Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Tue Oct 26 2021

Valid 00Z Wed Oct 27 2021 - 00Z Fri Oct 29 2021

...The second phase of a nor'easter expected to bring high winds and heavy
rain to coastal New England tonight into Wednesday...

...An anomalous low pressure system expected to trigger severe weather
over the Plains on Wednesday, and stormy conditions across the South,
Southeast, and the Midwest on Thursday...

...Wet pattern continues across the Pacific Northwest and Rockies through
midweek...

An active weather pattern will bring rainy to stormy conditions across
many parts of the country during the next couple of days. First of all, a
complex low pressure system that brought heavy rain near and around the
New York City area this morning is forecast to intensify rapidly as it
rotates back toward the southern New England coast tonight. This second
phase of the nor'easter is expected to pack quite a punch of very strong
winds together with bands of heavy rain as they rotate around the cyclone
center and push onshore tonight. Winds gusting to hurricane force is
possible later tonight near Cape Cod as the center of the nor'easter edges
closer. All of southern New England westward to New York City and
northern New Jersey will be under the threat of flash flooding and
coastal flooding through tonight into early Wednesday. The cyclone center
is forecast to begin moving back out to sea later on Wednesday with winds
and rain gradually tapering off Wednesday night into early Thursday over
much of New England.

Meanwhile, another energetic low pressure system is developing over the
High Plains. This system will first ingest moisture and unstable air from
the Gulf of Mexico to trigger rounds of severe thunderstorms across the
central and southern Plains ahead of a vigorous cold front and a dry line
later tonight into Wednesday. By Wednesday night, the threat of severe
weather will shift into the Deep South and down near the Gulf Coast as an
anomalously large and deep low pressure system continues to develop in the
mid-section of the country. An axis of moderate to heavy rain is also
expected to extend north of the system across the Mid-Mississippi Valley
into the Midwest. By Thursday evening, widespread moderate to locally
heavy rain is forecast to engulf the Tennessee and Ohio Valleys, the lower
Great Lakes, as well as into the Mid-Atlantic region. Some locations in
the lower to mid-Mississippi Valley could see the barometer dipping to
record low levels for the month of October.

For the Pacific Northwest, a series of low pressure systems moving onshore
will prolong the wet pattern for several more days. Low elevation rain
and high elevation heavy snow will continue to impact much of the coastal
Pacific Northwest and Cascades respectfully. A couple inches of rain are
possible for the low elevations while 1-2 feet of snow is likely across
much of the highest peaks of the northern Cascades. Lighter snow amounts
are expected over much of the high Rocky Mountain peaks. Cool conditions
across much of the West today will return to seasonal levels on Wednesday
behind the developing cyclone over the central U.S. Meanwhile, dry
conditions will prevail over the Southwest.

Kong


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Tue Oct 26 2021

Valid 12Z Fri Oct 29 2021 - 12Z Tue Nov 02 2021

...Major storm to focus a heavy rainfall/flood threat for parts of
the Mid-Atlantic and the Northeast Friday into Saturday...


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

An anomalously deep cyclone, capable of threatening October low
pressure records for some places in the Ohio Valley, will be
tracking into the Ohio Valley and Mid-Atlantic by the start of the
medium range period on Friday. This will likely bring a period of
active weather across the eastern U.S. for the latter part of this
week and into the weekend. Model guidance over the past few cycles
has generally shown good agreement on the large scale evolution
and track of this system, with some lingering uncertainties for
timing and details. A 00Z/06Z model composite represents consensus
well for this system and maintains good continuity with the
previous forecasts.

Behind this system the large scale flow aloft will trend toward a
strong ridge over western Canada and a mean trough eventually
covering the eastern half of Canada along with the northern lower
48. At the leading edge of the advancing upper trough, a frontal
system reaching the Pacific Northwest by early Friday will reach
the Plains this weekend. The models have recently come into better
agreement with the associated upper-level shortwave as it crosses
the northern tier of the U.S. and the upper trough core deepens
over central Canada. The front should reach the Northeast by early
next week.

Greater uncertainty exists over the eastern Pacific and western
U.S. A compact upper low should track into the
central/north-central West Coast by Saturday but the models are
still in the process of resolving the specifics. Latest GFS runs
have been a bit on the fast and/or south side of the guidance
spread. At least into Saturday the 12Z UKMET/CMC have gravitated
closer to the 00Z ECMWF versus their old 00Z runs that were
farther north. On the other hand the new 12Z ECMWF has adjusted
north of the new UKMET/CMC. The complexities increase by early
next week. Not only will there be lingering issues for the
weakening energy that remains from this upper low, but also
important detail differences for a reinforcing shortwave dropping
into the northern U.S. (some combination of the two possibly
supporting frontal waviness east of the Rockies). There is also
uncertainty over what proportion of eastern Pacific shortwave
energy may ultimately cut through a moderate upper ridge that the
ensemble means build along or inland from the West Coast--which
will play a significant role in the coverage and intensity of any
precipitation over the West. These issues tend to have lower
predictability by 6-7 days out in time, favoring a transition to a
model/ensemble mean blend that provides reasonable continuity
while awaiting more confident trends/clustering.

...Weather/Hazards Highlights...

The biggest weather threat during the medium range period will be
the deep low pressure system working across the Eastern U.S. later
this week into next weekend, bringing widespread moderate to heavy
rain. By Friday, the heaviest rainfall should be impacting much of
the Mid-Atlantic and into the lower Great Lakes, lifting into the
Northeast by Saturday. A sustained Atlantic moisture feed ahead of
the front anchored by the surface low will likely focus a possible
flood/runoff threat across parts of these regions.

The front moving through the Northwest will continue organized
rainfall and mountain snow across parts of the northern
Rockies/Pacific Northwest on Friday, with conditions finally
beginning to dry out by next weekend as upper ridging builds
behind the departing trough. This weekend the moisture should sink
southward into the Great Basin and the central Plains, with some
snow possible during the colder overnight hours even at the lower
elevations of the High Plains. From the weekend into early next
week there is lower confidence with the precise coverage and
intensity of precipitation from the West into the Plains, based on
the uncertainty of important details of flow aloft. Locations from
the southern Plains into the Midwest and Ohio Valley should see
expanding coverage of rainfall along the potentially wavy front by
early next week. Most of the rain should be on the light to
moderate side but a few pockets of heavier activity could be
possible. Some Pacific moisture may reach the northern half of the
West Coast early next week but again with low confidence in the
details at this time.

For temperatures, much of the Southeast/Central Gulf region should
see highs up to 10-15F below normal beneath the trough extending
south from the upper low tracking into the central Appalachians by
Saturday. Highs should trend back towards normal as the low lifts
into the Northeast. The central U.S. will see a wide variation of
temperatures over the course of the period. First an area of plus
10-15F or so anomalies for highs will drop from the northern
Plains/central High Plains into the southern High Plains and then
far southern Texas Friday-Sunday. This warmth will be replaced by
an expanding area of below normal readings--initially over Montana
on Saturday and then reaching at least as far as the central
Plains and Midwest by early next week, aided by advancing surface
high pressure. Areas from the northern High Plains through the
south-central Plains and east into the Middle Mississippi Valley
should see one or more days with highs 5-15F below climatological
values for this time of year. Anomalies for min temperatures
should be less extreme but could still be noteworthy for producing
the first freeze in some areas from the central Plains into the
Midwest around next Tuesday.

Rausch/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

Hazards:
- Heavy rain across portions of the Mid-Atlantic, the Northeast,
and the Great Lakes, Fri, Oct 29.
- Heavy rain across portions of New England, Sat, Oct 30.
- Flooding possible across portions of the Central Plains, the
Mid-Atlantic, and the Northeast.
- Flooding likely across portions of the Middle Mississippi
Valley, the Great Lakes, and the Ohio Valley.
- Much below normal temperatures across portions of the Central
Plains and Midwest, Tue, Nov 2.

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 Tue Oct 26 2021

Valid 00Z Wed Oct 27 2021 - 00Z Wed Nov 03 2021

Trade winds should remain weak through this week and into this
week allowing for a drier and more stable pattern over Hawaii. Any
showers are likely to stay confined to windward facing terrain
areas. The guidance continues to show that upstream upper ridge
amplification over the west-central Pacific should allow rounding
shortwaves to dig southeastward towards Hawaii, which could bring
brief disruptions and shifts towards more northerly trades later
this week and next weekend. 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
840 PM EDT Tue Oct 26 2021

Day 1
Valid 01Z Wed Oct 27 2021 - 12Z Wed Oct 27 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHEAST...

01Z Update...
The powerful storm that has developed southeast of New England
this evening is forecast track to the west and then southwest
overnight. Upper level energy along with low-to-mid level
frontogenesis will support some additional bands of moderate to
heavy rain moving southwest across southern New England,
southeastern New York, and northern New Jersey through the evening
into the overnight. A band of intense rainfall currently moving
west across Cape Cod is expected to contribute to heavy rainfall
amounts this evening across portions of eastern Massachusetts and
Rhode Island. It is unclear how far west this band will persist,
however recent runs of the HRRR do show it producing another
period of moderate rain overnight for Connecticut and the NYC
metro. Meanwhile, strong onshore flow north of the low is
expected to support periods of at least moderate rain through the
overnight across portions eastern Massachusetts, including the
Boston metro, and Rhode Island. Therefore, a Slight Risk was
maintained for parts of eastern Massachusetts, Rhode Island, as
well as western Long Island and the NYC metro. A Marginal Risk
was maintained for areas farther north and west, including
portions of Upstate New York and western New England. While
widespread additional heavy amounts are not expected across these
areas, recent runs of the HRRR have shown some signal for locally
moderate to heavy amounts, which may result in additional runoff
concerns given the heavy rains that have already occurred today.
Latest 3-hr flash flood guidance values are an inch or less for
most areas along and west of the Hudson Valley into the Catskills
and northern New Jersey.

Pereira

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Tue Oct 26 2021

Day 2
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...

21Z Update...

Southwest/Lower Mississippi into Northern Mississippi Valley...
As an upper low digs across the central US, modest instability and
increasing PW values leads to chances of flash flooding. Much of
the activity will likely be more on the progressive side of
things, but flooding potential still exists with guidance showing
chances to see brief periods of 1 to 2 inches per hour rainfall
rates. Did extend the Marginal Risk northward to encompass
portions of northern MS valley where QPF values have increased
steadily in past 24hrs.

Pacific Northwest...
Latest QPF tends to remain on the lower end, but with recent
rainfall and soils saturated to well above normal, have opted to
leave in the Marginal Risk that was previously in play.

Chiari

Previous Discussion...

ArkLaTex/Central Gulf coast...
A couple batches of convection are expected in the vicinity of the
ArkLaTex and 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,
with some upward trends noted in the guidance since this time last
night. 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). Two week rainfall through Monday
morning is showing some small patches of 200% of average.
Rainfall on Tuesday across the Olympic Peninsula should lead to
somewhat more saturation. A Marginal Risk remains in the area as
a precaution.

Roth

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Tue Oct 26 2021

Day 3
Valid 12Z Thu Oct 28 2021 - 12Z Fri Oct 29 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE PACIFIC NORTHWEST COAST & BLUE RIDGE MOUNTAINS...

21Z Update...
Latest QPF guidance has increased through the Blue Ridge Mountains
of NC/VA over the past 12 hours leading to an increase in the WPC
forecast for the period. This area has been on the drier side over
the past 2 weeks, but given the terrain isolated flash flooding is
certainly possible. Have kept the Marginal Risk area in place to
cover this threat. No changes were made to the Marginal Risk area
for WA either as soils remain well above normal and additional
rainfall may lead to isolated flash flooding.

Chiari

Previous Discussion...

Pacific Northwest...
Onshore flow of up to 50 kts, precipitable water values at or
above 1.25", with a smidge of instability should weaken and shift
south later in the period (Thursday afternoon onward) as a cold
front shifts south and a low crosses southwest Canada.
Considering the degree of inflow and modicum of instability,
believe hourly rain totals of 0.5"+ are possible so included
western WA and far northwest OR in a Marginal Risk area. Two week
precipitation anomalies are slowly increasing with some areas of
200%+ of average as of Monday morning and rainfall from previous
days (Tuesday and Wednesday) should continue that trend.

Blue Ridge Mountains of NC/VA...
Inflow at 850 hPa of 30-60 kts will come in perpendicular to the
terrain for roughly 12 hours ahead of a near-record deep for
October occluded cyclone moving across the Mid-South and Lower OH
Valley. Precipitable water values of 1.25"+ should be available,
so when the instability is there (500+ J/kg), hourly rain totals
to 1.5" are possible. However, the guidance advertises this
window to be quite short so overall totals are forecast to be
modest. Two week precipitation anomalies are well below average
but the terrain is rugged. Autumn leaf fall could cause clogging
drainage as well. Thought it prudent to raise a Marginal Risk due
to the above.

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
433 PM EDT Tue Oct 26 2021

Valid 00Z Wed Oct 27 2021 - 00Z Sat Oct 30 2021

...North-Central Rockies...
Day 1...

Longwave trough and front-side vort max will exit Colorado this
evening with rising heights in its wake early Wednesday. Lingering
snow will wind down over elevations generally above 8000-9000 ft
as surface low pressure moves into the Plains. WPC probabilities
for 4 inches or more are around 30-50% for the highest peaks in
the CO Rockies, especially early on Day 1.


...Pacific Northwest...
Days 1-3...

Active North Pacific Jet will maintain a rather steady fetch of
moisture into the Pacific Northwest in quasi-zonal flow. 130+ kt
jet will lift northward Wednesday into early Thursday bringing in
anomalous precipitable water values of +1 to +2.5 sigma ahead of a
cold front. Combined with favorable onshore flow and broad-scale
forcing for ascent, widespread precipitation is expected over the
region for the next few days. Snow levels will start modestly low
around 5000 ft day 1 but then rise to around 8000-9000 ft as
milder air moves in from the WSW Thursday. As the cold front moves
ashore by Thursday afternoon, snow levels will drop back to near
or below 5000 ft as the precipitation moves southeastward out of
the Cascades.

The highest peaks of the Washington Cascades will likely see
significant snow accumulations of several feet owing to plentiful
QPF. WPC probabilities for at least 8 inches of snow are 30-80% in
higher elevations day 1 but 10-50% day 2 and below 10% day 3.
Farther east, day 1 probabilities of at least 4 inches of snow are
30-60% over parts of the Bitterroots and northwestern Montana as
moisture with the lead system today streams eastward.

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


Fracasso

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
424 PM EDT Tue Oct 26 2021

Valid 12Z Fri Oct 29 2021 - 12Z Tue Nov 02 2021

...Major storm to focus a heavy rainfall/flood threat for parts of
the Mid-Atlantic and the Northeast Friday into Saturday...


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

An anomalously deep cyclone, capable of threatening October low
pressure records for some places in the Ohio Valley, will be
tracking into the Ohio Valley and Mid-Atlantic by the start of the
medium range period on Friday. This will likely bring a period of
active weather across the eastern U.S. for the latter part of this
week and into the weekend. Model guidance over the past few cycles
has generally shown good agreement on the large scale evolution
and track of this system, with some lingering uncertainties for
timing and details. A 00Z/06Z model composite represents consensus
well for this system and maintains good continuity with the
previous forecasts.

Behind this system the large scale flow aloft will trend toward a
strong ridge over western Canada and a mean trough eventually
covering the eastern half of Canada along with the northern lower
48. At the leading edge of the advancing upper trough, a frontal
system reaching the Pacific Northwest by early Friday will reach
the Plains this weekend. The models have recently come into better
agreement with the associated upper-level shortwave as it crosses
the northern tier of the U.S. and the upper trough core deepens
over central Canada. The front should reach the Northeast by early
next week.

Greater uncertainty exists over the eastern Pacific and western
U.S. A compact upper low should track into the
central/north-central West Coast by Saturday but the models are
still in the process of resolving the specifics. Latest GFS runs
have been a bit on the fast and/or south side of the guidance
spread. At least into Saturday the 12Z UKMET/CMC have gravitated
closer to the 00Z ECMWF versus their old 00Z runs that were
farther north. On the other hand the new 12Z ECMWF has adjusted
north of the new UKMET/CMC. The complexities increase by early
next week. Not only will there be lingering issues for the
weakening energy that remains from this upper low, but also
important detail differences for a reinforcing shortwave dropping
into the northern U.S. (some combination of the two possibly
supporting frontal waviness east of the Rockies). There is also
uncertainty over what proportion of eastern Pacific shortwave
energy may ultimately cut through a moderate upper ridge that the
ensemble means build along or inland from the West Coast--which
will play a significant role in the coverage and intensity of any
precipitation over the West. These issues tend to have lower
predictability by 6-7 days out in time, favoring a transition to a
model/ensemble mean blend that provides reasonable continuity
while awaiting more confident trends/clustering.

...Weather/Hazards Highlights...

The biggest weather threat during the medium range period will be
the deep low pressure system working across the Eastern U.S. later
this week into next weekend, bringing widespread moderate to heavy
rain. By Friday, the heaviest rainfall should be impacting much of
the Mid-Atlantic and into the lower Great Lakes, lifting into the
Northeast by Saturday. A sustained Atlantic moisture feed ahead of
the front anchored by the surface low will likely focus a possible
flood/runoff threat across parts of these regions.

The front moving through the Northwest will continue organized
rainfall and mountain snow across parts of the northern
Rockies/Pacific Northwest on Friday, with conditions finally
beginning to dry out by next weekend as upper ridging builds
behind the departing trough. This weekend the moisture should sink
southward into the Great Basin and the central Plains, with some
snow possible during the colder overnight hours even at the lower
elevations of the High Plains. From the weekend into early next
week there is lower confidence with the precise coverage and
intensity of precipitation from the West into the Plains, based on
the uncertainty of important details of flow aloft. Locations from
the southern Plains into the Midwest and Ohio Valley should see
expanding coverage of rainfall along the potentially wavy front by
early next week. Most of the rain should be on the light to
moderate side but a few pockets of heavier activity could be
possible. Some Pacific moisture may reach the northern half of the
West Coast early next week but again with low confidence in the
details at this time.

For temperatures, much of the Southeast/Central Gulf region should
see highs up to 10-15F below normal beneath the trough extending
south from the upper low tracking into the central Appalachians by
Saturday. Highs should trend back towards normal as the low lifts
into the Northeast. The central U.S. will see a wide variation of
temperatures over the course of the period. First an area of plus
10-15F or so anomalies for highs will drop from the northern
Plains/central High Plains into the southern High Plains and then
far southern Texas Friday-Sunday. This warmth will be replaced by
an expanding area of below normal readings--initially over Montana
on Saturday and then reaching at least as far as the central
Plains and Midwest by early next week, aided by advancing surface
high pressure. Areas from the northern High Plains through the
south-central Plains and east into the Middle Mississippi Valley
should see one or more days with highs 5-15F below climatological
values for this time of year. Anomalies for min temperatures
should be less extreme but could still be noteworthy for producing
the first freeze in some areas from the central Plains into the
Midwest around next Tuesday.

Rausch/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

Hazards:
- Heavy rain across portions of the Mid-Atlantic, the Northeast,
and the Great Lakes, Fri, Oct 29.
- Heavy rain across portions of New England, Sat, Oct 30.
- Flooding possible across portions of the Central Plains, the
Mid-Atlantic, and the Northeast.
- Flooding likely across portions of the Middle Mississippi
Valley, the Great Lakes, and the Ohio Valley.
- Much below normal temperatures across portions of the Central
Plains and Midwest, Tue, Nov 2.

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