Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Tue May 14 2024

Valid 12Z Tue May 14 2024 - 12Z Thu May 16 2024

...Severe thunderstorms and flash flooding will impact portions of the
Gulf Coast and the Southeast on Tuesday...

...Above-average temperatures expected for South Florida and the West...

Wet and unsettled weather is in store for the Gulf Coast and Southeast
today thanks to a slow-moving frontal boundary. Showers and thunderstorms
across this region have the potential to turn severe and will be capable
of producing heavy rain. SPC has a Slight Risk for severe weather,
indicating the potential exists for multiple corridors of damaging winds,
very large hail, and a one or two tornadoes covering central/northern
Florida and southern Georgia. Additionally, WPC has a Slight Risk for
excessive rainfall and local flash flooding highlighted for much of the
same area as the severe weather threat while a broader Marginal Risk spans
northward to the Ohio Valley and Mid-Atlantic regions. As the system
advances eastward the threats for severe storms and heavy rainfall will
reduce for Wednesday to a Marginal Risk.

Showers and storms over the lower Missouri an Mid-Mississippi valleys to
the Ohio Valley will spread east through the Mid-Atlantic and the
Northeast today. A brief window of drier weather over the Mississippi
Valley into early Wednesday will be followed by the next round of showers
and storms that will begin to spread into the region by late Wednesday.
Before impacting the central U.S. on Wednesday, this previously noted
system will spread showers and storms followed by much cooler temperatures
from the Northern Rockies into Central Rockies and High Plains on Tuesday.


South Florida will continue to have well above-normal, with near to record
high temperatures through Wednesday. Meanwhile, warmer temperatures will
spread across the western states reaching well-above normal highs across
the region by midweek.

Campbell

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Tue May 14 2024

Valid 12Z Fri May 17 2024 - 12Z Tue May 21 2024

...Another southern tier heavy rainfall threat during the latter
half of the week...


...Overview...

Downstream from a reorienting eastern Pacific upper ridge, guidance
is generally suggesting flat to broadly cyclonic mean flow late
this week into the weekend, followed by a tendency toward more
upper troughing over the West early next week. There are still a
lot of lower-predictability detail questions about flow rounding
the ridge and reaching the lower 48. There is also considerable
spread for the details/timing of an upper shortwave and associated
surface system crossing the eastern half of the country late this
week into the weekend. Continue to expect the heaviest convective
emphasis to be across the southern tier late this week into the
weekend, with some areas of less extreme but potentially meaningful
rainfall farther north.

...Guidance/Predictability Assessment...

Over recent days the dynamical models/ensembles and machine
learning (ML) models have been showing difficulty in resolving
important but medium to smaller scale details within progressive
Northeast Pacific into lower 48/southern Canada flow, and then how
potentially better defined western troughing may evolve
(influencing when an upper low drifting toward California
accelerates inland). There has been plenty of spread and run-to-run
variability over the East as well, with significant differences for
timing/details of shortwave energy and associated surface
reflection late this week onward plus what the larger scale flow
will look like by early next week.

Based on comparisons of 12Z/18Z guidance, a composite of
operational model solutions provided the most palatable starting
point. At the start of the period Friday the 12Z ML models offered
more support toward flatter flow over the Northwest than the
GFS/GEFS. By mid-late period, the ML models generally leaned toward
the cluster of 12Z ECMWF/CMC and their ensemble means which were
more progressive than the GFS/GEFS for the system crossing the East
and moving into the Atlantic, and showed less pronounced digging
south of an upper low that may reach southwestern Canada and the
northwestern U.S. (but with spread for incorporation of the
southern stream Pacific into California upper low) This majority
cluster also suggested that by next Tuesday there should be some
degree of upper troughing over eastern North America with ridging
over the Plains, in contrast to a flat ridge reaching the East with
southwesterly flow over the Plains. Thus the forecast leaned more
toward the 12Z ECMWF-ECens/CMC-CMCens scenario. The one
reservation was that incorporation of the Pacific/California low
could have been a little slow.

New 00Z guidance continues to highlight the volatility of the
pattern, with the UKMET/CMC trending toward the GFS over the
Northwest as of early Friday. Then the average of guidance
trends slower with more separation for the eastern U.S. upper
shortwave late week into the weekend, while the ECMWF has flipped
somewhat in the GFS direction for the pattern over the East by next
Tuesday.

...Weather/Hazards Highlights...

Another southern tier convective event should be in progress at the
start of the period early Friday. The Day 4 Excessive Rainfall
Outlook that covers the Friday-Friday night time frame continues to
depict a Slight Risk area that extends from Louisiana into Alabama,
with some adjustment from continuity to reflect latest guidance.
This area will be sensitive to additional rainfall given wet soil
conditions. Frontal/wave focus with available moisture and
instability along with potential for repeat rains may enhance
totals. The surrounding Marginal Risk area accounts for some
potential of a convective axis hanging back even farther to the
west (still most pronounced in GFS runs). Farther north, latest
ECMWF/GFS/CMC/ICON runs are starting to cluster toward a potential
axis of less extreme but locally heavy rainfall over the Ohio
Valley in association with a front affecting the region. Short-term
rainfall should lead to somewhat wetter soil than initially
observed. The Day 4 ERO introduces a Marginal Risk area to account
for the developing guidance cluster. By Day 5 Saturday, the only
coherent area of potentially heavy rainfall is over portions of the
Southeast as the southern tier surface front/waviness continue
onward. A Marginal Risk provides a reasonable starting point given
the guidance scatter within this region. Confidence is much lower
for rainfall coverage and totals farther northward over the East.

Otherwise, some rainfall could linger along parts of the East Coast
into Sunday if the slower side of the envelope verifies for the
system affecting the region. Some areas of rainfall may develop
over portions of the northern-central Plains into the Midwest by
the weekend or early next week depending on shortwave
energy/surface details, with low confidence for specifics of
intensity at any particular location. Portions of the West may
begin to see scattered precipitation by the weekend and early next
week with the tendency toward more upper troughing and arrival of
the southern stream upper low.

The southern two-thirds of the West will likely see above normal
temperatures continue through the weekend with some areas seeing
highs 10-15F or so above normal. Northern areas will already be
moderating close to normal Friday-Sunday while additional lowering
of heights aloft over the West early next week should bring
temperatures toward normal over the remainder of the West. Some of
the western warmth will extend into portions of the Plains and
Mississippi Valley with plus 5-15F anomalies. The Florida
Peninsula and South Texas should see several days with highs up to
5-10F above normal, in a warm pattern to the south of a couple
fronts expected to focus upon reaching more northern parts of these
states. Some daily records for highs/warm lows will be possible.
These temperatures may bring heat indices as high as 105-110
degrees for south Florida and upwards to 115 degrees for south
Texas, highlighting a threat of hazardous heat.

Rausch


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
358 AM EDT Tue May 14 2024

Valid 00Z Wed May 15 2024 - 00Z Wed May 22 2024

...Heavy rain threat mid-late week and likely extending into the
weekend...

The forecast remains on track for an unseasonably deep/south upper
low to settle just west of 30N 160W by Wednesday-Thursday,
followed by only a slight north or northwest drift thereafter.
Consensus of latest guidance continues to show the associated
deepening surface low near the upper center pushing a front as far
east as the central islands mid-late week, then retreating
gradually westward as upper heights over the state slowly rise.
Expect the developing deep-layer southerly flow to bring an
initial increase of precipitable water values (PWATs), to at least
1.50-1.75 inches, with the best focus for heavy rain and
thunderstorms over the central islands mid-late week. The
dominant clustering of dynamical and machine learning guidance
still depicts a shift back to the western islands by the weekend
with additional moisture streaming up from the south to bring
PWATs up to at least 2.00 inches for a time. Flash flooding will
be a threat during this multi-day heavy rainfall event. By
Monday-Tuesday most solutions suggest that the surface boundary
should drift a little farther west while the moisture/rainfall
axis becomes less pronounced, leading to a drier trend over the
main islands. The one model diagnostic comparison of note is that
from Sunday or Monday onward the 00Z GFS flattens the southeast
side of its upper trough/builds in more ridging from the east
versus most other dynamical models/means and machine learning
models. As a result this GFS run may be a little quick with its
eventual westward shift of the heavy rain axis. The prior 18Z GFS
compared better into early Monday but then also started to stray
from the majority somewhat. Continue to monitor forecasts over
the coming days for any potential adjustments in specific details.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue May 14 2024
Day 1
Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH FLORIDA AND FAR SOUTHERN GEORGIA...

...North Florida and Far Southern Georgia...

The combination of an MCS ongoing as of the time of this writing
over southeast Alabama, southwest Georgia, and the Florida
Panhandle, followed by a trailing cold front tonight will bring at
least 2 rounds of rain to northern Florida. In between, daytime
heating and abundant atmospheric moisture will support potential
isolated to widely scattered convection across north Florida this
afternoon. All these chances of rain will all occur in the Slight
Risk area of north Florida through tonight.

Antecedent soil conditions from NASA Sport imagery have well above
normal soil moisture for portions of the eastern Florida Panhandle,
and hugging the Florida/Georgia Line up to the coast. For much of
the Florida Peninsula, even the northern Peninsula, soils are well
below normal for saturation. Thus, the Slight Risk was adjusted
northward to cover areas with more saturated soils, and were
conservative on the southern side in central Florida, which should
be able to handle all of the forecasted rain with only isolated
instances of flash flooding.

A bit of uncertainty persists as to how much rain will fall after
12Z, with the MCS rapidly traversing the Panhandle now. A faster
movement would reduce overall rainfall expected today. However,
given very favorable antecedent atmospheric conditions...namely
MUCAPE values between 1,000 and 2,000 J/kg and PWATs to 2 inches,
which is 2.5 sigma above normal for this time of year and above the
90th percentile climatologically, any storms that form will have no
trouble producing rates up to 2.5 inches per hour. Despite the
sandy soils and flat terrain of north Florida, these rates could
overwhelm local drainage quickly. The Slight is considered a
lower-end Slight, given the very high FFGs in place across most of
north Florida.

...Mid-Atlantic and Ohio/Tennessee Valleys...

For the Mid-Atlantic, the nose of the LLJ will advect abundant Gulf
moisture north up the Carolina Coast and into Virginia today. As a
low over the Ohio Valley approaches, convection is expected to
become commonplace across the Southeast and southern Mid-Atlantic.
For most of the day, ridging in place aloft should work to prevent
most organization of the storms, though cell mergers and localized
outflow boundaries could cause isolated organization that could
prolong heavy rainfall rates. The area most likely to see
convection today into tonight...eastern North Carolina and southern
Virginia have generally average soil moisture conditions, and so
should be able to handle most of the rainfall, even if briefly
heavy with only isolated instances of flash flooding possible. The
Marginal was trimmed south a bit from inherited to remove the DC
area and much of the central Appalachians, as easterly flow should
prevent most convection from getting to the west side of the Blue
Ridge.

For the Ohio/Tennessee Valleys, a vertically stacked low with
associated upper level cold air will take advantage of about normal
atmospheric moisture and abundant solar heating to result in some
organization to any convection that forms. Expect most of the
convection to be diurnally driven...so the most likely time
heavy rainfall will occur will be this afternoon...though due to
continued forcing, convection may persist into the overnight hours.
Since the vertically stacked low will be moving overhead of western
areas of the lower Ohio Valley...the convection may be able to
organize a bit better there. This will be generally in the vicinity
of southern Illinois and Indiana. Despite this, a relative lack of
moisture should limit any heavy rainfall, so the Marginal remains
largely unchanged.

Wegman

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue May 14 2024
Day 2
Valid 12Z Wed May 15 2024 - 12Z Thu May 16 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
CENTRAL AND SOUTHERN PLAINS AS WELL AS FOR THE NORTHERN FLORIDA
PENINSULA...

...Central and Southern Plains...

Return flow developing in advance of an approaching positively
tilted longwave trough will begin to re-moisten the atmosphere
across the central and especially southern Plains Wednesday and
Wednesday night. Fortunately, all the ingredients for heavy rain
and storms will not be coincident with each other. Thus, the
signals for storms and heavy rain are very sparse and in poor
agreement. From inherited, the focus for more widespread convection
is favoring Oklahoma and into southeastern Kansas, while signals
across hard-hit areas of Texas have been decreasing. Since much of
Oklahoma and Kansas have had time to recover from heavy rains the
past couple weeks, soil moisture conditions are at or below
average for this time of year. Thus, any organized convection,
besides not having a direct connection to deep Gulf moisture will
also be occurring over areas that would likely find the rainfall to
be beneficial. Thus, the Marginal Risk in place across the area
remains unchanged.

For hard hit Texas, the decreasing signal for storms indicates any
convection that develops will struggle to develop, and will remain
isolated. This should keep any resultant flash flooding as very
isolated. However, given how saturated the soils are over almost
the entire northeastern half of the state, even isolated convection
if it moves over a particularly sensitive area could cause flash
flooding. Increasingly abundant Gulf moisture returning north over
south Texas could support a storm or two capable of heavy rainfall
rates. Fortunately most of south Texas hasn't been as wet as areas
further north and east, so once again the flash flooding threat
should be isolated.

...Northern Florida Peninsula...

The cold front moving south down the Florida Peninsula Wednesday is
expected to stall out somewhere over the central part of the state,
generally near Tampa and Orlando. This front will act as a focal
point for convection...mostly Wednesday morning and early afternoon
as the front makes its final push south before stalling out. Soil
moisture levels in this area are well below normal for this time of
year, and given the resistance to flooding typical of this area
represented by very high FFGs, it will take training heavy
thunderstorms to cause isolated flash flooding. However, given the
front will act as a "train track" of sorts in the fast westerly
flow south of the front, training is possible. The Marginal risk
remains in place albeit shifted a little bit south with the latest
model trends.

Wegman

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue May 14 2024
Day 3
Valid 12Z Thu May 16 2024 - 12Z Fri May 17 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EAST TEXAS AND NORTHERN AND WESTERN LOUISIANA...

...Portions of East Texas and Northern and Western Louisiana...

Yet another round of very heavy rainfall is expected Thursday and
Thursday night across the many of the same very hard hit areas of
east Texas through northern and western Louisiana that keep getting
hit with very heavy rain, including just yesterday in some areas.

A positively tilted longwave trough with attendant 100+ kt jet
streak rounding the southern/eastern sides of the trough and
attendant shortwave will all eject out of the southern Rockies and
across Texas Thursday and approach Louisiana Thursday night. Return
flow off the Gulf will tap into impressive amounts of atmospheric
moisture for any time of year. PWATs will climb above 2.25 inches
for the upper Texas Coast, southwestern Louisiana, and extending
well inland. This is above the 95th percentile for this time of
year compared with climatology, and more than 3 sigma above normal.
In addition to atmospheric moisture, abundant surface moisture will
be available as moderate to major flooding is already ongoing
across much of this area. The aforementioned trough, jet streak,
shortwave, and enhanced upper level diffluence supporting
additional lift, and will all combine forces over this already
exceptionally hard hit area.

With weak surface forcing synoptically, outflow boundaries will
likely be the primary forcing at the surface...but given all the
other very favorable ingredients in place that should be all that
it takes to result in MCS development. The abundant moisture will
allow the MCS to persist as it moves across eastern Texas and into
western Louisiana through Thursday night. The embedded convection
will be capable of producing rainfall rates of 3 inches per hour,
with the strongest storms capable of higher. Instability will not
be a problem either, as the abnormally moist atmosphere and strong
May sun will advect MUCAPE values over 3,000 J/kg into the
southern/western sides of the MCS. This will support backbuilding
convection, slowing the overall movement of the MCS. West-pointing Corfidi
Vectors will be orthogonal to the southerly flow of moisture and
instability into the MCS. This will support convection developing
along the outflow boundaries that are part of the MCS, and training
with the MCS over the same areas, locally enhancing storm total
rainfall.

In coordination with SHV/Shreveport, LA; FWD/Dallas-Fort Worth,
TX; HGX/Houston, TX; LCH/Lake Charles, LA; and JAN/Jackson, MS
forecast offices, a Moderate Risk upgrade was introduced with this
update. Given the above synoptic setup, it seems probable that a
High Risk consideration may need to be given in future forecasts,
especially if the "bulls-eye" of heaviest rain remains over this
hard hit areas of Texas and Louisiana. Moderate to Major flooding
is already ongoing across much of east Texas even before the start
of this event, and drainage has been slow...such that the much
needed 2 day break is unlikely to alleviate potential flash
flooding concerns much. Considerable/significant flash flooding is
likely in the Moderate Risk area. That said, this event still
being on forecast Day 3, there remains quite a bit of time for the
forecast to change, and almost any movement of the heaviest
rainfall would result in lessening flooding concerns. That said,
guidance has been in very good agreement on the placement of this
heaviest rain for several days already, which lends confidence to
this scenario unfolding for the Moderate Risk upgrade.

Despite very good agreement in most of the guidance for a Day 3
forecast, the American models (NAM/GFS) have been distinct
outliers in suggesting this event will not occur at all, with any
heavy convection staying well north and west into Oklahoma. These
models have been discarded as part of the forecast given the very
good agreement in all of the rest of the guidance. It should be
noted however that as persistent as all of the foreign guidance has
been in forecasting this event, the NAM and GFS have been equally
persistent that it will not happen.

Wegman

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Tue May 14 2024


Another southern tier convective event should be in progress at the
start of the period early Friday. The Day 4 Excessive Rainfall
Outlook that covers the Friday-Friday night time frame continues to
depict a Slight Risk area that extends from Louisiana into Alabama,
with some adjustment from continuity to reflect latest guidance.
This area will be sensitive to additional rainfall given wet soil
conditions. Frontal/wave focus with available moisture and
instability along with potential for repeat rains may enhance
totals. The surrounding Marginal Risk area accounts for some
potential of a convective axis hanging back even farther to the
west (still most pronounced in GFS runs). Farther north, latest
ECMWF/GFS/CMC/ICON runs are starting to cluster toward a potential
axis of less extreme but locally heavy rainfall over the Ohio
Valley in association with a front affecting the region. Short-term
rainfall should lead to somewhat wetter soil than initially
observed. The Day 4 ERO introduces a Marginal Risk area to account
for the developing guidance cluster. By Day 5 Saturday, the only
coherent area of potentially heavy rainfall is over portions of the
Southeast as the southern tier surface front/waviness continue
onward. A Marginal Risk provides a reasonable starting point given
the guidance scatter within this region. Confidence is much lower
for rainfall coverage and totals farther northward over the East.

Otherwise, some rainfall could linger along parts of the East Coast
into Sunday if the slower side of the envelope verifies for the
system affecting the region. Some areas of rainfall may develop
over portions of the northern-central Plains into the Midwest by
the weekend or early next week depending on shortwave
energy/surface details, with low confidence for specifics of
intensity at any particular location. Portions of the West may
begin to see scattered precipitation by the weekend and early next
week with the tendency toward more upper troughing and arrival of
the southern stream upper low.

The southern two-thirds of the West will likely see above normal
temperatures continue through the weekend with some areas seeing
highs 10-15F or so above normal. Northern areas will already be
moderating close to normal Friday-Sunday while additional lowering
of heights aloft over the West early next week should bring
temperatures toward normal over the remainder of the West. Some of
the western warmth will extend into portions of the Plains and
Mississippi Valley with plus 5-15F anomalies. The Florida
Peninsula and South Texas should see several days with highs up to
5-10F above normal, in a warm pattern to the south of a couple
fronts expected to focus upon reaching more northern parts of these
states. Some daily records for highs/warm lows will be possible.
These temperatures may bring heat indices as high as 105-110
degrees for south Florida and upwards to 115 degrees for south
Texas, highlighting a threat of hazardous heat.

Rausch

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Tue May 14 2024


Another southern tier convective event should be in progress at the
start of the period early Friday. The Day 4 Excessive Rainfall
Outlook that covers the Friday-Friday night time frame continues to
depict a Slight Risk area that extends from Louisiana into Alabama,
with some adjustment from continuity to reflect latest guidance.
This area will be sensitive to additional rainfall given wet soil
conditions. Frontal/wave focus with available moisture and
instability along with potential for repeat rains may enhance
totals. The surrounding Marginal Risk area accounts for some
potential of a convective axis hanging back even farther to the
west (still most pronounced in GFS runs). Farther north, latest
ECMWF/GFS/CMC/ICON runs are starting to cluster toward a potential
axis of less extreme but locally heavy rainfall over the Ohio
Valley in association with a front affecting the region. Short-term
rainfall should lead to somewhat wetter soil than initially
observed. The Day 4 ERO introduces a Marginal Risk area to account
for the developing guidance cluster. By Day 5 Saturday, the only
coherent area of potentially heavy rainfall is over portions of the
Southeast as the southern tier surface front/waviness continue
onward. A Marginal Risk provides a reasonable starting point given
the guidance scatter within this region. Confidence is much lower
for rainfall coverage and totals farther northward over the East.

Otherwise, some rainfall could linger along parts of the East Coast
into Sunday if the slower side of the envelope verifies for the
system affecting the region. Some areas of rainfall may develop
over portions of the northern-central Plains into the Midwest by
the weekend or early next week depending on shortwave
energy/surface details, with low confidence for specifics of
intensity at any particular location. Portions of the West may
begin to see scattered precipitation by the weekend and early next
week with the tendency toward more upper troughing and arrival of
the southern stream upper low.

The southern two-thirds of the West will likely see above normal
temperatures continue through the weekend with some areas seeing
highs 10-15F or so above normal. Northern areas will already be
moderating close to normal Friday-Sunday while additional lowering
of heights aloft over the West early next week should bring
temperatures toward normal over the remainder of the West. Some of
the western warmth will extend into portions of the Plains and
Mississippi Valley with plus 5-15F anomalies. The Florida
Peninsula and South Texas should see several days with highs up to
5-10F above normal, in a warm pattern to the south of a couple
fronts expected to focus upon reaching more northern parts of these
states. Some daily records for highs/warm lows will be possible.
These temperatures may bring heat indices as high as 105-110
degrees for south Florida and upwards to 115 degrees for south
Texas, highlighting a threat of hazardous heat.

Rausch

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
1031 AM EDT Mon May 13 2024

Valid 00Z Tue May 14 2024 - 00Z Fri May 17 2024

...WPC Winter Weather Desk no longer routinely staffed this
summer...

Pending any significant winter weather, the WPC Winter Weather Desk
will not be staffed this summer. The desk will resume continuous
staffing starting in late September 2024.

Most of the winter products from WPC will continue to be produced,
including the Winter Storm Severity Index (WSSI), Probabilistic
Winter Storm Severity Index (WSSI-P), and the Probabilistic Winter
Precipitation Forecasts (PWPF).

WPC

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Tue May 14 2024

Valid 12Z Fri May 17 2024 - 12Z Tue May 21 2024

...Another southern tier heavy rainfall threat during the latter
half of the week...


...Overview...

Downstream from a reorienting eastern Pacific upper ridge, guidance
is generally suggesting flat to broadly cyclonic mean flow late
this week into the weekend, followed by a tendency toward more
upper troughing over the West early next week. There are still a
lot of lower-predictability detail questions about flow rounding
the ridge and reaching the lower 48. There is also considerable
spread for the details/timing of an upper shortwave and associated
surface system crossing the eastern half of the country late this
week into the weekend. Continue to expect the heaviest convective
emphasis to be across the southern tier late this week into the
weekend, with some areas of less extreme but potentially meaningful
rainfall farther north.

...Guidance/Predictability Assessment...

Over recent days the dynamical models/ensembles and machine
learning (ML) models have been showing difficulty in resolving
important but medium to smaller scale details within progressive
Northeast Pacific into lower 48/southern Canada flow, and then how
potentially better defined western troughing may evolve
(influencing when an upper low drifting toward California
accelerates inland). There has been plenty of spread and run-to-run
variability over the East as well, with significant differences for
timing/details of shortwave energy and associated surface
reflection late this week onward plus what the larger scale flow
will look like by early next week.

Based on comparisons of 12Z/18Z guidance, a composite of
operational model solutions provided the most palatable starting
point. At the start of the period Friday the 12Z ML models offered
more support toward flatter flow over the Northwest than the
GFS/GEFS. By mid-late period, the ML models generally leaned toward
the cluster of 12Z ECMWF/CMC and their ensemble means which were
more progressive than the GFS/GEFS for the system crossing the East
and moving into the Atlantic, and showed less pronounced digging
south of an upper low that may reach southwestern Canada and the
northwestern U.S. (but with spread for incorporation of the
southern stream Pacific into California upper low) This majority
cluster also suggested that by next Tuesday there should be some
degree of upper troughing over eastern North America with ridging
over the Plains, in contrast to a flat ridge reaching the East with
southwesterly flow over the Plains. Thus the forecast leaned more
toward the 12Z ECMWF-ECens/CMC-CMCens scenario. The one
reservation was that incorporation of the Pacific/California low
could have been a little slow.

New 00Z guidance continues to highlight the volatility of the
pattern, with the UKMET/CMC trending toward the GFS over the
Northwest as of early Friday. Then the average of guidance
trends slower with more separation for the eastern U.S. upper
shortwave late week into the weekend, while the ECMWF has flipped
somewhat in the GFS direction for the pattern over the East by next
Tuesday.

...Weather/Hazards Highlights...

Another southern tier convective event should be in progress at the
start of the period early Friday. The Day 4 Excessive Rainfall
Outlook that covers the Friday-Friday night time frame continues to
depict a Slight Risk area that extends from Louisiana into Alabama,
with some adjustment from continuity to reflect latest guidance.
This area will be sensitive to additional rainfall given wet soil
conditions. Frontal/wave focus with available moisture and
instability along with potential for repeat rains may enhance
totals. The surrounding Marginal Risk area accounts for some
potential of a convective axis hanging back even farther to the
west (still most pronounced in GFS runs). Farther north, latest
ECMWF/GFS/CMC/ICON runs are starting to cluster toward a potential
axis of less extreme but locally heavy rainfall over the Ohio
Valley in association with a front affecting the region. Short-term
rainfall should lead to somewhat wetter soil than initially
observed. The Day 4 ERO introduces a Marginal Risk area to account
for the developing guidance cluster. By Day 5 Saturday, the only
coherent area of potentially heavy rainfall is over portions of the
Southeast as the southern tier surface front/waviness continue
onward. A Marginal Risk provides a reasonable starting point given
the guidance scatter within this region. Confidence is much lower
for rainfall coverage and totals farther northward over the East.

Otherwise, some rainfall could linger along parts of the East Coast
into Sunday if the slower side of the envelope verifies for the
system affecting the region. Some areas of rainfall may develop
over portions of the northern-central Plains into the Midwest by
the weekend or early next week depending on shortwave
energy/surface details, with low confidence for specifics of
intensity at any particular location. Portions of the West may
begin to see scattered precipitation by the weekend and early next
week with the tendency toward more upper troughing and arrival of
the southern stream upper low.

The southern two-thirds of the West will likely see above normal
temperatures continue through the weekend with some areas seeing
highs 10-15F or so above normal. Northern areas will already be
moderating close to normal Friday-Sunday while additional lowering
of heights aloft over the West early next week should bring
temperatures toward normal over the remainder of the West. Some of
the western warmth will extend into portions of the Plains and
Mississippi Valley with plus 5-15F anomalies. The Florida
Peninsula and South Texas should see several days with highs up to
5-10F above normal, in a warm pattern to the south of a couple
fronts expected to focus upon reaching more northern parts of these
states. Some daily records for highs/warm lows will be possible.
These temperatures may bring heat indices as high as 105-110
degrees for south Florida and upwards to 115 degrees for south
Texas, highlighting a threat of hazardous heat.

Rausch


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