Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
1201 PM EST Sun Dec 15 2019

Valid 00Z Mon Dec 16 2019 - 00Z Wed Dec 18 2019

...Widespread wintry weather expected from the central Plains to the
Northeast today through Tuesday...

...Strong to severe thunderstorms likely across the Deep South on Monday...

...Santa Ana winds impact southern California Monday into Tuesday...

Heavy snow is developing and expanding across the central Plains this
Sunday northeast of a surface low moving across the southern Plains, where
more than a foot of snow is expected for today over the higher elevations
of southern Colorado. As the associated upper-level disturbance quickly
moves across the southern Rockies, snow, freezing rain and mixed
precipitation will spread eastward into the Midwest later today and into
the Ohio Valley tonight. By Monday morning, a wide swath of snow is
expected to impact areas from the central Plains eastward through the
Midwest, the lower Great Lakes, central Appalachians, and into the
northern Mid-Atlantic states. By Monday night into Tuesday morning, much
of New England will see snow spreading in while mild air aloft will likely
transition the snow to freezing rain across southern New England. Cold
air moving in behind the system combined with a weaker system moving in
from the west should lead to lake effect snows downwind of all the Great
Lakes on Tuesday. The coastal sections of the Northeast should see the
snow changing over to plain rain by Tuesday morning. Meanwhile,
thunderstorms will become likely across the Deep South as the low pressure
system intensifies along a sharp front. Fueled by increasing moisture,
warmth and instability from the Gulf of Mexico, the thunderstorms will
likely become strong to severe as they sweep across the central and
eastern Gulf states Monday and Monday night.

In the wake of the front, a strong high pressure system builds into the
Great Basin will set up a dry, gusty northeast flow across California on
Monday and Tuesday, with the promise of cool, Santa Ana winds which appear
to be moderate in strength. Due to recent rains wetting area vegetation,
fire weather danger looks low per guidance from the Storm Prediction
Center.

Kong/Roth


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
131 PM EST Sun Dec 15 2019

Valid 12Z Wed Dec 18 2019 - 12Z Sun Dec 22 2019

...Overview and Guidance/Predictability Assessment...

Stormy period for the Pacific Northwest into Northern California
with successive systems moving inland and a possible atmospheric
river event type set-up. The East will be quieter for the end of
the week but a weak system in the Central Plains may become more
developed off the Southeast coast next weekend.

Models and ensembles are starting to show better agreement with
the incoming system into the West on Wednesday and how it travels
across the CONUS through at least day 5. After that, big questions
remain on interactions between northern/southern stream energies.
Recent runs, like the ECMWF, tend to suggest limited seperation of
the energy, while some like the GFS keep the southern stream
seperate with rather significant low development off the Southeast
coast next weekend. Run to run continuity in most of the
deterministic runs is quite poor by day 6-7, though ensemble
members do suggest at least the potential for low development
somewhere off the East Coast. 00z low track plots show East Coast
lows anywhere from Florida to New England at 12z next Sunday.
Given the spread, WPC preferred a mostly ensemble mean blend, with
some contribution from the ECMWF and GFS for a bit more
definition. This resulted in a rather large shift northward with
the surface low to off the Mid-Atlantic coast (which is roughly in
the middle of the spread) on this cycle of the WPC progs, though
it's important to note that the day 7 forecast is of very low
confidence at this point.


...Weather Highlights/Threats...

Precipitation will be centered on the West, particularly coastal
OR and northern California. Upper trough in the eastern Pacific
will allow a fetch of moisture from near Hawai'i to reach the
OR/CA coast with a north/south oscillation between fronts as the
trough changes shape upstream. Several inches of precipitation are
expected but over a 2-3 day period with a likely break in between.
Daytime temperatures will likely be below average with the
rainfall but otherwise near average elsewhere and near to above
average for overnight lows. East of the Rockies, temperatures may
be more variable with a quick cold shot skirting through the
Northeast on Thursday (10s/20s for highs and -0s/0s/10s for lows).
East Coast low development may bring a period of snowfall to
someone from the Mid-Atlantic to the Northeast which of course is
highly variable on actual low track/existence. At this point, WPC
winter weather probabilities show nothing more than a 10% chance
of accumulations along the I-95 corridor.

Santorelli/Fracasso


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
226 AM EST Sun Dec 15 2019

Valid 00Z Mon Dec 16 2019 - 00Z Mon Dec 23 2019

Upper ridging will build into the area Monday as a cold front dips
southward toward the region around Tuesday. Trades will weaken a
bit on Sunday only to increase in advance of the front Monday.
Generally light windward/mauka showers are likely. Trades will
slowly then decrease through Thursday. By next Fri/Sat, another
front may approach the region and perhaps sink through the
islands, enhancing some windward showers. Surface high pressure
behind it may be quite strong per some models and even the ECMWF
ensemble mean showed PMSL above 1032mb which is above the 99th
percentile for the third week of December. Trades may become quite
breezy with the strong pressure gradient, with additional showers
both windward and leeward on strong easterly to ENE flow. By late
Sat into Sun, GFS remains bullish on developing and tracking low
pressure off the ITCZ near 180 farther east than most of the other
models and ensembles, but still keeps the plume of heavier rain
west of Kauai through 00Z/23.

Fracasso

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
710 PM EST Sun Dec 15 2019

Day 1
Valid 01Z Mon Dec 16 2019 - 12Z Mon Dec 16 2019

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

Bann



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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Sun Dec 15 2019

Day 2
Valid 12Z Mon Dec 16 2019 - 12Z Tue Dec 17 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL EXTENDING ACROSS
THE TENNESSEE VALLEY INTO THE OHIO VALLEY..


2100 UTC update

The 1200 utc model cycle is showing fairly good model to model
agreement on the qpf axis for this period. Given this and good
model continuity...there were only a few minor changes made to the
marginal and slight risk areas from the previous issuance. The
western portion of the slight risk area over central Tennessee was
extended westward by approximately 50 to 75 nm. This was to pick
up the NAM conest heavy precip across this region. The conest was
showing a several hour period between 1900 utc Mon and 0000 utc
Tue when .50"+ hourly rates are possible over western TN. A
slight south push to the eastern edge of the marginal risk also
done across the Mid-Atlantic. This was to pick up several of the
models that shifted southward with their qpf axes across this
area.

Oravec

0900 UTC discussion


...Central Gulf Coast across the Tennessee and Ohio Valleys into
the northern Mid Atlantic...
Deep moisture on the nose of a strong low level jet ahead of a
cold front crossing the region during Day 2 is expected to result
in an axis of heavy rain extending from the central Gulf Coast
into the eastern OH Valley. There has been a unanimous shift to
the south with the axis of heaviest rainfall in the 00z guidance,
so the axis was dropped south when compared to the previous
forecast. Due to the change, the WPC QPF and Excessive Rainfall
outlook were based on a blend of the 00z
ECWMF/GFS/UKMET.

Deep moisture (with precipitable water values near 1.25 inches,
which is between two and three standard deviations above the mean)
is transported northeast on the nose of a 45/55 knot low level jet
ahead of the cold front crossing the region at the beginning of
Day 2. In advance of a developing long wave trough over the
central US, bands of heavy rainfall are expected to develop from
AL across central and eastern TN/KY into WV prior to 17/00z. While
the instability profile across the TN Valley into the OH Valley
showed almost no surface based instability, model soundings showed
the potential for 100/250 J/KG of MUCAPE reaching from central TN
into far eastern KY (with the instability rooted above 900 mb).
This could allow convective elements to develop in the mainly
frontogenetically forced rain bands (supported by synoptic ascent
in the right entrance region of a 130 jet streak extending from IL
into northern New England).

Even with some instability elevated in the column, hourly rainfall
rates this far north probably do not exceed an inch. The exception
could be if training of the rain bands occurs, especially from
central TN into northern and central WV. In any event, the
combination of deep moisture and synoptic scale ascent could
produce an axis of 2.00/3.00 inches of rainfall extending from
north central TN across eastern KY into central WV. These rainfall
amounts could cause problems across KY into WV, where three hour
flash flood guidance values are as low as an inch.

Based on this, a Slight Risk was stretched from central TN across
central and eastern KY into WV (and this was collaborated with
WFOs LMK/JKL/RLX/OHX). Some consideration was given to a small
Moderate Risk across portions of central WV. where flash flood
guidance are below an inch. Given the general lack of instability
this far north, it is not clear if hourly rainfall rates exceed an
inch. After collaborating with WFO RLX, it was decided to leave
the Slight Risk in place for now. While there is better agreement
in the 00z model suite, there are still some latitudinal
differences with respect to the placement of axis of highest
rainfall. Because of the remaining differences, there could be
some changes to the shape and latitudinal extent of the Slight
Risk in later forecasts.

Further south across the central Gulf coast into the lower TN
Valley, the influx of instability from the Gulf of Mexico is more
substantial, with 500/1000 J/KG of MUCAPE extending into southern
MS/AL between 18/18z and 17/03z. The low level southwest flow
transports 1.50/1.75 inch precipitable water air ahead of the
frontal boundary, and the combination of moisture and instability
should be sufficient for a more expansive convective footprint
that further north. Unlike the TN and OH Valleys, the convection
appears as though it will largely be progressive (with short term
training possible near stronger cells). The 14 day rainfall
amounts have been close to normal, so a Marginal Risk was extended
across this region to account for the convection.

Hayes


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
226 PM EST Sun Dec 15 2019

Day 3
Valid 12Z Tue Dec 17 2019 - 12Z Wed Dec 18 2019

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

Oravec

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
329 PM EST Sun Dec 15 2019

Valid 00Z Mon Dec 16 2019 - 00Z Thu Dec 19 2019


Day 1...

...Colorado/New Mexico...
A well-defined mid-level shortwave will continue to support snows
as it moves across the south-central Colorado and far northern New
Mexico Rockies Sunday evening. Areas impacted by the heaviest
amounts will likely include the Sangre de Cristo Mountains, where
upslope flow and low level convergence is expected to carry the
threat for heavy snows through the evening hours on Sunday.

...Central Plains to the Northern Mid Atlantic...
Initial wave impacting the region will move east from the
Mississippi valley/Ozark region into the the Ohio valley Sunday
evening. Moist southerly flow into a low-to-mid level baroclinic
zone will support a wintry mix advancing east ahead of the wave.
While widespread heavy snowfall accumulations are not expected,
there is the potential for locally heavier amounts. WPC guidance
shows a narrow axis of higher probabilities for amounts of
4-inches or more centered across central Indiana. Low-to-mid
level frontogenesis may support banded precipitation, generating
higher totals across this region Sunday evening. Latest HREF
guidance shows several members producing 0.05-0.10 in/hr liquid
precipitation rates across this region during the 0-6 UTC Monday
time period. A wintry mix of snow, sleet and ice will then
spread east into the central Appalachians overnight and then east
of the mountains into the northern Mid Atlantic by morning.

Meanwhile, the second wave will have moved east of the Rockies by
Monday morning, producing another round of winter precipitation
across portions of the Plains and mid Mississippi valley. The
general trend in the models over the past day has been drier, with
lighter snow and ice accumulations expected from the Oklahoma and
Texas Panhandle regions northeastward across the mid Mississippi
valley on Monday.

Day 2...

...Mid Mississippi Valley to the Northeast...
Models continue to show the second shortwave amplifying over the
central Plains and mid Mississippi valley late Monday before
lifting through the Ohio valley ahead of an upper trough digging
into the Upper Midwest overnight. Similar to the preceding wave,
widespread heavy snow accumulations are not expected. However,
this system is expected to produce another widespread swath of
snow, sleet and freezing rain as it moves through the Ohio valley
Monday night and into the northern Mid Atlantic and Northeast on
Tuesday. Significant icing may be more of a concern with this
second wave, with several guidance members indicating ice
accumulations of 0.10 inch or more from Potomac Highlands in West
Virginia northward into the Allegheny Mountains in western
Pennsylvania and then eastward as far east as northern New Jersey
and the Catskills. While areas farther to the north may remain
mostly snow, models show the system remaining fairly suppressed
and sliding quickly off of the southern New England coast Tuesday
evening. This will help to limit snowfall accumulations across
the Adirondacks and northern New England.

Day 3...

...Great Lakes...
The previously noted shortwave digging into the Upper Midwest will
continue to amplify, with an upper low closing off and moving east
from the Great Lakes into the Northeast on Wednesday. This will
drive a strong cold front southeast through the Upper Great Lakes
late Tuesday and the Lower Great Lakes by early Wednesday. Behind
the front, strong northwest winds across the Upper Lakes and
west-northwest winds across the Lower Lakes will support lake
effect snow showers, with significant accumulations possible over
the eastern U.P. and northern Lower Michigan, as well as western
New York and the Tug Hill region before showers diminish late
Wednesday.

Pereira

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
131 PM EST Sun Dec 15 2019

Valid 12Z Wed Dec 18 2019 - 12Z Sun Dec 22 2019

...Overview and Guidance/Predictability Assessment...

Stormy period for the Pacific Northwest into Northern California
with successive systems moving inland and a possible atmospheric
river event type set-up. The East will be quieter for the end of
the week but a weak system in the Central Plains may become more
developed off the Southeast coast next weekend.

Models and ensembles are starting to show better agreement with
the incoming system into the West on Wednesday and how it travels
across the CONUS through at least day 5. After that, big questions
remain on interactions between northern/southern stream energies.
Recent runs, like the ECMWF, tend to suggest limited seperation of
the energy, while some like the GFS keep the southern stream
seperate with rather significant low development off the Southeast
coast next weekend. Run to run continuity in most of the
deterministic runs is quite poor by day 6-7, though ensemble
members do suggest at least the potential for low development
somewhere off the East Coast. 00z low track plots show East Coast
lows anywhere from Florida to New England at 12z next Sunday.
Given the spread, WPC preferred a mostly ensemble mean blend, with
some contribution from the ECMWF and GFS for a bit more
definition. This resulted in a rather large shift northward with
the surface low to off the Mid-Atlantic coast (which is roughly in
the middle of the spread) on this cycle of the WPC progs, though
it's important to note that the day 7 forecast is of very low
confidence at this point.


...Weather Highlights/Threats...

Precipitation will be centered on the West, particularly coastal
OR and northern California. Upper trough in the eastern Pacific
will allow a fetch of moisture from near Hawai'i to reach the
OR/CA coast with a north/south oscillation between fronts as the
trough changes shape upstream. Several inches of precipitation are
expected but over a 2-3 day period with a likely break in between.
Daytime temperatures will likely be below average with the
rainfall but otherwise near average elsewhere and near to above
average for overnight lows. East of the Rockies, temperatures may
be more variable with a quick cold shot skirting through the
Northeast on Thursday (10s/20s for highs and -0s/0s/10s for lows).
East Coast low development may bring a period of snowfall to
someone from the Mid-Atlantic to the Northeast which of course is
highly variable on actual low track/existence. At this point, WPC
winter weather probabilities show nothing more than a 10% chance
of accumulations along the I-95 corridor.

Santorelli/Fracasso


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