Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Fri Dec 27 2019
Valid 12Z Mon Dec 30 2019 - 12Z Fri Jan 03 2020
...Significant winter storm expected to impact parts of the Upper
Great Lakes into Upper Great Lakes/Northeast early next week...
...Heavy rainfall potential from the western-central Gulf Coast
region northeastward mid-late next week...
...Overview and Guidance/Predictability Assessment...
The guidance continues to show fairly good agreement at least with
respect to the large scale mean flow. The upper low over the Upper
Midwest early day 3/Monday will track gradually northeast while
another vigorous upper system drops southward along or just inland
from the West Coast. As the West Coast feature reaches over or
near northwest Mexico, strong North Pacific flow should reach into
western North America. Models show upper ridging over the east
Pacific to strengthen by the middle of next week, which eventually
should lead to amplified troughing over the central U.S./northern
Mexico by next Thursday and Friday.
Though large scale flow evolution shows good agreement through the
period, there remains some fairly notable differences with respect
to timing of the various systems. For the initial Midwest upper
low, some timing differences remain, but there seems to be a trend
towards the slower ECMWF/CMC. This would allow for more high
pressure to build into eastern Canada for a time, leading to a bit
slower northward progress of triple point wave development
tracking from off the northern Mid-Atlantic coast into the
Canadian Maritimes by Wednesday.
Meanwhile, fairly significant differences continue for the energy
dropping southward along the West Coast. The GFS/GEFS mean remain
slower than the ECMWF/ECENS mean but have trended slightly faster
with the past couple of runs. The latest CMC runs are slower than
the ECMWF, but still faster than the GFS, while the latest UKMET
sits right in between the ECMWF and CMC. This difference has
significant influence downstream involving how quickly moisture
spreads from near the Gulf Coast and northeastward Tuesday night
onward with the GFS scenario the slowest.
There remains some differences in the evolution of reinforced
amplified troughing across the Central U.S. at the end of the
period with respect to the North American energy dropping into the
West. The CMC is much faster to push this trough eastward next
Friday/day 7 than the remainder of the guidance and the means
(even its own).
Based on the above evaluation, the WPC blend for days 3-5 used a
majority deterministic guidance blend between the 12z
ECMWF/CMC/UKMET with smaller contributions from the 18z GEFS/12z
ECENS means. After day 5, increased the weighting of the ensemble
means and removed the CMC from the blend (and the UKMET since it
is not available past 12z day 5). Was able to incorporate some of
the 18z GFS past day 5 though as it became more similar to the
ECMWF and the ensemble means with the larger scale pattern. This
blend resulted in good continuity from the previous WPC shift.
...Weather Highlights/Threats...
By the start of the medium range period (Monday), the storm system
initially over the Upper Great Lakes should weaken in favor of a
developing coastal wave off the Northeast coast. Precipitation
with this system will be ongoing across the Upper Lakes into the
Northeast Monday into Tuesday with snow likely across northern
areas. The latest WPC medium range winter weather outlook shows
the best chance for accumulating significant snowfall across far
northeast Minnesota/northern Wisconsin, as well as across interior
New England.
Precipitation (lower elevation rain and mountain snows) across the
Pacific Northwest will be ongoing through much of the period as
subsequent waves of energy move onshore. Some moisture should
reach into the Intermountain West, though with lesser intensity.
Ahead of the upper low in northern Mexico, ample moisture from the
Gulf of Mexico is likely to surge northward across the eastern
half of the country by the middle to later part of next week.
Expect a period of widespread showers and thunderstorms across
portions of the western/central Gulf Coast and lower Mississippi
Valley, which should eventually spread northeastward into the
Tennessee Valley/central Appalachians as well. While confidence
remains low for precise timing and duration, the models continue
to show a decent signal for a meaningful rainfall event over this
general area.
The warm sector ahead of the initial Upper Great Lakes storm will
contain much above normal temperatures to start next week, with
some readings as much as 20F above normal over the Mid-Atlantic.
Frontal passage will bring a cooling trend but only to
near/moderately above normal levels. Central U.S. trough
amplification aloft late in the period should encourage a warmer
trend again over the East by next Thu. Areas from the
central/southern Rockies into the Southwest are likely to see the
most persistent below average temperatures with some areas
averaging 5-10F below normal for highs through the Mon-Fri period.
Santorelli
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