Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1101 AM EST Wed Feb 20 2019
Valid 12Z Sat Feb 23 2019 - 12Z Wed Feb 27 2019
...Significant winter storm from the south-central Plains to the
Upper Midwest/Great Lakes and interior New England this weekend...
...Heavy rainfall/flooding threat in the Southeast Saturday and
again next week...
...Pattern Overview...
The Pacific Northwest will see additional/reloading troughing out
of western Canada downstream of a very strong upper high over
Alaska next week. To the east, a potent shortwave will deepen and
lift northeastward into the Great Lakes as the upper ridge over
the Bahamas remains in place. This will keep the storm track
inland well west of the coastal plain to the south of a Hudson Bay
upper low.
...Guidance/Predictability Assessment and Preferences...
Guidance was reasonably well-clustered at mid-larger scales
through the medium range period until about Monday. A composite
blend of the latest GFS/ECMWF/UKMET/Canadian offered a reasonable
starting point for Sat-Mon as a rapidly deepening system (central
pressure fall of about 20mb/24 hrs between Sat and Sun morning)
lifts into the Great Lakes and then exits out the St. Lawrence
Valley Sunday into Monday. This system has the potential to deepen
into the upper 970s mb or at least low 980s mb which may be close
to monthly record low pressure values for portions of Lower
Michigan Sunday morning.
In the West, the models/ensembles diverge on how to handle the
upper high over Alaska and downstream flow over southwestern
Canada/Pacific Northwest -- ECMWF and many of its ensembles take
shortwave vorticity back westward underneath the upper high while
the GFS and many of its ensembles (as well as the Canadian/UKMET)
took it southward then slowly eastward along the Oregon coast. The
parallel 06Z FV3-GFS was less aggressive than the GFS. Pattern
would allow a westward/retrograding shortwave so opted to keep
some ECMWF influence to the forecast but otherwise trended toward
the ensembles which would give way to at least flatter flow by
next Wed into OR/NorCal. In the east, some semblance of a system
should lift toward the Great Lakes next Tuesday then off the coast
next Wednesday but how it evolves is uncertain.
...Weather Highlights/Hazards...
Weekend system will bring a threat for significant accumulating
snow with some icing possible to the northwest/north/northeast of
the surface low with strong/gusty winds and potential blizzard
conditions across the Upper Midwest and Great Lakes. Mild air will
surge northward east of the front with a brief spike in
temperatures into the 60s and 70s across the Mid-Atlantic. Farther
south, heavy rain from the end of the short range will be possible
into the start of the medium range aided by convection in the warm
sector and plenty of Gulf moisture. SPC has denoted a chance of
severe weather on Saturday from Arkansas to KY/TN and northern
Alabama. Florida may escape most of the rainfall but record warm
temperatures will be possible with the mild upper pattern.
A long-standing and wintry mean upper trough over the West will
again be reinforced Sat-Mon as energy digs to the lee of a
blocking Alaska-centered upper high. These systems will be less
amplified than prior systems in this recent pattern, but will
bring unsettled weather through the Northwest and north-central
Intermountain West and Rockies, with the best chance for
accumulating snowfall over the Cascades into especially northern
California. Focus of the precipitation may remain concentrated
over the same area depending on how the pattern evolves offshore
into the West Coast, which would result in several days of
rain/mountain snow.
Fracasso/Schichtel
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found 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