Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1032 AM EST Thu Feb 21 2019
Valid 12Z Sun Feb 24 2019 - 12Z Thu Feb 28 2019
...Major Upper Midwest/Great Lakes/Interior New England Winter
Storm Sunday/Monday...
...Pattern Overview.
The Rex block pattern across the central/north Pacific extending
from near Hawaii to Alaska will persist or even strengthen during
the medium range. The strongest positive height anomaly center
will shift northward into the arctic by the early to middle
portion of next week. Teleconnections relative to this shift in
the anomaly support an eastward shift of the mean trough across
the CONUS/Canada, from a position which as been across the western
U.S. to a mean position from Hudson Bay to the Upper Midwest by
the middle of next week. A shift to a more zonal flow regime
across the CONUS southern tier will also be a consequence of this
modulation across the Pacific, as shortwave energy take a more
northern track and reinforces the mean trough/upper low.
...Guidance/Predictability Assessment and Preferences...
Guidance was quite well clustered early in the medium range at the
mid and larger scales, and a blend of the latest ECMWF/GFS/UKMET
offered a reasonable starting point for the forecast during days
3-4 (Sun-Mon). One issue that emerges by Mon-Tue is exactly what
happens as shortwave energy initially across the Pacific Northwest
shears apart, with solutions differing on just how much energy
moves east across the CONUS and Canada versus westward into the
Pacific underneath the ridge. At this time, confidence is low in
this aspect of the forecast, and the outcome does have downstream
implications such as the amplitude of eastern U.S. troughing by
next Tue-Wed. Despite these disagreements, model/ensemble
solutions do all show a consistent picture at the larger scales
even through day 7 as energy consolidates into an upper near/south
of Hudson Bay, with shortwaves traversing the southern periphery
through broad cyclonic flow from the Midwest to the Northeast.
Given the increased uncertainty at mid/small scales, weighting of
ECENS/GEFS ensemble means was gradually increased during days 5-7
(Tue-Thu).
...Weather Highlights/Hazards...
A threat for significant snows/some icing exists around a deep
surface low. The 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 which may be close to monthly record low pressure values
for portions of Lower Michigan Sunday. Expect strong/gusty winds
and potential blizzard conditions over the Upper Midwest/Great
Lakes and into Canada Sunday-Monday. Mild air will surge northward
east of a trailing cold front with a brief spike in temps to the
60/70s as far north as the Mid-Atlantic. Farther south, a
prolonged heavy rain pattern will finally end with cold frontal
passage, with lead record warm temps possible over Florida.
A long-standing and wintry mean upper trough over the West
deamplifies next week, but will still be reinforced over the
Northwest into Sun-Mon as energy undercuts and digs to the lee of
a blocking Alaska-centered closing upper high. This will bring
unsettled weather across the Northwest/Northern CA and the
north-central Intermountain West/Rockies. The best chance for
heavy snow will be from the Cascades to especially northern CA
with emergence of uncertain Pacific flow undercutting the northern
stream.
Flatter flow downstream over central and eastern U.S. next week
reduces potential for big storms, but a series of smaller/less
predictable systems will progress over the region. This will still
favor cold air surges/anomalously cold temps through the n-central
U.S. and snow swaths over the northern tier states. Rainfall also
redevelops across the Gulf Coast/FL with the fronts, but guidance
has backed off with amounts.
Ryan/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