Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
324 AM EDT Wed Apr 24 2019

Valid 12Z Wed Apr 24 2019 - 12Z Fri Apr 26 2019

...Severe Thunderstorms and heavy to excessive rainfall possible across
Texas on Wednesday, and into the Lower Mississippi Valley/Gulf Coast by
Thursday...

...Near record temperatures possible across parts of California and the
Desert Southwest...

Ongoing heavy rainfall and strong to severe thunderstorms across central
Texas this morning will continue drifting slowly south and eastward today
(Wednesday). The greatest chance for severe weather and heavy to excessive
rainfall will be across central and eastern Texas where both WPC and SPC
have highlighted this region within a slight risk for flash flooding and
severe thunderstorms, respectively. This system will shift into the Lower
Mississippi Valley by Thursday with again a threat for severe weather and
heavy rainfall, mainly concentrated along the central Gulf Coast. Showers
and thunderstorms, with locally heavy rainfall, will also extend northward
Wednesday and Thursday into parts of the Tennessee and Ohio Valleys along
a slow moving frontal boundary to the north. Temperatures across much of
Texas on Wednesday will be relatively chilly with afternoon highs 10 to 20
degrees below average (particularly across western and central Texas),
with warmer air arriving by Thursday.

Elsewhere, a low pressure system lifting through New England will bring
rain or a rain/snow mix to the region on Wednesday, while a cold front
dropping through the Northern Rockies will allow for showers and
thunderstorms to spread across the Northern/Central Rockies on Wednesday,
and into the Northern Plains and Upper Midwest by Thursday. Temperatures
across much of the West and into the Northern and Central Plains will be
very warm through the short range as an upper level ridge expands across
the region. Daytime and nighttime temperatures averaging 10 to 20+ degrees
above normal would approach record values (mainly across parts of
California and the Desert Southwest).

Santorelli


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EDT Wed Apr 24 2019

Valid 12Z Sat Apr 27 2019 - 12Z Wed May 01 2019

...Overview and Guidance Preferences...

Models suggest the potential for at least some degree of pattern
change across the CONUS during the medium range. Initially
separate streams with a progressive northern stream and less
progressive southern stream appear set to consolidate by the
middle of next week. This is expected to occur as the northern
stream undergoes amplification in response to a persistent
upstream Rex Block extending from Alaska to Hawaii. Amplification
of an upper trough across the western U.S. by the middle of next
week is expected along with an expansion of the subtropical ridge
northward into the Southeast and southern Mid-Atlantic.

A negatively tilted upper trough is expected to be in place on day
3 (Sat) across the Northeast, with a phased surface low pressure
system expected to be deepening as it pulls away from New England.
Farther west, the northern stream remains active with one compact
but energetic shortwave crossing the Midwest/Great Lakes Sat-Sun
and yet another diving into the Pacific Northwest/northern
Rockies. Meanwhile, a cutoff upper-level low should approach
California by Sun night/Mon before moving inland and interacting
in some fashion with the broader upper trough amplification across
the West by early-mid next week.

The forecast during days 3-5 (Sat-Mon) was based on a multi-model
deterministic blend including the 12Z ECMWF/UKMET and 18Z FV3-GFS.
Model solutions differed on the evolution of the negatively tilted
upper trough and deepening surface low as they pull away from the
Northeast Sat-Sun, but a blended approach appeared best at this
time. Preferred the GFS/FV3/UKMET idea of a more amplified
shortwave crossing the Great Lakes/Ohio Valley Sat-Sun over the
weaker/more suppressed ECMWF solution. Farther west, consensus has
improved with respect to amplified shortwave energy reaching the
Northwest/northern Rockies Sat/Sun, but some timing differences
remain, and models also differ as to whether the wave quickly
moves east into the Northern Plains by late Sun or shears out with
more energy remaining across the interior Northwest. The 12Z UKMET
suggested that a compromise between these ideas was feasible and
thus this was the preference. Ensembles do provide a fair degree
of support for the idea of keeping a significant portion of the
shortwave energy back across the interior West into early/mid next
week, with the arrival of additional energy helping to lower
heights and amplify a broader upper-level trough. The evolution of
the upper trough will also impact what happens farther south as
the initially cutoff Pacific energy moves inland across the Four
Corners region, with the eventual forward speed of this system as
it begins to become absorbed tied to the degree of northern stream
amplification. While models differ as to exactly how this occurs,
ensemble means agree in principle, this leaned heavily toward
ECENS and GEFS means by days 6-7 (Tue-Wed).


...Weather Highlights/Threats...

Widespread rain is expected across New England on Sat as the
deepening low pressure system moves away from the region. Low
pressure moving from the Central/Northern Plains to the
Midwest/Great Lakes/Northeast Sat/Sun should be accompanied by
showers and thunderstorms, with the potential for locally heavy
rainfall. A few areas on the northern fringe of this precipitation
area may see the potential for snow or a rain/snow mix,
particularly portions of northern Wisconsin and Michigan on Sat.
Through the reminder of the forecast period, into the middle of
next week, the focus for scattered to numerous showers and
thunderstorms will shift back to the central U.S., where a
stationary surface front along with arrival of additional
upper-level shortwaves will focus multiple rounds of showers and
thunderstorms. Models currently show the greatest potential for
heavy rainfall across areas from the Central Plains to the Upper
Midwest Mon-Tue. The central/northern Rockies will also see an
increase in precipitation (rain and mountain snow) by early to mid
next week as the upper trough amplified across the West and
additional southern stream Pacific energy arrives.

Below average temperatures are expected through much of the
forecast period from the northern Rockies to the Upper Midwest and
Great Lakes, where high temperatures of 5 to 15 deg F below
average may be common. Above average temperatures initially in
place from California to the southern Rockies and adjacent Plains
should shift east to the Southeast and southern Mid-Atlantic
regions by early to mid next week as the western trough amplifies
and ridging builds into the Southeast.


Ryan


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

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
809 AM EDT Wed Apr 24 2019

Valid 00Z Thu Apr 25 2019 - 00Z Thu May 02 2019

Upper ridging just to the west of the region will slide over
Hawai'i Thursday and east of the area Friday as it weakens. This
will favor generally light windward showers amid a typical trade
pattern. By the weekend, an upper low to the south of the
Aleutians is forecast to drop southeastward to near 40N/160W
around 12Z Sunday. The 00Z Canadian was displaced from the
consensus. For next Mon-Wed, the upper low is forecast to wobble
east-northeastward but trailing shortwave on its western side may
move southeastward just to the north of the region, ending up as
another closed low to the northeast of the state next Wednesday.
The 00Z GFS/ECMWF and 06Z GFS along with most ensemble members
showed this evolution, but with east-west spread. Preferred a
solution between the 00Z GFS and ECMWF for now given the
uncertainty in the upstream trough/ridge configuration. This was
also between their respective ensemble means. At the surface, main
upper low will carry a cold front to near Kauai on Sunday as winds
veer from easterly to southeasterly. Precipitable water values
will rise from below average to near and just above average next
week (to about 1.25 - 1.50 inches) but deep tropical moisture
should remain to the south of the area. Rainfall will increase in
coverage but may likely only be modest in amounts. Front may then
push across the island chain as the southern upper trough moves to
the southeast, which would bring in northerly to then
northeasterly flow in its wake and a drying trend from NW to SE.


Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Wed Apr 24 2019

Day 1
Valid 12Z Wed Apr 24 2019 - 12Z Thu Apr 25 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL TO NORTHEAST TEXAS INTO WESTERN LOUISIANA...

...Central to Northeast Texas into Southwest Louisiana...
Another day of active convection likely across central to
northeast TX...into the Lower Mississippi Valley region...along
and ahead of the cold front expected to push eastward across these
regions. There are no significant changes expected day 1 to the
recent overall favorable pattern for active convection in the
vicinity of this front. The upstream closed low moving across
northern Mexico into southwest Texas will maintain a very difluent
upper flow pattern ahead of it across much of Texas into the Lower
Mississippi Valley in an axis of pw values 1.5+ standard
deviations above the mean. There are some qpf axes differences
among the latest suite of guidance...with WPC favoring the more
southern max solution given the gradually lowering mid to upper
level heights which should support the next round of convection
taking a more southern track across Central to Northeast Texas.
While some of the shorter term qpf details are lower
confidence...the latest WPC qpf axis for day 1 is closer to the
solution offered by the HREF mean. While simulated radar
reflectivity from the hi res guidance are often very
poor...typically being underdone versus reality...the expected
movement is often better. The latest simulated radars from the hi
res guidance suggests the slowest moving convection possible in
the 1800 to 0000 utc time period over central Texas...with
increasing progression to the east after this during the overnight
hours across northeast Texas into Louisiana. Much of the latest
qpf guidance reflects this with generally lesser amounts across
northeast TX into southwest LA...compared to central TX. At the
moment...one broad slight risk was denoted given uncertainties
with convective details.

...Lower Arkansas into the Lower Ohio Valleys...
While we favored an overall farther south max in qpf for the day 1
period...moderate to locally heavy rains still likely to extend
northeastward into the Lower Arkansas and Lower Ohio Valley region
as above average pw values stream northeastward this period across
these areas. With stream flows generally above normal across
these regions from heavy precip over the past few weeks...isolated
runoff issues possible...with a marginal risk area maintained.

Oravec

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Apr 24 2019

Day 2
Valid 12Z Thu Apr 25 2019 - 12Z Fri Apr 26 2019

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHWESTERN
LOUISIANA INTO SOUTHERN ALABAMA AND THE FLORIDA PANHANDLE...

The mid-level low will transition from closed to an open wave
through the day on Friday. The training heavy rain that will be
observed over Texas and Louisiana on Day 1 will carry over into
Day 2 for the Gulf Coast states. High resolution guidances, such
as the ARW and NMMB, has rainfall rates of 1-2+ inches/hr in an
organized line that tracks from central Louisiana to Alabama and
western Florida while a secondary line, albeit weaker, initiates
over west-central Louisana shortly after 14/15Z and tracks
northeast into Mississippi. Strong surface convergence, modest
lift and instability in the low/middle atmosphere and continuous
supply of deep Gulf moisture will increased precipitation
efficency. Excessive rainfall will likely be problematic and
result in

Areal averages of 1.5 to 3 inches with isolated higher amounts is
certainly probable given that the precipitable water values
feeding northward into the system are in excess of 1.75 inches are
forecast along the Gulf Coast with values near 1.5 inches farther
north toward Tennessee. The region has been abnormally wet over
the past 2 weeks, with pockets of 200+ percent of average rainfall
extending from the Lower Mississippi Valley to the Alabama/Georgia
line.

Campbell

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Wed Apr 24 2019

Day 3
Valid 12Z Fri Apr 26 2019 - 12Z Sat Apr 27 2019

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM SOUTHWESTERN
LOUISIANA INTO FAR SOUTHWESTERN MISSISSIPPI...

The mid-level low near northeastern Texas at the start of the
period will weaken/open up and move eastward into the southeastern
U.S. by 12Z/26. A carry-over of training heavy rain supports a
Slight Risk over southern/southwestern Louisiana just beyond
12Z/25, but it appears this region should be in the clear by 18Z,
even in the slowest solution depicted by the 12Z NAM. Precipitable
water values in excess of 1.75 inches are forecast along the Gulf
Coast with values near 1.5 inches farther north toward Tennessee
as seen in the 12Z GFS/ECMWF. While heavy rain appears likely
across the Gulf Coast states into the Tennessee Valley, with
widespread rainfall of 1-2 inches, locally 3-4 inches through
12Z/26, the overall nature of the system appears progressive
enough to preclude a Slight Risk at this time outside of
Louisiana. The region has been abnormally wet over the past 2
weeks though, with pockets of 200+ percent of average rainfall
extending from the Lower Mississippi Valley to the Alabama/Georgia
line. As the event draws nearer and access to hi-res guidance
becomes available, more precision to the Day 3 outlook will likely
be needed.

Otto

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 AM EDT Wed Apr 24 2019

Valid 12Z Wed Apr 24 2019 - 12Z Sat Apr 27 2019

...Day 1...Northern Maine...

Low pressure over southeastern Ontario will cross southern Quebec
across northern New England Wednesday morning, and then across the
coastal waters of Maine.
Initial temperatures aloft are too warm for snow. A brief change
over is possible as the 850 mb low passes during the day
Wednesday, with cold advection commencing. Day 1 probabilities
for two inches are low over far northern Maine.

The probability of 4 or more inches of snow is less than 10
percent day 2.


...Day 3...Northern Rockies...

A northern stream shortwave trough crosses the Pacific Northwest
and northern Rockies Friday and Friday night. Gulf moisture up the
Great Plains and Pacific moisture from the Great Basin converge to
allow precip to break out as the frontal circulation provides
ascent. Low to moderate probabilities for four inches are shown in
MT Rocky Mountain Front and Absarokas.

As the system move east across the northern Plains, a low risk is
indicated near the North-South Dakota border as cooling once the
low level circulation passes eventually allows rain to have a
chance of changing over to snow. The 00z NAM was most aggressive
but the GFS/ECMWF and most SREF/GEFS members have less snow
potential than the NAM, so probabilities are low given initial
conditions too warm for snow, leaving a limited window for snow to
occur.

The probability of significant freezing rain is less than 10
percent days 1-3.

Petersen

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EDT Wed Apr 24 2019

Valid 12Z Sat Apr 27 2019 - 12Z Wed May 01 2019

...Overview and Guidance Preferences...

Models suggest the potential for at least some degree of pattern
change across the CONUS during the medium range. Initially
separate streams with a progressive northern stream and less
progressive southern stream appear set to consolidate by the
middle of next week. This is expected to occur as the northern
stream undergoes amplification in response to a persistent
upstream Rex Block extending from Alaska to Hawaii. Amplification
of an upper trough across the western U.S. by the middle of next
week is expected along with an expansion of the subtropical ridge
northward into the Southeast and southern Mid-Atlantic.

A negatively tilted upper trough is expected to be in place on day
3 (Sat) across the Northeast, with a phased surface low pressure
system expected to be deepening as it pulls away from New England.
Farther west, the northern stream remains active with one compact
but energetic shortwave crossing the Midwest/Great Lakes Sat-Sun
and yet another diving into the Pacific Northwest/northern
Rockies. Meanwhile, a cutoff upper-level low should approach
California by Sun night/Mon before moving inland and interacting
in some fashion with the broader upper trough amplification across
the West by early-mid next week.

The forecast during days 3-5 (Sat-Mon) was based on a multi-model
deterministic blend including the 12Z ECMWF/UKMET and 18Z FV3-GFS.
Model solutions differed on the evolution of the negatively tilted
upper trough and deepening surface low as they pull away from the
Northeast Sat-Sun, but a blended approach appeared best at this
time. Preferred the GFS/FV3/UKMET idea of a more amplified
shortwave crossing the Great Lakes/Ohio Valley Sat-Sun over the
weaker/more suppressed ECMWF solution. Farther west, consensus has
improved with respect to amplified shortwave energy reaching the
Northwest/northern Rockies Sat/Sun, but some timing differences
remain, and models also differ as to whether the wave quickly
moves east into the Northern Plains by late Sun or shears out with
more energy remaining across the interior Northwest. The 12Z UKMET
suggested that a compromise between these ideas was feasible and
thus this was the preference. Ensembles do provide a fair degree
of support for the idea of keeping a significant portion of the
shortwave energy back across the interior West into early/mid next
week, with the arrival of additional energy helping to lower
heights and amplify a broader upper-level trough. The evolution of
the upper trough will also impact what happens farther south as
the initially cutoff Pacific energy moves inland across the Four
Corners region, with the eventual forward speed of this system as
it begins to become absorbed tied to the degree of northern stream
amplification. While models differ as to exactly how this occurs,
ensemble means agree in principle, this leaned heavily toward
ECENS and GEFS means by days 6-7 (Tue-Wed).


...Weather Highlights/Threats...

Widespread rain is expected across New England on Sat as the
deepening low pressure system moves away from the region. Low
pressure moving from the Central/Northern Plains to the
Midwest/Great Lakes/Northeast Sat/Sun should be accompanied by
showers and thunderstorms, with the potential for locally heavy
rainfall. A few areas on the northern fringe of this precipitation
area may see the potential for snow or a rain/snow mix,
particularly portions of northern Wisconsin and Michigan on Sat.
Through the reminder of the forecast period, into the middle of
next week, the focus for scattered to numerous showers and
thunderstorms will shift back to the central U.S., where a
stationary surface front along with arrival of additional
upper-level shortwaves will focus multiple rounds of showers and
thunderstorms. Models currently show the greatest potential for
heavy rainfall across areas from the Central Plains to the Upper
Midwest Mon-Tue. The central/northern Rockies will also see an
increase in precipitation (rain and mountain snow) by early to mid
next week as the upper trough amplified across the West and
additional southern stream Pacific energy arrives.

Below average temperatures are expected through much of the
forecast period from the northern Rockies to the Upper Midwest and
Great Lakes, where high temperatures of 5 to 15 deg F below
average may be common. Above average temperatures initially in
place from California to the southern Rockies and adjacent Plains
should shift east to the Southeast and southern Mid-Atlantic
regions by early to mid next week as the western trough amplifies
and ridging builds into the Southeast.


Ryan


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

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml