Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Sun Oct 24 2021

Valid 00Z Mon Oct 25 2021 - 00Z Wed Oct 27 2021

...There is a High Risk of excessive rainfall over the Sierra Nevada
Mountains and a Moderate Risk of excessive rainfall over the Middle
Mississippi and Ohio Valleys through Monday morning...

...Heavy snow for the Sierra Nevada Mountains...

...There is an Enhanced Risk of severe thunderstorms over parts of the
Middle/Lower Mississippi and Ohio Valleys through Monday morning...

A strong storm just west of the Pacific Northwest Coast will impact the
West Coast over the next several days. Strong onshore flow will stream
moisture into Northern/Central California, producing heavy rain and snow
over parts of California. Therefore, the WPC has issued a High Risk of
excessive rainfall over the Sierra Nevada Mountains through Monday
morning. Significant and potentially life-threatening flash flooding
flash is expected with the storm. Some areas that normally do not
experience flash flooding will flood. Heavy snow is also forecast for the
Sierra Nevada Mountains through late Monday afternoon. The excessive
rainfall threat lessens to a Slight Risk over parts of the Sierra Nevada
Mountains with a Marginal Risk over other parts of the Sierra Nevada
Mountains and parts of the Southern California Coast from Monday into
Tuesday morning. The excessive rainfall threat ends on Tuesday for
California.

Moreover, rain and highest elevation snow will develop over parts of the
Pacific Northwest into the Northern Intermountain Region and continue into
Monday. On Monday, after the front passes, the snow levels will lower over
the Northwest portion of the country. On Tuesday, rain and higher
elevation snow move into the Rockies, with rain developing over parts of
the Northern/Central High Plains by Tuesday evening.

Meanwhile, an area of low pressure over the Central Plains and associated
front will interact with moisture flowing northward over the
Plains/Mississippi Valley to produce showers and thunderstorms over the
region through Monday afternoon, with some storms becoming severe.
Therefore, the SPC has issued an Enhanced Risk of severe thunderstorms
over parts of the Middle/Lower Mississippi and Ohio Valleys through Monday
morning. The hazards associated with these thunderstorms are frequent
lightning, severe thunderstorm wind gusts, hail, and a few tornadoes.
Furthermore, there is a 10% or greater probability of EF2-EF5 tornadoes
over parts of the region and a 10% or greater probability of wind gust of
65 knots or greater. Additionally, there is a 10% or greater probability
of two-inch or great hail over parts of northeastern Missouri through
Monday morning. However, the severe thunderstorm threat is not the only
threat over the Midwest. Heavy rain will develop along the front,
therefore, the WPC has issued a Moderate Risk of excessive rainfall with
the rain and thunderstorms over parts of the Ohio Valley through Monday
Morning. The associated heavy rain will create numerous areas of flash
flooding. Furthermore, many streams may flood, potentially affecting
larger rivers.

As the system moves eastward, the risk of severe thunderstorms weakens
slightly to a Slight Risk over parts of the Mid-Atlantic and the
Central/Southern Appalachians on Monday. Also, rain will develop over
parts of the Great Lakes Sunday evening into Monday. The rain moves into
parts of the Northeast early on Monday. On Tuesday morning, the area of
low pressure moves to the northern Mid-Atlantic Coast and off the New
England Coast by Tuesday evening. Overnight Monday into Tuesday, the low
aids in streaming moisture into the Northeast, producing heavy rain.
Therefore, the WPC has issued a Slight Risk of excessive rainfall with the
rain. The associated heavy rain will create mainly localized areas of
flash flooding, with urban areas, roads, and small streams the most
vulnerable starting on Tuesday and going through Tuesday night.

Lastly, the front associated with the Pacific Northwest system moves out
of the Rockies and onto the Plains by Tuesday evening. The system will
produce rain and thunderstorms over the Plains on Tuesday evening.

Ziegenfelder

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Mon Oct 25 2021

Valid 12Z Thu Oct 28 2021 - 12Z Mon Nov 01 2021

...Major storm to focus a heavy rainfall/runoff threat from the
Midwest/Southeast Thursday to the Mid-Atlantic/Northeast into next
weekend...


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

A deep cyclone lifting from the Mississippi Valley to the
Northeast this weekend will keep much of the medium range quite
active. The guidance with this system continues to show good
agreement on the large scale evolution/track, increasing
confidence a bit, but do offer some uncertainties in the details,
which may take until the short range to resolve. A general model
blend seemed a good starting point for this system, maintaining
good continuity with the previous shift. This system may threaten
October low pressure records across parts of the mid-Mississippi
to Ohio Valley.

Elsewhere, the last couple of model runs show the next trough
moving towards the West Coast by the middle of this week may split
sending a northern stream shortwave across the northern Rockies
and northern Plains into next weekend. The guidance has trended
more north and stronger with this system suggesting the Northeast
U.S. low may eventually get wrapped up into this system late
period. The last few model runs hint at a possible southern stream
upper low near California next Sat-Mon, but the ensembles offer
plenty of uncertainty on whether this feature even exists at all.

This cycle of the WPC medium range progs used a blend of the
deterministic models days 3-4 transitioning towards and almost
equal weighting of the GFS/ECMWF with the GEFS/ECENS means by day
7. This helps to mitigate the late period differences.


...Weather/Hazards Highlights...

The biggest weather threat during the medium range period will be
the deep low pressure system working across the eastern third of
the U.S. later this week into next weekend bringing widespread
moderate to locally heavy rains. Closed upper low development and
a slowed frontal system progression will allow rainfall to expand
across the Ohio Valley/Appalachians/Southeast later this week then
the Mid-Atlantic/Northeast into next weekend as aided by a
sustained Atlantic moisture feed. Repeat/duration activity and
favored orographic lift should focus a possible flood/runoff
threat for parts of the central Appalachians.

Out West, shortwave energy progressing towards the West Coast will
direct a plume of moisture to support another round of heavy
rain/mountain snows to mainly parts of the Pacific Northwest, but
also farther inland across the northern Rockies on Thursday into
Friday. Conditions should finally dry out by next weekend though
as upper ridging builds over the region.

For temperatures, the Midwest/South to the East Coast should
remain near or more likely below normal through much of the period
underneath cloudy and unstable skies. Humidity levels though
should allow overnight mins to be more above normal. Behind this,
upper ridging shifts across the central U.S. bringing a period of
above normal (+5-10F) days before the next shortwave moderates
temperatures back towards or even below normal. Out West,
temperatures should generally stay within a few degrees on either
side of normal.

Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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
316 AM EDT Mon Oct 25 2021

Valid 00Z Tue Oct 26 2021 - 00Z Tue Nov 02 2021

Trade winds should relax this week, ushering in a drier and more
stable pattern over Hawaii. Any showers are likely to stay
confined to windward facing terrain areas as high pressure builds
north of the islands. The guidance continues to show that upstream
upper ridge amplification over the west-central Pacific should
allow rounding shortwaves to dig southeastward towards Hawaii the
latter half of the next week, which could bring brief disruptions
and shift towards more northerly trades later this week. The
latest models suggest the associated cold fronts with these
systems should weaken and die out north of the islands though, so
limited to no moisture/QPF impacts are expected.

Santorelli

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
849 PM EDT Sun Oct 24 2021

Day 1
Valid 01Z Mon Oct 25 2021 - 12Z Mon Oct 25 2021

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL IN THE UPSLOPE
REGION OF THE SIERRA NEVADA RANGE IN NORTHERN CALIFORNIA...

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ILLINOIS AND INDIANA...

...California...
Significant and potentially life threatening flash flooding will
continue across portions of central and northern CA through the
overnight hours as the atmospheric river continues to slowly shift
southward. Hourly rainfall magnitudes are generally between
0.25-0.5" within the core of heaviest rains...although locally as
high as ~0.75". Additional rainfall of 1-4" across valley
locations is expected through 12z...with 3-5" across both the
coastal ranges and the Sierra...locally as high as 5-7" in the
most favored upslope areas.

...Illinois and Indiana...
Areas of flash flooding are likely across portions of north
central IL and IN through ~06z tonight. Good overlap of upper
level divergence and low level convergence has resulted in a
rather rapid expansion of convection over the area. Part of the
convective line has propagated off to the east...which has
decreased the flash flood risk to some degree along the forward
propagating portion of the line. However heavy short duration
rates may still result in some flooding of susceptible locations.
The greatest flash flood risk appears to be along the southern
edge of the convective line where cells are currently backbuilding
and orienting more in a west/east training fashion. This portion
of the line also has the greatest upstream instability advecting
in and is oriented parallel to the convergence axis...thus for
many reasons it seems like this portion of the ongoing convection
will pose the greatest flash flood threat over the next several
hours. The 18z HREF indicates rather high probabilities of
exceeding FFG through 06z across this region (although probably
displaced a bit too far north in the HREF)...which combined with
ongoing radar/satellite trends...warrants a continuation of the
Moderate risk.

The risk looks to drop considerably after 06z, as both upper level
divergence and lower level convergence weaken...along with
decreased instability. The 18Z HREF also indicates little to no
FFG exceedance after 06z. Thus the main event looks to be over the
next several hours, with lessening impacts thereafter.

Chenard

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Mon Oct 25 2021

Day 1
Valid 12Z Mon Oct 25 2021 - 12Z Tue Oct 26 2021

...THERE IS A SLIGHT RISK IN PORTIONS OF CENTRAL CALIFORNIA...

...California...
The Marginal and Slight Risk areas in California appear ideal
given the most recent WPC QPF and 00z HREF probabilities of 0.5"
an hour and 3"+ in 24 hours. Scattered to widespread coverage of
precipitation will shift south, focusing more on central and
southern parts of the state. There has been a gradually shift in
model guidance to delay the end of the precipitation. There will
be some overlap with heavy/excessive precipitation which occurred
Sunday and Sunday night and additional rainfall on Monday. Burn
scars still present the greatest risk of excessive rainfall.


...Olympic Peninsula of Washington...
Enough moisture, inflow from the Pacific, and instability leads to
a modest chance of 0.5" an hour totals. The HREF supports a high
chance of 3"+ totals in the Olympic Peninsula, but snow levels
appear low, which limits the threat across its higher elevations.


...Northeast Mid-Atlantic States and Southern New England...
Slowing trends have led to a southwest shift in the existing
Marginal Risk area. Rainfall associated with the comma head of a
low pressure system off the East coast moves northward and starts
to draw moisture on-shore from the Atlantic Ocean, with
precipitable water values around 1.25". Instability should be
greatest across the central Mid-Atlantic states, which along with
frontogenesis could aid hourly rain totals. There is some spread
in terms of placement and associated rainfall amounts. As a
result, confidence is not high...but there is a signal for the
potential of locally moderate to heavy rainfall in the area. Two
week precipitation has been minimal, but recent leaf fall could
clog area drainage and lead to standing water with rainfall below
flash flood guidance, so the Marginal Risk remains in place as a
precaution.

Roth/Campbell/Bann

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
350 AM EDT Mon Oct 25 2021

Day 2
Valid 12Z Tue Oct 26 2021 - 12Z Wed Oct 27 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN MID-ATLANTIC INTO SOUTHERN NEW ENGLAND...

There is agreement that low pressure off the East Coast will be
the major player in producing precipitation across the
region...but the degree of development and some model mass field
differences result in uncertainty regarding maximum values and
placement. There remains some overlapping of solutions over
portions of southern New England and the eastern portions of the
Northern Mid-Atlantic states get 1.5-3" areal average rainfall,
with enough frontogenesis and instability to expect the possibilty
of 1" an hour rain totals, which would be most problematic in New
York City and nearby northeast NJ. Adding to the concern is
ongoing leaf fall across the area, which could cause drainage
issues. Areas depicted from continuity remain relatively similar,
with some southwest adjustment made into eastern PA and NJ to
match model trends.

Roth/Campbell/Bann

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Mon Oct 25 2021

Day 3
Valid 12Z Wed Oct 27 2021 - 12Z Thu Oct 28 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE ARKLATEX, CENTRAL GULF COAST, & PACIFIC NORTHWEST COAST...

ArkLaTex/Central Gulf coast...
A couple batches of convection are expected in the vicinity of
Louisiana near and ahead of a cold front. Inflow at 850
hPa/effective bulk shear of 30-40 kts is expected to organize
convection, most likely into lines within the unidirectional
southwest flow. Instability should be sufficient from eastern TX
down to the central Gulf coast for potential heavy rain issues.
Overall, the threat should be minimal as much of the region has
had below average rainfall over the past couple of weeks. Should
any mesocyclones form in this environment, they could hold up the
line for an hour or two and briefly pose a flash flood threat as
hourly rain totals to 2.5" appear possible in the expected
environment.

Coastal Pacific Northwest...
A returning warm front ahead of a surface low brings precipitable
water values of 1-1.25" on the heels of 850 hPa inflow of 35-50
kts. The 21z SREF indicates that there could be a smidge of
instability, which when combined with the above factors could lead
to some chance of 0.5" an hour totals, particularly late in the
period (Wednesday night). While two week rainfall through Sunday
morning was modest, ongoing rains and what is expected on Monday
across the Olympic Peninsula should lead to some degree of
saturation. A Marginal Risk was raised as a precaution.

Roth


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
352 PM EDT Sun Oct 24 2021

Valid 00Z Mon Oct 25 2021 - 00Z Thu Oct 28 2021

...Pacific Northwest and California to the Central Rockies...
Days 1-3

The deep closed low which is clearly evident on satellite imagery
today will continue to lift ENE into British Columbia Monday,
while an impressive longwave trough shifts onshore the West Coast.
This trough will then move rapidly eastward towards the Central
Plains by Wednesday, with a subtle negative tilt occurring late in
the forecast period in response to amplification of a shortwave
rotating through the base. As this feature moves eastward, ascent
will remain impressive through height falls and PVA, in addition
to intense upper diffluence as a robust Pacific jet streak,
progged to reach 150 kts, shifts onshore downstream of the primary
trough axis, with a secondary jet max following in its wake.
Pronounced low-level warm and moist advection will drive IVT to
above the climatological maximum as far east as Utah by D2, and
the overlap of intense ascent and anomalous moisture will lead to
widespread precipitation across the West, with snow occurring in
the higher elevations.

The heaviest snow through the period is likely D1 /Sunday night
and Monday/ across the Sierra where the most intense moisture
plume (atmospheric river) pivots orthogonally into the terrain.
This will drive intense upslope enhancement in addition to the
deep layer synoptic ascent, and with PWs progged to be more than 3
standard deviations above the climo mean, snowfall rates of 3"/hr
are likely which will accumulate heavily above 6000 ft. The WSE
10th percentile on D1 is over 20" in the highest peaks above
10,000 ft, and WPC probabilities for 18 inches are more than 90%.
It is likely that some of the higher peaks will exceed 3 feet of
snow D1, with widespread 1-2 ft. Additional heavy snow is likely
in some of the other favored upslope regions including the
Sawtooth, Wind Rivers, and WA Cascades where WPC probabilities for
8 inches are more than 60%.

By D2, the strongest ascent shifts eastward into the Great Basin
and Central Rockies as the trough axis pivots eastward and the
accompanying shortwave amplifies. With still pronounced IVT in
place, this strong ascent has led to an increase in guidance
snowfall, generally above 7000 ft. WPC probabilities have
increased for 6 inches D2, now reaching 50% in parts of the
Wasatch, Uintas, Wind Rivers, Tetons, and San Juans.

With the secondary /upstream/ jet streak moving into the Pacific
NW D2, renewed moist advection and robust diffluence will produce
heavy snow in the WA and OR Cascades where WPC probabilities for 6
inches are above 70%, and locally more than 12 inches of snowfall
is likely. By D3 /Tuesday night and Wednesday/ the snow shuts down
across the Central Rockies, but continued moisture and upper level
ascent in the WA Cascades will create another day of moderate to
heavy snow possibly exceeding 6 inches above 5000 ft. A few inches
of snow is also possible at Washington Pass and Stevens Pass
during the forecast period.


For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
238 AM EDT Mon Oct 25 2021

Valid 12Z Thu Oct 28 2021 - 12Z Mon Nov 01 2021

...Major storm to focus a heavy rainfall/runoff threat from the
Midwest/Southeast Thursday to the Mid-Atlantic/Northeast into next
weekend...


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

A deep cyclone lifting from the Mississippi Valley to the
Northeast this weekend will keep much of the medium range quite
active. The guidance with this system continues to show good
agreement on the large scale evolution/track, increasing
confidence a bit, but do offer some uncertainties in the details,
which may take until the short range to resolve. A general model
blend seemed a good starting point for this system, maintaining
good continuity with the previous shift. This system may threaten
October low pressure records across parts of the mid-Mississippi
to Ohio Valley.

Elsewhere, the last couple of model runs show the next trough
moving towards the West Coast by the middle of this week may split
sending a northern stream shortwave across the northern Rockies
and northern Plains into next weekend. The guidance has trended
more north and stronger with this system suggesting the Northeast
U.S. low may eventually get wrapped up into this system late
period. The last few model runs hint at a possible southern stream
upper low near California next Sat-Mon, but the ensembles offer
plenty of uncertainty on whether this feature even exists at all.

This cycle of the WPC medium range progs used a blend of the
deterministic models days 3-4 transitioning towards and almost
equal weighting of the GFS/ECMWF with the GEFS/ECENS means by day
7. This helps to mitigate the late period differences.


...Weather/Hazards Highlights...

The biggest weather threat during the medium range period will be
the deep low pressure system working across the eastern third of
the U.S. later this week into next weekend bringing widespread
moderate to locally heavy rains. Closed upper low development and
a slowed frontal system progression will allow rainfall to expand
across the Ohio Valley/Appalachians/Southeast later this week then
the Mid-Atlantic/Northeast into next weekend as aided by a
sustained Atlantic moisture feed. Repeat/duration activity and
favored orographic lift should focus a possible flood/runoff
threat for parts of the central Appalachians.

Out West, shortwave energy progressing towards the West Coast will
direct a plume of moisture to support another round of heavy
rain/mountain snows to mainly parts of the Pacific Northwest, but
also farther inland across the northern Rockies on Thursday into
Friday. Conditions should finally dry out by next weekend though
as upper ridging builds over the region.

For temperatures, the Midwest/South to the East Coast should
remain near or more likely below normal through much of the period
underneath cloudy and unstable skies. Humidity levels though
should allow overnight mins to be more above normal. Behind this,
upper ridging shifts across the central U.S. bringing a period of
above normal (+5-10F) days before the next shortwave moderates
temperatures back towards or even below normal. Out West,
temperatures should generally stay within a few degrees on either
side of normal.

Santorelli


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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