Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
202 PM EDT Wed Apr 07 2021
Valid 12Z Sat Apr 10 2021 - 12Z Wed Apr 14 2021
...Overview and Guidance Evaluation/Predictability Assessment...
Models and means continue to show a blocky pattern covering
portions of North America into the Atlantic, with upper ridging
likely from eastern Canada through southern Greenland while a
couple of upper lows/surface systems head into the Great Lakes and
another upper low meanders near the Canadian Maritimes. Farther
west, as ridging builds into western Canada (consistent with
teleconnections relative to the eastern Canada/southern Greenland
ridge as well as the Pacific trough to the west) solutions
continue to diverge by early-mid week for possible western U.S.
troughing.
During the weekend there is good agreement in principle that a
compact Midwest into Great Lakes shortwave/upper low will promote
fairly strong and concentrated surface low development from the
Mississippi Valley into Great Lakes. There is still some spread
for track though. The front anchored by this low should continue
across the East with embedded low pressure continuing over the
western Atlantic. After this, a majority of guidance including the
ensemble means would have a larger scale shortwave reaching into
western North America by early Saturday, leading to an upper low
that crosses the northern tier states and eventually reaches the
Great Lakes by next Tuesday. The corresponding surface low would
bring another frontal system across the lower 48 during the
weekend and the first half of next week. Both upper lows exhibit
some sensitivity in terms of whether they stay in the Great Lakes
region or some/all of their energy may get carried farther
eastward by more progressive mean flow to the south. Some recent
GFS/CMC runs have tended to be the solutions exhibiting the latter
idea while their ensemble means have been holding back closer to
the ECMWF. On the other hand, the means could be lagging the
models in their depiction of the Canadian Maritimes upper low.
Confidence is low for precise movement of this low, with the
GFS/CMC actually bringing it back to or near New England by next
Wednesday versus remaining guidance that is farther east.
As for the western troughing, teleconnections relative to the
strong positive height anomalies over or just east of eastern
Canada continue to support the more modest depictions of the
GEFS/ECMWF/CMC means versus the deeper trough and embedded closed
low scenario that the GFS has been showing over the past couple
days and now joined by the 00Z CMC.
A general model blend served as a solid starting point for days
3-5 where large scale agreement is good. Thereafter, trended
towards the means while still incorporating about a third of the
ECMWF which seemed best in line with the ensembles. This approach
maintains excellent continuity with previous WPC continuity.
Uncertainty, especially past day 5 is low to average at best given
the possibility for pattern change should the guidance start
clustering towards any of the current minority ideas.
...Sensible Weather and Potential Hazards...
During the weekend expect the strengthening Mississippi Valley
into Great Lakes system and its associated fronts to focus areas
of moderate to heavy rainfall over portions of the eastern U.S.
Heaviest rain should be over and near the central and eastern Gulf
Coast, with a severe weather threat too as highlighted by the
Storm Prediction Center convective outlook for Saturday from the
Florida Panhandle into central Georgia. Significant rain could
also be possible closer to the upper/surface low in the Upper
Great Lakes depending on training within any rain bands. The next
system emerging from the West will bring mostly light rain/higher
elevation snow to northern parts of that region and then some
rain/thunderstorms to the eastern half of the country but in most
cases with lighter amounts than expected for the preceding system.
Some more organized rains could also focus north of a trailing
frontal boundary from the Southern Plains to lower Mississippi
Valley/Gulf Coast. There is increased potential for another round
of moderate to heavy rainfall across parts of the central Gulf
Coast by next Tuesday-Wednesday. Some of the precipitation could
be in the form of snow over parts of the extreme northern tier
from late weekend into next week. During the early to middle part
of next week the combination of cyclonic flow aloft and low level
upslope flow could produce some precipitation over the
central/south-central Rockies, though with considerable
uncertainty over coverage and amounts.
Warm flow ahead of the developing storm tracking into the Great
Lakes this weekend will bring widespread lows of 10-20F or so
above normal over the East Saturday-Sunday and plus 10-25F
anomalies for highs in a more focused area from the Great
Lakes/Upper Ohio Valley into New England on Saturday. Temperatures
will drop to within a few degrees on either side of normal behind
this system's cold front. The next system initially coming into
western North America will spread below normal highs (minus 5-15F
anomalies) across the Northwest/northern Rockies during the
weekend and then into the central/east-central U.S. Monday into
Wednesday. Southern parts of the West will remain above normal
through the period and an upper ridge approaching from the Pacific
may expand coverage of plus 10-20F anomalies for highs over
northern California and the Northwest early-mid week.
Santorelli/Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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