Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
329 AM EDT Sat Apr 20 2024

Valid 12Z Sat Apr 20 2024 - 12Z Mon Apr 22 2024

...Heavy rain and scattered instances of flash flooding possible across
the southern Plains and lower Mississippi Valley today...

...Light snow continues across parts of the central Rockies and central
High Plains...

...Below average temperatures for much of the Nation east of the Rockies
this weekend before spring warmth returns to the Plains on Monday...

A lingering quasi-stationary front draped across the Southeast, Gulf Coast
States, and southern Plains will be the focus for numerous showers and
thunderstorms this weekend. The greatest impacts associated with this
system are expected to occur today as heavy rain could lead to scattered
flash flooding issues from Texas to Mississippi. Multiple rounds of
potentially intense rainfall developing over saturated terrain has
prompted a Slight Risk (level 2/4) of Excessive Rainfall through tonight.
This area includes much of central and eastern Texas, northern Louisiana,
southern Arkansas, and central Mississippi. A few isolated storms could
also contain damaging wind gusts and hail throughout central/southern
Texas and the Southeast. The greatest coverage of showers and
thunderstorms are expected to progress eastward tonight and eventually
confine to the central Gulf Coast and Southeast on Sunday, with lessening
chances for flash flooding and severe thunderstorms.

Light precipitation chances are also forecast to continue across other
parts of the Nation through Monday. Light snow will linger throughout
parts of Colorado today, where a few additional inches of accumulating
snowfall are expected along the Front Range. Winter Weather Advisories
remain in effect. Meanwhile, an exiting cold front today followed by a
stronger cold front on Sunday night will lead to rain and snow showers
traversing New England. Lastly, the next frontal system to impact the
Pacific Northwest will rapidly spread showers throughout the region
tonight before swiftly pushing rain chances into the northern Plains and
Upper Midwest on Monday.

Temperatures are anticipated to remain below average for much of the
country behind the cold front extending from the Southeast to the southern
Plains. This leaves the West, Southwest, and Southeast as the warm spots
this weekend with highs into the 70s and 80s. As temperatures moderate on
Monday and return flow enters the central U.S., below normal temperatures
are expected to erode as highs into the 70s surge into the central Plains,
with cooler weather remaining across the Northeast. Additionally, lows
into the 30s over the next few mornings could lead to frost/freeze
concerns for locations already in the growing season from the Midwest to
the upper Ohio Valley.

Snell


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sat Apr 20 2024

Valid 12Z Tue Apr 23 2024 - 12Z Sat Apr 27 2024


...General Overview...

The forecast period begins on Tuesday with an amplifying upper
trough over the Great Lakes and then over the Northeast U.S., and
this will propel a cold front across much of the Eastern U.S. for
the middle of the week. Surface high pressure then builds in across
the Midwest and then across the East to close out the week along
with cooler temperatures. Meanwhile, a synoptic scale trough
develops across the Western U.S. by Friday and into Saturday with a
couple of shortwaves ejecting out across the Plains, and
supporting surface cyclogenesis across the western High Plains.
This will likely result in developing showers and storms for the
Central U.S., with some heavy rain and strong storms possible by
the end of the week.

...Guidance/Predictability Assessment...

One of the main forecasting challenges early on will be across the
Northeast U.S., and this regards the degree of phasing between an
incoming shortwave from the Midwest and an arctic trough dropping
from the Hudson Bay region. The GFS and to a slightly lesser extent
the GEFS mean support a stronger system with a closed low
developing across portions of New England, whereas the non-NCEP
guidance and the machine learning guidance from the ECMWF supports
the idea of a broader trough. Otherwise the models are in pretty
good agreement for the Tuesday-Wednesday time period, and a multi-
deterministic model blend with slightly more weighting towards the
ECMWF works well as a starting point in the forecast process. There
is growing consensus for a trough developing across the West to
close out the week, although timing differences remain for the
shortwave crossing the Southwest and then the Southern Plains (GFS
is most progressive), and also for the shortwave/closed low
originating from the Gulf of Alaska. The use of the ensemble means
accounted for about 40-50% of the forecast blend by Friday-
Saturday.

...Weather/Hazards Highlights...

Upper trough and frontal passages through the Midwest/Great Lakes
and then the Northeast will be accompanied by some widespread light
to moderate rains early-mid next week. Given the overall
progressive nature of this storm system, and the lack of
appreciable instability and QPF most places in this period, no
flash flood threat areas are currently warranted for the WPC
Excessive Rainfall Outlook for Tuesday. On Wednesday, warm air
advection across Texas and Oklahoma will bring a return to
scattered showers and storms across this region, with some of the
model guidance suggesting the potential for some local one inch
rainfall totals. However, the guidance is not locked in on the
location of these scattered QPF maxima, and therefore it seems
prudent to hold off on any risk areas for the Day 5 outlook on
Wednesday until a better model signal develops for placement. More
widespread showers and storms are then expected to develop across
the Plains and Midwest states into later next week as moisture and
instability increase with surface low pressure system development,
and the potential exists for some strong storms ahead of the
dryline in the Southern Plains, so this will continue to be
monitored.

Initial upper level ridging out West should support above normal
temperatures through about Wednesday, and then a return to near or
slightly below normal levels for much of the West Coast and
Intermountain West starting Thursday. Warmer temperatures and
increasing humidity are forecast for the south-central U.S. by
next Friday and beyond as the upper ridge axis builds back, and
southerly flow from the western Gulf advects moisture northward
ahead of a developing surface low. Highs could get over 100 degrees
for portions of southern Texas by Saturday.

Hamrick


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation 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
317 AM EDT Sat Apr 20 2024

Valid 00Z Sun Apr 21 2024 - 00Z Sun Apr 28 2024

The Hawaii domain is expected to have a relatively uneventful
weather pattern through the upcoming week. This will generally
equate to east-northeasterly trades on most days. Some slight
veering to the east is likely by Wednesday-Friday as the main
surface high becomes centered northeast of the state.

There will likely be a trough that approaches the northwestern
Islands by the end of the week, based on the latest ensemble
guidance. The accompanying surface front will likely remain
northwest of the state through the end of the week, and the
weather impacts to the state will likely be minimal. Light to
moderate trade wind showers are likely each day for the favored
terrain, and mainly dry for southwestern portions of the Islands.
Some locally heavier showers are possible through this weekend
with a weak upper trough passing by the region.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Apr 20 2024
Day 1
Valid 12Z Sat Apr 20 2024 - 12Z Sun Apr 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND
EAST TEXAS THROUGH PORTIONS OF THE ARKLAMISS...

The primary change to the ERO from the previous forecast was to
expand the Slight a bit to cover all of central Mississippi all the
way to the Alabama border. The latest guidance and CAMs suggest
that convection that forms at the nose of a robust LLJ will track
along to the east a bit more quickly than previous forecasts.
Training convection will be likely in this scenario as the storms
track along a nearly stationary boundary. Unfortunately in the
Arklamiss, the convection will track near or over an area that was
quite hard hit a few days ago, covering the AR/LA/MS tripoint along
the Mississippi River and extending across central Mississippi.
Thus, despite the fact the forecast remains wetter across portions
of east Texas, the antecedent conditions and saturated soils in the
Arklamiss should make up for the lesser amounts of rainfall
expected to still result in widely scattered instances of flash
flooding. Depending on how well the convection holds together in
this area, localized considerable flash flooding is possible,
especially if the stronger storms track in the same areas hard hit
a few days ago.

Across Texas, the Slight risk area is largely unchanged. As a
strong shortwave ejects out into Texas today, increasing Gulf
moisture moving north along a strong LLJ will raise PWATs above 1.5
inches in many areas, which is about 2-2.5 sigma above normal. This
will support a large area of rain over much of north Texas, which
will quickly expand east with time. The shortwave and resultant
cyclogenesis will shift the LLJ east with time. Meanwhile, strong
Canadian high pressure will dive south down the Plains, increasing
the thermal gradient...along which the convection will form and
move east. The shortwave and strongest moisture flux will support
the strongest convection across east Texas this afternoon at peak
heating, where a higher-end Slight is considered in effect. As the
rainfall pushes east into the Arklamiss this evening, the
thunderstorms should gradually weaken, resulting in slightly less
rainfall, but as mentioned above the antecedent conditions are more
favorable for flash flooding development.

While changes to the Slight and surrounding Marginal Risk areas
were minimal, they were nudged southward with this update in
keeping with both guidance trends and typical climatology of these
kinds of events, where stronger storms and heavier rains tend to
form a bit south of guidance where both instability and moisture
availability are both greater.

Wegman

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Apr 20 2024
Day 2
Valid 12Z Sun Apr 21 2024 - 12Z Mon Apr 22 2024

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

The inherited Marginal Risk area for the central Gulf Coast was
removed with this update, in coordination with LIX/Slidell, LA and
MOB/Mobile, AL forecast offices. A bit faster evolution of the
rainfall and dry air encroachment from the central Plains will
allow remaining rainfall and potentially some embedded
thunderstorms to move east quickly by Sunday morning. As a result,
forecast rainfall in this area continues to rapidly decrease, as
has been the trend in the guidance for the past few days.

Wegman

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Apr 20 2024
Day 3
Valid 12Z Mon Apr 22 2024 - 12Z Tue Apr 23 2024

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

Wegman

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sat Apr 20 2024


Upper trough and frontal passages through the Midwest/Great Lakes
and then the Northeast will be accompanied by some widespread light
to moderate rains early-mid next week. Given the overall
progressive nature of this storm system, and the lack of
appreciable instability and QPF most places in this period, no
flash flood threat areas are currently warranted for the WPC
Excessive Rainfall Outlook for Tuesday. On Wednesday, warm air
advection across Texas and Oklahoma will bring a return to
scattered showers and storms across this region, with some of the
model guidance suggesting the potential for some local one inch
rainfall totals. However, the guidance is not locked in on the
location of these scattered QPF maxima, and therefore it seems
prudent to hold off on any risk areas for the Day 5 outlook on
Wednesday until a better model signal develops for placement. More
widespread showers and storms are then expected to develop across
the Plains and Midwest states into later next week as moisture and
instability increase with surface low pressure system development,
and the potential exists for some strong storms ahead of the
dryline in the Southern Plains, so this will continue to be
monitored.

Initial upper level ridging out West should support above normal
temperatures through about Wednesday, and then a return to near or
slightly below normal levels for much of the West Coast and
Intermountain West starting Thursday. Warmer temperatures and
increasing humidity are forecast for the south-central U.S. by
next Friday and beyond as the upper ridge axis builds back, and
southerly flow from the western Gulf advects moisture northward
ahead of a developing surface low. Highs could get over 100 degrees
for portions of southern Texas by Saturday.

Hamrick

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sat Apr 20 2024


Upper trough and frontal passages through the Midwest/Great Lakes
and then the Northeast will be accompanied by some widespread light
to moderate rains early-mid next week. Given the overall
progressive nature of this storm system, and the lack of
appreciable instability and QPF most places in this period, no
flash flood threat areas are currently warranted for the WPC
Excessive Rainfall Outlook for Tuesday. On Wednesday, warm air
advection across Texas and Oklahoma will bring a return to
scattered showers and storms across this region, with some of the
model guidance suggesting the potential for some local one inch
rainfall totals. However, the guidance is not locked in on the
location of these scattered QPF maxima, and therefore it seems
prudent to hold off on any risk areas for the Day 5 outlook on
Wednesday until a better model signal develops for placement. More
widespread showers and storms are then expected to develop across
the Plains and Midwest states into later next week as moisture and
instability increase with surface low pressure system development,
and the potential exists for some strong storms ahead of the
dryline in the Southern Plains, so this will continue to be
monitored.

Initial upper level ridging out West should support above normal
temperatures through about Wednesday, and then a return to near or
slightly below normal levels for much of the West Coast and
Intermountain West starting Thursday. Warmer temperatures and
increasing humidity are forecast for the south-central U.S. by
next Friday and beyond as the upper ridge axis builds back, and
southerly flow from the western Gulf advects moisture northward
ahead of a developing surface low. Highs could get over 100 degrees
for portions of southern Texas by Saturday.

Hamrick

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 AM EDT Sat Apr 20 2024

Valid 12Z Sat Apr 20 2024 - 12Z Tue Apr 23 2024


The probability of significant snow and icing is less than 10
percent.


Mullinax

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sat Apr 20 2024

Valid 12Z Tue Apr 23 2024 - 12Z Sat Apr 27 2024


...General Overview...

The forecast period begins on Tuesday with an amplifying upper
trough over the Great Lakes and then over the Northeast U.S., and
this will propel a cold front across much of the Eastern U.S. for
the middle of the week. Surface high pressure then builds in across
the Midwest and then across the East to close out the week along
with cooler temperatures. Meanwhile, a synoptic scale trough
develops across the Western U.S. by Friday and into Saturday with a
couple of shortwaves ejecting out across the Plains, and
supporting surface cyclogenesis across the western High Plains.
This will likely result in developing showers and storms for the
Central U.S., with some heavy rain and strong storms possible by
the end of the week.

...Guidance/Predictability Assessment...

One of the main forecasting challenges early on will be across the
Northeast U.S., and this regards the degree of phasing between an
incoming shortwave from the Midwest and an arctic trough dropping
from the Hudson Bay region. The GFS and to a slightly lesser extent
the GEFS mean support a stronger system with a closed low
developing across portions of New England, whereas the non-NCEP
guidance and the machine learning guidance from the ECMWF supports
the idea of a broader trough. Otherwise the models are in pretty
good agreement for the Tuesday-Wednesday time period, and a multi-
deterministic model blend with slightly more weighting towards the
ECMWF works well as a starting point in the forecast process. There
is growing consensus for a trough developing across the West to
close out the week, although timing differences remain for the
shortwave crossing the Southwest and then the Southern Plains (GFS
is most progressive), and also for the shortwave/closed low
originating from the Gulf of Alaska. The use of the ensemble means
accounted for about 40-50% of the forecast blend by Friday-
Saturday.

...Weather/Hazards Highlights...

Upper trough and frontal passages through the Midwest/Great Lakes
and then the Northeast will be accompanied by some widespread light
to moderate rains early-mid next week. Given the overall
progressive nature of this storm system, and the lack of
appreciable instability and QPF most places in this period, no
flash flood threat areas are currently warranted for the WPC
Excessive Rainfall Outlook for Tuesday. On Wednesday, warm air
advection across Texas and Oklahoma will bring a return to
scattered showers and storms across this region, with some of the
model guidance suggesting the potential for some local one inch
rainfall totals. However, the guidance is not locked in on the
location of these scattered QPF maxima, and therefore it seems
prudent to hold off on any risk areas for the Day 5 outlook on
Wednesday until a better model signal develops for placement. More
widespread showers and storms are then expected to develop across
the Plains and Midwest states into later next week as moisture and
instability increase with surface low pressure system development,
and the potential exists for some strong storms ahead of the
dryline in the Southern Plains, so this will continue to be
monitored.

Initial upper level ridging out West should support above normal
temperatures through about Wednesday, and then a return to near or
slightly below normal levels for much of the West Coast and
Intermountain West starting Thursday. Warmer temperatures and
increasing humidity are forecast for the south-central U.S. by
next Friday and beyond as the upper ridge axis builds back, and
southerly flow from the western Gulf advects moisture northward
ahead of a developing surface low. Highs could get over 100 degrees
for portions of southern Texas by Saturday.

Hamrick


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation 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