Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Mon May 10 2021

Valid 00Z Tue May 11 2021 - 00Z Thu May 13 2021

...There is an Enhanced Risk of severe thunderstorms over parts of
south-central Texas through Tuesday morning...

...There is a Slight Risk of excessive rainfall over parts of the
Tennessee/Lower Mississippi Valleys and the Southern Plains through
Wednesday morning...

...Late-season snowfall is likely across the Northern/Central Rockies and
High Plains with much colder than average temperatures...

...Warm and dry conditions will cause fire danger in the southwestern U.S.
and California...

A front extending from the southern Mid-Atlantic southwestward to the
Central Gulf Coast, then westward into the Southern Plains will be
quasi-stationary through Wednesday. The boundary will move off the
Central/Western Gulf Coast into the Gulf of Mexico by Wednesday evening.
The system will produce severe thunderstorms and heavy rain over parts of
the Southern Plains and the southern Mid-Atlantic through Tuesday morning.
The SPC has issued an Enhanced Risk of severe thunderstorms over parts of
south-central Texas a Slight Risk of severe thunderstorms over the
Ark-La-Tex and the southern Mid-Atlantic through Tuesday morning. The
hazards associated with these severe thunderstorms are frequent lightning,
severe thunderstorm wind gust, hail, and a few tornadoes. Some of the
storms will have a 10% or greater probability of producing hail two inches
or greater over parts of south-central Texas through Tuesday morning. The
thunderstorms will also produce heavy rain. The showers and thunderstorms
over the southern Mid-Atlantic/Southeast will move off the coast overnight
Monday.

The showers and thunderstorms will continue over the Southern Plains and
the Lower Mississippi/Tennessee Valleys on Tuesday. Due to the heavy rain
over the region, the WPC has issued a Slight risk of excessive rainfall
over parts of the Tennessee/Lower Mississippi Valleys and the Southern
Plains through Wednesday morning. The hazards associated with the rain
are scattered flash flooding, where urban areas, roads, and small streams
are the most vulnerable. On Tuesday evening and into overnight Tuesday,
the showers and thunderstorms will return to the Southeast and continue
through Wednesday evening. Also, on Wednesday, the showers and
thunderstorms over the Southern Plains will retreat to the Gulf Coast by
Wednesday evening.

Meanwhile, energy from a weakening upper-level low over the Great Lakes
will move over the Northeast. Rain will develop over parts of the
Upper/Middle Mississippi Valley and the Great Lakes overnight Monday,
ending by Tuesday morning. On Tuesday, the rain will develop over parts
of the Northeast, continuing into Wednesday evening. Furthermore,
upper-level energy and cold temperatures will lead to snow across the
Northern and Central Rockies late Tuesday night. Temperatures are
expected to be below average for much of the country behind the potent
cold front, especially by Tuesday. High temperatures in the central parts
of the High Plains will be around 30 degrees below average for this time
of year. Freeze Warnings and Frost Advisories are in effect for portions
of the Great Lakes and Ohio Valley and Middle/Upper Mississippi Valley
through Tuesday morning in terms of cold low temperatures. In contrast,
temperatures will be warmer than average over the West Coast, especially
California. The warm temperatures and dry and windy conditions have led
to Elevated and Critical Risks of fire weather and Red Flag Warnings in
interior Northern California and the Southwest/Southern Rockies/Southern
High Plains through Tuesday morning.

Ziegenfelder

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
312 PM EDT Mon May 10 2021

Valid 12Z Thu May 13 2021 - 12Z Mon May 17 2021

1800 UTC update...

The models and ensembles continue to show fairly good agreement
(especially the first half of the period) with the overall large
scale pattern, but larger differences in the details. There is not
a whole lot to note with the new 00z/06z guidance, with the same
forecast concerns from the overnight shift as noted below. Given
this, maintained pretty close WPC continuity with no major changes
to the forecast needed. The updated guidance favored a blend
towards the deterministic ECMWF/GFS/CMC/UKMET days 3-5, with more
weight on the ensemble means/ECMWF days 6-7. See previous
discussion copied below for the details.

Santorelli


Previous discussion issued at 0655 UTC...

...Overview...

Latest guidance continues to show moderate progression to flow
aloft across the lower 48 and vicinity. Aside from fairly minor
details, there is reasonable consensus for a broad mean trough to
cross the eastern U.S. and emerge into the Atlantic while a
trailing ridge over the West as of Thursday should reach into the
Mississippi Valley by next Monday. There has been more spread and
variability for details of upstream Pacific energy that should
amplify into an upper trough that reaches/crosses the West during
the weekend, as well as for details of flow behind this feature.
Aside from a front dropping through the Gulf and Florida Peninsula
late this week, waves/fronts will primarily affect the
western/central U.S. as high pressure prevails over the East.
Thus the best focus for precipitation during the period will
likely extend from the northern Rockies through a majority of the
Plains and into the Middle/Upper Mississippi Valley. Contrasting
temperatures on Thursday, with parts of the West quite warm and
the South/East much cooler than normal, should steadily gravitate
toward climatology with time.


...Guidance Evaluation and Preferences...

With current model runs still showing minimal surface response to
the upper trough crossing the East aside from waviness well
offshore, the dominant forecast difficulties involve the shortwave
energy expected to move into the West during the weekend and
upstream flow that will affect the pattern behind this energy.
After a couple days of increased spread for where/when features
could try to separate, 12Z/18Z model/ensemble runs (and followed
up in the new 00Z runs) have returned to some variation of 2-3
days ago when there was better clustering toward the idea that any
separation that occurs would wait until energy reaches near the
West Coast. There are still significant differences for what
individual pieces of energy will do though, and predictability is
low due to their small scale. A blended approach (more models
than means into day 5 Saturday followed by a more even mix by day
7 Monday) continues to provide the best way to navigate the spread
and variability that the models/ensembles have exhibited so far.

Farther west, recent GFS runs trend out of phase with North
Pacific flow from the weekend onward--a result of western Pacific
flow separation which other guidance does not depict. In contrast
to the flat flow to ridging that the GFS shows near the West Coast
by next Monday, ECMWF/CMC runs and their ensemble means have been
fairly consistent so far in bringing another trough toward the
West--albeit with detail and amplitude differences. Latest GEFS
means fall between these two extremes, with a weaker and delayed
trough approaching the West. Current preference is to tilt the
forecast somewhat more in the direction of the ECMWF cluster
relative to the GFS/GEFS.


...Weather Highlights/Hazards...

Expect a gradual increase in coverage and amounts for rain and
thunderstorms over the central U.S. and northern Rockies from late
this week into early next week. Initial activity will be focused
mostly by weak northern stream shortwave impulses and there should
be a relative lack of deep moisture. Then Gulf moisture should
begin to contribute more and interact with one or more
waves/fronts as eastern U.S. high pressure nears the East Coast.
With uncertainty in the specifics, shortwave energy emerging from
the West late in the period may provide another ingredient in the
rainfall recipe. Currently expect the highest five-day totals
over portions of the Central/Southern Plains and Middle/Upper
Mississippi Valley. Some precipitation could be in the form of
snow over highest elevations of the northern Rockies. Any
precipitation farther west should be fairly light and scattered.
The upper trough crossing the East late this week may produce some
areas of mostly light rainfall over central/northern latitudes,
with greater coverage during the latter half of each day.
Locations near the Southeast coast and over the Florida Peninsula
could see somewhat more organized rainfall for a time before a
drier trend, as a wavy front initially over the Gulf and Florida
Peninsula pushes southward.

On Thursday the southern tier will see well below normal highs
with minus 10-20F anomalies over parts of the Southeast and minus
10-15F anomalies over parts of Texas. Some locations in the
Southeast could challenge daily records for cool highs. Less
extreme below normal readings should prevail late this week over
the remaining majority of the central/eastern U.S. Meanwhile
temperatures are likely to be up to 10-20F above normal over areas
from Oregon/California into the Great Basin late this week with
more moderate anomalies stretching into the central Rockies/High
Plains late this week into the weekend. During the weekend upper
troughing that moves into the West should bring temperatures back
down toward normal across the region from west to east. The
moderating trends for both cool and warm areas from late this week
may yield highs within a few degrees on either side of normal over
much of the lower 48 by next Monday.

Rausch


Hazards:
- Heavy rain across portions of the central and southern Plains,
Mon, May 17.
- Flooding possible across portions of eastern Texas to the
Louisiana border, eastern Kansas, as well as northern Ohio.

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
350 AM EDT Mon May 10 2021

Valid 00Z Tue May 11 2021 - 00Z Tue May 18 2021

Models and ensemble means continue to show precipitable water at
near to moderately below average values for the time of year, with
a tendency for a gradual decrease over the course of the week and
then stabilization into early next week. Thus expect most
rainfall to be in the lighter half of the spectrum. Trades are
likely to be in the light to moderate range into midweek, and in
some locations possibly close to being light enough at times for
land/sea breeze influence on showers while remaining rainfall
focuses over windward/mountain areas. Guidance consensus still
shows high pressure to the north strengthening somewhat after
Wednesday, promoting stronger trades and more pronounced windward
shower focus. An upper trough developing to the north/northeast
of the state mid-late week appears likely to deposit an upper low
that lingers northeast of the area through the weekend. The upper
trough/low and any shortwaves that rotate around the feature could
enhance rainfall a bit at times.

As for notable guidance comparisons, one item of note is that the
00Z ECMWF has its upper low drifting west of other models/means by
Sunday-Monday. Also from the weekend into next week the latest
GFS runs become out of phase with many aspects of the pattern over
the Pacific. This results in the GFS disrupting the surface high
north of the state late in the period and possibly ejecting the
nearby upper trough/low prematurely. The GEFS mean is closer to
the remaining solutions. These considerations lead to favoring
the non-GFS consensus for high pressure late in the period and the
CMC/CMC-GEFS-ECMWF means for the upper feature northeast of the
state once solutions diverge.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
455 PM EDT Mon May 10 2021

Day 1
Valid 2055Z Mon May 10 2021 - 12Z Tue May 11 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI VALLEY...

21 UTC Special Update...
Earlier convection that developed over the lower Mississippi
Valley/central Gulf Coast region has now propagated south into the
northern Gulf along with the associated outflow boundaries. With
organized redevelopment not expected through 12 UTC Tuesday, the
Marginal and Slight Risk areas were removed.

Still monitoring elevated convection developing near the ArkLaTex
region. While there is not a signal for widespread heavy amounts
across this region, locally heavier amounts may impact areas
impacted by recent heavy rains and with low FFG values.
Therefore, the Margial Risk was left in place.

Pereira

Previous Discussion...

Maintained the Marginal risk across portions of southwest TX from
approximately Del Rio south towards Zapata. Extreme instability is
forecast to develop over this region, with PWs increasing towards
the climatological 90th percentile as well. Convection should
initiate near the intersection of a stationary front and dry line
this afternoon over TX and also over the terrain of northern
Mexico. Given the degree of instability and southeasterly
inflow...these cells should survive eastward into southwest TX
this evening. Possible we see some upscale growth or at least some
convective clusters evolve. Localized 3-5" rainfall may pose an
isolated flash flood risk across this region.

Chenard/Jackson

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Mon May 10 2021

Day 2
Valid 12Z Tue May 11 2021 - 12Z Wed May 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE GULF COAST REGION TO EASTERN AND SOUTH TEXAS...

21z Update...Areas along the Gulf are still on track to see heavy
rainfall amounts associated with the frontal boundary. Latest
guidance has shifted the axis of heaviest amounts a bit further
eastward through portions of central MS/AL with upwards of 1 to 2
inches during this time frame with pockets of 2.5+ inches. These
areas are still sitting at 150 to 600 percent of normal of
rainfall over the past week. With this being said, have extended
the Marginal Risk area further eastward for MS and southward
through southern portions of AL per request of MOB CWA. Have also
extended the Slight Risk area a bit further eastward for these
areas as well as southward through southern TX with latest QPF
guidance showing heavy amounts of 2 to 4 inches for this area.

Chiari/Campbell

Previous Discussion...

Model agreement has gotten better in terms of the speed and
placement of the front approaching from the north...so only a
minor expansion was made around the western and southern/eastern
periphery of the previously-issued risk area.

The southward moving front over Texas and the central Gulf states
will slow its progression during this period which results in the
boundary becoming oriented from west-to-east. With very moist air
streaming in from the east/southeast over boundary, convection
will build westward and support training cells along and north of
this boundary. The signal remains for moderate to heavy rainfall
across parts of the area, although the GFS had backed off the
areal coverage and maximum rainfall amounts somewhat from 09/18Z.
Even so, am not inclined to make significant changes either to the
QPF or the Excessive Rainfall Outlook given the anomalous amount
of moisture in place (maximum anomalies in excess of 2 standard
deviations above climatology) which is sustained by continued
influx of moisture on low-level southeasterly winds and enough
instability (1000-1500 J/kg MLCAPE) to result in cells being able
to produce downpours.

Bann

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Mon May 10 2021

Day 3
Valid 12Z Wed May 12 2021 - 12Z Thu May 13 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE GULF
COAST REGION...

21Z Update...Models still in agreement on sending the deepest
moisture/instability out over the Gulf during this time period. No
significant changes made to day 3 QPF, thus no changes made to
previous Marginal Risk area.

Chiari/Campbell

Previous Discussion...

The threat of heavy to excessive rainfall will be lingering into
Wednesday along the Gulf coast given a moist airmass in place and
the presence of a front to help focus storms. However, the
approach and passage of a broad, low amplitude synoptic-scale
trough should usher the deepest moisture/instability out over the
Gulf ahead from west to east.

Before that happens, a corridor of precipitable water values in
excess on 1.75 inches should extend from coastal regions of the
South Texas to the Upper Texas coast and eastward into portions of
Mississippi/Alabama and Georgia. Given the resulting instability
and moisture convergence along the front, scattered storms capable
of producing heavy rainfall will linger until the front and
associated upper level trough moves through. With decreasing
precipitable water values and a less uniform low-level flow
pattern, thinking is that rainfall amounts/rates will not be as
high as in previous days. In due time, any convection should be
moving into areas which have higher FFG. Thus thinking is that a
Marginal Risk should be sufficient for now...although later shifts
can evaluate the need for a Slight Risk area given a clearer look
at how the pattern evolves.

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
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
312 PM EDT Mon May 10 2021

Valid 12Z Thu May 13 2021 - 12Z Mon May 17 2021

1800 UTC update...

The models and ensembles continue to show fairly good agreement
(especially the first half of the period) with the overall large
scale pattern, but larger differences in the details. There is not
a whole lot to note with the new 00z/06z guidance, with the same
forecast concerns from the overnight shift as noted below. Given
this, maintained pretty close WPC continuity with no major changes
to the forecast needed. The updated guidance favored a blend
towards the deterministic ECMWF/GFS/CMC/UKMET days 3-5, with more
weight on the ensemble means/ECMWF days 6-7. See previous
discussion copied below for the details.

Santorelli


Previous discussion issued at 0655 UTC...

...Overview...

Latest guidance continues to show moderate progression to flow
aloft across the lower 48 and vicinity. Aside from fairly minor
details, there is reasonable consensus for a broad mean trough to
cross the eastern U.S. and emerge into the Atlantic while a
trailing ridge over the West as of Thursday should reach into the
Mississippi Valley by next Monday. There has been more spread and
variability for details of upstream Pacific energy that should
amplify into an upper trough that reaches/crosses the West during
the weekend, as well as for details of flow behind this feature.
Aside from a front dropping through the Gulf and Florida Peninsula
late this week, waves/fronts will primarily affect the
western/central U.S. as high pressure prevails over the East.
Thus the best focus for precipitation during the period will
likely extend from the northern Rockies through a majority of the
Plains and into the Middle/Upper Mississippi Valley. Contrasting
temperatures on Thursday, with parts of the West quite warm and
the South/East much cooler than normal, should steadily gravitate
toward climatology with time.


...Guidance Evaluation and Preferences...

With current model runs still showing minimal surface response to
the upper trough crossing the East aside from waviness well
offshore, the dominant forecast difficulties involve the shortwave
energy expected to move into the West during the weekend and
upstream flow that will affect the pattern behind this energy.
After a couple days of increased spread for where/when features
could try to separate, 12Z/18Z model/ensemble runs (and followed
up in the new 00Z runs) have returned to some variation of 2-3
days ago when there was better clustering toward the idea that any
separation that occurs would wait until energy reaches near the
West Coast. There are still significant differences for what
individual pieces of energy will do though, and predictability is
low due to their small scale. A blended approach (more models
than means into day 5 Saturday followed by a more even mix by day
7 Monday) continues to provide the best way to navigate the spread
and variability that the models/ensembles have exhibited so far.

Farther west, recent GFS runs trend out of phase with North
Pacific flow from the weekend onward--a result of western Pacific
flow separation which other guidance does not depict. In contrast
to the flat flow to ridging that the GFS shows near the West Coast
by next Monday, ECMWF/CMC runs and their ensemble means have been
fairly consistent so far in bringing another trough toward the
West--albeit with detail and amplitude differences. Latest GEFS
means fall between these two extremes, with a weaker and delayed
trough approaching the West. Current preference is to tilt the
forecast somewhat more in the direction of the ECMWF cluster
relative to the GFS/GEFS.


...Weather Highlights/Hazards...

Expect a gradual increase in coverage and amounts for rain and
thunderstorms over the central U.S. and northern Rockies from late
this week into early next week. Initial activity will be focused
mostly by weak northern stream shortwave impulses and there should
be a relative lack of deep moisture. Then Gulf moisture should
begin to contribute more and interact with one or more
waves/fronts as eastern U.S. high pressure nears the East Coast.
With uncertainty in the specifics, shortwave energy emerging from
the West late in the period may provide another ingredient in the
rainfall recipe. Currently expect the highest five-day totals
over portions of the Central/Southern Plains and Middle/Upper
Mississippi Valley. Some precipitation could be in the form of
snow over highest elevations of the northern Rockies. Any
precipitation farther west should be fairly light and scattered.
The upper trough crossing the East late this week may produce some
areas of mostly light rainfall over central/northern latitudes,
with greater coverage during the latter half of each day.
Locations near the Southeast coast and over the Florida Peninsula
could see somewhat more organized rainfall for a time before a
drier trend, as a wavy front initially over the Gulf and Florida
Peninsula pushes southward.

On Thursday the southern tier will see well below normal highs
with minus 10-20F anomalies over parts of the Southeast and minus
10-15F anomalies over parts of Texas. Some locations in the
Southeast could challenge daily records for cool highs. Less
extreme below normal readings should prevail late this week over
the remaining majority of the central/eastern U.S. Meanwhile
temperatures are likely to be up to 10-20F above normal over areas
from Oregon/California into the Great Basin late this week with
more moderate anomalies stretching into the central Rockies/High
Plains late this week into the weekend. During the weekend upper
troughing that moves into the West should bring temperatures back
down toward normal across the region from west to east. The
moderating trends for both cool and warm areas from late this week
may yield highs within a few degrees on either side of normal over
much of the lower 48 by next Monday.

Rausch


Hazards:
- Heavy rain across portions of the central and southern Plains,
Mon, May 17.
- Flooding possible across portions of eastern Texas to the
Louisiana border, eastern Kansas, as well as northern Ohio.

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