Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Wed Jan 08 2020
Valid 12Z Sat Jan 11 2020 - 12Z Wed Jan 15 2020
...Eastern U.S. storm this weekend to bring a heavy rain threat to
lower half of the Mississippi Valley and areas to the
east/northeast, severe weather potential to parts of the South,
and wintry precipitation to the Midwest through Great
Lakes/northern New England...
...Active pattern over the Northwest...
...Overview/Guidance Assessment...
Into the first part of next week guidance is consistent and
agreeable in showing persistent and amplified western U.S. mean
troughing between strong ridges off the Southeast U.S. and over
the east-central Pacific. This pattern will support at least a
couple significant precipitation events over parts of the eastern
half of the lower 48. The first will be a widespread event
associated with low pressure tracking from the Mid-Lower
Mississippi Valley through the Northeast this weekend. Additional
events next week have a less-defined focus at this time and thus
have lower confidence in specifics, with best signals for highest
rainfall totals over the interior Southeast. Multiple systems
reaching the Northwest will produce significant precipitation over
favored terrain in terms of five-day totals. There will be a
pronounced temperature contrast between much above normal
temperatures over the East and much below normal temperatures
overspreading the northern Plains and to a lesser extent the
northern/central portions of the West.
Guidance is still in the process of resolving the evolution of the
shortwave/surface low tracking northeastward from the
Plains/Mississippi Valley through the Northeast this weekend. In
the latest runs through 12Z/18Z as well as the new 00Z cycle
solutions have generally sorted themselves into two clusters, with
the ECMWF/ECMWF mean/CMC/UKMET and some other models depicting an
occluded (or nearly so) surface system by early day 4 Sun with
timing slower than the sheared GFS/GEFS mean. Progression of the
southeasterly mean flow keeps the door open for timing that is a
bit faster than the ECMWF cluster but overall there is too much
consensus to incorporate the GFS scenario in the manual forecast.
Upstream flow, with another shortwave progressing into the West,
shows reasonable agreement into day 4 Sun. However after that
time the models/ensembles begin to go astray regarding the timing
and amplitude of this western energy once it continues beyond the
Rockies as well as for upstream impulses. Thus there is very poor
agreement over specifics of one or more surface waves that should
emerge over the Plains and continue to the east/northeast. Then
around Tue-Wed larger scale model discrepancies develop with
respect to how North Pacific energy could begin to dampen the
east-central Pacific upper ridge. Downstream effects may reach
the western U.S. by day 7 Wed.
Over recent days the guidance has varied with details of western
Canadian flow aloft that should dip into the northern parts of the
West/Plains and support the forecast trend toward colder
temperatures over this area. In differing ways the GFS and CMC
runs have tended to bring lower heights to areas near the
U.S.-Canadian border than most other guidance.
Forecast preference early in the period led to starting with a
blend of the 12Z and 00Z ECMWF along with the 12Z UKMET/CMC/ECMWF
mean for days 3-4 Sat-Sun. This blend toned down the specific 12Z
ECMWF solution that appeared too deep/westward within the
otherwise favored cluster for the eastern U.S. system. After this
system's departure the blend could then incorporate some 18Z
GFS/GEFS mean input along with maintaining the ECMWF runs and
ECMWF mean. Low confidence in details favored increasing total
ensemble weight to 50-70 percent by late in the period.
...Weather Highlights/Threats...
Heavy rainfall expected to be in progress over/near the Mid-Lower
Mississippi Valley at the start of the period early Sat should
continue to the east/northeast during the weekend as low pressure
tracks from the Mississippi Valley through the Northeast. In
addition to the heavy rainfall, expect this system to produce a
severe weather threat across portions of the South. Check latest
Storm Prediction Center products for updated information regarding
the severe weather forecast. Meanwhile winter weather will be
likely on the cold side of the system. The current winter weather
outlook highlights an axis from near Missouri through the central
Great Lakes and northern New England for the best potential of
meaningful snowfall.
Additional waves emerging from the Plains may promote the
stalling/northward return of the front trailing from the above
weekend system. However wave specifics are very unclear at this
time. As a result confidence is moderate to low regarding the
next possible heavy rain event. Currently the best signal extends
from the Lower Mississippi Valley into the southern Mid-Atlantic
with highest totals over the interior Southeast. Lighter
precipitation will be possible farther to the north/northeast,
snow over northern latitudes and mostly rain farther south.
Over the Northwest continue to expect repeated episodes of rain
and mountain snow with highest five-day totals focused along
favored terrain of the Cascades and Pacific Northwest
coast/northwest California. The northern Rockies will see a
secondary maximum of snow while areas from the Sierra Nevada into
the central Rockies will see somewhat lower totals. By the middle
of next week forecast specifics become more uncertain as eastern
Pacific flow differences range between persistence of the pattern
with another system reaching the Pacific Northwest (currently
preferred based on the ensemble means) or a farther offshore
system that would allow for a break in the precipitation.
Gradually lowering heights aloft along the central/western
U.S.-Canadian border will lead to much below normal temperatures
spreading over an increasing portion of the northern Plains and
West. The core of coldest air should be over/near Montana with
some readings 30-40F below normal by early next week. Northern
parts of the West may see highs 10-20F below normal by early-mid
week. Eastern U.S. warmth should be the most extreme this weekend
ahead of the Mississippi Valley through Northeast system, with
widespread plus 20-35F anomalies that may reach/exceed daily
records for highs and/or warm lows. Even after the system's
passage temperatures should remain 10-20F above normal for highs
with some higher anomalies for morning lows.
Rausch
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