Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Sat Dec 28 2019
Valid 12Z Tue Dec 31 2019 - 12Z Sat Jan 04 2020
...Significant winter storm expected to continue Tue across
northern New England...
...Heavy rainfall potential over the Lower Mississippi Valley to
the Southern Appalachians mid to late week...
...Overview...
An upper low will move from the Great Lakes region along the
U.S./Canadian border on Tue into Wed, causing snow to continue
across northern New England. Another upper low is expected to
track across Baja California Tue-Wed and combine with northern
stream energy to create a central U.S. trough by Fri. Rainfall,
possibly heavy, is forecast ahead of these features. Shortwaves
and a series of surface fronts should lead to persistent
precipitation in the Northwest.
...Guidance/Predictability Assessment...
Model guidance generally remains pretty agreeable in terms of the
large scale pattern early in the forecast, with the two
aforementioned upper lows. A multi-model blend of the 00Z and 06Z
GFS runs and the 00Z ECMWF, UKMET, and CMC was used through
midweek, as there were no significant model differences in the
East at that point. For the upper low moving through Baja
Califonia, the 00Z EC had a bit slower, more southwest track
compared to the GFS. Took a compromise of those solutions for its
placement, which seemed to work well, and the new 12Z GFS is in
between the two which is a good sign. There is uncertainty with
what time frame that low in the southern stream combines with the
northern stream, after troughing in the northern stream gets
created by energy dropping southward out of the eastern Pacific
Wed-Thu. The 06Z and 12Z GFS runs keep the streams separate into
Thu, while the other 00Z models have have one combined trough in
the central U.S. at that point. Leaned away from the GFS but did
not discount it completely, as having separate streams is not out
of the question. These differences could affect the timing of the
frontal movement at the surface, in turn affecting the placement
of the heaviest rain in Texas to the Southeast. Broad troughing
appears to set up over the CONUS by Sat, but no significant height
anomalies are expected by the end of the period.
...Weather Highlights/Threats...
As a surface low develops off the Northeast coast and the parent
upper low persists over the Great Lakes, snow will continue from
the short range period into Tue across northern New England, with
high probabilities for additional plowable snowfall amounts
especially over Maine. Precipitation is also likely for the
Northwest over the medium range period as persistent onshore flow
brings in some periods of higher moisture, while shortwaves act as
lift support and a couple of fronts move through. Lower elevation
rain and mountain snow are expected. As the upper low/trough lifts
northeast from Mexico, moisture from the Gulf will get pulled
northward, and rain should spread from the western Gulf Coast into
the Lower Mississippi Valley by Thu. The rain is currently
forecast to move into the Tennessee Valley and Southeast by
Fri/Sat, but the placement of the heaviest rainfall is not certain
at this time. Warmer than average temperatures are expected to
spread across most of the CONUS by midweek, with the Four Corners
region being the main cool spot.
Tate
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