Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Thu Sep 16 2021

Valid 00Z Fri Sep 17 2021 - 00Z Sun Sep 19 2021

...Post-Tropical Cyclone Nicholas will continue to bring a threat of
locally heavy rain and flash flooding to portions of the Gulf Coast states
over the next couple of days...

...Severe thunderstorms will be possible late this afternoon and this
evening across Minnesota into southeast South Dakota and parts of
Nebraska...

...First atmospheric river of the season will bring very beneficial and
welcomed rains to drought and wildfire-stricken areas of the Pacific
Northwest going into the weekend...

...Very warm, windy, and dry conditions will maintain a wildfire threat
across many areas of the Intermountain West and the northern High Plains
for the next couple of days...

Post-Tropical Cyclone Nicholas continue to linger over far southern
Louisiana this afternoon and should very slowly move northward over the
next couple of days before then dissipating. The system will continue to
bring areas of locally heavy showers and thunderstorms to portions of the
Gulf Coast states for the next couple of days. This will drive concerns
for some additional flash flooding, and the Weather Prediction Center has
depicted a Slight Risk of excessive rainfall for the central Gulf Coast
region stretching from southeast Louisiana east to the western Florida
Panhandle.

Meanwhile, a cold front currently settling southeast across the central
and northern Plains and adjacent areas of the upper Midwest will bring
scattered thunderstorms late this afternoon and evening across this
region, and there will be a threat for some of these thunderstorms to be
severe. In fact, the Storm Prediction Center has depicted a Slight Risk of
severe weather going into the evening hours across Minnesota, southeast
South Dakota, and parts of Nebraska with concerns for supercell
thunderstorms that will be capable of producing large hail, damaging
winds, and a few tornadoes. As the front moves through the Great Lakes
region on Friday and the Ohio Valley early this weekend, there will be
additional scattered areas of showers and thunderstorms associated with
it, but severe weather is generally not anticipated across these areas at
this time.

A well-defined atmospheric river associated with a strong low pressure
system advancing through the Gulf of Alaska will begin impacting the
Pacific Northwest by early Friday morning and will continue doing so
through the weekend. This is the first atmospheric river of the season and
it is expected to bring locally several inches to the coastal ranges of
Washington and Oregon, and also interior areas of the Cascades. Rain
should advance down into at least northern California including the
northern Sierra-Nevada by late in the weekend. This will be very
beneficial and welcomed rain given the current widespread drought
conditions and ongoing wildfire activity.

This system will move into the Rockies by Sunday and much colder
temperatures will settle in behind the frontal passage across the
Northwest which will yield temperatures of as much as 15 to 20 degrees
below average for this time of the year by the end of the weekend.
Conversely, the temperatures out ahead of the cold front will be quite
warm and this will result in a large area of the central and northern High
Plains seeing temperatures at least 15 to 20 degree above average. The
very warm, windy and dry conditions expected ahead of the cold front will
also yield an elevated threat of wildfire activity from areas of the
Intermountain West east into the northern High Plains, and there are
currently Red Flag Warnings in effect to address the wildfire concerns
over the next couple of days.

Orrison

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 AM EDT Fri Sep 17 2021

Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021

...Weather Pattern Overview...

The strong storm system that is currently approaching the Pacific
Northwest is expected to be over the central and northern Plains
by Monday morning, accompanied by a rather strong cold front that
should steadily progress across the central and then eastern U.S.
The large surface high and upper ridge across the eastern U.S.
will eventually give way to the approaching trough and cold front,
which will herald a pattern change for much of the eastern half of
the nation. In addition, a second storm system is forecast to
reach the northwestern U.S. by Wednesday and into Thursday as the
storm track across the northeast Pacific remains active next week.

...Overview and Guidance Evaluation/Preferences...

The deterministic guidance is in above average agreement for the
beginning of next week, with the 18/00Z GFS beginning to show a
faster solution by Tuesday night with the trough/cold front
crossing the Great Lakes and Midwest region. This is even more
apparent with the second trough crossing the northern Rockies and
northern Plains by Thursday, and also ahead of its own ensemble
mean. By next Friday, model spread becomes substantial across
much of the northern tier states and southern Canada, with much of
this owing to the eventual evolution of a possible closed upper
low over southern Manitoba as the second Pacific trough moves
inland.

The WPC front/pressures forecast was primarily based on a nearly
equal blend of the 12Z deterministic guidance through early
Wednesday, and then primarily the 12Z GFS/ECMWF and their
respective ensemble means through Thursday and Friday. The 12Z
run of the GFS was substituted for the 18Z run since the latter
was considerably faster with the cold front across the eastern
U.S. compared to the model consensus, whereas the former is more
in line with the CMC/UKMET/ECMWF.

...Weather and Potential Hazards...

Widespread showers and scattered thunderstorms are expected to
develop along and ahead of the cold front from the Midwest to the
Northeast and Mid-Atlantic states, with some locations getting on
the order of 1 to 2 inches of rainfall with locally higher totals
possible. Some strong to severe storms will also be possible
where favorable parameters of wind shear and instability exist,
although location specifics are uncertain. Winter weather will
also be likely across the higher terrain of northern Wyoming and
western Montana with accumulating snow likely on Monday, followed
by a drier weather pattern by Tuesday and beyond. There may also
be enhanced fire weather concerns for portions of the West,
depending on the magnitude of the pressure gradient and resulting
winds in the wake of the cold front.

It will still feel like summer across much of the southern Plains
and extending northward to the Great Lakes and the Ohio Valley to
begin the week, and more like late October across much of the
Rockies and Intermountain West with highs running 10 to 20 degrees
below average. These cooler temperatures are expected to reach
the Midwest by midweek and finally to the East Coast by Thursday
and Friday, along with much more pleasant humidity levels.

Hamrick


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
157 AM EDT Fri Sep 17 2021

Valid 00Z Sat Sep 18 2021 - 00Z Sat Sep 25 2021

The upper level ridge initially centered north of Hawaii late this
week will build in over the state through the upcoming weekend,
with the main subtropical surface high remaining anchored in place
to the north. A col in the ridge axis may develop early next week
to the east of the Big Island, with perhaps an inverted trough
axis moving westward across the state. Both the GEFS and ECENS
means are in good overall agreement across the Hawaii domain.

It appears the greatest moisture and rainfall associated with the
potential trough early next week should remain south of the
Islands, although the typical trade wind based showers are likely
each day. Easterly winds on the order of 15 to 25 mph should
prevail for most of the upcoming week.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
848 PM EDT Thu Sep 16 2021

Day 1
Valid 01Z Fri Sep 17 2021 - 12Z Fri Sep 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
CENTRAL GULF COAST...

0100 UTC Updates -- Minimal changes were made to the Marginal Risk
areas across the Mid Atlantic and Southeast regions, based on the
latest radar and satellite trends and recent CAM output. Will see
a reduction in the ERO threat in terms of areal coverage later
this evening and overnight across these areas (away from the
central Gulf coast), based on the relatively weak forcing/low-mid
level moisture flux, weak mid level lapse rates, and thus further
decay in deep-layer instability following sunset.

Hurley

Previous discussion below..

...Central Gulf Coast and Southeast...
Continued confluent flow east of the remnants of Nicholas will
allow for scattered convection to redevelop within the tropical
airmass, particularly during peak heating hours today, which could
have some organization based on the degree of lingering 850 hPa
inflow (continuing to exceed the mean 850-400 hPa wind) and
expected effective bulk shear. While rain totals are forecast to
be generally less than in recent days, convection will develop
across moist soils from prior rainfall (exceeding 8 inches in
spots) and low FFGs are expected, so flash flooding remains
possible. Hourly rain totals to 3" remain possible across the
east-central Gulf Coast per 12Z HREF probabilities where cells
organize, merge, or train which is where the Slight Risk remains.
The Marginal Risk was expanded east to include more of SC and
northern FL. 12Z CAM consensus is for merging boundaries in the
moist and unstable environment to trigger repeating heavy cells in
northeast FL, particularly around the Jacksonville metro. 12Z HREF
probabilities are above 70 percent for 5 inches in 6 hrs across
northeast FL which near the 6hr FFG for this area which has been
rather dry over the past two weeks.


...Upper Midwest...
Another round of low amplitude troughing will push another frontal
system across the Midwest on Thursday, which will provide lift for
convection, while 850-700mb moisture flux anomalies to around +4
to +5 standard deviations above the mean are in the forecast,
bringing precipitable water values up to 1.25-1.5". Scattered
bands of storms should eventually exhibit relatively fast storm
motions given 50-70 kt flow aloft, and antecedent conditions will
generally be dry, but isolated flash floods are possible before
forward propagation takes hold, especially tied to areas that
experience repeat convection or training amid localized low FFG
thresholds and/or hydrophobic/urban surfaces. Expanded the
northeast section to include the Keweenaw peninsula per recent
HRRR runs as well as removal of the northern Neb Sandhills which
have high FFG.


...Mid-Atlantic...
The competing influence of moisture (PWs around 1.75") and
instability (CAPE around 1500 to 2500 J/kg) flowing northeastward
associated with Nicholas and a potentially suppressive influence
aloft from a surface low moving offshore in the western Atlantic,
will be in place across the Mid-Atlantic region through tonight.
Scattered convection is likely to form with slow storm motions due
to low mean-layer flow. Flash flooding issues could occur
particularly if storms develop over the urban corridors and/or
areas of relatively low FFG or high two week rainfall departures
such as those in and near the urban megalopolis. Expanded the
Marginal Risk west to the Blue Ridge based on clearing trends in
the satellite imagery and the 12Z CAM consensus.

Jackson

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Thu Sep 16 2021

Day 2
Valid 12Z Fri Sep 17 2021 - 12Z Sat Sep 18 2021

...A SLIGHT RISK OF EXCESSIVE RAINFALL REMAINS IN PLACE ACROSS THE
CENTRAL GULF COAST...

...Central Gulf Coast...
While Nicholas will continue to weaken on Friday, moist and
convergent Gulf inflow is likely to continue across the Gulf Coast
into the Southeast, with scattered convection again expected.
Uncertainty is high with where convection will focus, but the
Slight Risk area for the Central Gulf Coast was maintained where
heavy rainfall is likely and soils are sensitive from a very
active pattern. Instability will be more expansive on Friday,
though convection should remain poorly organized like today. Also,
the 850 hPa inflow and mean 850-400 hPa wind are more comparable
than on previous days, implying the risk is no higher on Thursday.
Hourly rain totals to 3" remain possible where cells train or
merge. The Marginal Risk is in place over the lower MS Valley and
up to the TN Valley with an expansion farther southeast over
northern to central FL where heavy rain today could precondition
similar output Friday.


...Central Plains...
The cold front is now expected to progress to far southeast Neb by
late tonight and push into eastern KS/northern MO on Friday,
shifting the convergent focus for heavy rainfall farther south
than the previous forecast. PWs of 1.5" to 1.75" will be
immediately ahead of the front which will be roughly parallel to
the mean flow, allowing repeating activity. The Marginal Risk has
been shifted south quite a ways to capture the pre-frontal area
during the Friday diurnal peak. This risk area is just a segment
of a longer area of the front which extends southwest to the TX
Panhandle where localized runoff issues may occur. Further change
in the Marginal area may occur as confidence in the heavy rain
area increases.


...Pacific Northwest...
The first strong atmospheric river event of this cool season is
forecast to reach the WA coast Friday morning, with integrated
water vapor transport values at their climatological maximum for
this time of year per the NAEFS Ensemble Situational Awareness
Table. This should mainly be a beneficial rain for these areas
with the dry antecedent conditions a limiting factor for flooding.
However, 1 hour precip amounts of over 0.5" in the Olympics and
northern Cascades are likely given the ingredients of PWs
approaching 1.5", some instability, and 850 hPa inflow of 50+ kts.
The Marginal Risk area was confined to mainly western slopes in WA.

Jackson

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Thu Sep 16 2021

Day 3
Valid 12Z Sat Sep 18 2021 - 12Z Sun Sep 19 2021

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF
THE PACIFIC NORTHWEST AND SOUTHEAST...

...Central Gulf Coast...
Nicholas remnants will continue to pull northward up the lower MS
Valley. A high degree of moisture (PWs of 2"+), convergent though
light inflow near the magnitude of the mean layer (850-400 hPa)
wind, and instability patterns are similar to Friday. As the
system should be pulling farther inland, it is expected that
convective activity near the Gulf coast would be less organized
than previous days, so a Marginal Risk should suffice. Hourly rain
totals to 3" should remain possible where cells merge or train.
Given heavy rain potential over areas that may receive a fair
amount on the two previous days over northern FL, expanded the
Marginal Risk southeast.


...Pacific Northwest...
The Atmospheric River shifts its focus south to OR and far
northern CA. PWs remain in the 1.25-1.5" range near the OR/CA
border. Confined the Marginal Risk to the Cascades and higher
coast ranges (where several burn scars exist) near the CA/OR
border in coordination with WFOs MFR and EKA. The front may be
progressive enough to not need much more expansion to the risk
area.

Jackson

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
402 PM EDT Wed Apr 28 2021

Valid 00Z Thu Apr 29 2021 - 00Z Sun May 02 2021

...Southern New Mexico...
Day 1...
A closed upper-low drifting east along the U.S.-Mex border into
West Texas will maintain mid-level divergence across southern New
Mexico through Wednesday night into Thursday. This divergence
aloft coupled with low-level northeasterly flow will support
periods of light to moderate snow along the Sacramento Mountains.
While widespread heavy accumulations are not expected, WPC PWPF
does show some low probabilities for local amounts of 4-inches or
more across this region.


For Days 2-3, the probability of snow accumulations greater than 4
inches is less than 10 percent.

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


This is the final regularly scheduled heavy snow/significant icing
discussion of the season. Regularly scheduled discussions will
resume on or about October 1, 2021.

Pereira

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 AM EDT Fri Sep 17 2021

Valid 12Z Mon Sep 20 2021 - 12Z Fri Sep 24 2021

...Weather Pattern Overview...

The strong storm system that is currently approaching the Pacific
Northwest is expected to be over the central and northern Plains
by Monday morning, accompanied by a rather strong cold front that
should steadily progress across the central and then eastern U.S.
The large surface high and upper ridge across the eastern U.S.
will eventually give way to the approaching trough and cold front,
which will herald a pattern change for much of the eastern half of
the nation. In addition, a second storm system is forecast to
reach the northwestern U.S. by Wednesday and into Thursday as the
storm track across the northeast Pacific remains active next week.

...Overview and Guidance Evaluation/Preferences...

The deterministic guidance is in above average agreement for the
beginning of next week, with the 18/00Z GFS beginning to show a
faster solution by Tuesday night with the trough/cold front
crossing the Great Lakes and Midwest region. This is even more
apparent with the second trough crossing the northern Rockies and
northern Plains by Thursday, and also ahead of its own ensemble
mean. By next Friday, model spread becomes substantial across
much of the northern tier states and southern Canada, with much of
this owing to the eventual evolution of a possible closed upper
low over southern Manitoba as the second Pacific trough moves
inland.

The WPC front/pressures forecast was primarily based on a nearly
equal blend of the 12Z deterministic guidance through early
Wednesday, and then primarily the 12Z GFS/ECMWF and their
respective ensemble means through Thursday and Friday. The 12Z
run of the GFS was substituted for the 18Z run since the latter
was considerably faster with the cold front across the eastern
U.S. compared to the model consensus, whereas the former is more
in line with the CMC/UKMET/ECMWF.

...Weather and Potential Hazards...

Widespread showers and scattered thunderstorms are expected to
develop along and ahead of the cold front from the Midwest to the
Northeast and Mid-Atlantic states, with some locations getting on
the order of 1 to 2 inches of rainfall with locally higher totals
possible. Some strong to severe storms will also be possible
where favorable parameters of wind shear and instability exist,
although location specifics are uncertain. Winter weather will
also be likely across the higher terrain of northern Wyoming and
western Montana with accumulating snow likely on Monday, followed
by a drier weather pattern by Tuesday and beyond. There may also
be enhanced fire weather concerns for portions of the West,
depending on the magnitude of the pressure gradient and resulting
winds in the wake of the cold front.

It will still feel like summer across much of the southern Plains
and extending northward to the Great Lakes and the Ohio Valley to
begin the week, and more like late October across much of the
Rockies and Intermountain West with highs running 10 to 20 degrees
below average. These cooler temperatures are expected to reach
the Midwest by midweek and finally to the East Coast by Thursday
and Friday, along with much more pleasant humidity levels.

Hamrick


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