Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
204 PM EST Thu Dec 26 2019
Valid 12Z Sun Dec 29 2019 - 12Z Thu Jan 02 2020
...Significant winter storm expected to impact parts of the
northern Plains into Upper Great Lakes/Northeast into early next
week...
...Heavy rainfall potential from the western-central Gulf Coast
region northeastward mid-late next week...
...Overview and Guidance/Predictability Assessment...
Guidance agrees fairly well at least with respect to the large
scale mean flow. The upper low over the Plains as of early day 3
Sun will track gradually east-northeast while another vigorous
upper system drops southward along or just inland from the West
Coast. As the feature near the West Coast reaches over or near
northwest Mexico, strong North Pacific flow should reach into
western North America. Consensus expects ridging over the eastern
Pacific to strengthen by Wed-Thu and encourage amplification of
the incoming Pacific flow--leading to an amplified trough over the
central U.S./northern Mexico by next Thu and ejection of the
leading upper low.
Embedded details have exhibited notable spread and/or trending
though. Multi-run trends have generally favored the slower ECMWF
cluster for the initial Plains low and a corresponding longer
duration of precipitation across the northern tier of the central
and eastern U.S.. Ahead of this system the slower trend allows
for more high pressure to build into eastern Canada and the
Maritimes for a time, leading to slower northward progress of a
triple point wave forecast to track from off the northern
Mid-Atlantic coast into the Canadian Maritimes.
Meanwhile locally significant differences persist for the energy
dropping southward near the West Coast. Behavior of guidance thus
far seems to favor an intermediate track as it makes its descent,
with GFS runs a bit westward of the guidance majority. This is
opposite of historical biases so it will be interesting to see how
future runs cluster. After early Mon the westward position of GFS
runs becomes even more noticeable, though at least the 12Z run has
adjusted a bit east to near the 06Z/12Z GEFS mean runs. Latest
CMC runs are a bit slower than the ECMWF/ECMWF mean and UKMET but
faster than the GFS/GEFS mean. This difference has a significant
influence downstream, as GFS and to a lesser extent GEFS mean runs
bring lower heights through the eastern U.S. and into the western
Atlantic versus other guidance from early week onward. Resulting
sensible weather differences involve how quickly moisture will
spread from near the Gulf Coast northeastward Tue night onward
with the GFS scenario being slowest (but the 12Z GFS trending a
bit faster than the 06Z/00Z runs).
Differences appear more modest with the strong North Pacific flow
that comes into western North America and then amplifies in
response to the ridge off the West Coast, with most aspects of the
GFS/ECMWF and their means actually fairly similar from the eastern
Pacific into the western half of the U.S. by just after the end of
the forecast period.
Based on latest trends and clustering the updated forecast
emphasized the 00Z ECMWF/UKMET/CMC with only modest combined input
of the 06Z GFS/GEFS mean into mid-period. Thereafter a
model/ensemble blend consisting of even weight of the 00Z
ECMWF/CMC and 06Z GEFS/00Z ECMWF mean provided the best
representation of the most likely evolution relative to data
available through the 06Z cycle.
...Weather Highlights/Threats...
The storm system initially over the Upper Midwest should make slow
progress into the Upper Great Lakes while a developing coastal
wave tracks near the northeastern coast. This evolution will
support a lengthy period of precipitation (wintry weather in
northern areas) from portions of the northern Plains into the
Northeast. Please see the medium range winter weather outlook
products for more details on areas that will see the greatest
probability of significant snowfall. Other areas in the East will
see a more brief episode of rain as the cold front passes through.
Some locally heavy rainfall may be possible over/near the
Tennessee Valley on Sun.
Farther west, expect the energy dropping southward near the West
Coast to bring areas of rain and higher elevation snow of varying
intensity from Sun into early next week. Exact amounts at some
locations will be very sensitive to exact track of the upper
system, with additional time needed to refine this fine-scale
detail. Then the strong jet energy moving into the Northwest and
pushing southeastward Tue-Thu may bring a burst of fairly intense
terrain-enhanced rain/mountain snow to areas across the Pacific
Northwest/northern Rockies and to a lesser extent into the central
Rockies.
As the West Coast U.S. into northwest Mexico upper low energy
comes under the influence of the strong flow moving into the
Northwest/Rockies, ample moisture from the Gulf of Mexico is
likely to surge northward across the eastern half of the country
by the mid-late part of next week. Thus expect a period of
widespread showers and thunderstorms across portions of the
western/central Gulf Coast and lower Mississippi Valley, with the
potential for areas of heavy rain extending northeast to some
degree as well. While confidence is not yet great for precise
timing and duration, there is a decent signal for a meaningful
rainfall event over this general area.
The warm sector ahead of the initial Upper Midwest storm will
contain much above normal temperatures into the start of next
week, with some readings as much as 20-35F above normal over the
eastern half of the lower 48. 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 Sun-Thu period.
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