Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sun Jan 05 2020
Valid 12Z Wed Jan 08 2020 - 12Z Sun Jan 12 2020
...Heavy rain potential Fri into the weekend from the Lower
Mississippi Valley to the Ohio Valley...
...Overview/Guidance Assessment...
Latest guidance continues to advertise an amplified mean trough
aloft settling over the western U.S., between strengthening ridges
over the east-central Pacific and off the Southeast U.S.. The
Climate Prediction Center 8-14 Day forecast still suggests some
variation of this pattern may persist well beyond the end of the
medium range period. Evolution aloft and associated surface
features will lead to a heavy rain threat most likely from the
Lower Mississippi Valley through the Tennessee Valley and Ohio
Valley. Teleconnections relative to the positive height anomaly
center which multi-day means show near or offshore the East Coast
later in the period also target this area for greatest
precipitation potential. Meanwhile North Pacific energy feeding
into the mean trough will bring episodes of enhanced precipitation
with best focus over the Pacific Northwest.
Near the start of the period on Wed the models and ensemble means
have gravitated to the idea started by some solutions yesterday
that deepening low pressure off the New England coast will trace a
wide enough arc to keep a majority of the associated moisture just
offshore. Guidance has not behaved well for this system thus far
due to evolution being dependent on important small scale details
aloft so further adjustments could still be possible.
Farther west there is reasonable agreement that a shortwave will
reach the western states by early Wed, progressing eastward while
additional energy arrives Thu. Ejection of the leading energy and
possible interaction with southern Canada flow should lead to a
surface wave that tracks from the Plains northeastward into Canada
Thu onward. The latter shortwave will progress across the
Southwest/southern Rockies late in the week with solutions
diverging for timing especially after early Fri. Although the new
00Z GFS has slowed a bit, it continues to bring the shortwave more
aggressively into the eastern U.S. than most other guidance
including the GEFS mean. Preference remains with a non-GFS
evolution that maintains greater integrity of positive height
anomalies near the East Coast. On the other hand yesterday's 00Z
and 12Z ECMWF runs may become a bit slow and confidence is low
that the ejecting energy will consolidate in a way that ECMWF/CMC
runs show by day 7 Sun. As a result the ensemble means become the
best option for the manual forecast valid next weekend.
At least into day 6 Sat the guidance is becoming better clustered
in principle for upper trough energy that drops into the Northwest
by early Sat and expands over the West through Sun. Interestingly
by day 7 Sun the new 00Z GFS shows less amplitude on the southwest
side of the trough versus consensus, in contrast to a number of
previous runs that had been digging the trough a bit westward of
the majority. GEFS/ECMWF means, the 12Z ECMWF, and new 00Z CMC
cluster fairly well. At the surface there is still a moderate
amount of model/ensemble spread (similar to yesterday) for low
pressure that tracks into southern British Columbia and/or the
Pacific Northwest Fri/Sat--favoring maintenance of an intermediate
path.
Early in the period a model blend with more 18Z GFS/12Z ECMWF
weight than the 12Z CMC/UKMET best represented consensus for
significant features with day 5 Fri being the transition day that
started to incorporate the 18Z GEFS/12Z ECMWF means. Days 6-7
Sat-Sun trended increasingly toward the means given questionable
attributes of the GFS after early Fri and the ECMWF by late in the
weekend. The overall theme of the forecast maintains good
continuity with some typical run-to-run detail adjustments.
...Weather Highlights/Threats...
In spite of lingering uncertainties in the specifics, confidence
is increasing for the potential of a heavy rainfall event from the
Lower Mississippi Valley into the Tennessee Valley and Ohio Valley
late this week into the weekend. The cold front trailing from
late week Plains-southern Canada system should have one or more
embedded waves and may progress fairly slowly for a time while
awaiting upstream western energy aloft. Two or three days of Gulf
inflow ahead of this wavy front would provide considerable
moisture to enhance rainfall amounts and some embedded convection
is possible. This area encompasses some locations that recently
received significant rainfall so effects from this upcoming event
will require close monitoring. There is still a signal for some
wintry precipitation on the northern periphery of the moisture
shield but so far with fairly low probabilities of significant
accumulations.
Over the West, continue to expect highest totals of rain/mountain
snow over the Pacific Northwest coast/northwest corner of
California as well as the Cascades, with a secondary maximum over
the northern Rockies. Some moisture will extend southeastward but
with lighter amounts in most cases. Heaviest precipitation should
occur from Fri or Fri night into the weekend, though amounts are
likely not to be extreme given fairly modest precipitable water
anomalies.
The aforementioned Plains-southern Canada system may produce some
precipitation over parts of the Upper Midwest and Great Lakes but
with some dependence on system strength that has yet to be fully
resolved. Some activity should accompany the trailing front as it
reaches the Northeast. Earlier in the period, the system tracking
off the New England coast on Wed will strengthen as it heads into
the Canadian Maritimes. Based on current forecast most areas
should see modest amounts of precipitation but there will be a
brief period of breezy conditions in its wake.
Expect highs to be 5-15F below normal on average from the northern
Plains through much of the West during the period. Colder air may
drop into Montana toward the end of the period next Sun. Warmth
will overspread the eastern half of the country with some
anomalies likely to exceed plus 20F, especially 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