Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Thu May 06 2021

Valid 12Z Thu May 06 2021 - 12Z Sat May 08 2021

...Unsettled weather enters the Pacific Northwest and Northern Great Basin
today before shifting into the Northern High Plains on Friday...

...Scattered showers and thunderstorms found across the Lower Great Lakes,
Ohio Valley, and Mid-Atlantic through the beginning of the weekend...

...Warm temperatures in the West spread into the Rockies and High Plains
on Friday as cooler temperatures remain throughout the East...

A relative lull in the recent active weather pattern is forecast to the
end the workweek. For the eastern half of the country, a lingering cold
front across Florida will produce scattered showers and thunderstorms
until exiting the region on Friday morning. A few storms could turn severe
along eastern sections of the peninsula today. A separate system swinging
through the Ohio and Tennessee valleys this afternoon is expected to spark
thunderstorms of its own, with low chances for damaging wind gusts and
large hail. By Friday, this same low pressure system and associated cold
front will reach the Mid-Atlantic, bringing additional rain and isolated
thunderstorms. Behind the cold front, cool and below average temperatures
will be found throughout much of the eastern U.S. over the next couple of
days. Highs will only reach the 50s and 60s across the Great Lakes, Ohio
Valley, Mid-Atlantic, and Northeast. This equates to temperatures around
10 to 20 degrees below average.

In the West, a cold front off the coast of the Pacific Northwest will push
inland this afternoon and evening through the Northwest and Northern Great
Basin. Light rain and higher elevation snow, as well as scattered
thunderstorms can be expected. Precipitation is then forecast to reach the
Northern Rockies and Northern High Plains by Friday while an upper-level
low drops southeastward into the region. As colder air aloft filters into
the Northern Rockies, snow could potentially mix into the lower elevations
of western Montana on Friday night.

While much of the Southwest and Central Great Basin dealing with
exceptional drought could use rain, little to no precipitation is
anticipated across this area with the approaching storm system.
Temperatures are also expected to remain well above average and top out
near the triple digits in the typical southwestern hot spots. Above
average temperatures will also be felt throughout much of the central and
southern Rockies/High Plains as well.

Snell

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
147 AM EDT Thu May 6 2021

Valid 12Z Sun May 9 2021 - 12Z Thu May 13 2021

***Heavy rain and more storms for portions of the South and the
Ohio Valley***

***Unseasonably cool for the western High Plains with some snow
for the Rockies***

...Weather Pattern and Model Guidance...

A split flow upper level pattern is expected across the U.S. and
southern Canada for the medium range period, with embedded
shortwave impulses in the southern stream providing an active
weather pattern across the south-central U.S. with multiple rounds
of showers and storms. Farther north, a broad trough over the
Intermountain West and northern Rockies, and a large Canadian
surface high sinking southward across the north-central U.S., will
result in below average temperatures from the Rockies to the East
Coast. A moderation trend may commence by the middle of next week
across the West as an upper level ridge tries to build back in
after the trough lifts out of the region.

The deterministic model guidance is in decent synoptic scale
agreement for the Sunday-Monday time period, with the exception of
the 12Z CMC across the northwestern U.S. where it is farther north
with the placement of the upper trough, and it is slower with the
progression of the surface low across the Ohio Valley. By Monday
night, the UKMET becomes stronger than the other guidance with a
shortwave tracking towards the Pacific Northwest, and considerably
slower and stronger with the trough crossing the Great Lakes
region when compared to the ensemble means. Towards the end of
the forecast period, the GFS/GEFS is slower with the ejection of
the western U.S. trough across the Plains and Midwest, whereas the
ECMWF/ECENS are more progressive, which is normally opposite of
what is typically seen at day 5-7 time scales.


...Weather Highlights/Hazards...

The best rainfall prospects will likely be from the Ohio Valley to
the central Appalachians on Mothers Day, associated with a surface
low tracking eastward across the region. The potential exists for
1 to 3 inches of rain from southern Indiana to West Virginia.
Another zone of heavy rain, along with strong thunderstorms, is
also probable across portions of the Deep South for next Tuesday
and Wednesday with similar rainfall totals possible. Some
training of thunderstorms may lead to instances of flooding, and
this is something that will continue to be monitored in the days
ahead. Light to moderate rain can also be expected for the
western High Plains early next week, and it will be cold enough
for accumulating snow for the higher terrain of Colorado and
Wyoming. Dry conditions should persist across most of the
Intermountain West and the Desert Southwest.

Temperatures will likely be more reminiscent of March across the
central and western High Plains, and adjacent portions of the
northern Rockies for the Sunday to Tuesday time period, with highs
up to 20 degrees below early May averages. This equates to
readings maxing out in the mid 40s to mid 50s for many of those
areas, and even colder across the central and northern Rockies.
Some of these cooler temperatures should also be realized across
Great Lakes and Northeast U.S. courtesy of the upper trough and
Canadian surface high in place, although not as anomalous. By the
middle of next week, above normal temperatures are likely to make
a return to the West Coast region ahead of the next cold front.

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
213 AM EDT Thu May 6 2021

Valid 00Z Fri May 7 2021 - 00Z Fri May 14 2021

The cut-off upper low currently near 165 degrees west will very
slowly track eastward across the western Islands through the end
of the week as suggested by the ensemble means. This will result
in an inverted surface trough and a col in the subtropical ridge
that will tend to weaken the Trades some. This feature should
lift out as a second trough approaches from the northwest over the
weekend. Meanwhile, a large surface high builds in across the
northeast Pacific by the end of the week and into the weekend near
140 degrees west, and this should increase the Trades some by the
weekend. By the beginning of next week, both the GEFS and EC
means are suggesting the potential for another upper trough north
of the state.

In terms of rainfall, some enhanced showers and perhaps some
thunderstorms can be expected across the central and western
Islands by Friday and into Saturday as the upper low slowly drifts
overhead, and creates some additional instability. Elsewhere,
typical trade wind enhanced showers can be expected, with heavier
showers likely across the Big Island.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
836 PM EDT Wed May 05 2021

Day 1
Valid 01Z Thu May 06 2021 - 12Z Thu May 06 2021

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

Hurley

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Thu May 06 2021

Day 1
Valid 12Z Thu May 06 2021 - 12Z Fri May 07 2021

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

Locally heavy rain is likely today along the east coast of FL.
Cell motions off to the east should generally push storms offshore
with time, although some brief westward backbuilding is possible.
Generally looking at 1-3" of rain on a localized basis...and the
00z HREFv3 shows little to no FFG exceedance probabilities. With
that said, rainfall rates will be high enough to potentially
result in localized instances of urban flooding, particularly
along the western fringes of the Miami metro area. However, not
expecting enough coverage or magnitude to warrant a Marginal risk
at this time.

Chenard

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu May 06 2021

Day 2
Valid 12Z Fri May 07 2021 - 12Z Sat May 08 2021

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

Jackson

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu May 06 2021

Day 3
Valid 12Z Sat May 08 2021 - 12Z Sun May 09 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GREAT PLAINS AND NORTHERN OZARKS...

An upper level trough approaching the Rockies Friday night
promotes lee-side surface cyclogenesis in southeast CO that tracks
east near the KS/OK border Saturday afternoon/night. Ample Gulf
moisture streams up the southern Plains beginning early Friday in
return flow that increases with the surface low development,
reaching 55kt SSWly 850mb flow Saturday evening over eastern OK,
with 1.25" to 1.5" PWs over eastern KS and southern MO (2 standard
deviations above normal). Sufficient instability (generally 1000
to 1500 J/kg) accompany this warm conveyor belt with upper level
divergence from the left exit region of a WNWly jet streak over OK
and the right entrance over IA. 30kt SWly mean layer flow promotes
progressive storm motion, but southeastward upwind propagation
vectors promote localized cell mergers and repeating activity from
Saturday afternoon from initial activity near the western KS/NE
border across eastern KS and well into MO Saturday night. Much of
east-central KS has been rather dry over the past two weeks with
increasing recent rainfall southeast toward the terrain of the
Ozarks. Mesoscale processes may develop in organized activity and
result in enhanced rainfall in the northern Ozarks. Therefore, a
sizable SLIGHT RISK was introduced from the eastern KS/NE border
southeast through southwest MO in agreement with WFOs OAX TOP ICT
EAX and SGF. Of note is expected convective activity in eastern
KS/southwest MO early Saturday morning that may locally saturate
soil as well as leave boundaries for focused activity Saturday
night.

Farther north over southeast Neb, northern MO to southern IL
strong low-level frontogenesis promotes enhanced
rainfall/potentially locally heavy which is encompassed in a broad
Marginal Risk. Again, more southern areas have received more
rainfall in the past week with increased sensitivity to runoff in
that direction.

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
147 AM EDT Thu May 6 2021

Valid 12Z Sun May 9 2021 - 12Z Thu May 13 2021

***Heavy rain and more storms for portions of the South and the
Ohio Valley***

***Unseasonably cool for the western High Plains with some snow
for the Rockies***

...Weather Pattern and Model Guidance...

A split flow upper level pattern is expected across the U.S. and
southern Canada for the medium range period, with embedded
shortwave impulses in the southern stream providing an active
weather pattern across the south-central U.S. with multiple rounds
of showers and storms. Farther north, a broad trough over the
Intermountain West and northern Rockies, and a large Canadian
surface high sinking southward across the north-central U.S., will
result in below average temperatures from the Rockies to the East
Coast. A moderation trend may commence by the middle of next week
across the West as an upper level ridge tries to build back in
after the trough lifts out of the region.

The deterministic model guidance is in decent synoptic scale
agreement for the Sunday-Monday time period, with the exception of
the 12Z CMC across the northwestern U.S. where it is farther north
with the placement of the upper trough, and it is slower with the
progression of the surface low across the Ohio Valley. By Monday
night, the UKMET becomes stronger than the other guidance with a
shortwave tracking towards the Pacific Northwest, and considerably
slower and stronger with the trough crossing the Great Lakes
region when compared to the ensemble means. Towards the end of
the forecast period, the GFS/GEFS is slower with the ejection of
the western U.S. trough across the Plains and Midwest, whereas the
ECMWF/ECENS are more progressive, which is normally opposite of
what is typically seen at day 5-7 time scales.


...Weather Highlights/Hazards...

The best rainfall prospects will likely be from the Ohio Valley to
the central Appalachians on Mothers Day, associated with a surface
low tracking eastward across the region. The potential exists for
1 to 3 inches of rain from southern Indiana to West Virginia.
Another zone of heavy rain, along with strong thunderstorms, is
also probable across portions of the Deep South for next Tuesday
and Wednesday with similar rainfall totals possible. Some
training of thunderstorms may lead to instances of flooding, and
this is something that will continue to be monitored in the days
ahead. Light to moderate rain can also be expected for the
western High Plains early next week, and it will be cold enough
for accumulating snow for the higher terrain of Colorado and
Wyoming. Dry conditions should persist across most of the
Intermountain West and the Desert Southwest.

Temperatures will likely be more reminiscent of March across the
central and western High Plains, and adjacent portions of the
northern Rockies for the Sunday to Tuesday time period, with highs
up to 20 degrees below early May averages. This equates to
readings maxing out in the mid 40s to mid 50s for many of those
areas, and even colder across the central and northern Rockies.
Some of these cooler temperatures should also be realized across
Great Lakes and Northeast U.S. courtesy of the upper trough and
Canadian surface high in place, although not as anomalous. By the
middle of next week, above normal temperatures are likely to make
a return to the West Coast region ahead of the next cold front.

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