Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Wed May 08 2024

Valid 00Z Thu May 09 2024 - 00Z Sat May 11 2024

...Severe thunderstorms and flash flooding are likely from parts of the
Ozarks to the Tennessee Valley tonight before the severe weather threat
shifts south on Thursday toward the Southern Plains and Southeast...

...Heavy mountain snow and and moderate lower elevation rain gradually
wanes across the Northern Rockies on Thursday...

...Above average temperatures remain across southern Texas and the
Southeast, while warmth also begins to build into the Northwest
late-week...

Active weather is ongoing and expected to continue through tonight across
parts of the middle/lower Mississippi valleys, Tennessee Valley, and into
the southern Appalachians. The atmospheric setup includes an upper low
over the Northern Plains with associated troughing extending into the
Southwest, allowing for moist southerly flow into the Mid-South. A
quasi-stationary boundary extending into the Ohio Valley and detached
outflow boundary in the Tennessee Valley will provide a focus for storms
to develop and track along. Numerous thunderstorms are expected to become
severe and possibly contain very large to giant hail, damaging wind gusts,
and some strong tornadoes possible. The Storm Prediction Center has issued
a Moderate Risk (level 4/5) of severe weather between south-central
Missouri to Middle Tennessee and south-central Kentucky. Additionally, as
storms overlap with saturated soil from earlier rainfall and train behind
one another, numerous instances of flash flooding are possible. The
greatest risk for flash flooding is across southwestern Kentucky and
Middle Tennessee, where a Moderate Risk (level 3/4) of Excessive Rainfall
is in effect. By Thursday, the overnight thunderstorm activity is
anticipated to sink southward and impact areas from the Southern Plains to
the Southeast. Very large hail appears probable across parts of central
Texas and into the ArkLaTex region, with a broader region of severe hail
and damaging wind potential extending through the lower Mississippi Valley
into the Southeast. Areas of flash flooding are also possible along this
corridor stretching between eastern Texas and central Georgia. As the
associated cold front continues to push south and east on Friday, the
scattered thunderstorms threat is expected to linger, but remain confined
to the Southeast.

A long duration heavy precipitation event is also ongoing throughout the
Northern Rockies and parts of the northern High Plains through at least
Thursday. The upper low over the Northern Plains continues to funnel
moisture into favorable upslope regions of Montana and Wyoming. Heavy snow
remains probable into tonight across the higher elevated ranges, with
moderate rainfall in the lower elevations. As the upper low weakens and
becomes more elongated, precipitation is also expected to weaken while
also sliding southward into the central/southern Rockies. Additionally,
the well below average temperatures are forecast to remain throughout the
Rockies with a warmer trend beginning by the end of the week.

Warm weather for the final few days of the workweek can be expected across
the Northwest, Southeast, Gulf Coast, and southern Texas. Locations in
these regions can anticipate highs around 10 to 15 degrees above average,
with a few daily record highs potentially being tied/broken. As far as
actual high temperatures go, southern Texas will be by far the hottest
with highs into the triple digits and upper 90s. Low-to-mid 90s are
forecast across Florida on Thursday, with 80s and 90s throughout the rest
of the Gulf Coast and Southeast. Meanwhile, the Pacific Northwest will
also need to break out the summer clothing and stay hydrated as highs soar
into the 80s by Friday, with low 90s possible in localized interior valley
locations from central California to Oregon.


Snell


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed May 8 2024

Valid 12Z Sat May 11 2024 - 12Z Wed May 15 2024

...Emerging Texas/Gulf Coast/Southeast Heavy Rainfall Threat...


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

Guidance continues to show pretty good agreement that mean
troughing will linger over the Great Lakes/Eastern U.S. into early
next week as a blocky pattern out west takes some time to de-
amplify. By early next week, shortwave energy into western Canada
will amplify over the Northwestern U.S. allowing a closed low over
the Great Basin to shift eastward into the Plains/Midwest Monday-
Wednesday. This should also help push troughing over the East out
and upper ridging to build across the Southeast. Additional
shortwave energy may move into the Southwest late period as well.

Overall, the latest models and ensembles show good agreement on the
large scale pattern, but plenty of uncertainty in the timing and
details of individual systems. The differences in the central U.S.
shortwave in particular could have bigger implications on heavy
rainfall across parts of the southern Plains into the Gulf
Coast/Southeast this weekend and into next week, including areas
that have been very wet in the past week. So these areas may be
more sensitive than normal to smaller changes in the
pattern/details.

Despite the differences, it seemed that a general model compromise
worked well as a starting point and offered the best continuity
with the previous WPC forecast. For days 3-5, was able to use an
equal blend of the deterministic solutions, incorporating
increasing amounts of the ensemble means into the blend for days
6-7 to mitigate the smaller scale details that are impossible to
resolve at these longer time scales.


...Weather/Hazards Highlights...

Moderate rainfall will linger this weekend over the Northeast with
wrapback moisture and upper trough support as downstream energy
transference leads to coastal low genesis, but details remain
uncharacteristically uncertain even as we enter short range time
frames. Upstream northern stream systems will then periodically
offer bouts of mainly lighter rains from the north-central U.S. to
the Great Lakes/Northeast this weekend to next midweek, albeit
also with uncertainty in focus/timing, but with limited impact.

Meanwhile to the south, a cooling/slowing trailing front along
with a reinforcing front and upper shortwaves are expected to
periodically focus precipitation from the southern Rockies/Plains
out through the Gulf of Mexico and Gulf Coast/Florida this weekend
through early-mid next week. A growing but still not solidified
guidance signal offers support for moisture to increase along the
Gulf Coast and vicinity ahead of the returning warm front. Heavy
rain may work inland over time as the front edges northward. The
WPC Day 5/Sunday into Monday Excessive Rainfall Outlook update this
afternoon expanded the previous marginal risk area significantly
northward into eastern Texas to far southern Oklahoma. Given
soil/streamflow sensitivities across especially southeast Texas,
also expanded the slight risk farther north. Heavy rain and a
threat of runoff issues may also spread through the central Gulf
Coast to the Southeast early-mid next week to monitor.

Santorelli/Schichtel


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 forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 AM EDT Wed May 08 2024

Valid 00Z Thu May 09 2024 - 00Z Thu May 16 2024

...A wet pattern emerging for Hawaii...

Trade flow into the islands has less support through the rest of
the week with weakening and shift of mid-latitude high pressure
with onset of ample mid-latitude upper troughing/height falls and
lead surface frontal approach. This also prompts a more moist
island trade response/inverted surface troughing. Forecast spread
continues to decrease as guidance continues a trend toward
development of an ampified upper trough/low over the state late
week and weekend that seems reasonable considering amplified
upstream upper ridging. There is then a longer term guidance trend
for upstream digging of amplified kicker/replacement upper trough
energy toward the state early-mid next week to rejuvinate wet
pattern support. A favored composite of decently clustered
guidance solutions bolsters confidence in development of an
emerging wetter pattern for the state with potential for locally
heavier rainfall, with some focus into favored terrain.

Schichtel

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Wed May 8 2024
Day 1
Valid 16Z Wed May 08 2024 - 12Z Thu May 09 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO AND TENNESSEE VALLEYS...

...16Z Update...

Current radar/sat composite shows two distinct areas of impact this
morning; a west-east elongated boundary with training bouts of
convection across southern KY into northern TN, and a prolific
supercell propagating to the east-southeast across MO. The
supercell over MO is important in the overall evolution of flooding
prospects across the Ohio and Tennessee Valleys as the mesocyclone
from this cell will pivot southeast, becoming the main focal point
for congealing convection with expected upscale growth into a
significant MCS that is well-stated with the latest SPC D1 Moderate
Risk for severe. Rainfall across southern KY into northern TN has
already produced localized totals of 1-2" with another 1-3"
anticipated over the course of the morning and early afternoon
before the main precip shield makes headway into the region this
evening through the overnight time frame (For more information on
the current rainfall, see MPD #0239). The compounding factor
creates a significant threat for flash flooding potential as areal
priming from the first wave of rainfall will degrade the regional
FFG's leading to a proned zone for further impact. This generally
lies within the confines of the remnant boundary where the current
precip is ongoing, and where the MCS will become rooted later in
the period.

Theta-E gradient is well-documented given the latest RAP surface
analysis with some modest advection anticipated across the Mid-
Mississippi Valley within the lee of a surface low analyzed near
the KS/MO border. This will create a general zone of recovery over
western TN into far southwest KY down through the Lower
Mississippi Valley with MLCAPE indices likely between 2500-4000
J/kg across the aforementioned area, and 2000-3000 J/kg for points
further east across TN and northern MS/AL. The western locations
will see an increase in convective coverage with locally heavy
rainfall initiating between 18-21z allowing for a modest flash
flood threat based on discrete convection alone. As we move into
the early evening hours, general convective pattern will shift from
discrete convection to more multi-cell and organized cell
structures with aid from the primary mesocyclone diving southeast
out of MO along the theta-E gradient. That's when the convective
mode will favor upscale growth and a significant swath of very
heavy rainfall is anticipated through the path of the expected MCS.

Probability fields reflect the anticipated impacts this evening
within the bounds of the latest HREF EAS signals for at least 3" of
total precip the next 24 hrs, as well as a significant output from
the 10 and 100-year ARI exceedance probabilities. The former is now
running between 60-80% across the border of KY/TN to the northeast
of I-65 with a large swath of at least 40% probability expanding
west-northwest to east-southeast between Paducah to about 75 miles
east of Nashville. The 10-year ARI exceedance probabilities are
fairly similar to the 3" EAS markers with 100-year ARI settling
between 20-35% in that zone to the northeast of I-65. This
correlates to the area that will see dual impact from the morning
wave of convection and the expected MCS overnight leading to HREF
blended mean totals approaching 6-8" locally. An expanse of 3-4.5"
mean totals encompass the entire area from southwest KY through
northern and central TN leading to a robust areal coverage of heavy
rainfall and the necessity to expand the MDT risk a bit further
south to reflect the trends. The southern periphery of the SLGT was
also expanded to account for recent trends in guidance bringing
the southern and southwest extent of the primary MCS into northern
MS/AL, as well as northern GA as the bowing segment is forecast to
propagate to the southeast within that sharp theta-E gradient,
tapping into the favorable environment on the southern extent of
the complex. A secondary MCS is possible given the latest HRRR as a
shortwave ejects out of TX behind the primary shortwave
responsible for the setup. This will be a little less severe and
doesn't favor the prolific rainfall totals from it's predecessor,
however additional rainfall over areas that have been impacted
would add, "Insult to injury" and could exacerbate the flood threat
that is forecast to occur across TN.

The previous MDT was maintained with minor expansion on the western
and southern edges of the risk area. There is a non-zero chance for
a targeted High Risk somewhere within the confines of TN, but this
will be more of a nowcast as we monitor the evolution of the
prospectus MCS and the QPF footprint left in wake of the
disturbance. Regardless, a significant flash flood threat is likely
within the bounds of the MDT risk and could be locally significant
even within areas bounded by the SLGT risk. Elsewhere, the threat
across the Midwest did not feature any significant chances that
would allow from much deviation from the previous forecast.
Continuity was maintained for the locally heavy rainfall threat
under the guide of the inverted trough and upper forcing situated
across IA/MN/SD.

Kleebauer

...Previous Discussion...

A Moderate Risk has been introduced with this update across
portions of the Ohio and Tennessee Valleys for the potential of
multiple rounds of thunderstorms through early Thursday morning
that may lead to several/numerous instances of flash flooding,
some of which could be significant across southern KY and middle
TN.

Current radar imagery shows a few lines of thunderstorms moving
south of the Ohio River into KY, generally along/ahead of a cold
front that is draped across portions of Indiana southwestward
through southern Illinois. Aloft, the deep closed upper low remains
centered over the Northern High Plains with a broad area of
southwesterly flow stretching from the Southern Plains to the lower
Great Lakes. Embedded in that flow is a weak shortwave analyzed
over portions of AR/MO, moving northeast.

The expectation is that the current convection will continue to sag
south/southeast through KY, settling across southern KY near the TN
border by the start of the period before stalling out with the
loss the best nocturnal LLJ. This is expected to lay out a quasi-
zonal boundary that will be the focus for additional thunderstorm
development through the day as the airmass reloads with the
deepening low pressure over the Mid-MS Valley.

The environment today will be characterized by high PWs (1.5"+)
and a zone of sufficient instability (MUCAPE > 2000 J/kg). Most
importantly will be the increasingly favorable setup for training
convection especially by later this evening/tonight across middle
TN and southern KY with the deep layer mean flow becoming oriented
parallel to the storm motions, helping to foster backbuilding and
training convection.

The probabilistic data continues to support the higher end rainfall
and rain rate potential with the 00Z HREF probabilities for 5"
increasing to near 50% straddling along the KY/TN border. This is
supported by moderate values of 2" hourly rainfall total
probabilities later today/this evening. Finally, the HREF EAS
probabilities for 3" have increased from the previous forecast to
near 40 percent, again mainly along the KY/TN border region. All
this together supports the potential for multi-inch totals through
the period and potential for more significant instances of flash
flooding.

Elsewhere, the Slight Risk encompasses areas further north into IN
and OH where some hi-res guidance suggests convection may develop
that far north and be robust enough to support isolated but intense
rain rates, and some of these areas are a bit more saturated due to
recent heavier rainfall. Further southwest along the frontal
boundary, deeper and more intense convection is likely to develop
along the frontal boundary into AR but the more isolated nature and
higher FFGs may limit scope of any flooding there.


...Upper Midwest...
Compact area of deeper convection is expected to develop along an
inverted trough underneath the core of the upper low in the
Midwest that will be capable of producing localized heavy rainfall
and flash flooding. A narrow axis of higher moisture is expected,
with PWs above 1-1.25" from far southeast South Dakota through
Iowa and southern Minnesota. Given this greater moisture, broad
forcing, and steepening lapse rates, pockets of slow-moving and
pivoting showers and thunderstorms are likely and the latest HREF
probabilities support isolated/scattered instances of hourly totals
in excess of 1" and 24-hr totals up to 2" in places. Given this,
there is at least some flash flood risk so the Marginal Risk for
the area continues to look good.


Taylor

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Wed May 8 2024
Day 2
Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS
THROUGH THE SOUTHEAST U.S...

...20Z Update...

The main addition with the new D2 ERO is the extension of the SLGT
risk across the Southeast U.S to account for a "two part" event
that is increasing in likelihood during the period. Prospects for
direct impact from a MCS upstream to the northwest are growing
across portions AL and GA with the remnant outflow boundary likely
playing a role in the evolution of the next disturbance growing
upscale and propagating out of east TX through the Deep South. HREF
probabilities have steadily grown with the latest update signaling
periods of 1-2"/hr rates embedded within the core of the complex as
it traverses AL/GA Thursday morning. Hi-res deterministic has
pivoted towards the idea over time with the upscale growth regime
likely to occur on the upwind side of the primary complex as it
moves through TN prior to the D2 period. 1-2"/hr rates are upwards
of 25-35% with a few 40+% bullseyes depicted over the course of
Thursday morning and early afternoon before the boundary finally
stalls in-of central MS/AL/GA. A solid 1-2" with locally higher
rainfall is plausible prior to the secondary convective complex
developing upstream over TX due to a the progression of a stout
mid-level shortwave ejecting out of the sub-tropical jet.

The convective development will likely cause some local flood
concerns over east TX where rainfall has been plentiful as of late
with NASA SPoRT soil moisture percentiles running upwards of 90-95%
within the confines of I-20 and I-10 extending eastward into LA.
This is the expected track of the next disturbance which will
likely move eastward across the Deep South over the course of
Thursday evening into early Friday morning. The forward propagation
is anticipated to be somewhat fast moving, but the flow is
forecasted to be more parallel to the stationary boundary lingering
from the previous complex, so the prospects for training and
backbuilding within the environmental bounds of the complex are
more probable leading to better flash flood potential. Areal
average QPF footprint within the HREF blended mean is a general
1.5-2" across east TX through central MS with a 2-3+" average
within central AL and GA thanks to the overlapping rainfall
prospects from both forecasted disturbances. HREF neighborhood
probability for at least 2" of total precip has a general 10-15%
outline across the Southeastern U.S with a bullseye of 25-30%
located over south-central GA to the AL line. Deterministic QPF
maxima are generally 3-5" within any area between east TX through
GA indicating the forecasted limit of the convective impacts. This
will be enough to garner some localized flash flood prospects
across many areas in the Southeast, especially any urban corridors
and places that see a repeating convective regime or any kind. A
SLGT risk was maintained and expanded to the east to reflect the
potential.

Further to the north, scattered convection across the Midwest and
Mid-Atlantic along and west of the Piedmont will allow for isolated
heavy rainfall concerns with some urban and terrain influenced
flash flooding prospects within any stronger rain cores. Elevated
PWATs and modest mid and low-level instability forecast will
maintain an environment that is favorable for a low-end MRGL
concern with the best prospects across the central Appalachians
where the overall QPF forecast and low-end exceedance probabilities
for 1"/hr rates based on the 12z HREF signal. Thus, have kept
continuity with the previous MRGL risk forecast.

Kleebauer

...Previous Discussion...

...Southeast...
The strong spring system over the Central U.S. will begin to
elongate/stretch in a lengthy, positively tilted shortwave trough
by the start of the forecast period and sink southward toward the
region. Earlier convection from the previous day is likely to push
outflow boundaries through the region while another embedded
shortwave trough is expected to move through in the southern
stream. Altogether, this should put the region in a favorable
region of forcing for ascent. Deeper moisture is expected to pool
along/ahead of the surface boundaries and will be characterized by
PWs greater than 1.5", which is around 2 std above the
climatological normal for this time of year. With sufficient
instability developing that will overlap with the better forcing
and moisture, rounds of thunderstorms are expected to develop along
the boundary initially over eastern TX before slipping
south/southeast toward the Gulf Coast across portions of LA/MS/AL.
By this point in the storm system evolution, the flow is likely to
be more parallel to the overall storm motions and this could favor
some repeating rounds or training convection particularly over
portions of MS/AL. The environmental ingredients will support
intense rain rates over 1-2"/hr at times and the latest
probabilistic data shows potential for isolated 2-4" totals in a
relatively short period of time. While there remains some
uncertainty in the placement of the heaviest, no significant
changes were made to the Slight Risk area and the message for
scattered instances of flash flooding.

...Central Appalachians to the Mid-Atlantic...
Low pressure is expected to track from the lower Ohio Valley to
the eastern Great Lakes over the forecast period as shortwave
trough energy aloft and height falls over the region. A strong
southwesterly flow in the low levels will bring higher moisture,
characterized by dewpoints in the upper 50s to lower 60s and PWs
above 1", especially from the Central Appalachians and southern
areas of the Northeast. With the uncapped and warm/moist
environment, there should be scattered to numerous showers and
thunderstorms that develop that will be capable of producing
isolated/localized rain rates in excess of 1-1.5"/hr at times. This
could cause isolated flash flooding, particularly for some of the
areas that have seen recent heavy rainfall and more saturated
ground conditions as well as urban locations.

...Midwest to Western Great Lakes...
Deeper moisture pooled along an inverted trough / TROWAL slowly
moving through the region will be the primary focus for embedded
heavy rain cores that may lead to flash flooding. Tall/skinny CAPE
profiles, an axis of higher/deeper moisture, and warm cloud depths
will promote efficient rain producing showers and thunderstorms
that could total 1-2" in spots and a Marginal Risk was introduced
for portions of this region.

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Wed May 8 2024
Day 3
Valid 12Z Fri May 10 2024 - 12Z Sat May 11 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST U.S....

...20Z Update...

Only minor adjustments were necessary from the previous forecast
issuance as guidance continues to favor a window of opportunity for
locally heavy rainfall across southern GA and northeast FL in wake
of a passing MCS that is forecast to traverse the region on Friday
morning and exit off the coast during the afternoon hours. The
location of the boundary responsible for the focal point of the MCS
propagation will be the main point for the forecast with the
forward speed of the MCS the second point of interest with regards
to heavy rain potential. Guidance is around the 1-2" range with
locally higher amounts embedded across southern GA, but there are
hints that a bigger event is possible pending the speed of the MCS
as it approaches the southeastern coastal plain. Ensemble bias
corrected QPF is around 1.5-2.5" across the southern GA/AL lines,
in-line with the expected positioning of the MCS when averaged
across global deterministic. The complex will be in a slow,
weakening state late Friday morning into the afternoon, but a
pronounced theta-E gradient positioned over northeastern FL and
southeast GA signals some potential for redeveloping convection
across those areas based on the premise of targeted low-level
convergence within a pre-frontal trough pattern before instability
gets swept out of the area from the trailing cold front to the
northwest. This area has notoriously higher FFG's due to the soil
types and limited impact from significant rainfall anytime
recently. This will lead to a lower end flash flood risk outside
any urbanized zones and places already seeing impact at the back
end of D2 into the new period (Southwest GA and Southeast AL). The
MRGL risk was maintained as a result.

Kleebauer

...Previous Forecast...

The advancing frontal boundary across the Southeast U.S. will be
the primary focus for deep convection Friday into Friday afternoon
and will be capable of producing rain rates in excess of 1-1.5"/hr
at times. Showers and thunderstorms are expected to be ongoing at
the start of the period, lingering from the deep, organized
convective line from the previous day. This activity will be in the
presence of a sufficiently high moisture pool, characterized by
dewpoints well into the 60s and low 70s and a PW axis of 1.5-1.75".
A subtle shortwave trough noted in the flow will be the main large
scale forcing mechanism to drive the convection that looks to
track through southeast Alabama, southern Georgia, northern
Florida and then eventually off the Southeast coast. Some
localized heavier rain totals and rain rates will be possible that
could pose an isolated flash flood risk.

Taylor

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed May 8 2024


Moderate rainfall will linger this weekend over the Northeast with
wrapback moisture and upper trough support as downstream energy
transference leads to coastal low genesis, but details remain
uncharacteristically uncertain even as we enter short range time
frames. Upstream northern stream systems will then periodically
offer bouts of mainly lighter rains from the north-central U.S. to
the Great Lakes/Northeast this weekend to next midweek, albeit
also with uncertainty in focus/timing, but with limited impact.

Meanwhile to the south, a cooling/slowing trailing front along
with a reinforcing front and upper shortwaves are expected to
periodically focus precipitation from the southern Rockies/Plains
out through the Gulf of Mexico and Gulf Coast/Florida this weekend
through early-mid next week. A growing but still not solidified
guidance signal offers support for moisture to increase along the
Gulf Coast and vicinity ahead of the returning warm front. Heavy
rain may work inland over time as the front edges northward. The
WPC Day 5/Sunday into Monday Excessive Rainfall Outlook update this
afternoon expanded the previous marginal risk area significantly
northward into eastern Texas to far southern Oklahoma. Given
soil/streamflow sensitivities across especially southeast Texas,
also expanded the slight risk farther north. Heavy rain and a
threat of runoff issues may also spread through the central Gulf
Coast to the Southeast early-mid next week to monitor.

Santorelli/Schichtel

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed May 8 2024


Moderate rainfall will linger this weekend over the Northeast with
wrapback moisture and upper trough support as downstream energy
transference leads to coastal low genesis, but details remain
uncharacteristically uncertain even as we enter short range time
frames. Upstream northern stream systems will then periodically
offer bouts of mainly lighter rains from the north-central U.S. to
the Great Lakes/Northeast this weekend to next midweek, albeit
also with uncertainty in focus/timing, but with limited impact.

Meanwhile to the south, a cooling/slowing trailing front along
with a reinforcing front and upper shortwaves are expected to
periodically focus precipitation from the southern Rockies/Plains
out through the Gulf of Mexico and Gulf Coast/Florida this weekend
through early-mid next week. A growing but still not solidified
guidance signal offers support for moisture to increase along the
Gulf Coast and vicinity ahead of the returning warm front. Heavy
rain may work inland over time as the front edges northward. The
WPC Day 5/Sunday into Monday Excessive Rainfall Outlook update this
afternoon expanded the previous marginal risk area significantly
northward into eastern Texas to far southern Oklahoma. Given
soil/streamflow sensitivities across especially southeast Texas,
also expanded the slight risk farther north. Heavy rain and a
threat of runoff issues may also spread through the central Gulf
Coast to the Southeast early-mid next week to monitor.

Santorelli/Schichtel

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EDT Wed May 8 2024

Valid 00Z Thu May 09 2024 - 00Z Sun May 12 2024

...Northern to Central Rockies...
Days 1-3...

As the upper low continues to sink south and weaken throughout the
night, the northerly IVT that prompted the anomalous supply of
moisture over the Northern Rockies and the enhanced upslope flow
into favorably oriented mountain ranges will diminish. The mountain
ranges favored for periods of moderate-to-heavy snow through
tonight are the Absaroka, Big Horns, Wind River, and Laramie
Ranges. By Thursday, high pressure building in over Montana will
prompt easterly upslope flow into the southern WY, the CO Rockies,
and as far south as the Sangre De Cristo and San Juans of northern
NM. Latest 12Z HREF shows the potential for 1-2"/hr snowfall rates
Thursday afternoon and into Thursday evening in the central CO
Rockies. Some light mountain snow is expected as far west as the
Uinta and Wasatch of UT as a 500mb cuts off beneath an amplifying
ridge over British Columbia. This cut off 500mb low will keep
mountain snow (generally at/above 9,000ft) over these mountains
ranges in the Central Rockies through Friday and even into
Saturday, but snowfall rates are not forecast to be exceptional and
should result in minor impacts at worst for elevations >9,000ft.

WPC PWPF depicts high chances (>70%) for snowfall >12" at
elevations >9,000ft in the Absaroka and Wind River Ranges through
Thursday afternoon. Farther east, the Big Horns and Laramie Range
have moderate-to-high chances (50-70%) for snowfall totals >8".
Then,
between Thursday afternoon and Friday afternoon, WPC PWPF sports
moderate-to-high chances (50-70%) for snowfall totals >8" in the
San Juans and Sangre De Cristo above 10,000ft. These listed
mountain ranges are forecast to witness Minor Impacts according to
the latest WSSI with elevations above 9,000ft having the best odds
of witnessing localized Moderate Impacts.

The probability of significant icing across the CONUS is less than
10 percent.

--

In other news, this is the final day the WPC Winter Weather Desk
is scheduled to be staffed for the 2023-2024 cold season. WPC PWPF
will still run automatically each night and afternoon for users to
review. Should a late season winter storm occur in the coming
weeks, we will quickly spin the desk up to provide support. Thank
you to all who have read our discussions and utilized our products
this past winter season! We will be back to fully staffing the
winter desk in late September.

Mullinax

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
236 PM EDT Wed May 8 2024

Valid 12Z Sat May 11 2024 - 12Z Wed May 15 2024

...Emerging Texas/Gulf Coast/Southeast Heavy Rainfall Threat...


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

Guidance continues to show pretty good agreement that mean
troughing will linger over the Great Lakes/Eastern U.S. into early
next week as a blocky pattern out west takes some time to de-
amplify. By early next week, shortwave energy into western Canada
will amplify over the Northwestern U.S. allowing a closed low over
the Great Basin to shift eastward into the Plains/Midwest Monday-
Wednesday. This should also help push troughing over the East out
and upper ridging to build across the Southeast. Additional
shortwave energy may move into the Southwest late period as well.

Overall, the latest models and ensembles show good agreement on the
large scale pattern, but plenty of uncertainty in the timing and
details of individual systems. The differences in the central U.S.
shortwave in particular could have bigger implications on heavy
rainfall across parts of the southern Plains into the Gulf
Coast/Southeast this weekend and into next week, including areas
that have been very wet in the past week. So these areas may be
more sensitive than normal to smaller changes in the
pattern/details.

Despite the differences, it seemed that a general model compromise
worked well as a starting point and offered the best continuity
with the previous WPC forecast. For days 3-5, was able to use an
equal blend of the deterministic solutions, incorporating
increasing amounts of the ensemble means into the blend for days
6-7 to mitigate the smaller scale details that are impossible to
resolve at these longer time scales.


...Weather/Hazards Highlights...

Moderate rainfall will linger this weekend over the Northeast with
wrapback moisture and upper trough support as downstream energy
transference leads to coastal low genesis, but details remain
uncharacteristically uncertain even as we enter short range time
frames. Upstream northern stream systems will then periodically
offer bouts of mainly lighter rains from the north-central U.S. to
the Great Lakes/Northeast this weekend to next midweek, albeit
also with uncertainty in focus/timing, but with limited impact.

Meanwhile to the south, a cooling/slowing trailing front along
with a reinforcing front and upper shortwaves are expected to
periodically focus precipitation from the southern Rockies/Plains
out through the Gulf of Mexico and Gulf Coast/Florida this weekend
through early-mid next week. A growing but still not solidified
guidance signal offers support for moisture to increase along the
Gulf Coast and vicinity ahead of the returning warm front. Heavy
rain may work inland over time as the front edges northward. The
WPC Day 5/Sunday into Monday Excessive Rainfall Outlook update this
afternoon expanded the previous marginal risk area significantly
northward into eastern Texas to far southern Oklahoma. Given
soil/streamflow sensitivities across especially southeast Texas,
also expanded the slight risk farther north. Heavy rain and a
threat of runoff issues may also spread through the central Gulf
Coast to the Southeast early-mid next week to monitor.

Santorelli/Schichtel


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 forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw