Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Sat Apr 04 2020

Valid 12Z Sat Apr 04 2020 - 12Z Mon Apr 06 2020

...Heavy showers and thunderstorms to persist across central and southern
Texas today...

...California braces for significant Pacific storm system to produce
copious amounts of rain and mountain snow this weekend and Monday...

...Nor'easter departs this morning, cool temperatures across the Plains on
Saturday will rebound to more spring-like levels by Sunday...

A potent cold front sweeping across south Texas and into northern Mexico
early this morning will help to diminish the severe threat on Saturday.
However, persistent southwesterly flow aloft with embedded upper level
disturbances will aid in additional showers and thunderstorms throughout
the from central and southern Texas to Arkansas and Louisiana. Rainfall
rates could be high within heavier showers and thunderstorms and may
promote the threat for localized flash flooding in parts of south Texas.
Shower and thunderstorm activity is expected to taper off Saturday night
with only a few stray showers and thunderstorms possible across Texas on
Sunday and into Sunday night.

The next weather headliner is a deep and anomalous upper trough that will
slam into California this weekend. Through Monday, rainfall totals in the
valleys and coastal areas of Oregon and California will ranger between 1
to as much as 3 inches. There is the potential for flash flooding along
the coast of Central and Southern California Sunday and into Monday. In
the higher elevations snow will start in the Sierra Nevada on Saturday,
then fall and accumulate at a rapid pace on Sunday. Travel is expected to
become treacherous with whiteout conditions possible. Heavy snow will
continue into Monday and begin to gradually taper off Monday night. By
Tuesday morning, portions of the Sierra Nevada are forecast to receive
anywhere from 1 to as much as 4 feet of snow with locally higher amounts
possible in the highest elevations. Winter Storm Warnings and Watches have
been issued for most of the Sierra Nevada. In addition to the heavy
precipitation totals, temperatures are likely to remain below normal
throughout the weekend and into the start of the upcoming week.

The nor'easter that has generated heavy showers, strong wind gusts, and
coastal flooding is heading east deeper into the northwest Atlantic this
morning. This should make for drier conditions along much of the East
Coast Saturday with seasonally mild temperatures on tap. Further west, the
Plains will start out the weekend on a cool note but moderating
temperatures are likely to take shape on Sunday. By Monday, temperatures
from the front range of the Rockies to the East Coast are forecast to feel
more like early May than early April. Temperatures will continue to warm
up across the Heartland and into most eastern regions on Tuesday with 80s
for highs possible in parts of the Midwest.


Mullinax


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
121 AM EDT Sat Apr 04 2020

Valid 12Z Tue Apr 07 2020 - 12Z Sat Apr 11 2020

...Significant precipitation potential over central and southern
California through the middle of next week...

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

An upper-level ridge is forecast to persist across the northeast
Pacific, west of British Columbia and south of Alaska, during the
extended forecast period. This blocking ridge will favor
persistent downstream troughing along the U.S. West Coast and
broad ridging/anticyclonic flow over the south central and
southeastern states. Embedded within the broad western trough,
models show strong consensus that an upper-level low will drop
southward along the West Coast Tue-Wed (days 3-4), before slowly
moving east across the interior West Thu-Fri (days 5-6), and into
the central U.S. by Sat (day 7). Farther north, an active northern
stream jet is expected from the Gulf of Alaska, north of the
Pacific ridge, east across western/central Canada. A fairly
significant shortwave is still expected to amplify across the
Great Lakes and Northeast Thu-Fri. The associated surface low
pressure system is forecast to cross the north central U.S.
Tue-Wed, reaching the eastern U.S. on Thu. Guidance continues to
suggest potential formation of a low off the coast of New England
Thu night into Fri.

The most significant model differences during the extended
forecast period continue to surround the speed with which the
western upper low begins to move eastward during the mid to late
part of next week. The GFS continues to be much faster than all
other deterministic guidance to move the system eastward Wed-Fri.
The 18Z GFS moved the wave east quickly enough to allow phasing
with northern stream energy across the central U.S., and the
formation of a deep cyclone in the Great Lakes by late in the
week. All other deterministic guidance remains slower than the
GFS, with the CMC and the UKMET on the slow end of the spread, and
the ECMWF in the middle. A look at trends among ensemble members
over the past 24 hours shows somewhat of a convergence toward the
center, providing continued support for the solution shown (fairly
consistently) by the ECMWF and ensemble means (even the GEFS
mean). Despite being faster with the western system, the GFS also
leaves a portion of the energy trailing behind, and eventually
amplifies it into a closed low near the Southwest U.S./Mexico
border late next week, something which is also reflected in the
GEFS ensemble mean to some extent. Guidance continues to show more
consensus with the northern stream feature at least through
Wed/early Thu (aside from the phasing shown by the GFS). Some
spread emerges by later next week with respect to how deep of a
trough gets carved out across the eastern U.S. by the wave. The
CMC and GFS have been more aggressive with amplifying a deep
trough in the East late next week, while the ECMWF solutions is
somewhat more moderate in that context, with support from ensemble
means as well.

Given the above considerations, chose to weight the ECMWF heavily
through days 3-4, with a gradual trend toward heavier weight
placed on ensemble means (especially the ECENS mean) during days
5-7.


...Weather/Hazard Highlights...

The western upper-level low is forecast to result in an extended
period of onshore flow for much of California, with the potential
for heavy rain (and snow in the Sierra) Tue-Wed. Farther east,
showers and thunderstorms with locally heavy rain are expected to
focus from the lower Mississippi Valley to the Southeast Tue in
association with a northward-moving warm front. The northern
stream system is expected to bring showers to portions of the
Upper Midwest and Great Lakes Tue-Wed. As a potential surface low
develops off the Northeast coast Thu into Fri, guidance shows the
potential for an area of enhanced precipitation across New
England, some of which may fall as snow, especially at
interior/higher elevation locations. By Thu-Sat, as the southern
stream upper low/trough moves east into the central U.S.,
increasing moisture return and a developing area of low pressure
in the Southern Plains/lower Mississippi Valley will result in
increasing coverage of showers and thunderstorms across much of
the Gulf Coast and into the Southeast.

A broad area of above average temperatures is expected across much
of the central and eastern U.S. Tue-Wed, with forecast highs 5 to
15 deg F above average expected (perhaps as high as 20 deg above
average for some areas). By later in the week as the northern
stream trough amplifies and the surface cold front advances, the
above average temperatures should become more confined to the
Southeast, with high temperatures 5 to 15 deg below average
overspreading areas from the Midwest to the Northeast. Meanwhile,
the slow-moving upper-level low will bring persistent below
average temperatures from much of California east into the
southwestern states, where high temperatures are forecast to be 10
to nearly 20 deg below average through much of next week -
moderating by next weekend as the system moves east.

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
205 AM EDT Sat Apr 04 2020

Valid 00Z Sun Apr 05 2020 - 00Z Sun Apr 12 2020

...Locally heavy rain possible next week...

A deep upper-level low initially centered near 32N/169W Saturday
morning is forecast to remain relatively stationary before
beginning a gradual northeastward movement Sun-Mon as it weakens
and interacts with a shortwave crossing the North Pacific. At
larger scales, model/ensemble consensus is relatively good for
this system. Models continue to indicate some potential for heavy
rainfall as the associated slow-moving cold front crosses Hawaii
and eventually stalls. The upper-level low will initially be
rather anomalous, with 500-hPa heights at least 4 standard
deviations below average, but guidance generally shows agreement
that the feature should become gradually less anomalous through
time as it moves east/northeast. The ECMWF and GFS both maximize
PWs in the 1.5-1.7 inch range along the slow-moving or possibly
stalled front across Hawaii next week. Low-level flow in the
vicinity of the front looks relatively weak, however, which may
serve to limit areal coverage of heavier rains. ECENS ensemble
precipitation probabilities show some signal for locally heavy
rains, primarily across the western/central Hawaiian islands
during the early to middle portion of next week. The GEFS was a
bit less enthusiastic, however. Nonetheless, given relatively good
moisture and the strong nature of the upper low north of the
state, would expect the potential for at least locally heavy rains
next week, especially during the Mon-Wed time frame. Model
consensus has improved on the idea that the front should stall
across Hawaii late next week, which would keep the potential for
scattered convection with locally heavy rains a possibility across
Hawaii through the end of the week.

Ryan

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 AM EDT Sat Apr 04 2020

Day 1
Valid 01Z Sat Apr 04 2020 - 12Z Sat Apr 04 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
CENTRAL TO SOUTH TEXAS...

The southeasterly 850-700 mb moisture flux is forecast to
strengthen Saturday morning from the lower Rio Grande Valley into
Central Texas along approximately 100W in association with
shortwave energy pushing across Southern Texas. This will lead to
increasing overrunning convection to the north of the strong front
that has pushed through most of Texas as of early Saturday
morning. While there is a lot of spread with the qpf details
among the 0000 UTC guidance, there is agreement that additional
moderate to heavy totals are possible across portions of Central
to South Texas Saturday. The model consensus qpf axis across the
southern portion of SJT's CWA, much of EWX and southern HGX CWAs
will overlap with the localized heavy totals from convection over
the past 24 hours. Hourly rates of .50-1"+ and additional
localized totals Saturday of 1-2"+ may produce runoff issues,
especially in the overlap areas with yesterday's heavy totals.
With respect to changes from the previous issuance for this
period, the marginal risk area was extended northward into
portions of central Texas, southern portions of SJT's CWA.

Oravec


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Sat Apr 04 2020

Day 2
Valid 12Z Sun Apr 05 2020 - 12Z Mon Apr 06 2020

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

...Central and Southwest California...
Ahead of a strengthening mid level low dropping toward the
Northern CA coast during Day 2, a deepening moisture plume and
limited instability in an upslope flow could result in excessive
rainfall over portions of Santa Barbara and Ventura counties in
southwest CA, mainly after 06/00z. Though the 00z GFS is probably
too quick moving the mid level low east during Day 2, there is
generally good model agreement, and a blend was used to construct
the Day 2 Excessive Rainfall Outlook.

As the closed mid level draws closer to the Northern and Central
CA coast around 06/00z, a 20/30 knot low level southwest focuses
0.75 inch precipitable water air (which is between two and three
standard deviations above the mean) on the terrain. There is a
multi model signal for IVT values of 500 kg/m/s in the moisture
plume, which should be sufficient to support hourly rainfall rates
of 0.50 inches, especially after 06/00z. Model soundings still are
not showing much in the way of instability in the plume ahead of a
surface cold front, but falling height should support steepening
lapse rates, and limited instability (less than 100 J/KG) could
materialize as the front approaches.

The instability could be enough to enhance rainfall rates,
especially after 06/06z. There is a solid model signal for
2.00/2.50 inch rainfall amounts in the favored upslope areas, as
the low level flow is nearly orthogonal to the terrain. Some of
the high resolution guidance (such as the 00z NAM CONUS Nest) is
suggesting that 3.00+ inch rainfall amounts are possible. For this
to occur, training would be needed. This is possible, as cells
move northeast while the front remains fairly progressive.

Three hour flash flood guidance values are as low as 1.50 inches
over portions of Santa Barbara and Ventura counties, despite
fairly dry conditions the last 7 to 14 days. The main threat is to
areas with relatively recent burn scars, as the higher rainfall
rates could result in mudslides and debris flows. Based on this,
a Slight Risk area was placed in the abovementioned areas. A
Marginal Risk was extended from the Central CA coast, where the
upslope flow could result in local 2.00+ inch rainfall amounts,
especially where hourly rainfall rates near 0.50 inches occur.


Hayes



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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Sat Apr 04 2020

Day 3
Valid 12Z Sun Apr 05 2020 - 12Z Mon Apr 06 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA...

...Southern California
As a strong closed mid level low crosses from Northern into
Central CA, a deep moisture plume becomes focused on the higher
terrain of Southern CA, resulting in a threat for heavy rainfall
and flash flooding below the snow level. There is a strong model
signal for 3.00+ inch rainfall amounts in the lower terrain of the
Transverse Range. Other than the 00z GFS looking too quick to move
the closed mid level low east, there was good model agreement, and
a blend was used to support the WPC Excessive Rainfall Outlook.

The closed mid level low trundles across Northern into Central CA
during Day 3, aiding in pushing a cold front across Southern CA.
Ahead of the front, a 25 knot low level southwest flow transports
0.75/1.00 inch precipitable water air (which is between two and
three standard deviations above the mean), which becomes focused
on the Transverse Mountains (specifically the San Gabriel and San
Bernardino Mountains), mainly before 07/00z. The combination of
moisture and synoptic scale lift in the left exit region of a 100
knot jet streak crossing Southern CA should be sufficient to
support hourly rainfall rates of 0.50 inches or greater,
especially in the favored upslope areas.

At this juncture, model soundings are showing almost no
instability in the column in the heart of the moisture plume, but
as heights fall, steepening lapse rates should add convective
elements to the precipitation pattern. Falling heights also means
falling snow levels, though much of the QPF is expected to fall in
the form of rain. As mentioned earlier, there is a solid model
signal for 3.00+ inch rainfall amounts, especially in areas that
experience hourly rainfall rates of 0.50 inches or better. The
best chance of reaching these levels is where short term training
occurs, as cells moving northeast along the eastward moving front.

The greatest threat for flash flooding is expected over areas with
relatively recent burn scars, which could lead to mudslides and
debris flows. Based on the above, and after collaborating with
WFOs LOX/SGX, a Slight Risk was placed here. It should be noted
that if there is a faster precipitation phase change, the areal
coverage of the Slight Risk could change with time.


...Texas into the Lower Mississippi Valley...
The persistent subtropical jet stream continues to feed moisture
from the eastern Pacific across TX into the Lower MS Valley during
Day 3. Coincidentally, a 20 knot low level southerly flow feeds
1.50/1.75 inch precipitable water air (which is between two and
three standard deviations above the mean) along a boundary
extending from central TX into south central MS.

As short waves in the quasi zonal mid level flow interact with the
deep moisture, they provide enough synoptic scale ascent to
promote the growth of convective cluster along and south of the
boundary. Model soundings indicated that the best instability
remains closer to the Gulf Coast, where 500/1000 J/KG of MUCAPE is
expected to reside. Because of this, there is some model spread
concerning where the axis of heaviest rainfall would reside. The
00z ECMWF is the most bullish with the QPF, suggesting local 2.50+
inch rainfall amounts. Other models are not as robust, though the
00z GFS does show something along the boundary.

Given the moisture in place, heavy rainfall is expected in areas
which have been wet for much of the winter into the early spring.
The model signal is weak, so for now, no excessive area was
assigned. However, if there is a better model signal signal in
later model runs, a Marginal Risk could be needed here in
subsequent forecasts.

Hayes

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
348 AM EDT Sat Apr 04 2020

Valid 12Z Sat Apr 04 2020 - 12Z Tue Apr 07 2020


...Western U.S....
...Late Season Heavy Snow for higher California Mountains...

Days 1-3...
Successive lows off the northern California coast (the latter of
which becomes cutoff and drifts with the coast) are forecast to
bring late season heavy snows to the southern Cascades, Klamath
and Shasta/Siskiyous, the entirety of the Sierra Nevada, and
higher southern California mountains through Monday night. The
highest amounts are expected in the Shasta region and along the
Sierra Nevada where 2 to 5 feet are likely.

A compact shortwave low swings east into the coastal border of
CA/OR today, weakening as it pushes through a ridge before
reaching the northern Rockies tonight. This initial system will
have an enhanced swath of Pacific moisture (PW above 0.75in)
across northern CA with a quick hitting heavy snow above the
4000ft snow level for the Klamath, Shasta/Siskiyous, Trinity Alps,
southern OR and CA Cascades today, spreading south to the northern
Sierra Nevada tonight. The combination of the weakening low,
higher snow levels, and much of the moisture precipitated in
CA/OR, little heavy snow risk exists over the northern Great Basin
with Day 1 snow probs limited to the aforementioned mountains with
high probabilities for 8 or more inches.

The second, stronger low approaches the far northern CA coast late
tonight, nearly stalling through Sunday night before drifting
south and slowly weakening as it reaches a point off the central
CA coast Monday night. The narrow corridor of enhanced Pacific
moisture plume is maintained south of this low with considerable
moisture flux into CA through the forecast period. Snow levels are
expected to be 3000 to 4000ft near the low/coast and 4000 to
5000ft over the Sierra Nevada and southern CA mountains (on Day
3). Day 2 snow probabilities for 8 or more inches are similar to
Day 1 (high) over far northern CA with the entirety of the Sierra
Nevada in heavy snow from the slow moving low and continued influx
of enhanced moisture. The Sierra Nevada have high probabilities
for 18 or more inches on Day 2 and 2 to 4 feet are likely over the
High Sierra. High Day 3 snow probabilities for 8 or more inches
are focused on the southern half of the Sierra Nevada and the San
Bernardino Mtns (particularly near Big Bear), along with the White
Mtns and the ranges of central Nevada (all within the main plume
of Pacific moisture). Speaking of this plume, the Day 2.5 time
frame is best for moisture intrusion across the Great Basin with
moderate probabilities for 8 or more inches for the Sawtooth Mtns
of ID and the Tetons in WY.

Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
121 AM EDT Sat Apr 04 2020

Valid 12Z Tue Apr 07 2020 - 12Z Sat Apr 11 2020

...Significant precipitation potential over central and southern
California through the middle of next week...

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

An upper-level ridge is forecast to persist across the northeast
Pacific, west of British Columbia and south of Alaska, during the
extended forecast period. This blocking ridge will favor
persistent downstream troughing along the U.S. West Coast and
broad ridging/anticyclonic flow over the south central and
southeastern states. Embedded within the broad western trough,
models show strong consensus that an upper-level low will drop
southward along the West Coast Tue-Wed (days 3-4), before slowly
moving east across the interior West Thu-Fri (days 5-6), and into
the central U.S. by Sat (day 7). Farther north, an active northern
stream jet is expected from the Gulf of Alaska, north of the
Pacific ridge, east across western/central Canada. A fairly
significant shortwave is still expected to amplify across the
Great Lakes and Northeast Thu-Fri. The associated surface low
pressure system is forecast to cross the north central U.S.
Tue-Wed, reaching the eastern U.S. on Thu. Guidance continues to
suggest potential formation of a low off the coast of New England
Thu night into Fri.

The most significant model differences during the extended
forecast period continue to surround the speed with which the
western upper low begins to move eastward during the mid to late
part of next week. The GFS continues to be much faster than all
other deterministic guidance to move the system eastward Wed-Fri.
The 18Z GFS moved the wave east quickly enough to allow phasing
with northern stream energy across the central U.S., and the
formation of a deep cyclone in the Great Lakes by late in the
week. All other deterministic guidance remains slower than the
GFS, with the CMC and the UKMET on the slow end of the spread, and
the ECMWF in the middle. A look at trends among ensemble members
over the past 24 hours shows somewhat of a convergence toward the
center, providing continued support for the solution shown (fairly
consistently) by the ECMWF and ensemble means (even the GEFS
mean). Despite being faster with the western system, the GFS also
leaves a portion of the energy trailing behind, and eventually
amplifies it into a closed low near the Southwest U.S./Mexico
border late next week, something which is also reflected in the
GEFS ensemble mean to some extent. Guidance continues to show more
consensus with the northern stream feature at least through
Wed/early Thu (aside from the phasing shown by the GFS). Some
spread emerges by later next week with respect to how deep of a
trough gets carved out across the eastern U.S. by the wave. The
CMC and GFS have been more aggressive with amplifying a deep
trough in the East late next week, while the ECMWF solutions is
somewhat more moderate in that context, with support from ensemble
means as well.

Given the above considerations, chose to weight the ECMWF heavily
through days 3-4, with a gradual trend toward heavier weight
placed on ensemble means (especially the ECENS mean) during days
5-7.


...Weather/Hazard Highlights...

The western upper-level low is forecast to result in an extended
period of onshore flow for much of California, with the potential
for heavy rain (and snow in the Sierra) Tue-Wed. Farther east,
showers and thunderstorms with locally heavy rain are expected to
focus from the lower Mississippi Valley to the Southeast Tue in
association with a northward-moving warm front. The northern
stream system is expected to bring showers to portions of the
Upper Midwest and Great Lakes Tue-Wed. As a potential surface low
develops off the Northeast coast Thu into Fri, guidance shows the
potential for an area of enhanced precipitation across New
England, some of which may fall as snow, especially at
interior/higher elevation locations. By Thu-Sat, as the southern
stream upper low/trough moves east into the central U.S.,
increasing moisture return and a developing area of low pressure
in the Southern Plains/lower Mississippi Valley will result in
increasing coverage of showers and thunderstorms across much of
the Gulf Coast and into the Southeast.

A broad area of above average temperatures is expected across much
of the central and eastern U.S. Tue-Wed, with forecast highs 5 to
15 deg F above average expected (perhaps as high as 20 deg above
average for some areas). By later in the week as the northern
stream trough amplifies and the surface cold front advances, the
above average temperatures should become more confined to the
Southeast, with high temperatures 5 to 15 deg below average
overspreading areas from the Midwest to the Northeast. Meanwhile,
the slow-moving upper-level low will bring persistent below
average temperatures from much of California east into the
southwestern states, where high temperatures are forecast to be 10
to nearly 20 deg below average through much of next week -
moderating by next weekend as the system moves east.

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