Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Tue Apr 20 2021

Valid 00Z Wed Apr 21 2021 - 00Z Fri Apr 23 2021

...Snow is expected to spread quickly from the Central U.S. to the Midwest
today, and then across northern New England on Wednesday into Thursday...

...Subfreezing record cold temperatures expected across much of the
south-central U.S. Wednesday morning, shifting to parts of the eastern
U.S. Thursday morning...

...Critical fire weather threat over Arizona through Friday...

Late season snow event continues to spread eastward across the Ozarks into
the Ohio Valley as a developing area of low pressure is forecast to race
along the Alleghenies into the Northeast later today into tomorrow
morning. As the low pressure system progresses northeastward, rain and
mixed precipitation will continue to envelop the Midwest, before a swath
of wet snow falls along the Canadian border from New York into Northern
New England beginning Wednesday morning through Friday. 6 to locally 8
inches is possible along the border through Friday. As such, "Moderate"
impacts are expected in these locations from snow-load, per the Weather
Prediction Center Winter Storm Severity Index, and Winter Weather
Advisories are in effect for the region. Further south in the
Mid-Atlantic, some rain and a few rumbles of thunder are possible as the
storm moves through.

In the wake of the developing storm system, a bout of unseasonably cold
air is forecast to spill into the Central U.S. today and tomorrow, leading
to widespread freeze and frost concerns tonight across the
central/southern Plains and the Midwest. Freeze Watches and Warnings
extend continuously from north-central Michigan into central Texas.
Numerous daily record low temperatures are forecast to be tied or broken
on Wednesday morning as temperatures dip to below freezing, around 20-25
degrees below average for late April. Residents in these areas are
encouraged to take preventative measures to mitigate the impacts of
sub-freezing temperatures on vulnerable vegetation if possible. By Friday,
the frigid airmass will moderate as it drifts east, although low
temperatures will remain around 10 degrees below normal in the Central
U.S., with lows hovering in the mid-30's to 40's.

Meanwhile, active weather across Florida has been edging very slowly
southward into southern Florida ahead of a slow-moving front. A low
pressure wave forming on the front could bring additional heavy showers
and thunderstorms today across central Florida. Improving conditions
should begin Wednesday morning as the wave moves off into the Atlantic and
rain chances decrease across southern Florida. While excessive rain may be
problematic in Florida, dry, windy conditions will set the stage for a
fire weather threat in Arizona over the next three days as low pressure
deepens in the region. Accordingly, the Storm Prediction Center has issued
an Elevated Risk of fire weather today, and a Critical Risk tomorrow, with
the potential for additional impactful fire weather also highlighted on
Friday.

Asherman/Kong

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Tue Apr 20 2021

Valid 12Z Fri Apr 23 2021 - 12Z Tue Apr 27 2021

...Heavy Rainfall/Flooding Threat across the South Friday into
this Weekend...

...Overview...

Two moderately progressive upper troughs will affect the lower 48,
between a blocky high-latitude regime featuring Alaska/Greenland
ridges plus a broad upper low over central-western Canada, and a
Greater Antilles ridge that eventually builds into southern
Mexico. The leading surface system will track from the southern
High Plains through the mid-South before turning the corner near
the coast as a nor'easter Sunday-Monday. Upper ridging will build
in behind this system as the upstream Pacific system moves into
and through the West during the first part of next week with low
pressure reaching the Plains by Tuesday. Aside from an area of
warmth ahead of the Pacific/western system, the pattern will be
rather chilly for late April with heaviest precipitation focused
over the South/East with the leading system and the West with the
feature emerging from the Pacific.


...Guidance/Predictability Assessment and Preferences...

The system tracking out of the southern High Plains and expected
to develop into a nor'easter still exhibits fine-scale shortwave
sensitivity that may temper confidence in specifics for a time.
There is improving consensus toward recent ECMWF/ECMWF mean runs
that have depicted formation of an upper low over New England
and/or the Canadian Maritimes by early next week. However spread
persists for the open shortwave that crosses the central/southern
U.S. and separate northern tier U.S./southern Canada energy that
will likely provide the greater input for the closed low. Precise
timing of closed low formation and the track will affect
precipitation coverage and amounts over New England. Overall
prefer a compromise approach with more ECMWF/GFS weighting
relative to the strong/inland 00Z UKMET and farther offshore 00Z
CMC. The 00Z ECMWF mean fit in with this idea a lot better than
the weak and suppressed 06Z GEFS mean. The 00Z GEFS mean was
closer to consensus so that run provided the minority GEFS input
for the forecast. 12Z model runs thus far maintain general support
for aforementioned preferences.

Models and ensembles have maintained better than average
clustering for the Pacific upper trough that reaches the West
early in the week and brings a fairly strong low pressure system
into the Plains by next Tuesday. The main trend over the past
24-36 hours has been for the GFS to trend a little faster with the
upper trough, though a slower trend in the new 12Z ECMWF shows
there may still be some shuffling of specifics even though the
overall feature exhibits better than average confidence. A blend
of operational guidance followed by some ensemble input along with
the 06 GFS/00Z ECMWF/00Z CMC late in the period represented
consensus well for this system.


...Weather/Hazards Highlights...

Rainfall will expand and intensify across the South from Friday
into Saturday as low pressure tracks out of the southern High
Plains. The leading warm front will interact with Gulf of Mexico
moisture and instability, likely producing areas of heavy
rainfall. Runoff and flooding issues are quite possible,
especially in areas that have received much above normal rainfall
this month. Currently expect the axis of highest rainfall to
extend from southern Arkansas/northern Louisiana into South
Carolina. There will be potential for severe weather as well so
check Storm Prediction Center outlooks for the latest information.
The system should then lift up the East Coast as trailing northern
stream energy from the Great Lakes region merges into the coastal
system and possibly closes an upper low. Enhanced rainfall is
possible from parts of the Mid-Atlantic into New England with
windy conditions region-wide as the low deepens. A little snow at
highest elevations could be possible depending on exactly how the
system evolves.

In the West, an amplifying Pacific upper trough/low and surface
front will work into the West during the weekend into early next
week. This system will spread lower elevation rain/mountain snow
focus from the Pacific Northwest and northern Rockies southward
into California and the Great Basin/central Rockies. The highest
totals with this system will likely be from the Pacific Northwest
into the Sierra Nevada. The majority of precipitation should be
light to moderate though parts of the central West Coast and
Sierra Nevada could see a period of locally enhanced activity.
Once the system moves to the Plains by Tuesday, potential will
increase for convectively-driven rainfall over parts of the Plains
and Mississippi Valley.

A chilly air mass Friday over the East and especially northern
High Plains (where highs will be 10-20F below normal) will
moderate some this weekend but there will still be decent coverage
of below normal highs over the eastern half of the country. An
area of above normal temperatures over parts of the West/Great
Basin ahead of the Pacific upper trough/low will shift eastward
across the Rockies and into the Plains by next week. Best
potential for one or more days with temperatures 10F or more above
normal will extend from near the Great Basin through the
central-southern Rockies into the Plains/Midwest. Trailing the
front that crosses the West, temperatures over the region will
decline to 5-15F colder than normal by next Monday-Tuesday.

Rausch/Fracasso


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 precipitation across portions of the Central Great Basin,
California, and the Southwest,
Sun-Mon, Apr 25-Apr 26.
- Heavy rain across portions of the Lower Mississippi Valley, the
Tennessee Valley, the
Mid-Atlantic, the Southern Appalachians, the Southeast, and the
Southern Plains, Fri-Sat, Apr
23-Apr 24.
- Heavy rain across portions of the Central Plains, the Lower
Mississippi Valley, the Northern
Plains, the Upper/Middle Mississippi Valley, and the Southern
Plains, Tue, Apr 27.
- Severe weather across portions of the Southeast, the Lower
Mississippi Valley, and the Southern
Plains, Fri, Apr 23.
- Severe weather across portions of the Southeast and the Lower
Mississippi Valley, Sat, Apr 24.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley.
- High winds across portions of the Great Lakes, the Mid-Atlantic,
the Northeast, the Central
Appalachians, and the Ohio Valley, Fri, Apr 23.
- High winds across portions of the Northeast, Mon, Apr 26.

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
310 AM EDT Tue Apr 20 2021

Valid 00Z Wed Apr 21 2021 - 00Z Wed Apr 28 2021

A cold front will approach the islands from the northwest,
stalling just north of Kauai. Precipitable water values are
expected to be near to below normal which supports generally light
windward showers. Heights will rise for the end of the week into
the weekend as an upper high builds to the northwest of the area.
This may allow weaknesses aloft to slip in from the east but a
mostly dry pattern is still expected with no organized areas of
heavy rainfall. Light and variable trades will become easterly as
the front approaches except for Kauai which may see more variable
trades out of the northeast at times. Trades may increase Fri-Sun
as high pressure slides by to the north and settles/weakens to the
ENE of the Big Island around 145W. Another front may be well
northwest of the area by next Tuesday.

Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
653 PM EDT Tue Apr 20 2021

Day 1
Valid 01Z Wed Apr 21 2021 - 12Z Wed Apr 21 2021

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

Pagano

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Tue Apr 20 2021

Day 2
Valid 12Z Wed Apr 21 2021 - 12Z Thu Apr 22 2021

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

Bann

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Tue Apr 20 2021

Day 3
Valid 12Z Thu Apr 22 2021 - 12Z Fri Apr 23 2021

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

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
417 PM EDT Tue Apr 20 2021

Valid 00Z Wed Apr 21 2021 - 00Z Sat Apr 24 2021

...Ohio Valley/Great Lakes/Northeast...
Days 1-3...
An amplified shortwave will continue to drive the leading edge of
an unseasonably cold airmass from the Midwest into the eastern
U.S., while supporting late-season accumulating snows from the
Ohio Valley northeastward into the interior Northeast. Heavy
accumulations are possible across portions of Upstate New York and
northern New England.

The upper level shortwave associated with the significant snowfall
that occurred across portions of the central Rockies and Plains
overnight and earlier today is forecast to lift from the Ohio
Valley into the Lower Lakes region overnight. Anomalously cold
air along with low-to-mid level frontogenesis west of a deepening
surface low will support a stripe of light to moderate snows from
the Ohio Valley into the Lower Lakes overnight, with the WPC PWPF
showing that at least an inch or two of accumulating snow is
likely from central Indiana to the shores of lakes Erie and
Ontario. As the shortwave continues to lift north, with a closed
low developing along the U.S./Canada border, snow will shift east
across the interior Northeast on Wednesday. The low is expected
to continue to deepen as it moves tracks into Atlantic Canada on
Thursday, with windy conditions and snow showers expected across
the Northeast on the backside of the departing system. Three day
snowfall totals of 6-inches or more are likely across portion of
western New York, the Finger Lakes and Tug Hill -- where lake
effect following the synoptic snows are expected to bolster
totals. Overall, the heaviest snowfall totals are expected to
occur across the Adirondacks and the New York North Country, as
well as far northern Vermont to Maine, where accumulations of
8-inches or more are likely.

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

Pereira

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Tue Apr 20 2021

Valid 12Z Fri Apr 23 2021 - 12Z Tue Apr 27 2021

...Heavy Rainfall/Flooding Threat across the South Friday into
this Weekend...

...Overview...

Two moderately progressive upper troughs will affect the lower 48,
between a blocky high-latitude regime featuring Alaska/Greenland
ridges plus a broad upper low over central-western Canada, and a
Greater Antilles ridge that eventually builds into southern
Mexico. The leading surface system will track from the southern
High Plains through the mid-South before turning the corner near
the coast as a nor'easter Sunday-Monday. Upper ridging will build
in behind this system as the upstream Pacific system moves into
and through the West during the first part of next week with low
pressure reaching the Plains by Tuesday. Aside from an area of
warmth ahead of the Pacific/western system, the pattern will be
rather chilly for late April with heaviest precipitation focused
over the South/East with the leading system and the West with the
feature emerging from the Pacific.


...Guidance/Predictability Assessment and Preferences...

The system tracking out of the southern High Plains and expected
to develop into a nor'easter still exhibits fine-scale shortwave
sensitivity that may temper confidence in specifics for a time.
There is improving consensus toward recent ECMWF/ECMWF mean runs
that have depicted formation of an upper low over New England
and/or the Canadian Maritimes by early next week. However spread
persists for the open shortwave that crosses the central/southern
U.S. and separate northern tier U.S./southern Canada energy that
will likely provide the greater input for the closed low. Precise
timing of closed low formation and the track will affect
precipitation coverage and amounts over New England. Overall
prefer a compromise approach with more ECMWF/GFS weighting
relative to the strong/inland 00Z UKMET and farther offshore 00Z
CMC. The 00Z ECMWF mean fit in with this idea a lot better than
the weak and suppressed 06Z GEFS mean. The 00Z GEFS mean was
closer to consensus so that run provided the minority GEFS input
for the forecast. 12Z model runs thus far maintain general support
for aforementioned preferences.

Models and ensembles have maintained better than average
clustering for the Pacific upper trough that reaches the West
early in the week and brings a fairly strong low pressure system
into the Plains by next Tuesday. The main trend over the past
24-36 hours has been for the GFS to trend a little faster with the
upper trough, though a slower trend in the new 12Z ECMWF shows
there may still be some shuffling of specifics even though the
overall feature exhibits better than average confidence. A blend
of operational guidance followed by some ensemble input along with
the 06 GFS/00Z ECMWF/00Z CMC late in the period represented
consensus well for this system.


...Weather/Hazards Highlights...

Rainfall will expand and intensify across the South from Friday
into Saturday as low pressure tracks out of the southern High
Plains. The leading warm front will interact with Gulf of Mexico
moisture and instability, likely producing areas of heavy
rainfall. Runoff and flooding issues are quite possible,
especially in areas that have received much above normal rainfall
this month. Currently expect the axis of highest rainfall to
extend from southern Arkansas/northern Louisiana into South
Carolina. There will be potential for severe weather as well so
check Storm Prediction Center outlooks for the latest information.
The system should then lift up the East Coast as trailing northern
stream energy from the Great Lakes region merges into the coastal
system and possibly closes an upper low. Enhanced rainfall is
possible from parts of the Mid-Atlantic into New England with
windy conditions region-wide as the low deepens. A little snow at
highest elevations could be possible depending on exactly how the
system evolves.

In the West, an amplifying Pacific upper trough/low and surface
front will work into the West during the weekend into early next
week. This system will spread lower elevation rain/mountain snow
focus from the Pacific Northwest and northern Rockies southward
into California and the Great Basin/central Rockies. The highest
totals with this system will likely be from the Pacific Northwest
into the Sierra Nevada. The majority of precipitation should be
light to moderate though parts of the central West Coast and
Sierra Nevada could see a period of locally enhanced activity.
Once the system moves to the Plains by Tuesday, potential will
increase for convectively-driven rainfall over parts of the Plains
and Mississippi Valley.

A chilly air mass Friday over the East and especially northern
High Plains (where highs will be 10-20F below normal) will
moderate some this weekend but there will still be decent coverage
of below normal highs over the eastern half of the country. An
area of above normal temperatures over parts of the West/Great
Basin ahead of the Pacific upper trough/low will shift eastward
across the Rockies and into the Plains by next week. Best
potential for one or more days with temperatures 10F or more above
normal will extend from near the Great Basin through the
central-southern Rockies into the Plains/Midwest. Trailing the
front that crosses the West, temperatures over the region will
decline to 5-15F colder than normal by next Monday-Tuesday.

Rausch/Fracasso


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 precipitation across portions of the Central Great Basin,
California, and the Southwest,
Sun-Mon, Apr 25-Apr 26.
- Heavy rain across portions of the Lower Mississippi Valley, the
Tennessee Valley, the
Mid-Atlantic, the Southern Appalachians, the Southeast, and the
Southern Plains, Fri-Sat, Apr
23-Apr 24.
- Heavy rain across portions of the Central Plains, the Lower
Mississippi Valley, the Northern
Plains, the Upper/Middle Mississippi Valley, and the Southern
Plains, Tue, Apr 27.
- Severe weather across portions of the Southeast, the Lower
Mississippi Valley, and the Southern
Plains, Fri, Apr 23.
- Severe weather across portions of the Southeast and the Lower
Mississippi Valley, Sat, Apr 24.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley.
- High winds across portions of the Great Lakes, the Mid-Atlantic,
the Northeast, the Central
Appalachians, and the Ohio Valley, Fri, Apr 23.
- High winds across portions of the Northeast, Mon, Apr 26.

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