Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1259 AM EST Thu Dec 12 2019
Valid 12Z Sun Dec 15 2019 - 12Z Thu Dec 19 2019
...Winter weather threat over the central Plains Sun/Mon will move
to the East Coast by midweek...
...Pattern Overview and Guidance/Predictability Assessment...
Broad troughing to start the period (Sunday) will shift focus to
the shortwave ejecting out of the Four Corners region and through
the southern/central Plains. This system will slide
east-northeastward to the Mid-Atlantic by Tuesday and then out to
sea. Another Pacific system will stumble into California Wed/Thu.
Models and ensembles diverge into mainly two unequally-weighted
camps with respect to the central/eastern system Sun-Tue. The
larger (and preferred) cluster of solutions revolved around the
12Z ECMWF/UKMET (and the ECMWF and Canadian ensemble means) with
the other smaller cluster around the 12-18Z GFS and 18Z GEFS mean.
The 12Z Canadian was closer to the ECMWF-led cluster but slower.
Ensemble trend supports a flatter solution though there are
members from all three ensemble systems that were more developed
and tracked farther to the northwest (west of the Appalachians)
like the GFS. Thereafter, GFS/ECMWF were fairly close with the
western trough moving into the West in line with the ensembles,
though the GFS was much flatter with its downstream ridge into the
Four Corners next Wed. With uncertainty in how the stretched
western system evolves coupled with the amount of cold air in the
Upper Midwest next Wed/Thu, trended toward a majority ensemble
weighting for the end of the period.
...Weather Highlights/Threats...
Wrapped-up system over northern Maine on Sunday will move into the
Canadian Maritimes on Monday with high pressure in its wake.
bringing in cooler air (20s/30s) to the Northeast after a brief
mild intrusion. Over the Plains, as low pressure exits the
Foothills it will spread rain (south) and snow (north) over the
area. There remains some north-south spread in amounts and
precipitation type (depending on the track of the low in relation
to the colder/warmer air mass interface) but a swath of at least
plowable snow seems fairly likely over parts of Kansas. Winter
weather outlook probabilities over 30% encompassed most of the
Sunflower State Sun-Mon. As the area of low pressure moves
eastward, rain may expand and increase over the Southeast from the
Lower MS Valley to the TN Valley and southern Appalachians. To the
north, snow is possible on the northern side of the precipitation
shield but this will depend on how far north the surface low may
track. To the West, precipitation with the incoming Pacific front
will focus along the coast and into the Sierra.
Fracasso
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