Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Tue Feb 26 2019
Valid 12Z Fri Mar 01 2019 - 12Z Tue Mar 05 2019
...Pattern Overview...
Guidance has been consistent with the forecast mean flow from the
multi-day mean perspective, showing broadly cyclonic flow over the
Lower 48--just to the south of a general east-west axis of
relatively low heights across southern Canada. This axis of low
heights will contain one or more upper lows that would help to
push very cold air into the central U.S. with the northern High
Plains possibly seeing a couple days of exceptionally low
anomalies of at least 30-40F below normal. At the same time flow
from the Pacific will bring moisture into parts of the West
(especially California into the central Rockies). Even with the
agreeable mean flow, some embedded details have been difficult to
resolve. In particular eastern U.S. details have varied with
trends over the past day showing a weaker potential storm over the
Great Lakes/Northeast this weekend but increased threat for areas
of locally heavy rainfall over the Gulf Coast/Southeast during the
period.
...Guidance Evaluation/Preferences...
The combination of the short range Pacific Northwest upper low and
initial southern Canada elongated upper low continue to provide
forecast difficulties. The past two runs of the ECMWF/ECMWF mean
(00Z/25 and 12Z/25) shifted more offshore with the western low and
eastward with the Canada low. The result is minimal Great
Lakes/Ohio Valley into New England surface development during the
weekend with lowest pressures tending to stay offshore. Other
solutions show varying degrees of faster ejection of the western
low energy and farther west southern Canada low--leading to
somewhat better defined low pressure than the ECMWF/ECMWF mean
albeit with weaker depth than seen in some previous guidance
(especially the GFS). Guidance behavior over the northern
U.S./southern Canada over recent days does not lend a lot of
confidence to any particular model system on its own. The one
common theme in latest solutions is a less amplified overall
trough aloft, leading to slower southward progression of the
strong cold front dropping through the Plains.
Meanwhile for the southern stream system heading into California
this weekend, models/ensemble means have settled into the expected
track fairly well but are remarkably persistent in their amount of
timing spread and in some cases run-to-run variability. This
favors maintaining a general compromise to help maintain stability
in the forecast. Into the weekend a blend of the 18Z GFS and 12Z
ECMWF/UKMET/CMC provides a reasonable intermediate starting point
for this system while representing only 30 percent inclusion of
the ECMWF for aforementioned issues to the north and east. New
00Z guidance thus far still supports leaning away from 12Z ECMWF
specifics for the northern latitudes during the first half of the
period.
By the latter half of the period the 18Z GEFS/12Z ECMWF means
provide a reasonable template for depicting the expected pattern
though operational runs provide some degree of relative agreement,
arguing for keeping as much as 50 percent of their ideas by day 6
Mon and 30 percent by day 7 Tue. In particular the operational
runs are suggesting that another deep upper low center could track
toward the Upper Great Lakes during the day next Tue. Farther
west there is fair agreement on an upper ridge building into the
West ahead of another Pacific system. Current means are similar
after recent trends in the GEFS mean toward the ECMWF mean. The
12Z GFS was the one notably less probable solution by the end of
next Tue as it was faster than other solutions to bring height
falls to the West Coast.
...Weather Highlights/Hazards...
After a brief period of drier weather over the West, expect the
arrival of another Pacific system to bring moisture into
California by around the start of the weekend with rain and higher
elevation snow then spreading eastward into the central Rockies.
Much of the precipitation with this system should depart by early
next week. The Sierra Nevada range should see highest totals with
other relative maxima over favored locations along the central
California coast and the Great Basin/central Rockies. Meanwhile
during the first half of the period low level upslope flow on the
western side of cold high pressure over the Plains along with
shearing shortwave energy should promote some snow along portions
of the northern Rockies.
From late this week into the weekend expect some snow to spread
across the northern Plains and eventually into the Northeast,
though recent weaker trends with surface low pressure have lowered
potential snowfall amounts somewhat. Behind the system there
should be a period of lake effect snow downwind from portions of
the Great Lakes without ice cover. Expect some areas of rain to
the south. Less amplified trends for flow aloft have increased
the possibility that the front trailing from the weekend eastern
U.S. system may hang up over Florida and the Gulf of Mexico, with
embedded southern stream shortwaves helping to enhance rainfall
along/north of the front. Some heavy rainfall may be possible
over the central/eastern Gulf Coast region and northern Florida
into the Southeast. With cold air pushing south/southeast with
time there will be increasing potential that the northern fringe
of precipitation across south-central latitudes could be in the
form of snow by early next week.
For temperatures the extremely cold anomalies over the central
U.S. will continue to be the dominant story with highs possibly at
least 30-40F below normal over the northern-central Plains from
Sat onward. Especially by Sun there may be fairly broad coverage
of locations nearing or reaching daily records for cold highs with
some scattered daily record lows also possible. Some of this cold
air will spread into the East by Sun-Tue with temperatures 10-20F
below normal even reaching the East Coast. The Northwest will be
chilly as well with some interior areas 10-20F below normal most
days. Above normal temperatures should be confined to mostly
morning lows over the southern half of the West and for some
min/max readings over the South/East ahead of the initial Plains
cold front.
Bullet points from medium range hazards chart:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Rausch
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