Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
202 PM EST Tue Dec 31 2019
Valid 12Z Fri Jan 03 2020 - 12Z Tue Jan 07 2020
...Heavy rainfall threat continues across the Southeast/southern
Appalachians on Friday...
...Overview/Guidance Assessment...
The pattern across the CONUS during the medium range looks to
remain progressive with an active stream of shortwave troughs from
the North Pacific eastward across North America. An amplified
upper trough initially in place across the central U.S. on day 3
(Fri) is forecast to move east by days 4-5 (Sat-Sun) resulting in
a deepening low pressure system across the Great Lakes and the
Northeast. Model consensus for this system was relatively good
with relatively minor timing/intensity differences noted. Farther
west, additional systems moving onshore along the West Coast and
tracking eastward through time show increasing spread among the
guidance, especially toward the end of the forecast period. A
relatively intense frontal system is forecast to move quickly into
the Pacific Northwest and across the Rockies days 3-4 before
moving into the central U.S. on day 5. Deterministic solutions
were reasonably well clustered with this feature, although some
timing differences began to show by Sun, even among the ensemble
means with the GEFS mean quite a bit slower than the ECENS mean.
By days 6-7 (Mon-Tue) spread becomes much larger with the next
shortwave approaching the West Coast. The ECMWF was quite a bit
slower and deeper than the GFS with the shortwave, and the GFS has
yet another shortwave (this one stronger) reaching the Pacific
Northwest shortly after the stronger/slower previous wave shown by
the ECMWF. These large differences by early next week lended to
heavy use of the ensemble means in the forecast.
The WPC forecast was initially based on a blend of the 00Z
ECMWF/06Z GFS (and gradually increasing ECENS/GEFS ensemble means)
during days 3-5. By days 6-7, the ECENS/GEFS means comprised a
majority of the forecast blend given the high spread by that time
frame.
...Weather Highlights/Threats...
Gulf of Mexico moisture will get drawn ahead of the trough lifting
through the mid-south continuing a threat for heavy rainfall from
the end of the short range period into parts of the
Southeast/southern Appalachians. Surface low across the Ohio
Valley deepens, and reforms off the Northeast coast by Saturday,
and depending on development of the upper system, could bring a
period of snow north and west of the low from the Upper Midwest to
northern/interior New England. Nearly daily bouts of precipitation
are likely for the Northwest into the Northern Rockies during the
period, focusing more over coastal locations and the Coastal
Range/Cascades. Above to well above normal temperatures across the
eastern third of the country Friday and Saturday should moderate
to near normal following passage of the cold front, with above
normal temperatures also moving across the Western U.S. in the
Central states this weekend.
Ryan/Santorelli
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
Hazards:
- Heavy precipitation across portions of the Pacific Northwest and
the Northern Great Basin,
Fri-Sun, Jan 3-Jan 5.
- Heavy rain across portions of the Southeast, the Mid-Atlantic,
and the Southern Appalachians,
Fri-Sat, Jan 3-Jan 4.
- Flooding possible across portions of the Middle Mississippi
Valley, the Great Lakes, the Ohio
Valley, and the Upper Mississippi Valley.
- Flooding occurring or imminent across portions of the Northern
Plains.
- High winds across portions of the Southern Appalachians, the
Mid-Atlantic, the Central
Appalachians, and the Ohio Valley, Sun, Jan 5.
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