Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Thu Mar 21 2019

Valid 12Z Thu Mar 21 2019 - 12Z Sat Mar 23 2019

...A developing coastal storm to bring rain, interior and higher elevation
snows, and gusty winds to parts of the Mid-Atlantic and Northeast...

...A couple rounds of rain and mountain snows likely across parts of the
West during the short range, with rain moving into the Southern and
Central Plains by Friday...

...Major to historic river flooding will continue across parts of the
Mississippi and Missouri River Basins with areal flooding a threat across
parts of the Northwest and Northern Plains as well...


The main sensible weather threat in the short range with be a developing
coastal low pressure system off the Carolina coasts this morning, tracking
up the east coast through Thursday and into Friday. Moderate to locally
heavy rainfall will accompany this system across the Mid-Atlantic and
Northeast with flash flooding a marginal and local threat particularly for
highly urbanized areas. A handful of flood watches are in effect across
parts of the Mid-Atlantic. Accumulating snowfall is also possible well
inland across portions of the interior Northeast. Gusty winds are also
likely across the Mid-Atlantic and Northeast into the weekend as the storm
deepens as it moves up the coast.

Meanwhile, a cold front moving into the Four Corners region this morning
will continue to produce rain and mountain snows across parts of the
Southwest and into the Southern Rockies on Thursday. As moisture get drawn
northward from the Pacific ahead of the cold front, precipitation is
expected to spread into parts of the southern and central Plains on Friday
as well. Another cold front entering the West Coast states by late Friday
will bring more rain and higher elevation snows to California, the Pacific
Northwest, and parts of the interior West this weekend.

Major to historic and catastrophic flooding across parts of the Missouri
and Mississippi River Basins will continue through the short range due to
ongoing effects following a period of rapid snow melt after heavy rainfall
last week. Flood warnings and advisories remain for parts of the region.
Areal flooding is also a concern across parts of the Northwest and into
the Northern Plains as warmer temperatures the end of this week lead to
accelerated snowmelt and the potential for ice jams.

Santorelli

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EDT Thu Mar 21 2019

Valid 12Z Sun Mar 24 2019 - 12Z Thu Mar 28 2019

...Overview and Guidance Assessment/Preferences...

Anomalous upper-level ridging extending from western Canada into
the north central U.S. this weekend is forecast by gradually break
down during the early to middle portion of next week. Some degree
of ridging appears likely to become reestablished from the Gulf
of Alaska north across Alaska - a position much more common in
recent months. Mean troughing should persist across the north
central Pacific, bringing a continuation of the recently seen
pattern with Pacific shortwave energy crossing the CONUS every
couple days. The reconfiguration of flow at the higher latitudes
will have some implications farther south, with less split flow
and greater likelihood of phased systems across the central U.S.
by mid to late next week. In the meantime, one significant
southern stream system remains during the medium range, affecting
the south central and southeastern U.S. Sun-Tue.

Models continue to struggle with the system crossing the southern
U.S. from Sun onward, and the potential evolution of a coastal low
along/off the Eastern Seaboard early-mid next week. In general,
guidance over the past day has trended toward a quicker eastward
progression, and somewhat less likelihood of a significant low
along the Southeast/Mid-Atlantic coast early next week. Ensemble
spread remains quite large, however. At 12Z on Tue, for example,
ensemble members range from a low still over the Mississippi
River, to others having energy already transferred to a new low
off the Southeast coast. While the consensus of solutions has
shifted faster, spread also seems to have increased over the past
few ensemble cycles. Thus, confidence is low, and would expect
continued run-to-run variability. Opted to tweak the forecast
toward a faster progression rather than making a dramatic change
given the low confidence. This resulted in a blend weighted a bit
more toward the slower solutions (12Z ECMWF/CMC) and not much
weight on the GFS which continues to lead the pack in terms of
eastward progression of the system and quickly moving any low out
into the open Atlantic early next week. Given significant
uncertainty as to how quickly any low along the Southeast coast by
the middle of next week moves out to sea or, perhaps up the coast
as the 12Z CMC and a handful of ensemble members show, maintained
at least low PoPs for portions of the Southeast/Mid-Atlantic into
midweek.

Elsewhere, models show better consensus that one weakening
shortwave should reach the West Coast late Mon, with a more
significant wave arriving late Tue, moving into the Great Basin or
Rockies Wed night-Thu. Models handle the first wave similarly, but
some larger differences emerge by the middle of next week with the
second system. Some timing differences emerge with the second
system Wed-Thu. Differences also exist as to structure of the
north Pacific ridge, with impacts on how/if the second shortwave
interacts with additional northern stream energy. In general, a
more ensemble-based approach was taken to the forecast by Wed-Thu,
with increased weight on the ECENS and NAEFS ensemble means.


...Weather Highlights and Hazards...

The southern stream system is expected to bring rain and
thunderstorms from portions of the Southern Plains and
lower/mid-Mississippi Valley to the Southeast. Locally heavy rain
is possible, but no strong signal for excessive rains is evident
in the guidance at this point given the trend toward increased
forward speed of the system. Cold temperatures on the northern end
of the precipitation shield may marginally support snow or a
rain/snow mix for some areas from the central Appalachians and
Mid-Atlantic, and perhaps into the Northeast. Meanwhile, the West
will see a couple rounds of precipitation from the incoming
Pacific systems, bringing rain and mountain snows to much of the
central/northern Rockies and from central California to the
Pacific Northwest. The arrival of the more significant system by
the middle of next week produces a stronger signal for potentially
heavy precip (rain and mountain snow) across portions of central
and northern California. Given the increasingly progressive flow
pattern across the CONUS, temperatures will be highly variable,
with much of the country seeing both above and below average
temperatures at different points during the extended period.


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
754 AM EDT Wed Mar 20 2019

Valid 00Z Thu Mar 21 2019 - 00Z Thu Mar 28 2019

Upper ridging and above average heights will slowly slide eastward
along 20N during the next week with troughing to its north and the
northeast. Heights may lower by the end of the period next Wed.
The tail end of a frontal system will stall and dissipate just
north of the area late Thu through the weekend as trades increase
for Friday until perhaps next Tue due to strong high pressure to
the north. Precipitable water values likely stay below average
with values near one inch with scattered windward/mauka showers
that may spill over to leeward areas. Models/ensembles were in
good synoptic agreement through the period.

Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1126 PM EDT Wed Mar 20 2019

Day 1
Valid 0325Z Thu Mar 21 2019 - 12Z Thu Mar 21 2019

...Coastal North Carolina...
Surface low pressure south of KILM is expected to track north
across eastern NC between 21/06z and 21/12z. As the low moves
inland (ahead of a negatively tilted trough over the TN Valley),
its associated warm front crosses eastern NC, possibly allowing
500-1000 J/KG of MUCAPE to comes ashore on a low level south
southeast flow during this time frame.

Developing low topped convection tracking across easternmost NC
and nearby far southeast VA could produce local 1.50/2.50 inch
rainfall amounts in the 21/06z to 21/12z time frame, as it taps
1.00/1.25 inch precipitable water air behind the warm front and
ahead of the cold front. Much of the high resolution guidance
keeps the greatest rainfall offshore or along the far eastern
portions of NC, closest to the best instability.

Because of this, there is uncertainty concerning the placement of
the highest rainfall amounts with the convection, should it make
it onshore over eastern NC. Three hour flash flood guidance values
are greater than 3.00 inches, as the region has been fairly dry in
the recent past. Finally, the entire synoptic system will begin to
accelerate late in the period (close to 21/12z), so the window of
opportunity could end up being yet another limiting factor for
flash flooding. Based on the above, no excessive area was assigned.

Hayes/Roth

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Thu Mar 21 2019

Day 1
Valid 12Z Thu Mar 21 2019 - 12Z Fri Mar 22 2019

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MID-ATLANTIC STATES...

...Mid-Atlantic States...
An area of overlapping low- to mid-level frontogenesis is expected
to slowly move through the Mid-Atlantic States during the day 1
period ahead of a negatively tilted trough within an area of
significant divergence aloft and in the proximity of a
strengthening low pressure area moving up the Fall Line. Areas of
MU CAPE available to the region are forecast to be no more than
100 J/kg with the more significant instability expected closer to
the Gulf Stream offshore the Eastern Seaboard which should mean
that the flash flood threat is low and that a longer duration
heavy rainfall event is more likely. From an hourly rainfall
perspective, rules of thumb suggest that with 0.75"+ precipitable
water values (PWs) available that hourly rain totals should max
out in the 0.3-0.4" range with six hour totals of 2" possible on a
local basis. The 00z HREF probabilities of 0.5" an hour are quite
low and appear to confirm this -- barely greater than 20% chance
between 2100 and 0100 UTC Thursday evening. Eastern PA appears to
be where the most precipitation is expected as the TROWAL of a
deepening low appears to pivot across the region.

Flash flood guidance values (FFGs) in this area are low (1-1.5" in
three hours) and the National Water Model shows or forecasts much
above average stream flow which bolsters confidence in the low
FFGs. Expanded the MRGL area from continuity to cover the heavy
rainfall footprint expected. Coordination with LWX/Sterling VA
and PHI/Mount Holly NJ helped confirm the broader area depicted.

Roth

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Wed Mar 20 2019

Day 2
Valid 12Z Thu Mar 21 2019 - 12Z Fri Mar 22 2019

...THERE IS A MARGINAL RISK FOR A PORTION OF THE MID-ATLANTIC...

Low pressure develops over eastern North Carolina tonight and
shifts north to the Chesapeake Bay Thursday. Abundant Gulf Stream
moisture ahead of this low on southwesterly 35kt flow at 850mb. PW
of 0.75 to 1 inch is one standard deviation above normal for the
central Mid-Atlantic. Orographic lift on the Catoctin/South
Mountain area of Maryland/Pennsylvania may enhance rainfall enough
to supersede the 2 inch per 6hr flash flood guidance for this area
despite a lack of instability Thursday. A marginal risk was
coordinated with WFOs LWX and CTP.

Jackson


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 PM EDT Wed Mar 20 2019

Day 3
Valid 12Z Fri Mar 22 2019 - 12Z Sat Mar 23 2019

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

Jackson



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
426 PM EDT Wed Mar 20 2019

Valid 00Z Thu Mar 21 2019 - 00Z Sun Mar 24 2019


...Ranges of CA/OR/Great Basin/Rockies...

On Day 1 tonight-Thu, Models continue to show a negatively-tilted
upper trough shifting east across California into Nevada. An
upper low is expected to close off within the base of the trough
and rotate east into the Great Basin on Thursday into early
Friday. Mountain snows are expected to taper in the Sierra and
move across the ranges of central NV to Utah, and then into
southwest CO in the San Juans. Some of the heaviest totals of the
Day 1 period are expected to fall across the southern Utah Wasatch
and Co San Juans, where Moderate Risk areas for accumulations of 8
inches are indicated.

On day 2 Thu night-Fri, additional mountain snows are forecast
across the central Nevada and Utah ranges, while extending farther
east into Colorado and northern New Mexico ranges. Upper jet
support along with orographic forcing will encourage heavy snows
along the windward slopes of the San Juan Mountains in southwest
Colorado, where WPC probabilities indicate a Moderate Risk for
accumulations of 8 inches and low risk of a foot or more on Day 2.
The snow winds down Fri night in CO with the departure of the 700
mb wave.

On day 3 Fri night-Sat, the next surge of 700 mb warm/moisture
advection and ascent leads to likely accumulating snows in the CA
Sierra Nevada, extending across the Siskyous and north into the
southern Cascades. As the wave moves further inland, a period of
snow is expected in the ranges of southern ID and then into
northwest WY, as well as the mountains of central NV.
Widespread heavy accumulations are not expected. WPC probabilities
indicate a chance of 8 inches or more in the CA Sierra Nevada and
as far north as southwest Montana and northwest WY on Day 3.

...Interior New York/New England...

Models continue to show a developing surface low, associated with
phasing shortwave troughs over the east, tracking north along the
Mid Atlantic to the New England coasts Thursday night through
Friday night. The low is expected to deepen quickly as it tracks
north along the New England coast on Friday.

Areas near the coast are expected to be rain with a transition to
snow in interior portions of New York and New England. Snow is
expected to change to rain along the low level cyclone track in
western Maine. The highest confidence for heavy snow is west of
the low track in the Adirondack Mountains of New York and points
north to the Canadian border. A foot of snow is possible in this
region.

Strong cold air advection on the backside of the system will
support a changeover to snow, spreading east into the northern
Vermont Green Mountains, where strong upslope flow could bolster
totals. For the two day period covering Fri and Sat, the latest
WPC probabilities indicate accumulations of 8 inches or more are
likely across the Adirondacks and northern Greens.

The probability of significant freezing rain is less than 10
percent.

Petersen

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EDT Thu Mar 21 2019

Valid 12Z Sun Mar 24 2019 - 12Z Thu Mar 28 2019

...Overview and Guidance Assessment/Preferences...

Anomalous upper-level ridging extending from western Canada into
the north central U.S. this weekend is forecast by gradually break
down during the early to middle portion of next week. Some degree
of ridging appears likely to become reestablished from the Gulf
of Alaska north across Alaska - a position much more common in
recent months. Mean troughing should persist across the north
central Pacific, bringing a continuation of the recently seen
pattern with Pacific shortwave energy crossing the CONUS every
couple days. The reconfiguration of flow at the higher latitudes
will have some implications farther south, with less split flow
and greater likelihood of phased systems across the central U.S.
by mid to late next week. In the meantime, one significant
southern stream system remains during the medium range, affecting
the south central and southeastern U.S. Sun-Tue.

Models continue to struggle with the system crossing the southern
U.S. from Sun onward, and the potential evolution of a coastal low
along/off the Eastern Seaboard early-mid next week. In general,
guidance over the past day has trended toward a quicker eastward
progression, and somewhat less likelihood of a significant low
along the Southeast/Mid-Atlantic coast early next week. Ensemble
spread remains quite large, however. At 12Z on Tue, for example,
ensemble members range from a low still over the Mississippi
River, to others having energy already transferred to a new low
off the Southeast coast. While the consensus of solutions has
shifted faster, spread also seems to have increased over the past
few ensemble cycles. Thus, confidence is low, and would expect
continued run-to-run variability. Opted to tweak the forecast
toward a faster progression rather than making a dramatic change
given the low confidence. This resulted in a blend weighted a bit
more toward the slower solutions (12Z ECMWF/CMC) and not much
weight on the GFS which continues to lead the pack in terms of
eastward progression of the system and quickly moving any low out
into the open Atlantic early next week. Given significant
uncertainty as to how quickly any low along the Southeast coast by
the middle of next week moves out to sea or, perhaps up the coast
as the 12Z CMC and a handful of ensemble members show, maintained
at least low PoPs for portions of the Southeast/Mid-Atlantic into
midweek.

Elsewhere, models show better consensus that one weakening
shortwave should reach the West Coast late Mon, with a more
significant wave arriving late Tue, moving into the Great Basin or
Rockies Wed night-Thu. Models handle the first wave similarly, but
some larger differences emerge by the middle of next week with the
second system. Some timing differences emerge with the second
system Wed-Thu. Differences also exist as to structure of the
north Pacific ridge, with impacts on how/if the second shortwave
interacts with additional northern stream energy. In general, a
more ensemble-based approach was taken to the forecast by Wed-Thu,
with increased weight on the ECENS and NAEFS ensemble means.


...Weather Highlights and Hazards...

The southern stream system is expected to bring rain and
thunderstorms from portions of the Southern Plains and
lower/mid-Mississippi Valley to the Southeast. Locally heavy rain
is possible, but no strong signal for excessive rains is evident
in the guidance at this point given the trend toward increased
forward speed of the system. Cold temperatures on the northern end
of the precipitation shield may marginally support snow or a
rain/snow mix for some areas from the central Appalachians and
Mid-Atlantic, and perhaps into the Northeast. Meanwhile, the West
will see a couple rounds of precipitation from the incoming
Pacific systems, bringing rain and mountain snows to much of the
central/northern Rockies and from central California to the
Pacific Northwest. The arrival of the more significant system by
the middle of next week produces a stronger signal for potentially
heavy precip (rain and mountain snow) across portions of central
and northern California. Given the increasingly progressive flow
pattern across the CONUS, temperatures will be highly variable,
with much of the country seeing both above and below average
temperatures at different points during the extended period.


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