Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Thu Apr 25 2019

Valid 12Z Thu Apr 25 2019 - 12Z Sat Apr 27 2019

...Severe weather and flash flooding possible across the Central Gulf
Coast on Friday, with heavy to excessive rain possible for the Ohio Valley
as well...

...Near record temperatures for parts of California and the Desert
Southwest Thursday and Friday...

...Accumulating snow possible for parts of the Northern Plains Friday into
Saturday...

Ongoing heavy rainfall and strong to severe thunderstorms across the Lower
Mississippi Valley this morning will continue drifting slowly eastward
today (Thursday). The greatest chance for severe weather and flash
flooding will be along the Central Gulf Coast where both SPC and WPC have
slight risks. Showers and thunderstorms will expand into the Tennessee and
Ohio Valleys as well along a cold front to the north. Heavy to excessive
rainfall and flash flooding will be possible with this activity too,
especially for parts of the Ohio Valley where WPC has a slight risk on
Thursday. As this system pushes east on Friday, showers and thunderstorms
will shift into the Eastern U.S. with locally heavy rainfall possible,
especially across parts of the Mid-Atlantic into the Northeast. The cold
front should clear the coast late Friday, with drier and cooler weather
expected to begin the weekend.

Temperatures across the West during the short range will be very warm
through the short range period as an upper level ridge expands across the
region. Daytime and nighttime temperatures averaging 10 to 20 degrees
above normal, especially across California and the Desert Southwest, could
approach record values for this time of the year. Meanwhile, showers and
thunderstorms this morning in the Central Plains will shift eastward along
a weak cold front into the Upper Midwest and Great Lakes on Thursday. A
surface low pressure system organizing across the Intermountain West on
Friday will allow for some rain and mountain snows in the Northern
Rockies, with showers and storms expanding into the Plains by the weekend.
Temperatures to the north of the surface low across the Northern Plains
may be cold enough to support a couple of inches of snow Friday into
Saturday for parts of the Dakotas.

Santorelli


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EDT Thu Apr 25 2019

Valid 12Z Sun Apr 28 2019 - 12Z Thu May 02 2019

...Overview and Guidance Preferences...

An expansive Rex Block across the north/central Pacific, extending
from Hawaii to Alaska, will support a downstream mean upper trough
centered across the northern Rockies through the extended forecast
period. A persistent upper low centered west of Hudson Bay, and
slowly retrograding through the period, will support southward
transport of chilly polar air into the north central U.S. Farther
east, the subtropical ridge is expected to gradually exert its
influence northward into the Southeast and southern Mid-Atlantic
early next week.

Preferred a multi-model blend approach initially during days 3-5
(Sun-Tue), incorporating the 12Z ECMWF/CMC and 18Z GFS (with
heaviest weight place toward the ECMWF). This blend leaned toward
the slower solutions (which were well supported by ensemble member
clustering) with the shortwave/surface low crossing the Ohio
Valley/Northeast on Sun. Additional northern stream shortwaves,
one crossing the north central U.S., and another diving into the
Pacific Northwest Sun-Mon were handled similarly by these
preferred solutions. Farther south, a cutoff southern stream upper
low approaching southern California late Sun-Mon continued to show
some timing differences, and the slower ECMWF was preferred over
the somewhat faster GFS timing (causing the GFS solution be
further deemphasized by Mon).

During days 6-7 (Wed), timing differences with the southern stream
feature continue to grow as the feature moves into the central
U.S. by Wed, with the GFS at least 18 hours faster than the
ECMWF/CMC. Farther north, model consensus has improved a bit that
the arrival of additional shortwave energy into the Pacific
Northwest early-mid next week should serve to further amplify a
broad upper trough axis centered over the northern Great
Basin/northern Rockies, with a consolidated area of shortwave
energy perhaps lifting out into the Northern Plains by Thu.
Ensemble means showed general agreement on this idea despite
increasing spread. Thus, the forecast by Wed-Thu placed increasing
weight on ECENS/GEFS ensemble means, with some continued use of
the deterministic ECMWF through the end of the period.


...Weather Highlights/Threats...

Showers and perhaps a few thunderstorms are expected to accompany
the low pressure system moving from the Ohio Valley/lower Great
Lakes to the Northeast on Sun. After that, the focus shifts to the
central U.S., where a persistent stalled frontal boundary along
with a few passing upper shortwaves will focus multiple rounds of
showers and thunderstorms, with the potential for areas of heavy
rainfall. The strongest signal in the guidance for heavy rain was
seen from portions of the Southern/Central Plains to the
Mid-Mississippi Valley from Mon through the middle of next week,
with QPF guidance suggesting the potential for multi-inch rainfall
totals over the course of a few days. On the northern fringe of
this area of precipitation in the central U.S. sufficient cold air
may be in place to support some snow on Sun across portions of the
Northern Plains. A second round of potential snow or rain/snow mix
will be possible across portions of the Northern High plains
Tue-Wed. Farther west, lowering heights and the passage of
multiple shortwaves will generate rain and mountain snow for
portions of the Great Basin and central/northern Rockies through
the period.

Below average temperatures are expected from Sun into next week,
from the northern Rockies to the Great Lakes. The greatest
anomalies are expected Sun-Wed from the northern Rockies to the
Northern High Plains, where high temperatures may be 10-20 deg F
below average. Warming temperatures are expected across the
Southeast and southern Mid-Atlantic by early next week as the
subtropical ridge builds. Highs across these areas may reach 10 to
15 deg above average Tue-Thu.

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
815 AM EDT Thu Apr 25 2019

Valid 00Z Fri Apr 26 2019 - 00Z Fri May 03 2019

Trades will trend weaker over the coming days as ridging at the
surface and aloft drifts eastward. Thus expect an initial
windward focus for showers to trend toward at least some sea/land
breeze focus. At the same time moisture will gradually increase,
with precipitable water values reaching above climatological
levels by Sunday-Monday. This will occur as shortwave energy on
the southwestern periphery of a mid-latitude system brings a front
toward/into the area. Guidance is not particularly enthusiastic
for heavy rainfall potential with this front but there may be at
least some enhancement of rainfall during the Sunday-Monday time
frame. Consensus shows a drying trend with northeasterly trades
becoming established Tuesday-Thursday as the upper shortwave
continues eastward away from the state and surface high pressure
tracks well north of the islands. Overall an average of latest
GFS/ECMWF runs and their ensemble means would provide a reasonable
starting point for the forecast, with most differences small
enough in scale to have fairly low predictability at extended time
frames. One note is that toward Wednesday-Thursday this solution
would lean away from the 00Z GEFS mean which becomes slowest with
the departing shortwave aloft.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Thu Apr 25 2019

Day 1
Valid 12Z Thu Apr 25 2019 - 12Z Fri Apr 26 2019

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
OHIO VALLEY AND CENTRAL GULF COAST...

Central Gulf coast/Deep South...
There was agreement that Gulf moisture coming northward to the
southeast of an MCV moving across central AR at 08z, leading to
heavy rains with the greatest amounts across portions of the
central Gulf coast/Deep South. The 00z GFS shows 700 hPa
temperatures (6C) in decline from here on out, with the effective
capping inversion sinking south to at least I-10 with time.
There's indication that the cap edge might be close to the Gulf
coast itself in recent radar imagery from southeast TX. The
guidance shows a significant amount of spread, which is not
particularly uncommon with precipitation within the warm sector
during the warm season. Precipitable water values (PWs) feeding
northward into the system are in excess of 1.75 inches along the
Gulf Coast. Portions of the South have been abnormally wet over
the past 2 weeks, with pockets of 200+ percent of average noted.
Due to the uncertainty, lowered amounts in the QPF somewhat and
attempted to keep some degree of continuity with the risk areas
inherited, other than to shift them somewhat north and east to
account for radar/model trends. Local amounts of 3-5" are
possible here, with as much as 2" in an hour anticipated given the
moisture and instability available. The heaviest rain could occur
due to cell training, cell mergers, or mesocyclone formation.
Leaned modestly towards the 00z HREF mean and 00z/06z high
resolution NAM in this area, as the mesoscale guidance has the
best chance of verifying in this sort of regime.

Ohio Valley...
There was a pronounced northward shift in the QPF, apparently due
to an MCV moving through central AR at 08z Thursday. Both the 00z
GFS and 00z ECMWF made the largest shifts to the north. This is
occurring despite mid-level height falling region wide. The
guidance does suggest that a northern stream shortwave is expected
to merge/phase with the MCV over time which maintains some
amplification to the mid-level flow pattern. Instability is
expected to ramp up to 1000-2000 J/kg, which when combined with
PWs rising just above 1.5" could lead to hourly rain totals to
1.75" where either cell training or mesocyclones develop. Recent
RAP runs and the 00z GFS show the best moisture to be present near
or just north of the Ohio river. At least two rounds of
precipitation are expected -- one with the outflow boundary near
the Ohio river and another with a front moving in from the
northwest. Local amounts of 2-3" are possible here. The fairly
broad range in the guidance has not been helpful -- thought the
00z Canadian Regional, 00/06z high resolution NAM, and 12z ECMWF
QPF looked the most reasonable when considering 00z GFS boundary
layer moisture convergence field which also allowed some degree of
continuity to be maintained. However, did broaden the marginal
risk area northward to account for the 00z GFS/00z ECMWF QPF
solutions as a precaution. A new slight risk area was added near
and north of the Ohio River where two week precipitation anomalies
are 200%+ of average and a recent batch of rain fell Wednesday and
Wednesday night. This was coordinated with the Paducah KY,
Indianapolis IN, Louisville KY, and Wilmington OH Weather Forecast
Offices.

Roth

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Thu Apr 25 2019

Day 2
Valid 12Z Fri Apr 26 2019 - 12Z Sat Apr 27 2019

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM
APPALACHIAN REGION TO THE NORTHEAST...

Showers and thunderstorms will be moving through the
Ohio/Tennessee Valleys and into the Appalachian region, northern
mid-Atlantic and the Northeast as a surface cyclone lifts into the
Great Lakes/Northeast. The deep moisture associated with the
recent heavy rain over the Gulf Coast and Lower Mississippi Valley
will continue to stream northward feeding into the leading edge of
the system, spreading rain from the southern Appalachian chain
northward into northern New England. The convection will mainly be
in ahead of, and within the proximity to the trailing cold front
and the lifting warm front; tracking over regions were 1-hr/3-hr
FFG is 0.50 to 1.00 inch in the mountains and 1.5 to 3 elsewhere.
Guidance continues to show a signal that this sector could receive
1 or more inches of rain, which would meet or exceed local FFG. A
Marginal Risk for excessive rainfall with an increased threat for
flash flooding was maintained but trimmed the southern boundary
back to southern West Virginia.

Campbell

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Thu Apr 25 2019

Day 3
Valid 12Z Sat Apr 27 2019 - 12Z Sun Apr 28 2019

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

Campbell

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
352 AM EDT Thu Apr 25 2019

Valid 12Z Thu Apr 25 2019 - 12Z Sun Apr 28 2019


...Northern Plains/Upper Midwest...
Days 2-3...

A shortwave digging southeast from the Pac NW will amplify into a
fast but potent closed mid-level circulation on the nose of a
90-100 kt upper level jet. As this feature moves rapidly across
the Northern Plains Saturday, it will catch up to a more slowly
departing but strong jet max across S Central Canada, and the
coupled jet structure will lead to surface cyclogenesis diving
from South Dakota into Ohio by the end of the period. Guidance has
come into better agreement with a more southerly track of this
surface low, and although the NAM is a strong/cold outlier, an
ECMWF/GFS/CMC compromise is favored and spreads a swath of heavy
snow from the western Dakotas into Michigan.

Some uncertainty still remains in both the placement of the
heaviest snow and the accumulation. Temperature profiles will be
marginal, but intense synoptic ascent through the aforementioned
coupled jet structure, as well as increasing mesoscale lift due to
strongly sloped frontogenesis embedded beneath a maturing TROWAL
suggests intense snowfall rates are likely which should overcome
the marginal profiles. Working against heavy snow accumulation is
that it is almost May, and afternoon snowfall with a high sun
angle will be tough to accumulate. There is a large spread in the
ensemble plumes for snowfall, but many ensemble members suggest a
swath of more than 4 inches of snow, with the highest likelihood
occurring from SE MN through southern WI where the best
deformation axis pivots, and WPC probabilities show a moderate
risk for 4 inches. Significantly higher amounts are possible in
isolated locations, and small probabilities do exist for more than
8 inches in this same area. However, confidence is lower than
average at this time and accumulations will be highly dependent on
snowfall rates, time of day, and if the best overlap of forcing
can occur concurrently for a prolonged period of time. Elsewhere,
from the ND/SD/MT border eastward into lower Michigan there is a
moderate chance for 2 inches of snowfall.


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

A shortwave and robust 120kt Pacific Jet will rotate onshore
Washington State late Saturday driving a surface low and
associated cold front into the northwest. Robust lift within the
LFQ of this potent jet max will produce precipitation across this
area, but initially the column will be too warm for snow outside
of the highest terrain of the Washington Cascades and Northern
Rockies. As the cold front shifts inland, snow levels will drop to
3000-4000 ft in the ranges of ID and MT, while mesoscale ascent
increases as a deformation band sets up NW of the developing
surface low. This combined with modest mid-level fgen in the
vicinity of the cold front will produce heavy snow before the low
exits to the east late on day 3. WPC probabilities are moderate
for 8 inches across the Northern Rockies through Sunday morning.

Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EDT Thu Apr 25 2019

Valid 12Z Sun Apr 28 2019 - 12Z Thu May 02 2019

...Overview and Guidance Preferences...

An expansive Rex Block across the north/central Pacific, extending
from Hawaii to Alaska, will support a downstream mean upper trough
centered across the northern Rockies through the extended forecast
period. A persistent upper low centered west of Hudson Bay, and
slowly retrograding through the period, will support southward
transport of chilly polar air into the north central U.S. Farther
east, the subtropical ridge is expected to gradually exert its
influence northward into the Southeast and southern Mid-Atlantic
early next week.

Preferred a multi-model blend approach initially during days 3-5
(Sun-Tue), incorporating the 12Z ECMWF/CMC and 18Z GFS (with
heaviest weight place toward the ECMWF). This blend leaned toward
the slower solutions (which were well supported by ensemble member
clustering) with the shortwave/surface low crossing the Ohio
Valley/Northeast on Sun. Additional northern stream shortwaves,
one crossing the north central U.S., and another diving into the
Pacific Northwest Sun-Mon were handled similarly by these
preferred solutions. Farther south, a cutoff southern stream upper
low approaching southern California late Sun-Mon continued to show
some timing differences, and the slower ECMWF was preferred over
the somewhat faster GFS timing (causing the GFS solution be
further deemphasized by Mon).

During days 6-7 (Wed), timing differences with the southern stream
feature continue to grow as the feature moves into the central
U.S. by Wed, with the GFS at least 18 hours faster than the
ECMWF/CMC. Farther north, model consensus has improved a bit that
the arrival of additional shortwave energy into the Pacific
Northwest early-mid next week should serve to further amplify a
broad upper trough axis centered over the northern Great
Basin/northern Rockies, with a consolidated area of shortwave
energy perhaps lifting out into the Northern Plains by Thu.
Ensemble means showed general agreement on this idea despite
increasing spread. Thus, the forecast by Wed-Thu placed increasing
weight on ECENS/GEFS ensemble means, with some continued use of
the deterministic ECMWF through the end of the period.


...Weather Highlights/Threats...

Showers and perhaps a few thunderstorms are expected to accompany
the low pressure system moving from the Ohio Valley/lower Great
Lakes to the Northeast on Sun. After that, the focus shifts to the
central U.S., where a persistent stalled frontal boundary along
with a few passing upper shortwaves will focus multiple rounds of
showers and thunderstorms, with the potential for areas of heavy
rainfall. The strongest signal in the guidance for heavy rain was
seen from portions of the Southern/Central Plains to the
Mid-Mississippi Valley from Mon through the middle of next week,
with QPF guidance suggesting the potential for multi-inch rainfall
totals over the course of a few days. On the northern fringe of
this area of precipitation in the central U.S. sufficient cold air
may be in place to support some snow on Sun across portions of the
Northern Plains. A second round of potential snow or rain/snow mix
will be possible across portions of the Northern High plains
Tue-Wed. Farther west, lowering heights and the passage of
multiple shortwaves will generate rain and mountain snow for
portions of the Great Basin and central/northern Rockies through
the period.

Below average temperatures are expected from Sun into next week,
from the northern Rockies to the Great Lakes. The greatest
anomalies are expected Sun-Wed from the northern Rockies to the
Northern High Plains, where high temperatures may be 10-20 deg F
below average. Warming temperatures are expected across the
Southeast and southern Mid-Atlantic by early next week as the
subtropical ridge builds. Highs across these areas may reach 10 to
15 deg above average Tue-Thu.

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