Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
148 PM 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 (straddling
the short-medium range overlap). 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. The
00Z ECMWF/ECMWF mean/CMC/UKMET were generally slower and depicted
an occluded (or nearly so) surface system by early day 4 Sun with
timing slower than the sheared 00Z/06Z GFS/GEFS mean. Shift in
the ensemble consensus a bit quicker than 12-24 hrs ago suggested
the quicker GFS was not to be discounted. Retained a 30% weighting
of the GFS solutions vs 70s% ECMWF-led solutions through day 5
Mon.
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. Consensus approach served as a best
start with increased weight toward the ensemble means by next Wed,
given the quite varied solutions (and low confidence) upstream
just south of Alaska by then.
...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 with a wintry mix in
between. 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 and
narrower swaths of sleet/ice.
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 far 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. With the
expected southward movement of the cold air from Canada, snow
levels may lower toward the lower elevations south of the
U.S./Canadian border. This was reflected on the Winter Weather
Outlook probabilities nearing sea-level next Tue-Wed.
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 mild/warm lows. Even after the system's
passage temperatures should remain 10-20F above normal for highs
with some higher anomalies for morning lows in the vicinity of the
frontal boundary.
Fracasso/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