generally reflect similar ideas
to their respective deterministic runs. Guidance does generally
show consensus, however, that a fairly broad region of cyclonic
upper-level flow will develop across the central/eastern U.S. by
next weekend, which along with the northeast Pacific ridge, will
favor southward transport of polar air into much of the CONUS.

The WPC forecast was based heavily on the 12Z ECMWF during days
3-4 (Wed-Thu). During days 5-7 (Fri-Sun), a gradual trend toward
more ECENS/GEFS ensemble mean weighting was shown, to compensate
for increasing spread in the northern stream late in the week. The
GFS was excluded based on its persistent fast solution with the
southern stream upper-level low/shortwave.


...Weather/Hazard Highlights...

The potential for heavy rain (and mountain snow) will persist
across southern California and portions of the Southwest through
Wed night as the upper low passes overhead. Farther east, the
trailing cold front south of the deepening eastern Canada surface
low is expected to focus showers and thunderstorms across portions
of the Gulf Coast, lower Mississippi Valley, and Southeast
Thu-Fri. The front is forecast to stall across the Gulf of Mexico
and the Florida Peninsula by next weekend, keeping a chance for
showers and thunderstorms in place along most of the Gulf Coast.
Farther north an area of enhanced precipitation appears
increasingly likely across New England Thu/Thu night as the
surface low develops off the coast of Maine. Some of the
precipitation across New England is expected to fall as snow,
especially at interior and higher elevation locations, where some
areas may see at least a couple inches of snow. The shortwave
expected to dive into the northwestern/north central U.S. late in
the week will be accompanied by a cold front and a polar air mass
attempting to nose southward in the lee of the Rockies. This
system will increase chances for mountain snow/valley rain across
the northern Rockies by next weekend.

Above average temperatures are expected across much of the central
and eastern U.S. on Wed, with forecast highs 5 to 15 deg F above
average. As the cold front sweeps southward and eastward, the
above average temperatures will be confined to the Southeast and
southern Mid-Atlantic region by Thu. A cooler air mass in the wake
of the front (highs 5 to 15 deg below average) is forecast to
overspread much of the central U.S. on Thu, reaching the eastern
U.S. on Fri and persisting into Sat. Farther west, highs are
forecast to be 10 to 20 deg below average across southern
California and much of the Southwest Wed-Thu as the upper low
passes. Temperatures across these areas should moderate some Fri
through the weekend.

Ryan


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 AM EDT Sun Apr 05 2020

Valid 00Z Mon Apr 06 2020 - 00Z Mon Apr 13 2020

...Locally heavy rain possible this week...

A deep upper-level low well north of Hawaii is forecast to drift
eastward through midweek. An associated surface cold front is
forecast to reach the western Hawaiian islands late Mon, and then
move east across the state Tue-Wed as it gradually weakens,
perhaps dissipating by Fri or Sat. Overall model/ensemble
consensus surrounding these features is relatively good among the
ECMWF/GFS/UKMET. Models continue to show PWs in the 1.5-1.7 inch
range spreading northward along the front, which should result in
the potential for at least localized heavy rainfall. One
inhibiting factor to more widespread heavy rains will be
relatively weak low-level flow in the vicinity of the front.
Ensemble guidance suggests the best probabilities for locally
heavy rains will occur across the western/central islands late Mon
into Tue as the front arrives. As the front weakens after that,
probabilities drop off, but fairly deep moisture in the vicinity
of the front should allow for isolated heavy rains through late in
the week even as far east as the Big Island as the front gradually
washes out. Models/ensemble show some degree of consensus that
another upper low may cut off north of Hawaii next weekend,
perhaps a bit closer to the state than the previous one. At this
time, models show the second upper low taking a moving in a more
easterly direction more quickly, which would keep the majority of
the deep moisture plume east of Hawaii. Another implication of
this somewhat repetitive synoptic setup is the relative lack of
much in the way of trades, which should largely remain south of
Hawaii through much of the upcoming week, with low-level flow
remaining consistently light and variable.


Ryan

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Sun Apr 05 2020

Day 1
Valid 12Z Sun Apr 05 2020 - 12Z Mon Apr 06 2020

...THERE IS A MODERATE RISK OF FLASH FLOODING OVER SANTA BARBARA
AND VENTURA COUNTIES IN CALIFORNIA...

...Central and Southwest California...
Ahead of a strong closed mid level low drops southeast toward the
Central CA coast during Day 1, an increasing low level southwest
flow transports deep moisture and instability into Central and
Southwest CA. The resulting convection is expected to support an
enhanced flash flood threat, especially in areas with relatively
recent burn scars. There was generally good model agreement with
the overall synoptic setup and timing, so the WPC QPF and
Excessive Rainfall Outlook was based on a multi model blend.

The GOES-17 IR loop showed moisture streams developing ahead of
the closed mid level low off the Pacific Northwest coast. Within
the moisture streams, the most recent RAP analysis showed 250 J/KG
of MUCAPE off the Northern CA coast, suggesting the convection
could be forming under the slow cooling tops. As the closed mid
level low tracks slowly southeast, the instability within the
deepening moisture plume (where the blend TPW showed an area of
0.90 inch precipitable water air, which is about two standard
deviations above the mean) is expected to impact the Central CA
coast during the morning hours. Much of the high resolution
guidance indicates the potential for hourly rainfall rates
approach 0.50 inches south of SFO with the storms.

The low level flow strengthens to 20/30 knot ahead of a cold
front/occlusion, becoming focused from southern Monterey county
south into San Luis county through the afternoon into the early
evening. HRRR/RAP soundings showed between 250/500 J/KG of MUCAPE
in the moisture plume, which could support hourly rainfall rates
of 0.50 inches (which is seen in the 00z NAM CONUS Nest). As the
850/300 mb mean flow becomes better aligned with the propagation
vectors, the potential for training increases. While the line of
convection is expected to move east, cells along the line track
northeast, resulting in training. In these areas, there is a multi
model signal for 2.00/2.50 inches of QPF, and the 00z HREF
probabilities showed a greater than 80 percent chance of the area
south of Monterey to the Santa Barbara line receiving 2.00/2.50
inches of rain. Based on this, a Slight Risk was placed in these
areas.

As the moisture plume drops south across Santa Barbara, Ventura
and Los Angeles counties, the IVT maximizes in the favored upslope
regions, mainly after 06/06z. Most of the high resolution models
still have 250/500
J/KG of MUCAPE in the plume, and hourly rainfall rates of
0.50/0.75 inches are possible, especially where training occurs.
There is a multi high resolution model signal for 2.50/3.50 inches
of rainfall here (below snow levels of about 5000 feet), and the
00z HREF probabilities indicated a greater than 80 percent chance
of 3.00+ inches in 24 hours, centered on Santa Barbara/Ventura
counties, with the greatest threat between 06/06z and 06/12z
(ahead of the front).

Three hour flash flood guidance values are as low as 1.50 inches
in this area, which is easily attainable in this moisture plume.
Though the area has been fairly dry over the last 7/14 days,
hourly rainfall rates of 0.50 inches or greater could cause
mudslides and debris flows over relatively recent burn scars.
Given the strong model consensus for the placement and amounts of
heavy rain, as well as the strength of the IVT ahead of the front,
a Moderate Risk was placed over portions of Santa Barbara and
Ventura counties. This was collaborated with WFO LOX.

Further east across the foothills of the Sierra Nevada Mountains,
the most recent RAP/HRRR indicated 250/500 J/KG of MUCAPE
developing as the low to mid level southwest flow transports 0.75
inch precipitable water air into the region later this morning
into this afternoon. The resulting convection could produce hourly
rainfall rates near 0.50 inches in the 05/22z to 06/02z time
frame, especially over the foothills southeast of SAC. Local
rainfall maxima of 1.50 inches are possible, which is close to the
three hour flash flood guidance. After collaborating with WFOs
STO/HNX, a Marginal Risk was placed here to cover the threat.


Hayes

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sun Apr 05 2020

Day 2
Valid 12Z Mon Apr 06 2020 - 12Z Tue Apr 07 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SANTA BARBARA AND VENTURA COUNTIES IN CALIFORNIA...


...Central and Southwest California...
No significant changes made to the marginal and slight risk areas
across Southwest California for the upcoming day 2 period. The
latest model guidance remains consistent in showing an axis of
heavy rains ahead of the very slow moving closed low pushing south
along the central to southern California coast during the day 2
period. The GFS which had been slightly more progressive with this
system is now much closer to the majority timing solutions. An
anomalous moisture plume, pw values 2.0 to 2.5+ standard
deviations above the mean, will continue to be directed onshore by
south southwesterly flow, enhancing upslope through much of the
Transverse Range. While instability remains limited, the overall
very favorable pattern will support hourly rainfall rates in
excess of .50" in this well defined onshore flow axis. There
continues to be a strong model consensus for 2 to 3" precip totals
resulting in flash flooding potential, especially across recent
burn regions where mudslide and debris flow concerns also exist.

Oravec


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Sun Apr 05 2020

Day 3
Valid 12Z Tue Apr 07 2020 - 12Z Wed Apr 08 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA AND THE UPPER OHIO VALLEY/CENTRAL APPALACHIAN
REGION...

...Southern California...
A marginal risk area was maintained across southwest California
for the upcoming day 3 period. The slow moving closed low off the
Central to Southern California coast day 2 will be pushing more
toward the southeast and to a position just off the northern Baja
Peninsula by the end of the day 3 period. With the closed moving
farther southeast day 3, the best onshore west southwest flow will
be directed into the Peninsular Range with much weaker easterly
low level flow into the Transverse Range for much of the day 3
period. The axis of the most anomalous PW values will be directed
into the Peninsular Range and points farther east into far
southeast California. Heaviest precip likely in the Peninsular
Range but can not rule out additional locally heavy rains farther
north and west through the Transverse Range. As mid level heights
lower day 3 as the closed low moves farther southeastward,
instability will increase across Southern California. This may
support some scattered convection across Southern California,
supporting additional locally heavy rains outside of the best
onshore flow and most anomalous PW axis farther to the southeast.

...Upper Ohio Valley into the Central Appalachians...
West southwesterly to westerly low level flow will be
strengthening late day 2 into day 3 across the Ohio Valley and
directed into a warm frontal boundary forecast to lie from the
Lower Great Lakes, southeastward into the Upper Ohio Valley.
Convection may form along and north of this boundary and push
southeastward from the Lower Lakes into the Upper Ohio
Valley/Central Appalachian region, supporting locally heavy rains.
There appears to be the potential of two rounds of convection
moving southeastward along this boundary. The initial one will be
early day 3, followed by another round forming after 0000 utc
April 8th and pushing southeast into the early hours of Wednesday.
There are some differences in the latitude of the max amounts in
the models, but a general consensus for the potential for two
episodes of organized convection and locally heavy totals with
each event. Precipitation totals across this region have been
slightly above average in spots over the past two weeks. Stream
flow anomalies as per the National Water Model, however, are
mostly average to below average across the risk area. Above
average flow is confined to the northern portion of the marginal
risk area across northeast Ohio and northwest Pennsylvania.
However, with the two event potential, believe at least a marginal
risk is warranted.

Oravec



Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
436 AM EDT Sun Apr 05 2020

Valid 12Z Sun Apr 05 2020 - 12Z Wed Apr 08 2020


...California...Great Basin...North-Central Rockies....

Days 1-3...
A closed low off the CA/OR border will become cutoff as it drifts
south off the CA coast through Monday night then shift east toward
the southern CA coast through Tuesday night. This system is on the
heels of the previous trough which brought heavy mountain snows
across northern California mountains Saturday. While the previous
trough is tracking to the northern Rockies as it weakens tonight,
this low becoming cutoff will drift south. Ample Pacific moisture
ahead of this low will stream into CA, spreading to southern CA by
Monday. Snow levels are expected to be 4000 to 5000ft in this
plume with the heaviest snows shifting south from the Shasta
Siskiyous and CA Cascades this morning and down the entirety of
the Sierra Nevada through Monday morning. Heavy snow shifts to the
southern CA Ranges (most notably the San Gabriel/San Bernardinos
with snow levels 5000 to 6000ft) Monday through Tuesday as the low
lingers offshore. Storm Totals of 2 to 5ft are likely for the
Sierra Nevada with isolated amounts of 2 to 3 ft for the highest
Transverse Ranges in southern CA.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
135 AM EDT Sun Apr 05 2020

Valid 12Z Wed Apr 08 2020 - 12Z Sun Apr 12 2020

...Significant precipitation potential over central and southern
California through Wed...

...Overview and Guidance/Predictability Assessment...

A broad upper-level ridge across the northeast Pacific Ocean is
forecast to expand during the extended forecast period, promoting
a broad region of downstream cyclonic flow across much of the
CONUS. An upper-level low initially off the coast of southern
California on Wed (day 3) is forecast to slowly move east across
the Southwest through late in the week, eventually getting
absorbed into a broad northern stream trough forecast to develop
across the central/eastern U.S. The speed with which the upper low
moves east and interacts with the northern stream remains the
point of greatest contention among the guidance. Models have
generally followed into the same camps as recent days, with the
GFS consistently quite a bit faster than other guidance, and the
UKMET on the slow side (the CMC has waffled between the two
extremes, but has spent more time on the slow side of the spread).
Trends among ensemble members suggest a slow convergence toward
the center, which has been consistently occupied by the ECMWF.

In the northern stream, model consensus remains relatively good on
a shortwave expected to dig across the Upper Midwest/Great Lakes
and Northeast Wed-Thu night, with a deepening low pressure system
expected to cross eastern Canada, and a surface low expected to
develop off the coast of Maine Thu night. Guidance spread is
higher with the next northern stream shortwave, forecast to dive
southeast across western Canada Fri/Fri night, and into the U.S.
northern tier on Sat. There is a quite a bit of spread on this
feature will amplify and dig a bit more southwesterly into the
eastern side of the Pacific ridge (GFS) or will be less amplified
and more quickly move east into the north central U.S. (ECMWF).
The ECENS and GEFS ensemble means generally reflect similar ideas
to their respective deterministic runs. Guidance does generally
show consensus, however, that a fairly broad region of cyclonic
upper-level flow will develop across the central/eastern U.S. by
next weekend, which along with the northeast Pacific ridge, will
favor southward transport of polar air into much of the CONUS.

The WPC forecast was based heavily on the 12Z ECMWF during days
3-4 (Wed-Thu). During days 5-7 (Fri-Sun), a gradual trend toward
more ECENS/GEFS ensemble mean weighting was shown, to compensate
for increasing spread in the northern stream late in the week. The
GFS was excluded based on its persistent fast solution with the
southern stream upper-level low/shortwave.


...Weather/Hazard Highlights...

The potential for heavy rain (and mountain snow) will persist
across southern California and portions of the Southwest through
Wed night as the upper low passes overhead. Farther east, the
trailing cold front south of the deepening eastern Canada surface
low is expected to focus showers and thunderstorms across portions
of the Gulf Coast, lower Mississippi Valley, and Southeast
Thu-Fri. The front is forecast to stall across the Gulf of Mexico
and the Florida Peninsula by next weekend, keeping a chance for
showers and thunderstorms in place along most of the Gulf Coast.
Farther north an area of enhanced precipitation appears
increasingly likely across New England Thu/Thu night as the
surface low develops off the coast of Maine. Some of the
precipitation across New England is expected to fall as snow,
especially at interior and higher elevation locations, where some
areas may see at least a couple inches of snow. The shortwave
expected to dive into the northwestern/north central U.S. late in
the week will be accompanied by a cold front and a polar air mass
attempting to nose southward in the lee of the Rockies. This
system will increase chances for mountain snow/valley rain across
the northern Rockies by next weekend.

Above average temperatures are expected across much of the central
and eastern U.S. on Wed, with forecast highs 5 to 15 deg F above
average. As the cold front sweeps southward and eastward, the
above average temperatures will be confined to the Southeast and
southern Mid-Atlantic region by Thu. A cooler air mass in the wake
of the front (highs 5 to 15 deg below average) is forecast to
overspread much of the central U.S. on Thu, reaching the eastern
U.S. on Fri and persisting into Sat. Farther west, highs are
forecast to be 10 to 20 deg below average across southern
California and much of the Southwest Wed-Thu as the upper low
passes. Temperatures across these areas should moderate some Fri
through the weekend.

Ryan


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml