Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Mon Jan 06 2020
Valid 12Z Thu Jan 09 2020 - 12Z Mon Jan 13 2020
...Heavy Rain Threat from the Lower-Mid Mississippi Valley into
Tennessee/Ohio Valleys Fri-Sat...
...Active pattern over the Northwest...
...Overview/Guidance Assessment...
There are embedded details that exhibit some spread and
variability but guidance agrees that strong ridges aloft over the
east-central Pacific and off the southeastern U.S. coast will hold
in place an amplified western U.S. trough at least into early next
week. Within this pattern, low level flow of Gulf moisture ahead
of an ejecting western shortwave/leading wavy cold front will
produce an episode of heavy rainfall from parts of the Lower-Mid
Mississippi Valley eastward/northeastward from late this week into
the weekend with some wintry weather on the northern side of the
precipitation shield. Meanwhile energy flowing around the
northeastern side of the Pacific ridge will bring a series of
systems into the Northwest. The trajectory of moisture across the
Northeast Pacific is not favorable for extreme precipitation
totals but the accumulations over several days may still be
significant.
One of the most problematic aspects of the forecast in recent days
has been the timing/evolution of shortwave energy reaching the
West Coast around the start of the period early Thu and ejecting
from the West after Fri. For the moment solutions are much closer
together than they had been, by way of the GFS hedging a bit
slower/less amplified as the shortwave crosses the eastern half of
the lower 48 and the 12Z ECMWF trending significantly faster/more
open than a number of previous runs. CMC runs continue to hold
onto a somewhat slower and more closed feature aloft until it
reaches past the Mississippi Valley. Ensemble means are still a
bit slower than the non-CMC cluster but provide added support for
an open wave. An early look at the new 00Z ECMWF reveals a
slightly slower trend, highlighting the ongoing uncertainty. The
latest guidance cluster would still produce a significant heavy
rain event over the east-central U.S. but a modestly shorter
duration would temper the highest totals somewhat.
Upstream the guidance spread appears to have narrowed somewhat for
the storm system forecast to track into Washington/southern
British Columbia Fri-Sat. There are still meaningful
medium/smaller scale shortwave differences yet to be resolved so
convergence of solutions may be a slow process. The energy aloft
will overspread the western U.S. with time with some of it
possibly reaching the Plains and supporting one or more surface
waves by the start of next week.
By the latter half of the period there is some spread over exactly
how western Canada energy may descend toward the western
U.S.-Canada border. The 00Z GFS backed off from the 516dm contour
upper low that the prior two runs had just north of Montana on day
7 Mon but is still deeper than most other models/means over that
area.
A multi-model blend with greater emphasis on the 12Z ECMWF and
12-18Z GFS relative to the 12Z UKMET/CMC provided a reasonable
depiction of consensus for the first half of the period, while
increasing detail uncertainties by days 6-7 Sun-Mon recommended
about 50-60 percent total weight of the 18Z GEFS/12Z ECMWF means
along with lingering input from the ECMWF/GFS runs.
...Weather Highlights/Threats...
A system tracking from the northern Plains into eastern Canada
late in the week will spread precipitation across the Great Lakes
and Northeast with some wintry weather possible across northern
parts of these regions.
Confidence is still above average for the overall heavy rainfall
event expected from parts of the Mid-Lower Mississippi Valley into
the Ohio/Tennessee Valleys during the late week into weekend time
frame. However uncertainty with the timing of supporting dynamics
ejecting from the West provides some challenge for resolving the
rainfall duration and highest totals. The scenario continues to
be favorable with a feed of Gulf moisture interacting with an
approaching wavy front, producing areas of heavy rain/embedded
convection. The Storm Prediction Center is also highlighting a
potential for severe weather across the southern tier of states
with this event. Some locations within the heavy rainfall threat
area have recently received significant rainfall so effects from
this upcoming event will require close monitoring. Meanwhile over
the past day the signal for wintry precipitation on the northern
periphery of the moisture shield has strengthened. Primary snow
potential should extend from parts of the Midwest through the
Great Lakes and Northeast. The wavy cold front will eventually
cross the East Coast and extend into the Gulf. This trailing part
of the front is likely to return north as a warm front around the
start of next week and become one focus for another potential
rainfall event near the end of the medium range period and beyond.
The series of systems coming into the Northwest will focus the
highest rain/mountain snow totals over the Pacific Northwest
coast/northwest corner of California along with the Cascades.
Expect a secondary maximum over the northern Rockies. At times
moisture will extend southeastward into the Sierra Nevada/Great
Basin/Central Rockies but with lighter amounts in most cases.
Amounts should not be extreme given that northwesterly flow should
promote only modest anomalies for deep moisture, but multi-day
totals over the most favored terrain could reach a few inches
liquid.
From late week into the weekend expect a surge of very warm air to
extend from the central through eastern U.S. with a broad area of
plus 10-25F anomalies for highs and plus 20-30F and locally higher
anomalies for morning lows. Some daily records are possible,
especially for warm lows. Most of the West will see moderately
below normal highs through the period. The extreme northern
Plains/Montana will be a focus for some colder anomalies,
including some readings 20-30F below normal Sun-Mon.
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