Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 PM EST Fri Dec 27 2019
Valid 12Z Mon Dec 30 2019 - 12Z Fri Jan 03 2020
...Significant winter storm expected to continue into Mon/Tue
across the Great Lakes to Northeast...
...Heavy rainfall potential from the western-central Gulf Coast
region northeastward mid-late next week...
...Overview...
An upper low should shift slowly northeastward from the Great
Lakes into southeastern Canada midweek, spreading precipitation
and potentially heavy snow across the Great Lakes and Northeast.
Meanwhile, another upper low is forecast to track southward across
the West Coast into northern Mexico through day 5/Wed, combining
with a northern stream trough around day 6/Thu. Ahead of the
latter, heavy rainfall is possible from the western Gulf Coast
northeastward.
...Guidance/Predictability Assessment...
Overall the large-scale pattern for the mass fields is in good
agreement in the models and ensembles. The upper low wrapping up
in the central U.S. during the short range period should place a
surface low in the Great Lakes region day 3/Mon, while a new
surface low develops in the far western Atlantic at the triple
point of the frontal system Mon-Tue. After yesterday's GFS runs
were a bit faster/farther north with these surface lows,
operational models now better agree on the general position and
strength of the lows, so a multi-model blend of the 06Z GFS and
00Z ECMWF, UKMET, and CMC was used for these features.
Model guidance also offers better agreement compared to previous
runs for the the upper low moving across the West early in the
week. While the 00Z operational ECMWF and its mean are still a bit
farther south and west of other guidance Mon-Wed, differences are
pretty minimal. By Thu, models have the upper low starting to
combine with the northern stream to create a positively-tilted
trough across the central U.S. Yesterday's EC runs were faster
than other guidance by Thu-Fri, as they had the energy splitting,
with one vort max quickly tracking east and then north through the
trough (which created major QPF differences). Today's EC runs are
now more in line with other guidance and its mean by hanging the
trough back west. So the main model outlier that remains appears
to be the 00Z CMC by Thu-Fri. Strong energy (as shown by 500 mb
vorticity) streams into the trough from the northwest in the CMC,
which wraps up an upper low as well as a strong surface low across
the central U.S. Thus, used a 00Z EC/EC mean and 06Z GFS/GEFS mean
blend for Thu-Fri.
...Weather Highlights/Threats...
The low in the Great Lakes and the subsequent surface low near the
Northeast coast will produce heavy precipitation across those
regions on Mon-Tue. Currently, the precipitation looks to begin as
rain and transition to snow in the Great Lakes region and interior
New England, with mixed precipitation also possible in the
transition. Locally heavy rainfall is possible farther south on
Monday across New York City and southern New England.
Ahead of the western upper low and central U.S. trough, heavy
rainfall is forecast to move west to east with the upper-level
support and as moisture flows in from the Gulf of Mexico.
Currently the heaviest rain looks to be along the west-central
Gulf Coast beginning Tue northeastward through the Lower
Mississippi and Tennessee Valleys into the Southeast. The exact
timing and placement of the heavy rainfall amounts cannot be
determined yet--the heaviest precipitation will likely occur along
the front, so any differences in the frontal position will cause
shifts in the precipitation forecast. Regardless, the potential is
there for heavy rainfall mid-late next week.
Waves of moisture and a few pulses of upper-level energy will lead
to persistent precipitation across the Northwest, with higher
elevation (coastal ranges, Cascades, and Northern Rockies) heavy
snow and lower elevation rain.
Ahead of the Great Lakes low in the warm sector, much above
average temperatures are likely across the Ohio Valley to the
Great Lakes region and Mid-Atlantic on Mon, with temperatures
still average or a bit above after a cold frontal passage. Warmer
than average temperatures are also expected in the central to
eastern U.S. Wed-Fri with some ridging ahead of the central U.S.
trough. The Southwest to Four Corners will be the main cool spot
through the period due to the anomalously low heights.
Tate
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