Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EST Tue Dec 10 2019

Valid 12Z Tue Dec 10 2019 - 12Z Thu Dec 12 2019

...Rain changing over to snow during the early morning on Wednesday a long
the I-95 corridor...

...Heavy lake effect snow along parts of Lake Superior Coastline...

...Wind chill advisory in effect over parts of the Upper Midwest on
Tuesday morning...

Temperatures will be 10 to 20 degrees above average over parts of the East
Coast in contrast, temperatures will be 10 to 25 degrees below average
over parts of the Upper Midwest...

A front extending from the Lower Great Lakes southwestward to the Central
Gulf Coast will move eastward off the East Coast and Gulf Coast by
Wednesday. The boundary will produce rain from parts of the Northeast
across the Central Appalachians into parts of the Central/Western Gulf
Coast moving off the Northeast Coast by Wednesday evening and the Gulf
Coast by Wednesday afternoon. The rain will change over to snow over
parts of the Southern Plains and the Lower Mississippi/Southwestern
Ohio/Western Tennessee Valleys by Tuesday morning. Likewise on the back
edge of the precipitation shield, rain will change over to snow over parts
of the Northern/Central Appalachians into parts of the Lower Mississippi
Valley by Tuesday afternoon. As the front continues to move eastward,
rain changing to snow will move into the I-95 corridor during the early
morning to late morning on Wednesday producing hazardous traveling
conditions for the morning rush hour. The rain and snow slowly end over
the Lower Mississippi Valley and Ohio/Tennessee Valleys overnight Tuesday.
The snow over the Northeast to the Mid-Atlantic will end by Wednesday
evening. Furthermore, lake effect and lake enhanced snow will develop
over the Upper Great Lakes through Wednesday evening.

Another front extending from the Middle Mississippi Valley roughly
northwestward to the Northern Rockies will remain quasi-stationary through
Thursday morning. The boundary will aid in producing scattered snow over
parts of the Northern Rockies/Northern High Plains through Wednesday
morning.

Meanwhile, upper-level energy will move onshore over the West Coast on
Tuesday evening and moves inland to the Plains by Thursday morning. The
energy will aid in producing rain over parts of the Pacific
Northwest/Northern California Coast on Tuesday. As the energy and
associated moisture move inland, higher elevation snow develops over parts
of the Northwest and Northern California overnight Tuesday. On
Wednesday, snow develops over parts of the Northern Intermountain Region
into the Great Basin. The snow will become more scattered on Wednesday
night over the Northern Rockies as snow develops over parts of the
Northern Plains overnight Wednesday. Additional energy and moisture move
onshore and move into the Pacific Northwest and Northern California
overnight Wednesday into Thursday morning producing more rain over the
area.


Ziegenfelder


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1234 AM EST Tue Dec 10 2019

Valid 12Z Fri Dec 13 2019 - 12Z Tue Dec 17 2019

...Eastern Seaboard heavy rain threat Friday/Saturday and lead
wintry precipitation risk for the interior
Mid-Atlantic/Appalachians & Northeast...

...Wet Pacific Northwest to snowy N-Central Great Basin/Rockies
flow into the weekend leads to Deep South low/rainfall development
early next week...


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

A cold upper vortex will reform and persist over Hudson Bay this
period. Over the lower 48, an active and complex pattern remains
as several streams laden with a series of impulses interact. Model
and ensemble solutions have become better clustered, but forecast
confidence remains somewhat tempered by recent run to run
continuity issues and flow complexity. Opted for a composite
GFS/ECMWF and GEFS/ECMWF ensemble mean solution blended with the
National Blend of Models as a basis for the WPC medium range
product suite.


...Weather Highlights/Threats...

Southern stream shortwave progression and subsequent northern
stream flow interaction should support low development and track
from the Gulf of Mexico/Gulf Coast to the Southeast then up the
East Coast Friday/Saturday. This will bring heavy rain up the East
Coast mainly from the Southeast to the Appalachians, Mid-Atlantic
then Northeast. System interaction with lower atmospheric cold air
damming offers an interior snow/ice threat to monitor. The degree
of phasing between two initially separate streams and baroclinic
zone/coastal details will be determining factors of eventual
evolution and potential coastal system threat.

The pattern will also become wet upstream over the Pacific
Northwest into late week as Pacific shortwave energies punch
onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored north-central Great
Basin/Rockies terrain through the weekend. By next Monday into
Tuesday, the exiting wave will eject and then interact with Gulf
moisture to spawn a rainfall focusing low pressure/frontal system
forecast to move over the Deep South, then potentially again up
the East Coast.

Schichtel


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
232 AM EST Tue Dec 10 2019

Valid 00Z Wed Dec 11 2019 - 00Z Wed Dec 18 2019

...Wet Flow Returns...

A favored composite of latest model and ensemble guidance supports
a generally dry stretch of weather through midweek in a pattern
with land/sea breezes under stabilizing ridging aloft. A stalling
front just northwest of Kauai should focus most moisture/rainfall
offshore.

Another front may reach the state from the northwest by Friday
with supporting upper trough amplification. This frontal approach
offers a risk of increased lead moisture pooling to fuel an
enhanced rainfall risk, with activity lingering over the weekend
into early next week as weakened upper trough/low energy settles
just west of the islands and as a lower atmospheric inverted
trough sets over the state. High pressure bridging to the north
then northeast of the islands will meanwhile usher in moderate to
breezy trades that would transition rainfall focus into favored
terrain.

Schichtel

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
240 AM EST Tue Dec 10 2019

Day 1
Valid 01Z Tue Dec 10 2019 - 12Z Tue Dec 10 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE DEEP SOUTH...

...Louisiana / Mississippi...
WPC has introduced a small Marginal Risk area running roughly from
Lafayette, LA, to Meridian, MS, and not quite including the city
of New Orleans. Although the recent weather has been dry, and the
streams can take a good amount of water, it is an intriguing setup
in which banded convective storms could train for several hours to
produce isolated rapid runoff from 2-3 inches of rain. Ahead of a
large scale weather system, deep layer ascent will be maximized
farther north, but a period of height falls this morning will
combine with frontal passage to produce several hours of well
defined synoptic lift, even as cooler and more stable air begins
to filter into the area at low levels. During this several hour
period, from about 10-16Z, heavy swaths of QPF show remarkable
similarity between the WRF-ARW2 and multiple recent runs of the
HRRR. The QPF signal has other support from the NAM, UKMET, and
ECMWF, giving us confidence in the location and at least the
potential for locally heavy amounts.

Low level inflow / 850 mb winds/ of 25 knots are not especially
strong, but should be sufficient to sustain convection, and the
predicted cell motions are ideal for training as the Corfidi
forward-propagation vectors show zero deviation from the mean 0-6
km wind. While the threat should be short lived and somewhat
low-end, the convergence of several pieces of evidence prompted us
to introduce the Marginal Risk area.

Burke

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 10 2019

Day 2
Valid 12Z Wed Dec 11 2019 - 12Z Thu Dec 12 2019

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

Pagano

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 10 2019

Day 3
Valid 12Z Thu Dec 12 2019 - 12Z Fri Dec 13 2019

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

Pagano

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
414 AM EST Tue Dec 10 2019

Valid 12Z Tue Dec 10 2019 - 12Z Fri Dec 13 2019

...Great Lakes...
Days 1-3...

Low pressure lifts away through eastern Canada today, leaving
strong CAA in its wake. This CAA moving across the Great Lakes
will produce periods of moderate to heavy Lake Effect Snow (LES)
on W to NW flow. The moisture is shallow, but forcing is robust
and prolonged with steep lapse rates and an 8kft inversion height.
The heaviest snow on D1 /Tuesday/ is likely in the Keweenaw
Peninsula where Lake Superior fetch and upslope combine, as well
as NW parts of the L.P. of MI where the effective fetch length
increases due to pre-saturation off Lake Superior. WPC
probabilities on D1 are low to moderate for 6 inches. By day 2,
the flow becomes more westerly, and the favored snow belt regions
downwind of Erie and Ontario feature the highest WPC
probabilities, which are high for 4 inches.

On D3 /Thursday/ a clipper and associated jet streak will race
southeast from central Canada and towards the Great Lakes. The
combination of a very cold column, jet level diffluence, and
subtle height falls will produce a swath of moderate snow despite
rapid progression of this system to the east. WPC probabilities
are moderate for 4 inches in northern WI and the U.P. of Michigan.


...Tennessee Valley through New England...
Days 1-3...

A strong cold front crossing the northeast and down through the
southeast will bring much colder air to these regions Tuesday and
Tuesday night. As this occurs, precipitation within a divergent
region ahead of a southern stream shortwave will continue to
spread northward in an anafrontal precipitation setup. Initially,
the precipitation will all be rain, but as strong CAA continues
the column will cool and p-type will transition to snow late
Tuesday in the Southeast, and early Wednesday into the Northeast.

While antecedent temperatures are very warm, and rainfall will be
widespread Tuesday, some snow accumulation is likely. This is due
to the likelihood for intense snowfall rates as a robust stripe of
7000-600mb fgen collocated with the RRQ of a 170+kt upper jet
streak to produce intense and deep layer ascent into the cold air.
The dynamic cooling of the column associated with heavy
precipitation will further enhance snowfall possibilities, and
HREF probabilities for 1"/hr have increased. The duration of
forcing and cold enough temperatures overlap is short, so total
snowfall should be limited. However, a few inches of snow are
possible as evidenced by WPC probabilities for 4 inches eclipsing
20% in the terrain of WV/VA, with 10-20% across southern New
England. In the I-95 corridor, a burst of heavy snow is likely,
potentially coinciding with Wednesday morning rush hour. While the
ground will be warm, heavy snow should overcome this briefly, and
WPC probabilities for 1" are as high as 50% for the urban corridor.

A secondary band of precip is progged by CAMs to cross the TN Vly
later this evening. This precip is mainly snow over TN, but mainly
ice on the higher portions of Smokey Mountains. This raises the
risk for a quarter inch of ice on those peaks to 10% tonight.

On day 3, a more pronounced southern stream low pressure may
blossom and spread precipitation from the Gulf of Mexico atop a
wedge of surface high pressure Thursday night. With cold surface
temps, especially in the western Carolinas and SW VA, the
precipitation may begin as freezing rain. There remains quite a
bit of spread in guidance in the thermals, but some freezing rain
is likely, and WPC probabilities for 1/4" are as high as 20%.


...Pacific Northwest...
Days 2-3...

Prolonged and robust Pacific Jet streak will angle into the
Pacific Northwest beginning Wednesday. This jet streak will drive
significant moist advection into the West, with small
perturbations embedded within the zonal flow accentuating synoptic
lift. Periods of heavy snow are likely on day 2 in the Washington
Cascades, with widespread heavy snow spreading eastward as far as
the Colorado Rockies and all ranges in between as the nose of the
jet shifts eastward. WPC probabilities feature a high risk for 12
inches from the Washington Cascades and Oregon Cascades, into the
Sawtooth, Northern Rockies, and Grand Tetons, with snow levels as
low as 1000 ft near the Canada border, to as high as 5000 ft in
Colorado. Total 2-day snowfall could eclipse 4 feet in the
Washington Cascades, with more than 2 ft possible in the other
highest terrain.


Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1234 AM EST Tue Dec 10 2019

Valid 12Z Fri Dec 13 2019 - 12Z Tue Dec 17 2019

...Eastern Seaboard heavy rain threat Friday/Saturday and lead
wintry precipitation risk for the interior
Mid-Atlantic/Appalachians & Northeast...

...Wet Pacific Northwest to snowy N-Central Great Basin/Rockies
flow into the weekend leads to Deep South low/rainfall development
early next week...


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

A cold upper vortex will reform and persist over Hudson Bay this
period. Over the lower 48, an active and complex pattern remains
as several streams laden with a series of impulses interact. Model
and ensemble solutions have become better clustered, but forecast
confidence remains somewhat tempered by recent run to run
continuity issues and flow complexity. Opted for a composite
GFS/ECMWF and GEFS/ECMWF ensemble mean solution blended with the
National Blend of Models as a basis for the WPC medium range
product suite.


...Weather Highlights/Threats...

Southern stream shortwave progression and subsequent northern
stream flow interaction should support low development and track
from the Gulf of Mexico/Gulf Coast to the Southeast then up the
East Coast Friday/Saturday. This will bring heavy rain up the East
Coast mainly from the Southeast to the Appalachians, Mid-Atlantic
then Northeast. System interaction with lower atmospheric cold air
damming offers an interior snow/ice threat to monitor. The degree
of phasing between two initially separate streams and baroclinic
zone/coastal details will be determining factors of eventual
evolution and potential coastal system threat.

The pattern will also become wet upstream over the Pacific
Northwest into late week as Pacific shortwave energies punch
onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored north-central Great
Basin/Rockies terrain through the weekend. By next Monday into
Tuesday, the exiting wave will eject and then interact with Gulf
moisture to spawn a rainfall focusing low pressure/frontal system
forecast to move over the Deep South, then potentially again up
the East Coast.

Schichtel


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