Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
456 AM EDT Sun Oct 25 2020

Valid 12Z Sun Oct 25 2020 - 12Z Tue Oct 27 2020

...Widespread snowfall across the northern Rockies and the northern Plains
today will push south into the central/southern Rockies and then turn into
an ice-storm for the southern High Plains on Monday and early Tuesday...

...Record cold temperatures continue to engulf the Rockies and the Plains
over the next few days...

...Critical to extreme fire risks and high wind threat over portions of
California; elevated to critical over the southern Rockies...

A low pressure system interacting with a surge of arctic air from western
Canada continues to deliver widespread snowfall from the northern Rockies
to the northern Plains early this Sunday morning. As the entire storm
system continues to push southward, moderate to heavy snow is expected to
develop in the central Rockies and central Plains today before turning
into mixed precipitation over the southern High Plains on Monday into
early Tuesday. More than a foot of snow with gusty winds and plummeting
temperatures is forecast for the higher terrain across the central Rockies
while a wide swatch of 6 inches or more of snow is expected across much of
the central to southern High Plains. Lighter amounts of snow and wintry
mix will also extend all the way into the upper Midwest. Temperatures
will fall to record levels from the interior western U.S. to a large
portion of the Great Plains for the next couple of days due to this early
season Arctic blast. Much of the northern and central Rockies are
forecast to witness sub-zero temperatures Sunday and Monday mornings. In
addition, strong winds behind the Arctic front will cause blowing and
drifting of snow with blizzard conditions possible in the windiest
locations. Bitterly cold wind chills ranging between -10 and -20 are also
expected in portions of the northern Rockies and northern Plains. This
will be in stark contrast with high temperatures still in the 90s over
western Texas today!

Dry air and gusty winds associated with the large arctic high pressure
system are expected to trigger Santa Ana winds that could raise fire
danger to extreme levels for parts of California. Conditions in
California will be most critical on Monday as the strong cold front is
forecast to bring high winds and low humidity down from the Great Basin.
Winds are forecast to be strong enough to result in downed trees and power
outages. Wind advisories have been issued for the entire Bay Area and
High Wind Watches along the higher elevations of southern California.
Farther east, elevated to critical fire risks can be expected over the
southern Rockies.

Meanwhile, colder air is filtering into the East Coast behind a cold
front. The front is forecast to stall across the Southeast, allowing
showers and some thunderstorms to ride up the front toward the
Mid-Atlantic today. The rain should be moving off the coast Sunday night
leading to mostly fine and warm weather over the Southeast on Monday.
However, some tropical downpours and thunderstorms are expected to
continue over southern Florida into early next week as Tropical Storm Zeta
forms over the western Caribbean Sea in this record-breaking hurricane
season. Residents along the Gulf Coast should continue to monitor the
latest forecasts for Zeta from the National Hurricane Center throughout
the weekend and into next week.

Kong/Mullinax


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sun Oct 25 2020

Valid 12Z Wed Oct 28 2020 - 12Z Sun Nov 01 2020

...Heavy precipitation threat from the Plains/Mid-Mississippi
Valley eastward to the Mid-Atlantic later this week...


...Guidance Evaluation/Preferences...

Deep closed low will exit out of New Mexico early Wed and travel
ENE to the Mid-Atlantic by Friday as it weakens into an open wave.
Just ahead of its path, newly formed Tropical Storm Zeta will
likely make landfall on Wednesday per the National Hurricane
Center just ahead of the upper low and its surface fronts. The two
sources of moisture will merge into/along the frontal boundary
over the Tennessee Valley Thursday and move to the East Coast
Friday. The GFS runs remained quicker with the timing of both the
upper low and Zeta while the UKMET was slowest. Preferred the more
stable guidance near the 12Z ECMWF/Canadian and ECMWF ensemble
mean along with the slightly quicker GEFS mean for the upper low.
Zeta lied closer to the GFS but also noted an unusually high
degree of uncertainty not uncommon in nascent storms. How the
interaction evolves will dictate the axis/orientation of heavy
rain but the event itself looks quite probable. Pattern becomes
much quieter by next weekend CONUS-wide.


...Weather/Hazard Highlights...

Very cold airmass in the Plains will moderate but many areas will
see see temperatures 10-30 degrees colder than normal on
Wednesday. To the east of the upper low, near to above normal
temperatures will trend toward near to below normal as the system
progresses eastward. In the Pacific Northwest, California and into
the Great Basin region, temperatures will be 5-15 degrees above
normal through most of the period.

Over the Southeast, moisture from Zeta will stream northward
toward the in situ frontal boundary on Wednesday into Thursday
just ahead of the closed low. Heavy rain and flooding are possible
from the mid-Mississippi Valley to the Tennessee Valley. As the
system moves eastward en masse, rainfall will continue to focus
along the boundary to the Mid-Atlantic. Guidance remains
inconsistent where this may be and will depend on the interaction
of the two features at the start of the period. On the northwest
side of the upper low, snow is likely over New Mexico to the TX/OK
panhandles and southwestern KS. With marginally cold air in parts
of the Northeast, snow is possible in higher elevations as the
precipitation streams eastward (primarily in a warmer air mass).
The Pacific Northwest will see a few bouts of mainly light rain
and higher elevation snow.

Fracasso

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
319 AM EDT Sun Oct 25 2020

Valid 00Z Mon Oct 26 2020 - 00Z Mon Nov 02 2020

...Heavy rain possible mid week...

Guidance indicates an upper low developing Monday to the northwest
of the area, dropping southward or southwestward toward Hawai'i.
The 00Z ECMWF was still the farthest west of the deterministic
models followed by the UKMET while the Canadian and GFS tool the
upper low southward along 160W over Kauai late Wed. The ensemble
spread has decreased a bit, mainly be eliminating farther east
members, but a much farther ECMWF-like scenario remains in play.
Prefer to stay somewhat in the middle or larger consensus near the
00Z GFS/Canadian until a larger shift in the ensembles develops.
Regardless, this would bring in deep tropical moisture over the
island chain (precipitable water values over 2 inches) around
Tuesday for at least a few days but perhaps continuing into next
weekend. Several inches of rain are possible each day in this
scenario which may result in localized flooding.

Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Sun Oct 25 2020

Day 1
Valid 16Z Sun Oct 25 2020 - 12Z Mon Oct 26 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHERNMOST AREAS IN SOUTH FLORIDA...

1600 UTC update

Only some small changes made to the previous Excessive Rainfall
Outlook across South Florida. The slight risk area was extended
slightly northward to the north of FLL and toward PBI. This was
to capture activity enhancing late morning along the coast. The
slight risk was also trimmed to the west of the urban areas across
South Florida where runoff issues should be less likely.

Oravec


0900 UTC discussion

Tropical moisture continues to expand well northeast of what is
now Tropical Storm Zeta, basically stretching northeast within a
mid-level Col. The environment over South Florida remains
conducive for additional heavy rainfall, with the persistent,
anomalous low-level moisture transport leading to periodic bands
of heavier showers and thunderstorms. PW values peaking between
2.3 to 2.4", along with mixed-layer CAPEs of 500-1000 j/kg, will
foster locally intense rainfall rates underneath the strongest
cells. Fortunately, as has been the case of late, these heavier
rain bands are expected to remain progressive and more
intermittent than otherwise. Additional localized rainfall of 3 to
6+ inches is expected through 12Z Monday, though the 00Z WRF-ARW
and NAM CONUS-Nest show pockets of 8+ inches.

WPC upgraded a small portion of the Marginal Risk to Slight Risk
across the southernmost area in South Florida -- including Miami
and Pompano Beach -- where rainfall over the past week-plus has
averaged 300-600+ percent of normal. Consequently, the current
flash flood guidance remains low -- with pockets of 1.5-2.0"/hr
and 2-2.5"/3hr FFGs within the Slight Risk area. Current HREF
probabilities of 3-hourly QPF exceeding 2.0" range between 40-60%
this morning and again this afternoon, which is certainly not
surprising (and likely on the low end) considering the favorable
thermodynamic environment.

Hurley


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sun Oct 25 2020

Day 2
Valid 12Z Mon Oct 26 2020 - 12Z Tue Oct 27 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN PLAINS...

...Southern Plains...
There will be the potential for some localized heavy rainfall over
parts of the southern Plains Monday and Monday night. The region
will be located between a broad positively tilted trough making
its way across the western United States and a mid-level ridge
positioned over northern Florida. The broad southerly flow should
result in decent moisture advection/moisture transport from the
Gulf despite the presence of a cold but shallow airmass already in
place near the surface. In addition, a front approaching from the
west combined with the upper level support suggest that there will
be good coverage of showers and some thunderstorms from parts of
the Red River Valley of Texas across southeast Oklahoma into
southern Missouri and northwestern Arkansas. Have been suspicious
of the operational GFS runs over the past few days given its speed
and the magnitude of moisture return the model brings to the
region...which results in the operational run being much wetter
compared to the GEFS ensemble mean and compared to other
operational models.

The limiting factors with regard to flooding include the lack of
any substantial instability to help drive the rainfall rates in a
region which has largely experienced 25 percent of normal rainfall
(or less) during the previous 14 days. Considering that forecast
soundings from the GFS and ECMWF show precipitable water values
peaking close to 1.5 inches...think that the isolated rainfall
maximum amounts will be close to a non-GFS consensus in the 1.5 to
2.5 inches. Given the Flash Flood Guidance in the area, still
feel that nothing more than Marginal Risk is needed at this point.

Bann


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Sun Oct 25 2020

Day 3
Valid 12Z Tue Oct 27 2020 - 12Z Wed Oct 28 2020

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Opted against continuing a Marginal Risk area from the Day 2
period into the Day 3 period across parts of Oklahoma on Tuesday.
The focus for heaviest rainfall should be shifting towards the
west in response to the approach of the upper low...and focusing
west of the area of heaviest rainfall expected on Monday. The GFS
continued to be very wet here compared to its own ensemble mean
and other numerical guidance...apparently ingesting more moisture
from the Gulf of Mexico into the mid-latitude system. A non-GFS
consensus yields precipitation amounts below the 1- and 3-hourly
Flash Flood Guidance. Later shifts can re-evaluate the need for
an ERO area.

Bann


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
425 AM EDT Sun Oct 25 2020

Valid 12Z Sun Oct 25 2020 - 12Z Wed Oct 28 2020


...Winter Storm impacting the western and central U.S....

An amplifying upper trough will continue to bring sharply colder
temperatures, along with moderate to heavy snows farther south
through the central Rockies and Plains on Sunday. A well-defined
shortwave currently centered over the northwestern U.S. will
continue to dig south into the Great Basin on Sunday. Increasing
lift and moisture ahead of the system, interacting with cold air
pushing south through the Plains and Rockies, will support
widespread snows across the central Rockies and Plains. While the
heaviest totals are expected across the higher elevations of the
Colorado Rockies, where WPC PWPF is indicating widespread amounts
of a foot or more, impactful accumulations are also expected
farther east, including along the I-25 corridor. Increasing low
level convergence/easterly flow, is expected to help support
heavier totals, with WPC PWPF continuing to show probabilities of
50 percent or greater for accumulations of 8-inches or more across
this region, including the Denver metro area.

On Monday, models show the upper wave continuing to drop farther
south, with a closed circulation beginning to develop near the
Four Corners region. By early Monday, drier conditions can be
expected across northern Colorado. However, snows are expected to
continue across southern Colorado while extending south into New
Mexico, with additional heavy accumulations likely across the San
Juan and Sangre de Cristo mountains. Meanwhile, snows will also
begin to develop farther southwest across northern and central
Arizona, with significant accumulations possible along the
Mogollon Rim and White Mountains.

By early Monday, southwesterly flow aloft, moving warm moist air
over shallow cold air sliding south, will set the stage for a
wintry mix, with accumulating ice becoming likely across portions
of Northwest Texas and western to central Oklahoma. Periods of
freezing rain are likely to continue through Tuesday into early
Wednesday across portions of western Texas and Oklahoma, while
spreading farther north into Kansas. While confidence in the
details is limited, latest WPC PWPF shows 50 percent of higher
probabilities for ice accumulations exceeding 0.10 inch from the
eastern Texas Panhandle through much of western Oklahoma.

Pereira

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sun Oct 25 2020

Valid 12Z Wed Oct 28 2020 - 12Z Sun Nov 01 2020

...Heavy precipitation threat from the Plains/Mid-Mississippi
Valley eastward to the Mid-Atlantic later this week...


...Guidance Evaluation/Preferences...

Deep closed low will exit out of New Mexico early Wed and travel
ENE to the Mid-Atlantic by Friday as it weakens into an open wave.
Just ahead of its path, newly formed Tropical Storm Zeta will
likely make landfall on Wednesday per the National Hurricane
Center just ahead of the upper low and its surface fronts. The two
sources of moisture will merge into/along the frontal boundary
over the Tennessee Valley Thursday and move to the East Coast
Friday. The GFS runs remained quicker with the timing of both the
upper low and Zeta while the UKMET was slowest. Preferred the more
stable guidance near the 12Z ECMWF/Canadian and ECMWF ensemble
mean along with the slightly quicker GEFS mean for the upper low.
Zeta lied closer to the GFS but also noted an unusually high
degree of uncertainty not uncommon in nascent storms. How the
interaction evolves will dictate the axis/orientation of heavy
rain but the event itself looks quite probable. Pattern becomes
much quieter by next weekend CONUS-wide.


...Weather/Hazard Highlights...

Very cold airmass in the Plains will moderate but many areas will
see see temperatures 10-30 degrees colder than normal on
Wednesday. To the east of the upper low, near to above normal
temperatures will trend toward near to below normal as the system
progresses eastward. In the Pacific Northwest, California and into
the Great Basin region, temperatures will be 5-15 degrees above
normal through most of the period.

Over the Southeast, moisture from Zeta will stream northward
toward the in situ frontal boundary on Wednesday into Thursday
just ahead of the closed low. Heavy rain and flooding are possible
from the mid-Mississippi Valley to the Tennessee Valley. As the
system moves eastward en masse, rainfall will continue to focus
along the boundary to the Mid-Atlantic. Guidance remains
inconsistent where this may be and will depend on the interaction
of the two features at the start of the period. On the northwest
side of the upper low, snow is likely over New Mexico to the TX/OK
panhandles and southwestern KS. With marginally cold air in parts
of the Northeast, snow is possible in higher elevations as the
precipitation streams eastward (primarily in a warmer air mass).
The Pacific Northwest will see a few bouts of mainly light rain
and higher elevation snow.

Fracasso

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